SERIAL PROCESSING OF VISUAL OBJECT-FEATURES IN THE POSTERIOR AND ANTERIOR PARTS OF THE INFEROTEMPORAL CORTEX
The macaque inferotemporal cortex(IT) has been thought essential for the
visual object discrimination and recognition, based on the previous lesion and
anatomical studies. IT contains many
cells that selectively respond to "moderately complex" visual
features of objects (Tanaka et al., J. Neurophysiol. in press). To investigate how the selectivity to
complex features is constructed and how cells with different critical features
are distributed within IT, we have made recordings from IT and related regions
of anesthetized, immobilized monkeys(Macaca
The critical feature for the activation of individual cells was determined by presenting many 3D objects, simplifying images of the effective objects with an image processing computer, and determining the simplest feature that evoked the maximal response. In a first series of experiments, the response to the critical feature was then compared with responses to simple stimuli. The simple stimuli included a fixed set of 32 spots and bars with different size, orientation, color, and contrast polarity and those prepared for individual cells in relation to the critical feature. Most cells in the anterior 2/3 of IT responded only to the complex critical feature and showed almost no (<25% of the maximum) responses to any of the simple stimuli. Their receptive fields were large (the square root of the area was 10 to 25 deg) and included the fovea. In the remaining posterior part of IT, three kinds of cells intermingled: cells that responded maximally to some of the simple stimuli, cells that showed some medium responses(25-75% of the response to the complex critical feature) to some of the simple stimuli, and cells that only responded to the complex critical feature. The posterior IT cells, regardless of the selectivity, had small receptive fields (1 to 9 deg), which occupied various positions within 15 deg from the fovea. Most cells in the prelunate gyrus (dorsal V4) responded maximally to some of the simple stimuli.
In a second series of experiments, the selectivity was compared among cells located at nearby positions within the anterior IT. After determining the critical feature of a cell, we made a set of 8 to 18 stimuli including the optimal, suboptimal and ineffective stimuli for the first cell, and examined with this set the responsiveness of cells sampled at 100 to 200 micron steps in the same penetration. In penetrations made vertical to the cortical surface, responses to the positive stimuli for the first cell were commonly shown by cells recorded over a distance of 0.5-1.4 mm, although the stimulus that evoked the maximum response or the tuning width often differed among the cells.
We suggest that the selective responses to complex features are first made in the posterior IT and the selectivity becomes more precise and positionally invariant in the anterior IT. The anterior IT is composed of modules in each of which cells with similar but slightly different selectivity cluster.