If I understand it correctly, humans can discriminate shades and colours based on their inherent contrast, as in we can see colors because there are different colors. Same for shades and for acoustic phenomens, because to differentiate two sounds they have to differ in some quality of sound: pitch, loudness, duration.
So the question is whether all perception is contrast based. One I would presume not to be contrast based is spatial perception, because it simply is there and always works in a standard manner.
A contrast effect is the enhancement or diminishment, relative to normal, of perception, cognition or related performance as a result of successive (immediately previous) or simultaneous exposure to a stimulus of lesser or greater value in the same dimension. (Here, normal perception, cognition or performance is that which would be obtained in the absence of the comparison stimulus—i.e., one based on all previous experience.)
Perception example: A neutral gray target will appear lighter or darker than it does in isolation when immediately preceded by, or simultaneously compared to, respectively, a dark gray or light gray target.
Cognition example: A person will appear more or less attractive than that person does in isolation when immediately preceded by, or simultaneously compared to, respectively, a less or more attractive person.
Performance example: A laboratory rat will work faster, or slower, during a stimulus predicting a given amount of reward when that stimulus and reward are immediately preceded by, or alternated with, respectively, different stimuli associated with either a lesser or greater amount of reward.
Background: When we reach out to pick up an object, not only do we direct our moving limb towards the location of the object, but the opening between our fingers and thumb is scaled in flight to the object's size. Evidence obtained from patients with neurological disorders has shown that the visual processing underlying the calibration of grip aperture and other movement parameters during grasping is mediated by visual mechanisms located in the cerebral cortex that are quite distinct from those underlying the experiential perception of object size and other object features. Under appropriate conditions, such dissociations can also be observed in individuals with normal vision. Here we present evidence that the calibration of grasp is quite refractory to pictorial illusions that have large effects on perceptual judgements of size.
Results We used a variation of the familiar ‘Titchener circles’ illusion in which two target circles of equal size, each surrounded by a circular array of either smaller or larger circles, are presented side by side. Subjects typically report that the target circle surrounded by the array of smaller circles appears to be larger than the target surrounded by larger circles. In our test, two thin ‘poker-chip’ discs were used as the target circles. The relative size of the two discs was randomly varied so that on some trials the discs appeared perceptually different but were physically equivalent in size, and on other trials they were physically different but appeared perceptually equivalent. The perceptual judgements made by the 14 subjects in our experiment were strongly affected by this size-contrast illusion. However, when asked to pick up a disc, the scaling of the subjects grip aperture (measured opto-electronically before contact with the disc) was largely determined by the true size of the target disc and not its illusory size.
Conclusion It would seem that the automatic and metrically accurate calibrations required for skilled actions are mediated by visual processes that are separate from those mediating our conscious experiential perception. Earlier studies on patients with neurological deficits suggest that these two types of processing may depend on quite separate, but interacting, visual pathways in the cerebral cortex.
Salvatore Aglioti, Dipartimento di Scienze Neurologiche, e Della Visione, Sezione di Fisologia Umana, Universita Degli Studi di Verona, 37134 Verona, Italy.
Joseph F.X. DeSouza, Department of Physiology , University of Western Ontario, London, Ontario N6A 5C1, Canada.
Melvyn A. Goodale (corresponding author), Department of Psychology, University of Western Ontario, London, Ontario N6A 5C2, Canada. E-mail address: [email protected]
The Neuroscience of Beauty
The notion of &ldquothe aesthetic&rdquo is a concept from the philosophy of art of the 18th century according to which the perception of beauty occurs by means of a special process distinct from the appraisal of ordinary objects. Hence, our appreciation of a sublime painting is presumed to be cognitively distinct from our appreciation of, say, an apple. The field of &ldquoneuroaesthetics&rdquo has adopted this distinction between art and non-art objects by seeking to identify brain areas that specifically mediate the aesthetic appreciation of artworks.
However, studies from neuroscience and evolutionary biology challenge this separation of art from non-art. Human neuroimaging studies have convincingly shown that the brain areas involved in aesthetic responses to artworks overlap with those that mediate the appraisal of objects of evolutionary importance, such as the desirability of foods or the attractiveness of potential mates. Hence, it is unlikely that there are brain systems specific to the appreciation of artworks instead there are general aesthetic systems that determine how appealing an object is, be that a piece of cake or a piece of music.
We set out to understand which parts of the brain are involved in aesthetic appraisal. We gathered 93 neuroimaging studies of vision, hearing, taste and smell, and used statistical analyses to determine which brain areas were most consistently activated across these 93 studies. We focused on studies of positive aesthetic responses, and left out the sense of touch, because there were not enough studies to arrive at reliable conclusions.
The results showed that the most important part of the brain for aesthetic appraisal was the anterior insula, a part of the brain that sits within one of the deep folds of the cerebral cortex. This was a surprise. The anterior insula is typically associated with emotions of negative quality, such as disgust and pain, making it an unusual candidate for being the brain&rsquos &ldquoaesthetic center.&rdquo Why would a part of the brain known to be important for the processing of pain and disgust turn out to the most important area for the appreciation of art?
Our interpretation of the result comes from cognitive theories of emotion that argue that aesthetic processing is, at its core, the appraisal of the value of an object -- in other words, an assessment of whether an object is &ldquogood for me&rdquo or &ldquobad for me.&rdquo The nature of this appraisal depends very strongly on what my current physiological state is. The sight of chocolate cake will lead to positive aesthetic emotions if I&rsquom famished but to feelings of disgust if I&rsquom sick to my stomach. Objects that satisfy current physiological needs will lead to positive aesthetic emotions (e.g., pleasure). Those that oppose these needs will lead to negative emotions (e.g., repulsion).
How does the anterior insula fit into this story? In thinking about the contrast between internal and external environments, the anterior insula seems to be much more associated with the former than the latter. It is part of the brain&rsquos &ldquointeroceptive&rdquo system, evaluating the state of the organs of our body. Other parts of the brain, then, respond directly to objects in the external environment: the sensory pathways of the brain. (One part of the cortex that seems particularly important for processing information across many sensory modalities is the orbitofrontal cortex.)
Brain areas such as the anterior insula and orbitofrontal cortex that are activated by pleasant smells or tastes are also the parts of the brain that are active when we are awed by Renaissance paintings or Baroque concertos. There is virtually no evidence that artworks activate emotion areas distinct from those involved in appraising everyday objects important for survival. Hence, the most reasonable evolutionary hypothesis is that the aesthetic system of the brain evolved first for the appraisal of objects of biological importance, including food sources and suitable mates, and was later co-opted for artworks such as paintings and music. As much as philosophers like to believe that our brains contain a specialized system for the appreciation of artworks, research suggests that our brain&rsquos responses to a piece of cake and a piece of music are in fact quite similar.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.
ABOUT THE AUTHOR(S)
Steven Brown is director of the NeuroArts Lab in the Department of Psychology, Neuroscience & Behaviour at McMaster University in Hamilton, Ontario. His research deals with the neural and evolutionary basis of the arts, including music, dance, acting and drawing. Xiaoqing Gao is a postdoctoral fellow at the Centre for Vision Research, York University in Toronto, Ontario. He studies the development and neural basis of face perception.
Illusions and Paradoxes: Seeing is Believing?
This page illustrates that our visual perception cannot always be trusted. The components of an object can distort the perception of the complete object. Our mind is the final arbiter of truth. Most optical illusions are the result of 1) incongruent design elements at opposite ends of parallel lines, 2) influence of background patterns on the overall design, 3) adjustment of our perception at the boundaries of areas of high contrast, 4) afterimages resulting from eye movements or from kinetic displays, or 5) inability to interpret the spatial structure of an object from the context provided by the picture.
Optical illusions have been studied for millenia. The ancient Greeks used a technique known as entasis which incorporates a slight convexity in the columns of the Parthenon to compensate for the illusion of concavity created by parallel lines. Many of the following illusions have been popularized by psychologists and artists like Hering, Ehrenstein, Meyer, Zöllner, Müller-Lyer, Poggendorf, and Escher.
The image in the lower right corner is upside-down, and the image to the right is rotating. Our interpretation of bumps and indentations is conditioned by the fact that objects are generally illuminated from the top. The rotating image may be interpreted as a wobbly elongated object viewed from the end (like a finger pointed in your direction) or as a ball rotating inside a washing machine viewed through the porthole. The ambiguity is caused because we don't have any clues to decide whether the bright portion of the image is above or below the display plane.
The circles appear to rotate when you move your head closer and further away from the screen while looking at the dot in the center. Our peripheral vision interprets the relative increase or decrease of the image in the retina as rotational motion of the slanted lines.
The retina is the part of the eye covered with receptors that respond to light. A small portion of the retina where the optic nerve connects to the brain has no receptors. An image that falls on this region will not be seen. Close your right eye. With your left eye, look at the L below. Slowly move your head closer or further away from the screen while looking at the L. The R will disappear when your head is approximately 50 cm (20 in) from the screen. You can repeat the experiment with your right eye by looking at the R.
An afterimage is a visual impression that remains in the retina after the initial stimulus is removed. The afterimage always has colors that are complementary to those of the original image. Look steadily at the cross in the center of the picture to see an afterimage.
Jeremy L. Hinton ca. 2005, "Lilac Chaser"
Take two pieces of heavy paper. On one of them make three holes with a pin spaced about 2 mm apart (1/16 inch) from each other forming a triangle. On the other one piece of paper, make a single hole with the pin. Place the card with the three holes next to your eye and look through the holes at the card with one hole. You will see three holes instead of one, and the pattern of holes will be upside down.
Stereoscopic vision makes depth perception possible. By crossing your eyes while looking at these pictures, the brain perceives a combined three-dimensional image. (Hint: Keep your eyes level with the pictures. Place your fingertip between the pictures just below the sun and look at your fingertip while you bring your finger toward your eyes. When your fingertip is approximately 7 inches (20 cm) from your eyes, the pictures in the background will combine into a 3-dimensional picture.)
Which of the following two images of the tower of Pisa seems to be leaning more?
The images are actually identical, but the tower on the right seems to lean more because the human visual system treats the two images as one scene. Our brains are conditioned to expect parallel towers to converge toward a common vanishing point, but because the tower on the right does not converge, our visual system interprets that it is leaning at a different angle. Below is a perspective drawing with three vanishing points of what our eyes expect.
Hold your head steady and fix your eyes on the dot in the center of the picture. The colored dots will seem to disappear in a few seconds. The effect is due to retinal fatigue which occurs when the afterimage of an object cancels the stimulus of the object on the retina. The effect is most pronounced when the objects do not have well-defined edges that are detectable by small eye movements.
The circles above appear distorted due to the black and white designs which are at various angles relative to the tangent of the circles.
The image below consists of circles formed from alternating black and white squares angled at 15 degrees relative to the tangent of the circles. The circles appear to form helical patterns because the squares in each of the adjacent concentric circles incline in opposite directions.
The squares labeled A and B are the same shade of gray. This can be verified by joining the squares marked A and B with two vertical stripes of the same shade of gray. The illusion that B is lighter than A is caused by the relative contrast of the surrounding dark squares and by the fact that our vision compensates for the shadow of the cylinder. Created by Edward H. Adelson, Professor of Vision Science at MIT.
Although there are only circles with sections taken out of them, our eyes strive to see triangles. The sides of the triangles may appear curved when the angles of the sections do not add up to 180 degrees.
A portion of misplaced lines can be clearly identified as forming a circle, even when there is no outline of a circle.
Our ability to reconstruct an image enables us to recognize a face even when half of the image has been blocked, including parts of the eyes, nose and mouth.
Animals which blend with the color and texture of their environment are more likely to survive either as prey or as predators. Camouflaged prey have a greater chance of surviving by avoiding detection, whereas camouflaged predators can hunt more successfully if they can approach the prey without being seen.
The coloration of zebras makes them very conspicuous in the African plains, but the pattern of black and white stripes makes it very hard for predators to distinguish one individual in the middle of the herd. Do you see eight or nine zebras?
In the 16th century, Giuseppe Arcimboldo became known for his portraits of human heads made entirely of fruits, vegetables and flowers. Some of his paintings of bowls of vegetables had to be inverted in order to see the bowl become a hat for the human head. This whimsical art form is still popular today. The following drawing of a frog turns into the head of a horse when the image is rotated.
|Place the cursor on the images to see the alternate interpretations.|
The figure in the top can be interpreted as a cube or as a corner. The darker shading of the bottom section reinforces the interpretation of a cube illuminated from the top. The figures below it add some elements that help us to disambiguate.
On what leg is the dancer standing? The direction of rotation of silhouettes may be ambiguous. This dancer created by Nobuyuki Kayahara stands on her left leg when she appears to be rotating clockwise, but on her right leg when she appears to rotate counter-clockwise.
People with normal color vision can perceive numbers formed by patterns of colored dots in every circle. If you do not see some of the numbers, you should have your eyes checked and consider working in a job where color discrimination is not critical.
Approximately 6%-8% of people of European descent, 4%-6% of people of Asian descent, and 2%-4% of people of African descent have some type of defective color vision. Images based on Tests for Colour Blindness by Dr. Shinobu Ishihara.
As an experiment, look at these circles with blue-red 3D glasses, first with one eye, and then with the other. Some of the numbers will not be visible! Also, use the glasses to look at the word color test below and explain the results.
This is a type of psycholinguistic test that poses some difficulty because the portion of the brain that handles language has the conflicting tasks of verbalizing the color of the written words while ignoring the meaning of words representing colors.
Dr. Marc Amsler developed the use of a grid of horizontal and vertical lines to monitor a person's central visual field. The test is performed by first covering one eye and looking at the center dot. The test is repeated by covering the other eye. Any distortions, wavy lines, blurred areas or blank spots may be an indication of macular degenertion.
On December 16, 1997 hundreds of Japanese children suffered seizures and convulsions following their viewing of a "Pocket Monsters" cartoon on television. Most children said they felt sick and had vision problems during a scene where the entire background was flashing red and blue. Additional children ended up in the hospital after the cartoon segment was replayed in the evening news. Neurologists believe that the children suffered photosensitive epilepsy induced by the flashing. Abnormal EEG can be triggered by flickering lights in a small percentage of persons when the flickering frequency is 5-10 hertz for children and 15-20 hertz for older people. Excessive TV watching can damage a child's development and education.
Warning: Do not place your mouse cursor here if you are subject to seizures.
Put Mouse Pointer Here to Animate
Moiré patterns are formed when two grids or line drawings are superimposed. The intersections of the lines create new patterns not present in the originals. This figure is created by overlapping two drawings consisting of lines radiating from a point. The interference pattern creates circles that cross both points.
The colored pieces of this puzzle can be rearranged to form two "13 by 5 right triangles" that have different surface areas. This is a visual paradox that can be explained mathematically.
Humpty Dumpty is about to take a great fall because he just found out that the two red lines are equal in length. Take a ruler and connect the tops or bottoms of the red lines. The brain interprets the converging lines as providing perspective. This interpretation is so powerful that it is virtually impossible to overcome its influence.
Fraunhofer diffraction is a type of optical wave diffraction that occurs when field waves are passed through an aperture or slit, causing the size of an observed aperture image to change due to the far-field location of observation. This image shows how the slits of vertical blinds in a window bend the rays of the sun and influence the shapes of the shadows projected on the wall. As the ears approach the shadows of the vertical blinds, the shadow of the ears stretches toward the shadow of the vertical blind to produce elongated ear shadows. The shadow of the head seems to grow horns at the points where the shadows of the blinds intersect the head.
These people are shifting places trying to hide. Sometimes you can count 13 people and sometimes 12. Who is missing when the count is 12?
Special effects produced by computer animation enhance many modern films. Click the link below to see some short clips of Star Wars light sabers in action.
What is a Contrast Effect? (with pictures)
The contrast effect is a phenomenon where people perceive greater or lesser differences than are actually present as a result of prior or simultaneous exposure to something with similar base characteristics, but different key qualities. In a simple example of how the contrast effect works with vision, a researcher can present a subject with a dark square and a light square, each enclosing a smaller square. Even if the smaller squares are actually the same color, the contrast effect will lead the viewer to think the square against the dark background is lighter than the one against the light background.
Visual perception is not the only thing the contrast effect can skew. This can also occur with human cognition, in an example of a cognitive bias. A teaching assistant might grade a mediocre essay more harshly after reading a very good writing sample, for example. People can utilize this effect in sales. A coffin salesperson can show people the same medium-range coffin in the midst of low-end products and high-end products, and they will perceive it differently depending on the surrounding comparison samples. This may encourage people to spend more than they would otherwise.
In the positive contrast effect, people will perceive something as better than it is as a result of exposure to a worse comparison sample, while in the negative version, people will think something is worse because they have a better comparison sample. This cognitive bias is extremely difficult to overcome, as it is naturally engrained in the brain and the way people think about and perceive the world around them.
Awareness of the contrast effect can lead people to try and take steps to compensate for it. In grading, for instance, people have a rubric they can use to create a more objective standard, and a teaching assistant may take a random sample of papers with various grades and ask another assistant to look them over and make sure the grades are fair. Concerns about visual contrast are especially important with signs and graphic design, where colors can appear abnormal depending on their surroundings and how people handle them.
Like other cognitive biases, the contrast effect can explain some seemingly contradictory human behaviors, and it is an important part of human psychology. People may engage in activities against their own best interests as a result of these biases, and could also do things that seem out of character, like spending more on a purchase than originally planned because of clever sales tactics exploiting known psychological vulnerabilities.
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
What is Sensation?
The term Sensation has to be understood as the process of using our sensory organs. Vision, hearing, smell, taste and touch are the main sensory organs that we use. In psychology, this is considered as one of the basic processes of human beings to make sense of the world around them. However, this is only a primary process. Now let us look at the term sensation in the general usage. It is interesting to note that the word ‘sensation’ has its adjectival form in the word ‘sensational’, whereas the word ‘perception’ has its adjectival form in the word ‘perceptive’.
Observe the two sentences:
1. He created a sensation among youngsters.
2. A leper has no sensation on his skin.
In both the sentences, you can find that the word ‘sensation’ is used in the sense of ‘feeling’ and hence, the meaning of the first sentence would be ‘he created a feeling among youngsters’, and the meaning of the second sentence would be ‘a leper has no feeling on his skin’. This highlights that the term sensation can be understood at different levels, which brings out various meanings.
Difference Between Sensation and Perception
We have five different sensory organs: eyes, nose, ears, tongue, and skin. These five sensory organs are responsible for receiving different stimulations around us through seeing, smelling, hearing, tasting, and, finally, feeling through the skin. The signals which are received through our sensory organs from the environment around us are called sensations. Simply put, sensations are what our sense organs receive and transmit to the brain. Once the brain receives the stimulus, it converts the whole signal into feelings, taste, sound, sight, and smell. On the other hand, perception is almost like a sixth sense. It is what we perceive or form an opinion on of anything and everything happening around us.
The perception of a person is an absolutely personal experience. It is what a person thinks about his or her environment, and it is how the person looks at the world around him. It is more of a psychological concept than anything physical like sensations. Two different people can have different perceptions about the same thing. For example, in body image, a healthy person has a different perception about his or her body. Healthy people, even if they are a little overweight, react and see themselves differently and accept who they are or work towards achieving what they want. Once they achieve it, they stop. While an anorexic person, no matter how slim, no matter how underweight, has a perception that they are still overweight and stops eating food altogether to achieve, what they perceive, to be the right body for them.
Perception is what a person wants to believe, their personal opinion. People of different generations or people of different religions or people from different backgrounds have a difference of opinion only because they perceive everything differently. Wise people try to understand the perceptions of other people whereas unwise people believe that what they perceive about a situation or about a person is the only correct perception.
Perception and sensation are different mostly because sensation is more physical. Sensations arise only because the body receives a stimulus, and the body reacts to it converting the stimulus into one of the things that one of the sensory organs of the body can identify. However, perception is absolutely psychological. Perceptions are individual thoughts of individual people.
Sensation is the process of hearing, smelling, feeling, tasting, and seeing as a result of external stimulations received by the five sensory organs of the body ears, eyes, nose, tongue, and skin. Perception, however, is the mental image of something or somebody made due to the different actions exhibited by the environment around us.
The Five Senses and the Nature of Perception
We perceive the world through our five senses—our eyes, ears, skin, nose, and mouth are all receptors. Everything that comes into the brain enters through one of these doors. Because most of us take the world in through our senses effortlessly, we don’t give much thought or attention to how we do this.
Even scientists were guilty of underappreciating the complexity of the senses. Back in the 1950s and 1960s, when computers were in their infancy, the thinking was that it would take a decade or so to build “perceiving machines” that could respond to sight, sound, touch and so on as well as a human being. Such a machine still doesn’t exist.
Lose a sense, however, and you will quickly appreciate what is missing. I know because that’s what happened to me when I found out my son was deaf. There was so much to learn about the way hearing works and the role of sound in the brain that I wrote a whole book about it. That was the long version.
This is the short version. What has to happen to put on the show that is our awareness of our environment? An awful lot. Neuroscientists have recently done some radical rethinking about the very nature of perception.
“Historically, the way we intuitively think about all perception is that we’re like a passive recording device with detectors that are specialized for certain things, like a retina for seeing, a cochlea for hearing, and so forth,” says David Poeppel, a professor of psychology and neural science at New York University and a director of the newly established Max Planck Institute for Empirical Aesthetics. “We’re kind of a camera or microphone that gets encoded somehow and then magically makes contact with the stuff in your head.”
At the same time, many of the big thinkers who pondered perception, as far back as the 19th-century German physician Hermann von Helmholtz, knew that couldn’t be quite right. If we reached for a glass or listened to a sentence, didn’t it help to be able to anticipate what might come next?
In the mid-to-late 20th century, a handful of prominent researchers proposed models of perception that suggested that we engaged in “active sensing,” seeking out what was possible as we went along. Such ideas didn’t gain much traction until the past decade, when they suddenly became a hot topic in the study of cognition. What everyone is talking about today is the brain’s power of prediction.
On one level, prediction is just common sense, which may be one reason it didn’t get much scientific respect for so long. If you see your doctor in the doctor’s office, you recognize her quickly. If you see her in the grocery store dressed in jeans, you’ll be slower to realize you know her.
Predictable events are easy for the brain unpredictable events require more effort. “Our expectations for what we’re going to perceive seem to be a critical part of the process,” says Greg Hickok, a neuroscientist at the University of California, Irvine. “It allows the system to make guesses as to what it might be seeing and to use computational shortcuts.”
In the old view of perception, a cascade of responses flows from the ear or the eye through the brain and ends with the ability to follow a complicated sentence or pick out the one person you are looking for in a crowded theater. That is known as bottom-up processing. It starts with basic input to any sense—raw data—and ends with such higher-level skills as reasoning and judgment and critical thinking—in other words, our expectations and knowledge.
But that is only half the story. Neuroscientists now believe that the process is also happening in reverse, that the cascade flows both ways, with information being prepared, treated, and converted in both directions simultaneously, from the bottom up and the top down.
This holds for simple responses as well as for complex thinking about philosophy or physics. If a sound is uncomfortably loud, for instance, it is the cortex that registers that fact and sends a message all the way back to the cochlea to stiffen hair cells as a protective measure. The same is true of the retina, adjusting for the amount of light available. It’s not your eye or ear doing that, it’s your brain.
Imagine someone beating rhythmically on a table with a pencil: tap, tap, tap, tap. By the third beat, you have anticipated the timing. By the fourth, scientists like Poeppel and Hickok could see activity in the brain that represents that prediction.
Perception then is an active process of constructing a reality, a conversation between the senses and the cortex that balances new information from the outside world with predictions from the interior world of our brain.
Parts of this post originally appeared in I Can Hear You Whisper: An Intimate Journey through the Science of Sound and Language (Dutton 2014).
The process of perception begins with an object in the real world, known as the distal stimulus or distal object.  By means of light, sound, or another physical process, the object stimulates the body's sensory organs. These sensory organs transform the input energy into neural activity—a process called transduction.   This raw pattern of neural activity is called the proximal stimulus.  These neural signals are then transmitted to the brain and processed.  The resulting mental re-creation of the distal stimulus is the percept.
To explain the process of perception, an example could be an ordinary shoe. The shoe itself is the distal stimulus. When light from the shoe enters a person's eye and stimulates the retina, that stimulation is the proximal stimulus.  The image of the shoe reconstructed by the brain of the person is the percept. Another example could be a ringing telephone. The ringing of the phone is the distal stimulus. The sound stimulating a person's auditory receptors is the proximal stimulus. The brain's interpretation of this as the "ringing of a telephone" is the percept.
The different kinds of sensation (such as warmth, sound, and taste) are called sensory modalities or stimulus modalities.  
Bruner's model of the perceptual process Edit
Psychologist Jerome Bruner developed a model of perception, in which people put "together the information contained in" a target and a situation to form "perceptions of ourselves and others based on social categories."   This model is composed of three states:
- When we encounter an unfamiliar target, we are very open to the informational cues contained in the target and the situation surrounding it.
- The first stage doesn't give us enough information on which to base perceptions of the target, so we will actively seek out cues to resolve this ambiguity. Gradually, we collect some familiar cues that enable us to make a rough categorization of the target. (see also Social Identity Theory)
- The cues become less open and selective. We try to search for more cues that confirm the categorization of the target. We also actively ignore and even distort cues that violate our initial perceptions. Our perception becomes more selective and we finally paint a consistent picture of the target.
Saks and John's three components to perception Edit
According to Alan Saks and Gary Johns, there are three components to perception: 
- The Perceiver: a person whose awareness is focused on the stimulus, and thus begins to perceive it. There are many factors that may influence the perceptions of the perceiver, while the three major ones include (1) motivational state, (2) emotional state, and (3) experience. All of these factors, especially the first two, greatly contribute to how the person perceives a situation. Oftentimes, the perceiver may employ what is called a "perceptual defense," where the person will only "see what they want to see"—i.e., they will only perceives what they want to perceive even though the stimulus acts on his or her senses.
- The Target: the object of perception something or someone who is being perceived. The amount of information gathered by the sensory organs of the perceiver affects the interpretation and understanding about the target.
- The Situation: the environmental factors, timing, and degree of stimulation that affect the process of perception. These factors may render a single stimulus to be left as merely a stimulus, not a percept that is subject for brain interpretation.
Multistable perception Edit
Stimuli are not necessarily translated into a percept and rarely does a single stimulus translate into a percept. An ambiguous stimulus may sometimes be transduced into one or more percepts, experienced randomly, one at a time, in a process termed "multistable perception." The same stimuli, or absence of them, may result in different percepts depending on subject's culture and previous experiences.
Ambiguous figures demonstrate that a single stimulus can result in more than one percept. For example, the Rubin vase can be interpreted either as a vase or as two faces. The percept can bind sensations from multiple senses into a whole. A picture of a talking person on a television screen, for example, is bound to the sound of speech from speakers to form a percept of a talking person.
In many ways, vision is the primary human sense. Light is taken in through each eye and focused in a way which sorts it on the retina according to direction of origin. A dense surface of photosensitive cells, including rods, cones, and intrinsically photosensitive retinal ganglion cells captures information about the intensity, color, and position of incoming light. Some processing of texture and movement occurs within the neurons on the retina before the information is sent to the brain. In total, about 15 differing types of information are then forwarded to the brain proper via the optic nerve. 
Hearing (or audition) is the ability to perceive sound by detecting vibrations (i.e., sonic detection). Frequencies capable of being heard by humans are called audio or audible frequencies, the range of which is typically considered to be between 20 Hz and 20,000 Hz.  Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic.
The auditory system includes the outer ears, which collect and filter sound waves the middle ear, which transforms the sound pressure (impedance matching) and the inner ear, which produces neural signals in response to the sound. By the ascending auditory pathway these are led to the primary auditory cortex within the temporal lobe of the human brain, from where the auditory information then goes to the cerebral cortex for further processing.
Sound does not usually come from a single source: in real situations, sounds from multiple sources and directions are superimposed as they arrive at the ears. Hearing involves the computationally complex task of separating out sources of interest, identifying them and often estimating their distance and direction. 
The process of recognizing objects through touch is known as haptic perception. It involves a combination of somatosensory perception of patterns on the skin surface (e.g., edges, curvature, and texture) and proprioception of hand position and conformation. People can rapidly and accurately identify three-dimensional objects by touch.  This involves exploratory procedures, such as moving the fingers over the outer surface of the object or holding the entire object in the hand.  Haptic perception relies on the forces experienced during touch. 
Gibson defined the haptic system as "the sensibility of the individual to the world adjacent to his body by use of his body."  Gibson and others emphasized the close link between body movement and haptic perception, where the latter is active exploration.
The concept of haptic perception is related to the concept of extended physiological proprioception according to which, when using a tool such as a stick, perceptual experience is transparently transferred to the end of the tool.
Taste (formally known as gustation) is the ability to perceive the flavor of substances, including, but not limited to, food. Humans receive tastes through sensory organs concentrated on the upper surface of the tongue, called taste buds or gustatory calyculi.  The human tongue has 100 to 150 taste receptor cells on each of its roughly-ten thousand taste buds. 
Traditionally, there have been four primary tastes: sweetness, bitterness, sourness, and saltiness. However, the recognition and awareness of umami, which is considered the fifth primary taste, is a relatively recent development in Western cuisine.   Other tastes can be mimicked by combining these basic tastes,   all of which contribute only partially to the sensation and flavor of food in the mouth. Other factors include smell, which is detected by the olfactory epithelium of the nose  texture, which is detected through a variety of mechanoreceptors, muscle nerves, etc.   and temperature, which is detected by thermoreceptors.  All basic tastes are classified as either appetitive or aversive, depending upon whether the things they sense are harmful or beneficial. 
Smell is the process of absorbing molecules through olfactory organs, which are absorbed by humans through the nose. These molecules diffuse through a thick layer of mucus come into contact with one of thousands of cilia that are projected from sensory neurons and are then absorbed into a receptor (one of 347 or so).  It is this process that causes humans to understand the concept of smell from a physical standpoint.
Smell is also a very interactive sense as scientists have begun to observe that olfaction comes into contact with the other sense in unexpected ways.  It is also the most primal of the senses, as it is known to be the first indicator of safety or danger, therefore being the sense that drives the most basic of human survival skills. As such, it can be a catalyst for human behavior on a subconscious and instinctive level. 
Social perception is the part of perception that allows people to understand the individuals and groups of their social world. Thus, it is an element of social cognition. 
Speech perception is the process by which spoken language is heard, interpreted and understood. Research in this field seeks to understand how human listeners recognize the sound of speech (or phonetics) and use such information to understand spoken language.
Listeners manage to perceive words across a wide range of conditions, as the sound of a word can vary widely according to words that surround it and the tempo of the speech, as well as the physical characteristics, accent, tone, and mood of the speaker. Reverberation, signifying the persistence of sound after the sound is produced, can also have a considerable impact on perception. Experiments have shown that people automatically compensate for this effect when hearing speech.  
The process of perceiving speech begins at the level of the sound within the auditory signal and the process of audition. The initial auditory signal is compared with visual information—primarily lip movement—to extract acoustic cues and phonetic information. It is possible other sensory modalities are integrated at this stage as well.  This speech information can then be used for higher-level language processes, such as word recognition.
Speech perception is not necessarily uni-directional. Higher-level language processes connected with morphology, syntax, and/or semantics may also interact with basic speech perception processes to aid in recognition of speech sounds.  It may be the case that it is not necessary (maybe not even possible) for a listener to recognize phonemes before recognizing higher units, such as words. In an experiment, Richard M. Warren replaced one phoneme of a word with a cough-like sound. His subjects restored the missing speech sound perceptually without any difficulty. Moreover, they were not able to accurately identify which phoneme had even been disturbed. 
Facial perception refers to cognitive processes specialized in handling human faces (including perceiving the identity of an individual) and facial expressions (such as emotional cues.)
Social touch Edit
The somatosensory cortex is a part of the brain that receives and encodes sensory information from receptors of the entire body. 
Affective touch is a type of sensory information that elicits an emotional reaction and is usually social in nature. Such information is actually coded differently than other sensory information. Though the intensity of affective touch is still encoded in the primary somatosensory cortex, the feeling of pleasantness associated with affective touch is activated more in the anterior cingulate cortex. Increased blood oxygen level-dependent (BOLD) contrast imaging, identified during functional magnetic resonance imaging (fMRI), shows that signals in the anterior cingulate cortex, as well as the prefrontal cortex, are highly correlated with pleasantness scores of affective touch. Inhibitory transcranial magnetic stimulation (TMS) of the primary somatosensory cortex inhibits the perception of affective touch intensity, but not affective touch pleasantness. Therefore, the S1 is not directly involved in processing socially affective touch pleasantness, but still plays a role in discriminating touch location and intensity. 
Multi-modal perception Edit
Multi-modal perception refers to concurrent stimulation in more than one sensory modality and the effect such has on the perception of events and objects in the world. 
Time (chronoception) Edit
Chronoception refers to how the passage of time is perceived and experienced. Although the sense of time is not associated with a specific sensory system, the work of psychologists and neuroscientists indicates that human brains do have a system governing the perception of time,   composed of a highly distributed system involving the cerebral cortex, cerebellum, and basal ganglia. One particular component of the brain, the suprachiasmatic nucleus, is responsible for the circadian rhythm (commonly known as one's "internal clock"), while other cell clusters appear to be capable of shorter-range timekeeping, known as an ultradian rhythm.
One or more dopaminergic pathways in the central nervous system appear to have a strong modulatory influence on mental chronometry, particularly interval timing. 
Sense of agency refers to the subjective feeling of having chosen a particular action. Some conditions, such as schizophrenia, can cause a loss of this sense, which may lead a person into delusions, such as feeling like a machine or like an outside source is controlling them. An opposite extreme can also occur, where people experience everything in their environment as though they had decided that it would happen. 
Even in non-pathological cases, there is a measurable difference between the making of a decision and the feeling of agency. Through methods such as the Libet experiment, a gap of half a second or more can be detected from the time when there are detectable neurological signs of a decision having been made to the time when the subject actually becomes conscious of the decision.
There are also experiments in which an illusion of agency is induced in psychologically normal subjects. In 1999, psychologists Wegner and Wheatley gave subjects instructions to move a mouse around a scene and point to an image about once every thirty seconds. However, a second person—acting as a test subject but actually a confederate—had their hand on the mouse at the same time, and controlled some of the movement. Experimenters were able to arrange for subjects to perceive certain "forced stops" as if they were their own choice.  
Recognition memory is sometimes divided into two functions by neuroscientists: familiarity and recollection.  A strong sense of familiarity can occur without any recollection, for example in cases of deja vu.
The temporal lobe (specifically the perirhinal cortex) responds differently to stimuli that feel novel compared to stimuli that feel familiar. Firing rates in the perirhinal cortex are connected with the sense of familiarity in humans and other mammals. In tests, stimulating this area at 10–15 Hz caused animals to treat even novel images as familiar, and stimulation at 30–40 Hz caused novel images to be partially treated as familiar.  In particular, stimulation at 30–40 Hz led to animals looking at a familiar image for longer periods, as they would for an unfamiliar one, though it did not lead to the same exploration behavior normally associated with novelty.
Recent studies on lesions in the area concluded that rats with a damaged perirhinal cortex were still more interested in exploring when novel objects were present, but seemed unable to tell novel objects from familiar ones—they examined both equally. Thus, other brain regions are involved with noticing unfamiliarity, while the perirhinal cortex is needed to associate the feeling with a specific source. 
Sexual stimulation Edit
Sexual stimulation is any stimulus (including bodily contact) that leads to, enhances, and maintains sexual arousal, possibly even leading to orgasm. Distinct from the general sense of touch, sexual stimulation is strongly tied to hormonal activity and chemical triggers in the body. Although sexual arousal may arise without physical stimulation, achieving orgasm usually requires physical sexual stimulation (stimulation of the Krause-Finger corpuscles  found in erogenous zones of the body.)
Other senses Edit
Other senses enable perception of body balance, acceleration, gravity, position of body parts, temperature, and pain. They can also enable perception of internal senses, such as suffocation, gag reflex, abdominal distension, fullness of rectum and urinary bladder, and sensations felt in the throat and lungs.
In the case of visual perception, some people can actually see the percept shift in their mind's eye.  Others, who are not picture thinkers, may not necessarily perceive the 'shape-shifting' as their world changes. This esemplastic nature has been demonstrated by an experiment that showed that ambiguous images have multiple interpretations on the perceptual level.
This confusing ambiguity of perception is exploited in human technologies such as camouflage and biological mimicry. For example, the wings of European peacock butterflies bear eyespots that birds respond to as though they were the eyes of a dangerous predator.
There is also evidence that the brain in some ways operates on a slight "delay" in order to allow nerve impulses from distant parts of the body to be integrated into simultaneous signals. 
Perception is one of the oldest fields in psychology. The oldest quantitative laws in psychology are Weber's law, which states that the smallest noticeable difference in stimulus intensity is proportional to the intensity of the reference and Fechner's law, which quantifies the relationship between the intensity of the physical stimulus and its perceptual counterpart (e.g., testing how much darker a computer screen can get before the viewer actually notices). The study of perception gave rise to the Gestalt School of Psychology, with an emphasis on holistic approach.
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic sensation (touch), taste and olfaction (smell), as listed above. It has been suggested that the immune system is an overlooked sensory modality.  In short, senses are transducers from the physical world to the realm of the mind.
The receptive field is the specific part of the world to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field the light that each rod or cone can see, is its receptive field.  Receptive fields have been identified for the visual system, auditory system and somatosensory system, so far. Research attention is currently focused not only on external perception processes, but also to "interoception", considered as the process of receiving, accessing and appraising internal bodily signals. Maintaining desired physiological states is critical for an organism's well-being and survival. Interoception is an iterative process, requiring the interplay between perception of body states and awareness of these states to generate proper self-regulation. Afferent sensory signals continuously interact with higher order cognitive representations of goals, history, and environment, shaping emotional experience and motivating regulatory behavior. 
Perceptual constancy is the ability of perceptual systems to recognize the same object from widely varying sensory inputs.  : 118–120  For example, individual people can be recognized from views, such as frontal and profile, which form very different shapes on the retina. A coin looked at face-on makes a circular image on the retina, but when held at angle it makes an elliptical image.  In normal perception these are recognized as a single three-dimensional object. Without this correction process, an animal approaching from the distance would appear to gain in size.   One kind of perceptual constancy is color constancy: for example, a white piece of paper can be recognized as such under different colors and intensities of light.  Another example is roughness constancy: when a hand is drawn quickly across a surface, the touch nerves are stimulated more intensely. The brain compensates for this, so the speed of contact does not affect the perceived roughness.  Other constancies include melody, odor, brightness and words.  These constancies are not always total, but the variation in the percept is much less than the variation in the physical stimulus.  The perceptual systems of the brain achieve perceptual constancy in a variety of ways, each specialized for the kind of information being processed,  with phonemic restoration as a notable example from hearing.
Grouping (Gestalt) Edit
The principles of grouping (or Gestalt laws of grouping) are a set of principles in psychology, first proposed by Gestalt psychologists, to explain how humans naturally perceive objects as organized patterns and objects. Gestalt psychologists argued that these principles exist because the mind has an innate disposition to perceive patterns in the stimulus based on certain rules. These principles are organized into six categories:
- Proximity: the principle of proximity states that, all else being equal, perception tends to group stimuli that are close together as part of the same object, and stimuli that are far apart as two separate objects.
- Similarity: the principle of similarity states that, all else being equal, perception lends itself to seeing stimuli that physically resemble each other as part of the same object and that are different as part of a separate object. This allows for people to distinguish between adjacent and overlapping objects based on their visual texture and resemblance.
- Closure: the principle of closure refers to the mind's tendency to see complete figures or forms even if a picture is incomplete, partially hidden by other objects, or if part of the information needed to make a complete picture in our minds is missing. For example, if part of a shape's border is missing people still tend to see the shape as completely enclosed by the border and ignore the gaps.
- Good Continuation: the principle of good continuation makes sense of stimuli that overlap: when there is an intersection between two or more objects, people tend to perceive each as a single uninterrupted object.
- Common Fate: the principle of common fate groups stimuli together on the basis of their movement. When visual elements are seen moving in the same direction at the same rate, perception associates the movement as part of the same stimulus. This allows people to make out moving objects even when other details, such as color or outline, are obscured.
- The principle of good form refers to the tendency to group together forms of similar shape, pattern, color, etc. 
Later research has identified additional grouping principles. 
Contrast effects Edit
A common finding across many different kinds of perception is that the perceived qualities of an object can be affected by the qualities of context. If one object is extreme on some dimension, then neighboring objects are perceived as further away from that extreme.
"Simultaneous contrast effect" is the term used when stimuli are presented at the same time, whereas successive contrast applies when stimuli are presented one after another. 
The contrast effect was noted by the 17th Century philosopher John Locke, who observed that lukewarm water can feel hot or cold depending on whether the hand touching it was previously in hot or cold water.  In the early 20th Century, Wilhelm Wundt identified contrast as a fundamental principle of perception, and since then the effect has been confirmed in many different areas.  These effects shape not only visual qualities like color and brightness, but other kinds of perception, including how heavy an object feels.  One experiment found that thinking of the name "Hitler" led to subjects rating a person as more hostile.  Whether a piece of music is perceived as good or bad can depend on whether the music heard before it was pleasant or unpleasant.  For the effect to work, the objects being compared need to be similar to each other: a television reporter can seem smaller when interviewing a tall basketball player, but not when standing next to a tall building.  In the brain, brightness contrast exerts effects on both neuronal firing rates and neuronal synchrony. 
Perception as direct perception (Gibson) Edit
Cognitive theories of perception assume there is a poverty of stimulus. This is the claim that sensations, by themselves, are unable to provide a unique description of the world.  Sensations require 'enriching', which is the role of the mental model.
The perceptual ecology approach was introduced by James J. Gibson, who rejected the assumption of a poverty of stimulus and the idea that perception is based upon sensations. Instead, Gibson investigated what information is actually presented to the perceptual systems. His theory "assumes the existence of stable, unbounded, and permanent stimulus-information in the ambient optic array. And it supposes that the visual system can explore and detect this information. The theory is information-based, not sensation-based."  He and the psychologists who work within this paradigm detailed how the world could be specified to a mobile, exploring organism via the lawful projection of information about the world into energy arrays.  "Specification" would be a 1:1 mapping of some aspect of the world into a perceptual array. Given such a mapping, no enrichment is required and perception is direct. 
From Gibson's early work derived an ecological understanding of perception known as perception-in-action, which argues that perception is a requisite property of animate action. It posits that, without perception, action would be unguided, and without action, perception would serve no purpose. Animate actions require both perception and motion, which can be described as "two sides of the same coin, the coin is action." Gibson works from the assumption that singular entities, which he calls invariants, already exist in the real world and that all that the perception process does is home in upon them.
The constructivist view, held by such philosophers as Ernst von Glasersfeld, regards the continual adjustment of perception and action to the external input as precisely what constitutes the "entity," which is therefore far from being invariant.  Glasersfeld considers an invariant as a target to be homed in upon, and a pragmatic necessity to allow an initial measure of understanding to be established prior to the updating that a statement aims to achieve. The invariant does not, and need not, represent an actuality. Glasersfeld describes it as extremely unlikely that what is desired or feared by an organism will never suffer change as time goes on. This social constructionist theory thus allows for a needful evolutionary adjustment. 
A mathematical theory of perception-in-action has been devised and investigated in many forms of controlled movement, and has been described in many different species of organism using the General Tau Theory. According to this theory, tau information, or time-to-goal information is the fundamental percept in perception.
Evolutionary psychology (EP) Edit
Many philosophers, such as Jerry Fodor, write that the purpose of perception is knowledge. However, evolutionary psychologists hold that the primary purpose of perception is to guide action.  They give the example of depth perception, which seems to have evolved not to help us know the distances to other objects but rather to help us move around in space. 
Evolutionary psychologists argue that animals ranging from fiddler crabs to humans use eyesight for collision avoidance, suggesting that vision is basically for directing action, not providing knowledge.  Neuropsychologists showed that perception systems evolved along the specifics of animals' activities. This explains why bats and worms can perceive different frequency of auditory and visual systems than, for example, humans.
Building and maintaining sense organs is metabolically expensive. More than half the brain is devoted to processing sensory information, and the brain itself consumes roughly one-fourth of one's metabolic resources. Thus, such organs evolve only when they provide exceptional benefits to an organism's fitness. 
Scientists who study perception and sensation have long understood the human senses as adaptations.  Depth perception consists of processing over half a dozen visual cues, each of which is based on a regularity of the physical world.  Vision evolved to respond to the narrow range of electromagnetic energy that is plentiful and that does not pass through objects.  Sound waves provide useful information about the sources of and distances to objects, with larger animals making and hearing lower-frequency sounds and smaller animals making and hearing higher-frequency sounds.  Taste and smell respond to chemicals in the environment that were significant for fitness in the environment of evolutionary adaptedness.  The sense of touch is actually many senses, including pressure, heat, cold, tickle, and pain.  Pain, while unpleasant, is adaptive.  An important adaptation for senses is range shifting, by which the organism becomes temporarily more or less sensitive to sensation.  For example, one's eyes automatically adjust to dim or bright ambient light.  Sensory abilities of different organisms often co-evolve, as is the case with the hearing of echolocating bats and that of the moths that have evolved to respond to the sounds that the bats make. 
Evolutionary psychologists claim that perception demonstrates the principle of modularity, with specialized mechanisms handling particular perception tasks.  For example, people with damage to a particular part of the brain suffer from the specific defect of not being able to recognize faces (prosopagnosia).  EP suggests that this indicates a so-called face-reading module. 
Closed-loop perception Edit
The theory of closed-loop perception proposes dynamic motor-sensory closed-loop process in which information flows through the environment and the brain in continuous loops.    
Feature Integration Theory Edit
Anne Treisman's Feature Integration Theory (FIT) attempts to explain how characteristics of a stimulus such as physical location in space, motion, color, and shape are merged to form one percept despite each of these characteristics activating separate areas of the cortex. FIT explains this through a two part system of perception involving the preattentive and focused attention stages.     
The preattentive stage of perception is largely unconscious, and analyzes an object by breaking it down into its basic features, such as the specific color, geometric shape, motion, depth, individual lines, and many others.  Studies have shown that, when small groups of objects with different features (e.g., red triangle, blue circle) are briefly flashed in front of human participants, many individuals later report seeing shapes made up of the combined features of two different stimuli, thereby referred to as illusory conjunctions.  
The unconnected features described in the preattentive stage are combined into the objects one normally sees during the focused attention stage.  The focused attention stage is based heavily around the idea of attention in perception and 'binds' the features together onto specific objects at specific spatial locations (see the binding problem).  
Other theories of perception Edit
Effect of experience Edit
With experience, organisms can learn to make finer perceptual distinctions, and learn new kinds of categorization. Wine-tasting, the reading of X-ray images and music appreciation are applications of this process in the human sphere. Research has focused on the relation of this to other kinds of learning, and whether it takes place in peripheral sensory systems or in the brain's processing of sense information.  Empirical research show that specific practices (such as yoga, mindfulness, Tai Chi, meditation, Daoshi and other mind-body disciplines) can modify human perceptual modality. Specifically, these practices enable perception skills to switch from the external (exteroceptive field) towards a higher ability to focus on internal signals (proprioception). Also, when asked to provide verticality judgments, highly self-transcendent yoga practitioners were significantly less influenced by a misleading visual context. Increasing self-transcendence may enable yoga practitioners to optimize verticality judgment tasks by relying more on internal (vestibular and proprioceptive) signals coming from their own body, rather than on exteroceptive, visual cues. 
Past actions and events that transpire right before an encounter or any form of stimulation have a strong degree of influence on how sensory stimuli are processed and perceived. On a basic level, the information our senses receive is often ambiguous and incomplete. However, they are grouped together in order for us to be able to understand the physical world around us. But it is these various forms of stimulation, combined with our previous knowledge and experience that allows us to create our overall perception. For example, when engaging in conversation, we attempt to understand their message and words by not only paying attention to what we hear through our ears but also from the previous shapes we have seen our mouths make. Another example would be if we had a similar topic come up in another conversation, we would use our previous knowledge to guess the direction the conversation is headed in. 
Effect of motivation and expectation Edit
A perceptual set, also called perceptual expectancy or just set is a predisposition to perceive things in a certain way.  It is an example of how perception can be shaped by "top-down" processes such as drives and expectations.  Perceptual sets occur in all the different senses.  They can be long term, such as a special sensitivity to hearing one's own name in a crowded room, or short term, as in the ease with which hungry people notice the smell of food.  A simple demonstration of the effect involved very brief presentations of non-words such as "sael". Subjects who were told to expect words about animals read it as "seal", but others who were expecting boat-related words read it as "sail". 
Sets can be created by motivation and so can result in people interpreting ambiguous figures so that they see what they want to see.  For instance, how someone perceives what unfolds during a sports game can be biased if they strongly support one of the teams.  In one experiment, students were allocated to pleasant or unpleasant tasks by a computer. They were told that either a number or a letter would flash on the screen to say whether they were going to taste an orange juice drink or an unpleasant-tasting health drink. In fact, an ambiguous figure was flashed on screen, which could either be read as the letter B or the number 13. When the letters were associated with the pleasant task, subjects were more likely to perceive a letter B, and when letters were associated with the unpleasant task they tended to perceive a number 13. 
Perceptual set has been demonstrated in many social contexts. When someone has a reputation for being funny, an audience is more likely to find them amusing.  Individual's perceptual sets reflect their own personality traits. For example, people with an aggressive personality are quicker to correctly identify aggressive words or situations. 
One classic psychological experiment showed slower reaction times and less accurate answers when a deck of playing cards reversed the color of the suit symbol for some cards (e.g. red spades and black hearts). 
Philosopher Andy Clark explains that perception, although it occurs quickly, is not simply a bottom-up process (where minute details are put together to form larger wholes). Instead, our brains use what he calls predictive coding. It starts with very broad constraints and expectations for the state of the world, and as expectations are met, it makes more detailed predictions (errors lead to new predictions, or learning processes). Clark says this research has various implications not only can there be no completely "unbiased, unfiltered" perception, but this means that there is a great deal of feedback between perception and expectation (perceptual experiences often shape our beliefs, but those perceptions were based on existing beliefs).  Indeed, predictive coding provides an account where this type of feedback assists in stabilizing our inference-making process about the physical world, such as with perceptual constancy examples.
Contrast is at the root of all perception. If we do not detect change then we perceive nothing. There is an apocryphal tale of a frog, placed in a gently warmed pan of water that eventually gets boiled alive. Yet if the frog is dropped in hot water it will leap out. When the frog cannot contrast the temperature increments, it does not realize it is getting hotter until it is too late.
We use contrast to perceive in other senses as well as our visual sense. We hence contrast loud and quiet sounds, hot and cold temperatures, bitter and sweet tastes, and so on.
Without contrast our senses wander, seeking anything of contrast to latch onto. Without contrast, such as in a dark night, we may hallucinate, creating our own perceptual contrasts. Put a person in sensory deprivation tank and they will soon start seeing and hearing things (and the longer they are there, the more real these imaginings become). Perhaps this related to imagination and dreaming. In the dark of nights or daytime musing, our mind needs to perceive something, so it just makes things up.