Grant: Discovery Grants Program - Individual

Humans explore their visual environment through orienting movements that rapidly change their line of sight. Small saccadic eye movements made without head movements are arguably the best understood volitional movement made by humans. However, because the eyes are nested within a mobile head, the oculomotor system evolved to control gaze, and large gaze shifts are brought about by coordinated movements of the eyes and head (gaze = eye-in-space = eye-in-head + head-in-space). The ubiquity, rapidity, and accuracy of eye-head gaze shifts make it a model system for understanding how the brain coordinates multi-segmental motion.We have hypothesized that the oculomotor system permits selective recruitment of head movements without gaze shifts by controlling neural activity relative to a high ""gaze"" threshold and a lower ""head"" threshold. Here, we test this hypothesis in humans by examining how well neck muscle recordings index covert oculomotor plans. Neck muscle recordings circumvent the biomechanical complexities inherent to head motion, and provide a direct measure of neural control on a trial-by-trial basis. One aim will be to record neck muscle activity during tasks that set up a competition between response alternatives, as the covert oculomotor plans developing during such tasks are well understood. Such an approach provides an opportunity to compare neck muscle activity with estimates of covert oculomotor processes derived from computational models. A second approach will be to examine neck muscle activity following brief pulses of transcranial magnetic stimulation (TMS). We have recently demonstrated that a neck muscle response is evoked by TMS of the frontal eye fields (FEF), and we now look to leverage this finding by using such evoked responses as an assay of FEF engagement during covert plans for large gaze shifts. The common theme throughout this proposal projects is that neck muscle activity in humans provides an objective measure into developing oculomotor projects that would otherwise be deemed ""covert"" by virtue of stability of the gaze axis.