- 1 Definition
- 2 History
- 3 Research on Perceptual Adaptation
- 4 References
- 5 Further reading
Perceptual adaptation is a unique function of the brain that accounts for the differences viewed in the world, as it relates to the senses. This phenomenon occurs in all senses, including vision, hearing, touch, and smell. An example is when images sensed through the eyes are relayed to the visual cortex of the brain, and if vision is altered slightly, the brain accounts for the difference and will allow one to perceive the world as "normal." This is a compensation mechanism the brain uses for the world to appear normal in our minds when our world has obviously been altered from its regular state. This is an important aspect in potential alterations to the visual field, effectiveness in compensating for alterations in the visual field, visual adaptation, and face recognition.
Herman Helmholtz, a distinguished scientist from the 1800s, thoroughly researched conscious sensations and how they converted into meaningful perceptions of events. He defined sensations as the "raw elements" of conscious experience that required no learning and perceptions contrarily as the meaningful interpretations he gave to senses . He studied the physical properties of the eye, color vision, and visual perception including perceptual adaptation. One of his classic experiments regarding how space perception could be altered by experience involved participants wearing certain glasses that distorted the visual field several degrees to the right of the normal location. Participants were asked to look at an object, close their eyes, and try to reach out and touch it. At first their responses were always misses to the right, but after a few trials participants became nearly flawless with this drill. At first, subjects started to place their hands to the left of the objects, but eventually it became natural to touch them even with their eyes closed. 
Helmholtz also theorized that perceptual adaptation might result from a process he referred to as unconscious inference. This is where the mind leads to the unconscious adoption of certain rules to make things follow in a logical order. An example of this phenomena is when a ball appears to be getting smaller and smaller, then mind will then infer that the ball is moving away from them. 
Research on Perceptual Adaptation
Psychologist George M. Stratton conducted, in the 1890s, experiments in which he tested the theory of perceptual adaptation. In one experiment, he wore a reversing glasses for 21½ hours over three days, with no change in his vision. After removing the glasses, "normal vision was restored instantaneously and without any disturbance in the natural appearance or position of objects."
On a later experiment, Stratton wore the glasses for eight whole days. By day four, the images seen through the instrument were still upside down. However, on day five, images appeared upright until he concentrated on them; then they became inverted again. By having to concentrate on his vision to turn it upside down again, especially when he knew images were hitting his retinas in the opposite orientation as normal, Stratton deduced his brain had reprocessed his vision and adapted to the changes in vision.
Potential alterations to the visual field
The visual field can be altered in many ways. The various ways that it can be altered is by angles, or by orientation. Optometrists alter the visual field by altering angles (i.e. fashion the subject with glasses that construe the visual field by differing angles). George Stratton conducted experiments where the visual field was altered by an angle of forty five degrees and his brain was able to adapt to the change and perceive the world as normal. Also, the field can be altered making the subject see the world upside down. But, as the brain adjusts to the change, the world appears "normal."
Effectiveness in compensating for alterations in the visual field
An altered visual field does not adversely affect one’s life. Our perceptual adaptation allows us to adapt to the world and live life as normal. Therefore, with an altered visual field, the limits are nonexistent. One is able to do whatever they please without limitations. In some extreme experiments, psychologists have tested to see if a pilot can fly a plane with altered vision. All of the pilots that were fitted with the goggles that alter their vision were able to safely navigate the aircraft with ease.
Visual Adaptation is defined as the aftereffects of exposure to a certain visual stimulus or visual pattern that caused one to lose sensitivity to that pattern and engage in stimulus bias. An example of this phenomenon is the "lilac chaser", introduced by Jeremy Hinton. The stimulus here are lilac circles, that once removed, leave green circles that then become the most prominent stimulus. The fading of the lilac circles is due to a loss of sensitivity to that stimulus and the adaptation to the new stimulus. To experience the "lilac chaser" effect, the subject needs to fixate their eyes on the cross in the middle of the image, and after a while the effect will settle in. Visual coding, a process involved in visual adaptation, is the means by which the brain adapts to certain stimuli, resulting in a biased perception of those stimuli. This phenomenon is referred to as visual plasticity; the brain's ability to change and adapt according to certain, repeated stimuli, altering the way information is perceived and processed. 
The rate and strength of visual adaptation depends heavily on the number of stimuli presented simultaneously, as well as the amount of time for which the stimulus is present. Visual adaptation was found to be weaker when there were more stimuli present, than when the stimuli were only a few. Moreover, studies have found that stimuli can rival each other, which explains why higher numbers of simultaneous stimuli lead to lower stimulus adaptation. Studies have also found that visual adaptation can have a reversing effect; if the stimulus is absent long enough, the aftereffects of visual adaptation will subside. Lastly, studies have also shown that visual adaptation occurs in the early stages of processing. 
Perceptual adaptation plays a big role in identifying faces. The phenomenon of aftereffects applies to faces as well. In an experiment conducted by Gillian Rhodes, the impact of face adaption was investigated, along with whether visual adaptation has an impact on recognizing faces. The experiment found that perceptual adaptation does, in fact, have an impact on face recognition. Individuals tend to adapt to common facial features as early as after five minutes of looking at them. This suggests that humans adapt to common facial features, leaving neural resources and space to identify uncommon characteristics and features, which is how humans identify specific faces on a case-by-case basis. 
Perceptual aftereffects for face recognition occur for several different stimuli, including gender, ethnicity, identity, emotion, and attractiveness of a face. The fact that this distinction occurs, implies that face recognition is a process that happens on a higher level and later on in the visual encoding, rather than early on within visual adaptation. The fact that the aftereffects in face recognition in particular are so strong, suggests that it is for the purpose of regulation of how processes work. This provides a sense of constancy in an individual's perception, while adapting to differences and possible versions of a stimulus allows for constancy and stability, and makes it easier to adapt to variations in a stimulus, while recognizing commonalities. These face perception cues are encoded in an individual's brain for extended periods of time, ensuring consistency over the individual's lifespan. A young person would perceive stimuli the same way as the an older individual. 
Auditory adaptation, as perceptual adaptation with other senses, is the process by which individuals adapt to sounds and noises. As research has shown, as time progresses, individuals tend to adapt to sounds and tend to distinguish them less frequently after a while. Sensory adaptation tends to blend sounds into one, variable sound, rather than having several separate sounds as a series. Moreover, after repeated perception, individuals tend to adapt to sounds to the point where they no longer consciously perceive it, or rather, "block it out". An individual that lives close to the train tracks, will eventually stop noticing the sounds of passing trains. Similarly, individuals living in larger cities no longer notice traffic sounds after a while. Moving to a completely different area, such as a quiet countryside, would then be aware of the silence, crickets, etc. 
Other Types of Sensory Adaptation
Perceptual adaptation is a phenomenon that occurs for all of the senses, including smell and touch. An individual can adapt to a certain smell with time. Smokers, or individuals living with smokers, tend to stop noticing the smell of cigarettes after some time, whereas people not exposed to smoke on a regular basis will notice the smell instantly. The same phenomenon can be observed with other types of smell, such as perfume, flowers, etc. The human brain can distinguish smells that are unfamiliar to the individual, while adapting to those it is used to and no longer require to be consciously recognized. This phenomenon also applies to the sense of touch. An unfamiliar piece of clothing that was just put on will be noticed instantly; however, once it has been work for a while, the body will instantly adapt to it and be no longer noticed. 
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