Impairment of extraretinal eye position signals after central thalamic lesions in humans.

Accuracy of four different types of memory-guided saccades was studied in two patients with a small central thalamic lesion, probably involving the region of the internal medullary lamina (IML), and in a control group. In the first paradigm, the eyes and head remained immobile between the time of the presentation of the visual target to be remembered and the memory-guided saccade. In the other three paradigms, the eyes were displaced during the same period (before the memory-guided saccade) by either visually-guided saccades, a smooth pursuit eye movement or a body movement (with vestibulo-ocular reflex suppression). Therefore, in these three paradigms, the initial eye displacement required the use of extraretinal eye position to produce accurate memory-guided saccades. Compared with the control group, the two patients had normal accuracy in the first memory-guided saccade paradigm, in which there was no initial eye displacement, but markedly impaired saccade accuracy in the other three paradigms. These results suggest that the cortical areas triggering saccades did not receive correct extraretinal eye position signals. They are consistent with an impairment of the efference copy, which could be distributed to the cortical ocular motor areas by the IML.