NEURONS WITH RELATED STIMULUS SELECTIVITY ARE CLUSTERED IN THE MACAQUE INFEROTEMPORAL CORTEX.

  Neurons in the anterior 2/3 of the inferotemporal cortex (IT) of the macaque are selectively activated by visual stimuli with particular 'complex' features: 'complex' shape or combinations of texture or color with shape.  Simultaneous recordings from multiple neurons with single electrodes have shown that adjacent cells have similar, but usually not identical, stimulus selectivity (Fujita et al., Soc Neurosci Abstr, 1990).  To assess the spatial arrangement of neurons with similar stimulus preferences we made 2 kinds of penetrations in IT, one directed vertically and the other tangentially to the cortical surface.
 Recordings were made in the crown of the anterior IT in anesthetized and immobilized monkeys (Macaca fuscata).  For vertical penetrations, electrodes were advanced from the side at right angles to the midline of the head and titled downward by 20‹.  For tangential penetrations, we directed electrodes from the side anteriorly at an angle of 45‹ to the midline and downward by 27‹.  We presented various paper cutouts and tens of 3-D objects to the money for the initial survey to find stimuli which activated a cell, and then determined the stimulus feature critical for the activation with the aid of a computer graphics system.  We made a set of 8 to 18 stimuli ('test set') including optimal, suboptimal and ineffective stimuli for that cell.  Other cells were sampled at 100 or 200 ƒÊm steps in that penetration and tested with this same set of stimuli.
  Over a distance of 0.6 - 1.4 mm in vertical penetrations we obtained neurons which responded to related stimuli in the test set, i.e., the optimal and suboptimal stimuli for the first tested cell in each penetration.  In tangential penetrations, cells with related stimulus selectivity were found within 0.2 - 0.5 mm.  In some tangential penetrations, there existed 2 or 3 separate clusters of neurons showing similar selectivity.  The gap between adjacent clusters was 0.4 - 1 mm.  We suggest that the anterior IT is composed of modules, in which neurons with related selectivity cluster across cortical layers.


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