The demonstrations reported below illustrated the process of sensory adaptation, where we become less sensitive to an unchanging stimulus over time (Davis & Palladino, 2006).
Demonstrations
Texture. In a demonstration of adaptation to a texture, an example of the sense of touch, I rubbed each of my index fingers back and forth several times over a coarse piece of sandpaper. I then rated its coarseness a “6,” (interpreted as “very coarse”) based on a 7-point scale, where increasing numbers corresponded with increasing feelings of coarseness. After 2 m, I repeated the procedure, but this time, my rating was “4” (interpreted as “fairly coarse”). The sensory receptors sensitive to coarseness, mechanoreceptors, became less sensitive (Hollins & Sliman, 2007), resulting in a reduction of perceived coarseness, after processing in the somatosensory association areas of the brain’s parietal lobes (Campbell, Reece, & Mitchell, 1999).
Water temperature. To demonstrate adaptation to water temperature, another example of the sense of touch, I first filled one bowl with hot water and another with cold. I poured an equal amount of water from each bowl into a third bowl, placed between the hot and cold ones. I immersed one hand in the hot water and, simultaneously, immersed the other in the cold water, where they remained for 3 m. I then placed both hands in the middle bowl. The same water felt cool on my left hand (previously immersed in hot water) and warm on my right one (previously immersed in cold water). Adaptation was a function of reduced sensitivities in the separate receptors for hot and cold stimuli (Patapoutian, Peier, & Viswanath, 2003), as perceived in the somatosensory regions of the parietal lobes (Campbell, Reece, & Mitchell, 1999).
Light. I placed 15 index cards over the brightly lit top of a flashlight, while in a dark room. After removing 6 cards, one at a time, I was able to see a small blurry patch of grayish-orange light in the middle of the remaining 9-card stack. Soon after staring at this blur, it began to get bigger, more distinctly circular, eventually appearing as a circle the same size as the circular top of the flashlight. The color became clearly orange, with a grayish tint only at the circle’s edges. After adding 1 card to the stack, I saw a blur similar to the one described above. While staring at it, the circle again appeared larger, and the orange became less grayish, except at the circle’s edges. I continued to see a smaller, less bright orange circle than the one I saw after removing the 7th card. After adding another card to what had become a 10-card stack, the blur I saw was gray, increased only slightly in size, and the gray color appeared to be mixed with only a touch of orange. After adding another card to the then 11-card stack, I stared until the 15 m allotted to the task ended, without seeing anything on the card. Seeing light that I hadn’t previously seen indicated visual adaptation had occurred, where the intensity of the light on the retina of each eye changed when I added individually 3 cards to the stack, in turn changing what was transmitted by the optic nerve, first to the thalamus and then to the visual association regions of the occipital lobes (Campbell, Reece, & Mitchell, 1999).
Theories of the Visual Sensory System
Sensation refers to processes that occur prior to awareness, or, regarding the sensation of vision, prior to awareness of light from any object that impinged on the eye (Davis & Palladino, 2006). Specifically, light first passes through the cornea, then the pupil, and then registers on the retina, interestingly as inverted from the visual stimulus, in other words, upside down. The retina has over a million receptor cells, rods (for black and white vision) and cones (for color vision), which are transmitted to ganglion cells which form the optic nerve. From the optic nerve, by means first of the visual area in the thalamus, then to the visual cortex, information reaches areas of the temporal and parietal lobes (Campbell, Reece, & Mitchell, 1999; Davis & Palladino, 2006). Prior to awareness, information from the stimulus must be interpreted or given meaning, the process of perception (Davis & Palladino, 2006). Visual perception begins when an exact replica or trace of the stimulus, an icon, is in the iconic sensory register (Sperling, 1960, as cited in Ashcraft, 1998). There are separate registers for each of the senses, where the trace is in the same format as the stimulus, or veridical, for example, as an echo in the echoic register (Ashcraft, 1998). The icon lasts for a brief duration, 1 sec at most, providing additional time for the brain to abstract information required for perception (Ashcraft, 1998). Two processes guide perception, “data driven” where features, such as angles, lines, edges, are extracted from the icon and then matched with long-term memory feature lists corresponding with visual patterns. There would, however, be many perceptual failures if only data-driven processing were used. For example, consider a “letter” that appears as follows: “/-.” Is it an “A” or an “H”? Without hesitation, if the letter appears between a “C” and a “T,” people read the word “cat,” but if between a “T” and an “E,” they read the word “the” (Ashcraft, 1998). Perception involved, as it frequently does, “conceptually driven” processing, processing guided by context or prior knowledge (Ashcraft, 1998).
Evolutionary Importance of Adaptation
The survival value of adaptation in terms of evolution refers to being able to withstand gradual environmental changes that occur “over the course of many generations” (Montagu, 1976, p. 41). For example, in the gradual drop of temperature culminating in the glacial age, members of a species with thicker body hair were more likely to survive than those with thinner hair, thus breeding with others of the species with thicker hair, thus “slowly over a number of generations – an entire species could change in its environment, from sparsely furred to thickly furred” (Montagu, 1976, p. 41). If the well-known small increase in temperature over the 20th century does result in a period characterized by the intense heat predicted by those warning us of the dangers of global warming, it’s plausible that the sensory adaptation experienced by many who have moved from colder areas of the country to the warmest areas (as when those used to living through Florida summers, for example, are comfortable, as opposed to the discomfort of visitors) would be at an advantage. On the other hand, unless scientists are able to develop replacements for the antibiotics that have become ineffective for many people, one might characterize the adaptation, or insensitivity, of bacterial cells to the ingredients in antibiotics as disadvantageous to survival. Indeed, one known form of adaptation to prolonged cold is deadly if one is unaware of the consequences. That is, the body adapts so that a person feels warm and comfortable – as he or she lies down and peacefully dies. Wiesel (1958/1982) wrote of his march in the snow and freezing cold with other Buchenwald inmates when he was 15. Despite his desire to live, when at last there were orders to stop marching, he “had neither the will nor the strength to get up” (p. 84). Had he not managed to obey his father and continue until they had found shelter, he would have died as others on the march did (possibly, in at least some cases, with intent). Thus, without knowledge of the consequences of sensory adaptation, surviving prolonged exposure to severe cold would not even be possible.
Nonetheless, the survival value of sensory adaptation has far exceeded the above exceptions. Sensory adaptation to the usual noises, smells, and other sensory stimuli in the environment enables us and other species to perceive the presence of unusual stimuli indicative of dangers, for example fire.
References
Ashcraft, M. (1998). Fundamentals of cognition. White Plains, NY: Addison-Wesley.
Campbell, N. A., Reece, J. B., & Mitchell, L. G. (1999). Biology (5th ed.). Menlo Park,
CA: Addison Wesley Longman.
Davis, S. F., & Palladino, J. J. (2006). Psychology (5th ed.). Upper Saddle River, NJ:
Prentice Hall.
Hollins, M., & Sliman, J. B. (2007). The coding of roughness. Canadian Journal of
Experimental Psychology, 61, 184-195.
Montagu, A. (1976). The nature of human aggression. New York: Oxford University Press.
Patapoutian, A., Peier, A. M., Story, G. M., & Visuanth, V. (2003). Mechanisms of temperature
perception. Nature Reviews, 4, 628-638.
during the eating of sweetened yogurt. Appetite, 34, 21-27.
Wiesel, E. (1958/1982). Night. (S. Rodway, Trans.). New York: Bantam Books.
