Relationship between neural responses and visual grouping in the monkey parietal cortex.

Visual grouping through the binding of multiple discrete elements is an important component of object perception, and neurological studies have shown that the posterior parietal cortex plays a vital role in that process. To study the neural mechanisms underlying visual grouping, we recorded neuronal activity from the lateral bank of the intraparietal sulcus (L-IPS) of monkeys while they performed a task that required them to discriminate among rapidly presented visual patterns composed of five black or white dots arranged in a cross. The monkeys had to detect the patterns in which dots with the same contrast were arranged either horizontally or vertically (target). Visual grouping was necessary for detection of the target, and we surmised that if L-IPS neurons are involved in visual grouping, they may selectively respond to the grouped objects. In addition, we manipulated the monkeys' attention to the grouping of the elements. We found that many L-IPS neurons showed selectivity for the orientation of the target stimuli, and that selectivity was enhanced by the top-down attention. Moreover, the selectivity correlated with behavioral performance. These results provide the first physiological evidence that L-IPS neurons make a crucial contribution to visual grouping by combining visual and attentional signals so as to bind discrete visual elements together.