SHAPE REPRESENTATION IN THE INFERIOR TEMPORAL CORTEX OF MONKEYS

Background: The inferior temporal cortex (IT) of the monkey has long b een known to play an essential role in visual object recognition. Dama ge to this area results in severe deficits in perceptual learning and object recognition, without significantly affecting basic visual capac ities. Consistent with these ablation studies is the discovery of IT n eurons that respond to complex two-dimensional visual patterns, or obj ects such as faces or body parts. What is the role of these neurons in object recognition! Is such a complex configurational selectivity spe cific to biologically meaningful objects, or does it develop as a resu lt of extensive exposure to any objects whose identification relies on subtle shape differences? If so, would IT neurons respond selectively to recently learned views or features oi novel objects? The present s tudy addresses this question by using combined psychophysical and elec trophysiological experiments, in which monkeys learned to classify and recognize computer-generated three-dimensional objects. Results: A po pulation of IT neurons was found that responded selectively to views o f previously unfamiliar objects. The cells discharged maximally to one view of an object, and their response declined gradually as the objec t was rotated away from this preferred view. No selective responses we re ever encountered-for views that the animal systematically failed to recognize. Most neurons also exhibited orientation-dependent response s during view-plane rotations. Some neurons were found to be tuned aro und two views of the same object, and a very small number of cells res ponded in a view-invariant manner. For the five different objects that were used extensively during the training of the animals, and for whi ch behavioral performance became view-independent, multiple cells were found that were tuned around different views of the same object. A nu mber of view-selective units showed response invariance for changes in the size of the object or the position of its image within the parafo vea. Conclusion: Our results suggest that IT neurons can develop a com plex receptive field organization as a consequence of extensive traini ng iii the discrimination and recognition of objects. None of these ob jects had any prior meaning for the animal, nor did they resemble anyt hing familiar in the monkey's environment. Simple geometric features d id not appear to account for the neurons' selective responses. These f indings support the idea that a population of neurons - each tuned to a different object aspect, and each showing a certain degree of invari ance to image transformations - may, as an ensemble, encode at least s ome types of complex three-dimensional objects. In such a system, seve ral neurons may be active for any given vantage point, with a single u nit acting like a blurred template for a limited neighborhood of a sin gle view.