Klaury Youchom-Tagheu¹, Philipp Kreyenmeier¹, Krystel R. Huxlin², Elisha P. Merriam¹; ¹National Institute of Mental Health, ²University of Rochester

Motor circuits in the brain have privileged access to visual information, allowing rapid responses to external stimuli. Whether such rapid visuomotor behaviors are mediated via subcortical pathways or rely on cortical visual processing is debated. Here we study oculomotor freezing, the reflexive cessation of saccades in response to the onset of visual or auditory stimuli, to test whether this short-latency oculomotor behavior relies on intact cortical visual processing in humans. We recorded eye movements of two patients (p01:65-year-old male; p02:42-year-old female) with unilateral stroke-induced damage to early visual cortex. The patients continuously fixated at the center of a screen while we presented visual gratings (1cycle/degree, 3degrees diameter, 100% contrast), auditory white noise bursts, or both combined. The grating was either presented inside the patients’ blind field (p01:[-18,-5]; p02:[7,5]) or at a corresponding location within the intact visual hemifield. The auditory stimulus was spatially unbiased. In both patients, stimulus-locked microsaccade rates exhibited a rapid decrease in response to the onset of the peripheral grating presented in the intact hemifield (indicating oculomotor freezing). The inhibition latency was ~100ms and lasted for ~300ms. In contrast, when iso-eccentric gratings were presented inside the blind field, the same saccade rate modulation was not observed. In trials where gratings were paired with the auditory stimulus, oculomotor freezing was enhanced for stimulus presentation in the intact hemifield, compared to auditory-only trials. No enhancement was observed when gratings were presented inside the blind field, indicating that multisensory enhancement only occurred when the visual stimulus was presented inside the intact field. Our results provide neuropsychological evidence that intact visual cortical processing of stimulus onsets is necessary to elicit oculomotor freezing. These findings are consistent with previously reported oculomotor abnormalities in cortically blind patients, and psychophysical studies suggesting that perception and oculomotor freezing rely on shared visual signals.