Dynamic Circuitry for Updating Spatial Representations. I. Behavioral Evidence for Interhemispheric Transfer in the Split-Brain Macaque

1 Department of Neuroscience and Center for the Neural Basis of Cognition, University of Pittsburgh, Pittsburgh, Pennsylvania; and 2 Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, Maryland Submitted 10 January 2005; accepted in final form 4 May 2005 Internal representations of the sensory world must be constantly adjusted to take movements into account. In the visual system, spatial updating provides a mechanism for maintaining a coherent map of salient locations as the eyes move. Little is known, however, about the pathways that produce updated spatial representations. In the present study, we asked whether direct cortico-cortical links are required for spatial updating. We addressed this question by investigating whether the forebrain commissures—the direct path between the two cortical hemispheres—are necessary for updating visual representations from one hemifield to the other. We assessed spatial updating in two split-brain monkeys using the double-step task, which involves saccades to two sequentially appearing targets. Accurate performance requires that the representation of the second target be updated to take the first saccade into account. We made two central discoveries regarding the pathways that underlie spatial updating. First, we found that split-brain monkeys exhibited a selective initial impairment on double-step sequences that required updating across visual hemifields. Second, and most surprisingly, these impairments were neither universal nor permanent: the monkeys were ultimately able to perform the across-hemifield sequences and, in some cases, this ability emerged rapidly. These findings indicate that direct cortical links provide the main substrate for updating visual representations, but they are not the sole substrate. Rather, a unified and stable representation of visual space is supported by a redundant cortico-subcortical network with a striking capacity for reorganization. Address for reprint requests and other correspondence: C. L. Colby, 115 Mellon Institute, 4400 5th Ave, Pittsburgh, PA 15213 (E-mail: ccolby{at}cnbc.cmu.edu )