CLUSTERING OF NEURONS RESPONSIVE TO SIMILAR OBJECT FEATURES IN THE INFEROTEMPORAL CORTEX

     The inferotemporal cortex (IT) is crucial for object vision in the monkey.  Lesions in IT selectively impair monkey's ability to distinguish different shapes and recognize objects seen before.  Neurons in IT, especially in its anterior part, preferentially respond to complex visual features of objects such as shapes or combinations of shape with color or texture(Tanaka et al., this volume).  In this presentation we show how neurons with similar stimulus selectivity are spatially arranged within anterior IT.
     Experiments were done in anesthetized and immobilized monkeys(Macaca fuscata).  We first compared the stimulus selectivities of nultiple neurons which were recorded simultaneously with a single electrode and were separated with 2 window discriminators.  Various paper cutouts and tens of 3-D objects were presented to the monkey for the initial survey to find stimuli which activated a neuron.  The stimulus features critical for activation was analyzed by a procedure involving step-by-step simplification of the images of effective objects with a computer graphics system.  After the optimal feature was determined for that neuron, we prepared a set of stimuli including optimal, suboptimal and ineffective ones, and examined the other units with this set.  At most (85%) of the recording sites, simultaneously recorded neurons responded to similar stimuli.  The exact optimal feature or the tuning width, however, often differed slightly between neighboring neurons.
     We next assessed the spatial extent of clustering of neurons with similar selectivity by inserting electrodes tangentially and at right angles to the cortical surface.  As in the experiments described above, we determined the critical stimulus feature for one neuron and then prepared a stimulus set.  We sampled other neurons at 100 or 200m steps in that penetration and tested with this stimulus set.  In penetrations directed normal to the cortical surface, we obtained neurons with similar selectivity over a distance of 0.6 - 1.4 mm, suggesting that neurons with similar stimulus preferences cluster across cortical layers.  In tangential penetrations, neurons with similar selectivity were grouped within 0.2 - 0.5 mm.  In some tangential penetrations, neurons showing similar selectivity were found in 2 or 3 separate clusters.  The gap between nearby clusters was 0.4 - 1.0 mm. 
     We suggest that the anterior IT consists of modules, each containing neurons with related, but often not identical, stimulus selectivity.  This organization can provide a substrate suitable for ensamble coding of diverse object features.


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