Visual adaptation

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 compensatory 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.

History

In the late 1800s, Hermann Helmholtz, a German physician and physicist, extensively researched conscious sensations and different types of perception . He defined sensations as the "raw elements" of conscious experience that required no learning, and perceptions as the meaningful interpretations derived from the senses. He studied the physical properties of the eye and vision, as well as acoustic sensation. In one of his classic experiments regarding how space perception could be altered by experience, participants wore glasses that distorted the visual field by several degrees to the right. Participants were asked to look at an object, close their eyes, and try to reach out and touch it. At first, the subjects reached for the object too far to the left, but after a few trials were able to correct themselves.

Helmholtz theorized that perceptual adaptation might result from a process he referred to as unconscious inference, where the mind unconsciously adopts certain rules in order to make sense of what is perceived of the world. An example of this phenomenon is when a ball appears to be getting smaller and smaller, the mind will then infer that the ball is moving away from them.

Research on perceptual adaptation

In the 1890s, psychologist George M. Stratton conducted experiments in which he tested the theory of perceptual adaptation. In one experiment, he wore a reversing glasses for 21½ hours over three days. After removing the glasses, "normal vision was restored instantaneously and without any disturbance in the natural appearance or position of objects."

Modern version of inverting mirrors with harness.

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 adapted to the changes in vision.

Stratton also conducted experiments where he wore glasses that altered his visual field by 45°. 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."

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 altered their vision were able to safely navigate the aircraft with ease.

Visual adaptation

Visual adaptation is defined as the aftereffects of exposure to a visual stimulus or pattern that causes loss of sensitivity to that pattern and induces 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.

Lilac Chaser from Jeremy Hinton's experiment

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. 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. Studies have also shown that visual adaptation occurs in the early stages of processing.

Face recognition

Perceptual adaptation plays a big role in identifying faces. In an experiment conducted by Gillian Rhodes, the impact of face adaptation 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 an older individual.

Further reading

• Stratton, George M. (1897). "Upright Vision and the Retinal Image". Psychological Review. 4: 182–7. doi:10.1037/h0064110.
• Richard L. Gregory (30 Oct 1997). Eye and Brain : The Psychology of Seeing. Oxford University Press. ISBN 978-0-19-852412-0.