Blindsight after hemidecortication: visual stimuli in blind hemifield influence anti-saccades directed there.
Patients missing a cortical hemisphere, removed surgically at adulthood, cannot consciously see a visual probe stimulus (P) flashed in their blind contralesional, hemifield. Nevertheless, they have a low-level form of blindsight wherein P can affect the reaction time of a manual response to the appearance of a visual target in their seeing hemifield. This ability is thought to require the pathway from retina-to-ipsilesional superior colliculus (SC) to cortex of the remaining hemisphere (Leh et al., 2006a, 2006b, 2009). Apart from emitting ascending signals, the SC normally sends saccade commands to the brainstem, a function seemingly conserved after hemidecortication because such patients can generate voluntary and accurate saccades bilaterally (Herter and Guitton, 2004). However, they cannot generate goal-directed saccades to P in their blind hemifield. We hypothesized that, in hemidecorticate patients, P might influence anti-saccades directed to the blind hemifield, to the mirror location of a visual cue presented in the seeing hemifield. We used anti-saccades because our patients could scale their anti-saccade amplitudes approximately according to different cue locations, thereby permitting us to control the end point of their anti-saccades to the blind hemifield. We identified in these patients a new form of blindsight wherein unseen P, if properly timed at the anti-saccade goal location in the blind hemifield, reduced the reaction time and improved the accuracy of anti-saccades directed to that general location. We hypothesize that P in the blind hemifield produced low-level signals in the ipsilesional SC that, if appropriately located and timed relative to anti-saccade goal and onset, interacted with anti-saccade motor preparatory activity - produced by descending commands to SC from the remaining hemisphere - so as to modify both anti-saccade reaction time and end point. Our results support normally encoded and functionally useful, but subliminal, signals in the retina-to-ipsilesional SC-to-reticular pathway of hemidecorticate patients.
Montreal Neurological Institute, McGill University, Canada.
Olga
O
Savina
Andre
A
Bergeron
Daniel
D
Guitton
D000328:Adult; D002540:Cerebral Cortex / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D010468:Perceptual Disorders / Q000503:physiopathology*; D010775:Photic Stimulation; D011930:Reaction Time / Q000502:physiology; D012438:Saccades / Q000502:physiology*; D014794:Visual Fields / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*