Head movements evoked by electrical stimulation in the frontal eye field of the monkey: evidence for independent eye and head control.

When the head is free to move, electrical stimulation in the frontal eye field (FEF) evokes eye and head movements. However, it is unclear whether FEF stimulation-evoked head movements contribute to shifting the line of sight, like visually guided coordinated eye-head gaze shifts. Here we investigated this issue by systematically varying initial eye (IEP) and head (IHP) positions at stimulation onset. Despite the large variability of IEP and IHP and the extent of stimulation-evoked gaze amplitudes, gaze displacement was entirely accounted for by eye (re head) displacement. Overall, the majority (3/4) of stimulation-evoked gaze shifts consisted of eye-alone movements, in which head movements were below the detection threshold. When head movements did occur, they often started late (re gaze shift onset) and coincided with rapid eye deceleration, resulting in little change in the ensuing gaze amplitudes. These head movements often reached their peak velocities over 100 ms after the end of gaze shifts, indicating that the head velocity profile was temporally dissociated from the gaze drive. Interestingly, head movements were sometimes evoked by FEF stimulation in the absence of gaze shifts, particularly when IEP was deviated contralaterally (re the stimulated side) at stimulation onset. Furthermore, head movements evoked by FEF stimulation resembled a subset of head movements occurring during visually guided gaze shifts. These unique head movements minimized the eye deviation from the center of the orbit and contributed little to gaze shifts. The results suggest that head motor control may be independent from eye control in the FEF.