Neural adaptation or sensory adaptation is a gradual decrease over time in the responsiveness of the sensory system to a constant stimulus. It is usually experienced as a change in the stimulus. For example, if a hand is rested on a table, the table's surface is immediately felt against the skin. Subsequently, however, the sensation of the table surface against the skin gradually diminishes until it is virtually unnoticeable. The sensory neurons that initially respond are no longer stimulated to respond; this is an example of neural adaptation. All sensory and neural systems have a form of adaptation to constantly detect changes in the environment. Neural receptor cells that process and receive stimulation go through constant changes for mammals and other living organisms to sense vital changes in their environment. Some key players in several neural systems include Ca2+ions (see Calcium in biology) that send negative feedback in second messenger pathways that allow the neural receptor cells to close or open channels in response to the changes of ion flow. There are also mechanoreception systems that use calcium inflow to physically affect certain proteins and move them to close or open channels. Functionally, it is highly possible that adaptation may enhance the limited response range of neurons to encode sensory signals with much larger dynamic ranges by shifting the range of stimulus amplitudes. Also, in neural adaptation there is a sense of returning to baseline from a stimulated response. Recent work suggests that these baseline states are actually determined by long-term adaptation to the environment. Varying rates or speed of adaptation is an important indicator for tracking different rates of change in the environment or the organism itself. Current research shows that although adaptation occurs at multiple stages of each sensory pathway, it is often stronger and more stimulus specific at "cortical" level rather than "subcortical stages". In short, neural adaptation is thought to happen at a more central level at the cortex.
Fast and slow adaptation There is fast adaptation and slow adaptation. Fast adaptation occurs immediately after a stimulus is presented i.e., within hundreds of milliseconds. Slow adaptive processes can take minutes, hours or even days. The two classes of neural adaptation may rely on very different physiological mechanisms. The time scale over which adaptation builds up and recovers depends on the time course of stimulation. Brief stimulation produces adaptation which occurs and recovers while more prolonged stimulation can produce slower and more lasting forms of adaptation. Also, repeated sensory stimulation appears to temporarily decrease the gain of thalamocortical synaptic transmission. Adaptation of cortical responses was stronger and recovered more slowly. Vastly different timescales of adaptation have also been shown to be implemented on the single neuron level, where they can give rise to time-scale free adaptation. At the very extreme of evolutionary timescales, neurons in different parts of retina have been found deploy differing amounts of lateral inhibition to compensate for the high dynamic range between the ground and the sky.
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. 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."
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.
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