SPOTTING THE CAT IN THE GRASS

Selective adaptation
An important early stage of vision is finding out which bits of the retinal image correspond to what kinds of physical thing ‘out there’ in the world. Our visual system first needs to discover the locations of objects, their colours, movements, shapes, and so on. This process can be demonstrated by the technique of selective adaptation. Whenever we enter a new environment, our sensory systems adjust their properties quite rapidly (over the course of a few seconds), optimizing their ability to detect any small change away from the steady background conditions. This is because interesting and important stimuli are usually ones that deviate suddenly in some way from the background (such as a tiger jumping out from behind a tree). Remember the cat in the grass: its tiny movements had to be extracted from the pattern of coherent movement on the retina produced by your movements as you walked past. By staring at something for a time (selective adaptation), we produce an unchanging pattern of stimulation on one reg on of the retina, and the visual system starts to treat this as the steady background, and lowers its sensitivity to it. When we stop staring at this same location, it takes a while for our vision to return to normal, and we can notice during this period of compensation that the world looks different. These differences represent the after-effects of adaptation. [adaptation decline in the response of a sensory or perceptual system that occurs if the stimulus remains constant] This whole process of adaptation is described as selective because only some perceptual properties are affected. The adaptations are restricted to stimuli similar to the one that has been stared at. Many kinds of visual after-effect have been discovered (as we can see in Everyday Psychology). These clear and robust phenomena are not confined to vision, but are found in touch, taste, smell and hearing also.
For example:
1. After running your fingers over fine sandpaper, medium sandpaper feels coarser (and vice versa).
2. After listening t a high tone for a while, a medium tone appears lower.
3. Musicians often build their music to a loud and cacophonous crescendo just before a sudden transition to a slow, quiet passage, which then seems even more mellow and tranquil than it otherwise would.
4. Holding your hand under running cold (or hot) water before testing the temperature of baby’s bath water will lead you to misperceive how comfortable the water will be for the baby. This is why you are always advised to test the temperature with your elbow.
5. After eating chocolate, orange juice tastes more tart.
6. When we enter a dark room, it takes a few minutes for our receptors to adapt, and we begin to notice things that had been simply too faint to activate those receptors at first