Accurate bidirectional saccade control by a single hemicortex

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Abstract
Anatomical, electrophysiological and lesion studies indicate that each cortical hemisphere normally generates saccades directed to the contralateral side. In contrast, in patients who had an entire cortical hemisphere removed surgically (hemidecortication), the remaining hemicortex can generate both contraversive and ipsiversive saccades. However, current evidence indicates that ipsiversive saccades are grossly inaccurate. The obvious reason for this is that hemidecorticate patients are blind in the hemifield ipsilateral to the remaining hemicortex, and therefore normal visual signals are not available to drive ipsiversive saccades. However, absent vision also implies that visual error signals are not available to calibrate ipsiversive movements. Furthermore, the innate anatomical substrate needed to support accurate ipsiversive saccade control, in addition to the normal contraversive control, appears sparse. We show here that, in spite of these obstacles, hemidecorticate patients could generate accurate ipsiversive saccades in a task that dissociated hemianopia from saccade direction. In this task, while the patients fixated a central fixation target (FT), saccade targets (STs) were briefly presented to the intact visual hemifield contralateral to the intact hemicortex. The FT was then moved towards and beyond the former location of the ST which evoked tracking eye movements that moved the eyes towards and then beyond the ST, thereby moving the goal, ST, into the blind visual hemifield ipsilateral to the intact hemicortex. When the FT was extinguished, the patients generated, in the dark, ipsiversive saccades that moved their eyes to the remembered location of the ST with the same accuracy as normal control subjects. This indicates that a single hemicortex can mediate accurate bidirectional saccade control via fully functional bilateral connections from cortex to brainstem oculomotor structures. The mechanisms whereby visual signals can calibrate ipsiversive saccades remain elusive.

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