Relationships between color, shape, and pattern selectivities of neurons in the inferior temporal cortex of the monkey

Abstract

1. To examine the way in which information from different visual submodalities is integrated in the inferior temporal (IT) cortex of the monkey, we studied the relationships between the color, shape, and pattern selectivities of individual neurons in IT cortex of two awake macaque monkeys. Neurons were recorded while each animal performed a visual fixation task. For each neuron, we analyzed selectivity for the visual submodalities of color, contour shape, and textural pattern using preselected standard sets of visual stimuli, namely colored geometrical shapes with certain patterns. 2. About two thirds (62%) of single neurons whose activities were recorded from the anterior part of IT cortex, which included the ventral bank of the superior temporal sulcus, responded to one or more of the stimuli in the standard sets of stimuli used. An index (stimulus selectivity index) was calculated for each neuron to quantify how well a cell discriminated the preferred stimulus from the least preferred stimulus in each set of stimuli. The stimulus selectivity index, as well as the statistical significance of the variation in the responses to the stimuli within a given set, was used to classify a cell as selective or not selective in a given submodality. Of the neurons whose responses were analyzed quantitatively, 69% were selective for color, 68% were selective for shape, and 82% were selective for pattern. 3. Of the neurons that were tested with respect to the selectivity for color and for shape, 45% were selective both for color and shape, 50% were selective for either color or shape, and only 5% were not selective for color or shape. These frequencies were not significantly different from those predicted from the occurrence of the selective and nonselective neurons in each submodality if the independence of the selectivities for color and shape is assumed. We also found that the color preference of individual neurons does not depend on the shape of the stimulus. These results indicate that there was no overt interaction between the selectivities for color and shape in these IT neurons. 4. Of the neurons that were compared with respect to the selectivity for color and for pattern, 58% were selective both for color and pattern, 38% were selective for either color or pattern, and only 4% were not selective for color or pattern. There was no correlation between the degree of color selectivity and the degree of pattern selectivity of individual neurons. 5. Of the neurons that were compared with respect to the selectivity for shape and for pattern, 65% were selective both for shape and pattern, 12% were selective for either shape or pattern, and 24% were not selective for shape or pattern. There was a close correlation between the degree of shape selectivity and the degree of pattern selectivity of individual neurons. 6. These results indicate that signals from different visual submodalities are integrated much more extensively in IT cortex than has previously been recognized. However, the fact that we did not find any evidence of an overt interaction between the selectivities for color and shape, as well as between those for color and pattern, seems to suggest that sensitivities to chromatic and spatial parameters of the stimuli were generated separately and assigned randomly to individual IT neurons. 7. Shape selectivity of IT neurons was relatively broad. The vast majority of the shape-selective neurons responded appreciably to more than one shape. This was also the case for the color selectivity of IT neurons. Most of the color-selective neurons responded appreciably to several different colors in the standard set. Most of the pattern-selective IT neurons also responded appreciably to more than one pattern. However, a minority of neurons responded only to one pattern, and some of these neurons seemed to be sensitive to a feature that characterized the stimulus.