DYNAMIC REGULATION OF FUNCTIONAL COLUMNSIN THE TEMPORAL VISUAL CORTEX
Department of Cognitive Neuroscience, Osaka University Medical School, and PRESTO, Research Development Corporation of Japan, Suita, Osaka 565,Japan
Area TE of the inferior temporal cortex occupies the final stage of the monkey visual cortical pathway crucial for object recognition. TE cells do not respond to views of every object, but are activated by a subset of, often only a few, objects among hundreds of objects tested. These cells are selective for visual features shared by the objects, such as a particular shape, color, texture or a combination of these. Although stimulus features critical for activating TE cells are more complex than those at earlier visual cortices, they are still simpler than the complex features of an object around us. Individual TE cells thus do not signal abstract representation of a particular object.
TE cells with similar selectivity cluster in multiple columns separated by those with different stimulus preferences. Stimuli which activate cells in one column do not excite cells in adjacent columns. The width of each column is 0.4-0.5 mm on average. Repeated recordings from the same site suggest that stimulus selectivity of columns is maintained for at least 30 days. The discreteness and temporal stability of TE columns provide us with the rationale to estimate the number of columns in area TE. The number is estimated to be 1300-2000. The number of distinct object features represented in TE may be smaller than this number, because there appear to be multiple columns selective for similar features.
Although stimulus selectivity of columns remains stable in the present experimental conditions, this does not exclude the possibility that the columns in TE can be reorganized under certain circumstances. Indeed, stimulus selectivity of cells in TE and an adjacent area in the perirhinal cortex has been shown to change when monkeys are trained to discriminate artificially synthesized shapes. The results, taken together, suggest that TE columns are maintained dynamically and, when necessary, can undergo changes.
When a monkey sees an object, a particular subset of columns, not the entire TE, or a single column, or a single "grandmother" cell, will be activated by the image. Different component features of the object activate different subsets of columns. The composition of the entire image may be represented by the combination of active columns. One TE may hold only a few hundreds of components expressed in columns, but the principle of combination allows TE to represent an almost infinite number of objects. Exactly how component features are bound to create a whole object image remains enigmatic.