Nervous Systems and Behaviour
Proceedings of the 4th International Congress of Neuroethology
September 3-8, 1995 Cambridge

How are object images represented in the monkey brain?

I. Fujita
Department of Cognitive Neuroscience, Osaka University Medical School, and PRESTO, Research Development Corporation of Japan, Suita, Osaka 565, Japan.

The retina transmits visual information to the brain in the form of activity in neurons arrayed on a 2-dimensional plane, each encoding local luminance an/or wavelength. This information is then processed step-by-step along the visual pathway, and a neuronal representation of objects should finally acquire characteristics that are useful and efficient for object recognition. We investigated how this is done in the monkey brain by studying area TE of the inferior temporal cortex. This area occupies the final stage of the macaque visual cortical pathway crucial for object recognition, and its functional architecture should reflect the final form of object representation in the brain.

Most TE cells do not respond to all objects, but to a subset, 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 objects around us. Individual TE cells 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 remains stable for at least 30 days. The discreteness and temporal stability of TE columns allow us 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 the 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 the TE can be reorganized under certain circumstances. Indeed, stimulus selectivity of cells in the TE and an adjacent area in the perirhinal cortex has been shown to change when monkeys are trained to memorize and discriminate artificially synthesized shapes. The results, taken together, suggest that TE columns are dynamically maintained 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 hundred of the components expressed in columns, but the principle of combination allows the TE to represent an almost infinite number of objects. Exactly how component features create an image of the whole object remains enigmatic.

Supported by grants from the Uehara Memorial Foundation, the Casio Science Promotion Foundation, the Ichiro Kanehara Foundation, and the Ministry of Education, Science and Culture, Japan (No. 07252218).