How does the brain keep track of relevant spatial locations when the eyes move? In extrastriate, parietal and frontal cortex, and in the superior colliculus, neurons update stimulus representations in conjunction with eye movements. This updating reflects a transfer of visual information, from neurons that encode a salient location before the saccade, to neurons that encode the location after the saccade. Copies of the oculomotor command - corollary discharge signals - likely initiate this transfer. Spatial updating, or remapping, is thought to contribute to the maintenance of stable spatial representations as the eyes move. We investigated the circuitry that supports spatial updating in the primate brain. Our central hypothesis was that the forebrain commissures provide the primary route for remapping spatial locations across visual hemifields, which entails the interhemispheric transfer of visual information. Further, we hypothesized that these commissures provide the primary route for corollary discharge signals, generated in one hemisphere, to initiate spatial updating in the opposite hemisphere. We tested these hypotheses by measuring spatial behavior and neural activity in two split-brain macaques. In behavioral experiments, we observed striking initial impairments in the monkeys' ability to update stimuli across visual hemifields. Surprisingly, however, we found that both animals were ultimately capable of performing these across-hemifield sequences. Both monkeys readily performed the same spatial task when updating required an interhemispheric transfer of corollary discharge signals, suggesting that these signals are transferred via subcortical pathways in the normal monkey. In physiological experiments, we found that neurons in lateral intraparietal cortex of the split-brain monkey can remap stimuli across visual hemifields, albeit with a reduction in the strength of remapping activity. These neurons were robustly active when within-hemifield updating was initiated by a saccade into the opposite hemifield. Our findings suggest that both visual and corollary discharge signals from opposite hemispheres can converge to update spatial representations in the absence of the forebrain commissures. These investigations provide new evidence that a unified and stable representation of visual space is supported by a redundant circuit, comprised of cortical as well as subcortical pathways, with a remarkable capacity for reorganization
