Adaptation (Campbell, Reece, & Mitchell, 1999; Davis & Palladino, 2006) is a process where over repeated exposure to the same stimulus, the intensity of activation in the sensory receptor cells and in areas of the midbrain or thalamus (Campbell, Reece, & Mitchell, 1999) weakens. These areas receive veridical (the same as the sensation) traces lasting less than about a second from the sensory receptors, traces referred to as the sensory registers (an iconic register for visual stimuli, Sperling, 1960, an echoic one for auditory stimuli, Darwin, Turvey, & Crowder, 1972, a tactile register for touch stimuli, Watkins & Watkins, 1974).
Perception occurs after sensory information is transmitted to areas of the cerebral cortex. Demonstrations Sensory vs. perceptual processing. Based on inferences drawn from previous research (as cited in Davis & Palladino, 2006), the demonstrations (described in a course handout) reported below were expected to result in adaptation. It should be noted that in these demonstrations, findings of adaptation could be attributable to expectancy effects influencing perception, i. e. , there is robust evidence that people often perceive what they expect to perceive (Harris, 1991).
Prior to the demonstrations, the class already had learned about adaptation and had expectations. For purposes of this paper, I ignored both possible expectancy effects and, more importantly, that there were no physiological, brain imaging, or brain wave monitoring measures. Texture (an example of touch). I rubbed both index fingers back and forth three times over a sheet of coarse sandpaper and then rated its coarseness a 7 (using a 7-point scale, where 7 = “very coarse,” though not necessarily the coarsest a stimulus could feel).
After 2 m, I rubbed my index fingers back and forth once over the same sandpaper, and rated its coarseness a 5 (meaning it felt “moderately coarse”). In both cases, mechanoreceptors under the skin that were sensitive to coarseness, different from those sensitive to smoothness, were activated (Hollins & Sliman, 2007), with less intensity after the 2 m delay. Awareness of adaptation was a result of processing in the somatosensory association areas of the parietal lobes in each hemisphere of the brain (Campbell, Reece, & Mitchell, 1999).
Water temperature (an example of touch). I simultaneously placed my left and right hands in bowls of hot and cold water respectively. After 3 m, I placed both hands in a middle bowl that was a mixture of water from the other two bowls. The water felt cold on my left hand (though not as cold as my right hand felt when it was in the cold water) and warm (though not hot) on my right hand.
Adaptation did not occur over the 3 m my hands were in the hot and cold water, but adaptation was reflected as a weakening of activation in the different receptors for hot and cold (Patapoutian, Peier, Story, & Viswanath, 2003) when the coldness and heat felt in the left and right hands respectively when in the mixed water were less intense than when the right and left hands were in the cold and hot water respectively. These perceptions were a result of processing in the somatosensory areas of the parietal lobes (Campbell, Reece, & Mitchell, 1999). Sweetness (an example of taste).
I swished a sip of bottled water that had been sweetened with sugar in my mouth and it took about 3 m until the water tasted and remained distinctively less sweet, i. e. , adaptation had occurred (consistent with Theunissen, Polet, Kroeze, & Schifferstein, 2000). After swallowing the sweet water, a sip from a glass of unsweetened bottled water (ordinarily a tasteless substance) had a taste best described as tinny or metallic. Since I had swished the sweet water until its sweetness was no longer weakening and the same sweetness remained in my mouth after swallowing, my best guess was that the fresh water would taste less sweet.
Since the taste buds on my tongue and in my mouth that responded to sweetness were still activated (the sweet taste remained in my mouth), the fresh water might have activated buds sensitive to a taste other than sweetness, resulting in a combination perceived as metallic or tinny in the parietal lobes of the cerebral cortex (Campbell, Reece, & Mitchell, 1999). Light. In a dark room, after placing 15 index cards on the circular lighted top of a flashlight, I removed one card at a time. After removing the fifth card, I saw a small blur of smoky orange light.
The light gradually increased in size until it stabilized as a circle about the same size as the top of the flashlight, and the orange color became less smoky except at the edges of the circle. When I added a card, I again saw a smoky orange blur. The blur gradually became a circle smaller than the previous one, the orange color didn’t become as bright as it previously had, and the smoky edges of the circle were wider than they previously were. When I added another card, all I ever was able to see was a small grayish light, and saw no light after adding another card.
Being able to detect light on two cards that I couldn’t previously detect was an example of visual adaptation (Campbell, Reece, & Mitchell, 1999; Davis & Palladino, 2006). There was a change in intensity of light registering on the retina of the eye, and transmitted by the optic nerve to the thalamus. My perceptions were a result of processing in the visual association area of the occipital lobes (Campbell, Reece, & Mitchell, 1999). Visual Processing Theories How does the brain convert visual sensations into meaning?
When we view, for example, a busy street from a window, we do not perceive individual features or fitting parts of a scene into a meaningful whole. The difficulty in developing theories of visual perception (reviewed in Treisman, 1986) is that we have no conscious awareness (implicit processing) of the complex operations that allow us to view visual scenes as a whole. Triesman described a first stage, creating a “primal sketch,” where information from sensory receptors is coded as lines, angles, orientations, colors, etc. The second stage is to combine the elements in the primal sketch into a meaningful whole.
Within each stage, parallel, i. e. , simultaneous, processing, accounts for seeing a whole so rapidly that we are unaware of recoding and combining operations. A third stage, when we need to find relationships between visual wholes, requires conscious awareness, i. e. , explicit processing. Conclusion Under most conditions, adaptation is advantageous. For example, if you live in an apartment where there are odors from different kinds of cooking in other apartments on the same floor as yours, without adaptation to these orders, detecting the faint smell of smoke signaling a fire would be more difficult.
Under unusual conditions, however, adaptation can be unfortunate, for example, hikers lost in the freezing cold for a prolonged period of time would die if they couldn’t deliberately prevent themselves from falling asleep when they felt warm because they had adapted to the cold. From an evolutionary perspective, adaptation has influenced the ability of a species to survive. Adapting so usual auditory and visual stimuli do not activate sensory receptors allows better detection of the sound and sight of threatening stimuli, for example, a member of another potentially dangerous species.
References
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