Switching of sensorimotor transformations: antisaccades and parietal cortex.
The sensorimotor processing necessary in complex realistic situations goes beyond straight-forward application of a given sensorimotor transformation. Contextual information may make it necessary to switch to another sensorimotor transformation. We studied the issue of switching by using the mixed memory prosaccade/antisaccade task. Neurons of the lateral intra-parietal area (LIP) might be involved in computing the sensorimotor transformations for both prosaccades and antisaccades. LIP neurons may also be involved in switching to the antisaccade sensorimotor transformation, when an antisaccade is requested. Some neurons in LIP show a paradoxical pattern of activity-motor in space but visual in time. Funahashi, Chafee and Goldman-Rakic reported in 1993 a complimentary pattern of activity in prefrontal cortex-visual in space but motor in time. These odd observations are explained by the hypothesis that (1) the parietal cortex contains a sensorimotor transformation module, and prefrontal cortex a context categorization module, and (2) following target onset, information flows from early visual system to parietal cortex and on to prefrontal cortex; then, a second wave of activation, contingent on a switching signal, arrives back at parietal cortex. The duration of this loop is less than 100 ms. Thus, the paradoxical activities are intermediate representations derived in the cognitive processing involved in switching sensorimotor transformations.