Story of Psychology - Morton Hunt [308]
His name and theory have faded from view, but the cues he was so enamored of have remained accepted components of contemporary accounts of depth perception.
Two Ways of Looking at Vision
“Visual perception,” Bela Julesz said fifteen years ago, “is in the same state as physics was prior to Galileo or biochemistry was prior to the discovery of the double helix by Watson and Crick.”73 Since then, a good deal more has been learned, and yet it remains true that each of the two major approaches—the neural and the cognitive—explains only some of the phenomena; there is not yet a comprehensive and unifying theory of visual perception. Perhaps some great organizing concept remains undiscovered, or perhaps visual perception is so complex that no one theory can embrace all of its concepts and that the two different approaches deal with events occurring at radically different levels of complexity.
We have seen something of each of these approaches. Here, to round out the picture, are brief sketches of how each explains visual perception in general.
The neural approach answers questions that preoccupied nineteenth-century physiologists: How can sensory nerves, though alike in structure, transmit different sensations to the brain? And how does the brain turn those incoming impulses into vision?
The answer, worked out in great detail over recent years,74 is that the nerve impulses themselves do not differ; rather, receptors that respond to specific stimuli send their messages separately to the striate or primary area of the visual cortex. The process begins on the retina, where rods are sensitive to low levels of illumination, cones to more intense levels; cones are of three types, each responsive to different wavelengths of visible light, and some, as we have already heard, sensitive to special shapes and motions.
From the rods and cones, the same kinds of nerve impulses travel along parallel pathways but end up in different areas of the brain—more than 90 percent of them in particular parts of the primary visual cortex and 10 percent in other subcortical structures. Thus the messages delivered to the brain have been analytically separated into color, shape, movement, and depth, and delivered to specialized receptive areas. By means of staining techniques that trace the neuron pathways in laboratory monkeys from retina to visual cortex, researchers have been able to identify more than thirty such distinct cortical visual areas.
What happens then? The brain puts it all together: Using single-cell recordings and two kinds of brain scans (PET, positron emission tomography, and fMRI, functional magnetic resonance imaging), perception researchers have puzzled out the extremely intricate architecture of the primary visual cortex and its wiring scheme (far too complex to take up here), which integrates the individual impulses and blends the information from the two eyes. The result is that the image cast on the retina winds up as the excitation of groups of complex neurons, but the pattern of these excitations in no way resembles the image on the retina or the scene outside the eye. Rather, as already mentioned, it is analogous to writing about a scene, which conveys what it consists of but does not in the least look like it.
It is not an image but a coded representation of the image, somewhat as the patterns of magnetism on a tape recording are not sounds but a coded representation of sounds. The representation, however, is not yet a perception; the primary visual cortex is in no sense the end of the visual path. It is just one stage in the processing of the information it handles.
From the striate region the partly assembled and integrated information is sent to other areas of the visual cortex and to higher areas of brain cortex beyond it. There, the information is finally seen by the mind and recognized as something familiar or something not seen before. How that