A biomechanical explanation for convergent head movements evoked by stimulation of the primate supplementary eye fields

Cortical microstimulation has played an important role in the investigation of movement coding in the frontal and supplementary eye fields (FEF and SEF). Recent evidence has shown that microstimulation of the SEF produces eye head gaze shifts whose kinematics depend on initial position. Across different stimulation sites in the SEF, evoked movements converge in a variety of reference frames, possibly reflecting the SEF’s role in complex sensory-motor transformations. Here, we examine neck muscle activity evoked by SEF stimulation while monkeys attained a range of different initial positions through electromyographic (EMG) recordings of the neck muscles. A similar approach targeting the FEF and superior colliculus (SC) has revealed a counter-intuitive trend where increasing levels of agonist (contralateral) neck muscle activity are associated with the smallest evoked movements. Monkeys were trained to look to one of nine different fixation points prior to SEF microstimulation (100 pA, 300 Hz, 200 ms) with stimulation was passed on half of all trials. SEF stimulation evoked a rapid facilitation (18 ± 5.5 ms ms) of EMG activity on contralateral agonist neck muscles and a simultaneous suppression of EMG activity on the antagonist neck muscles. Importantly, the expression of this generic evoked response depended on the positionally-dependent level of background EMG activity attained prior to stimulation. As in the SC and FEF, we observed a counter-intuitive trend where the smallest amplitude movements, which occurred for initial positions contralateral to the side of stimulation, were associated with the largest increases in evoked neck muscle activity. Our results suggest that the apparent convergence of the head following stimulation in a variety of oculomotor areas may result more because of biomechanical considerations consequent to the initial positions, as opposed to a centrally-programmed strategy reflective of reference frame coding