The primitive retino-tecto-reticular pathway is functional in hemidecorticate patients

Summary Normal vision requires the classic neural pathway from retina to lateral geniculate nucleus (LGN) to cortex. A lesion of visual cortex causes blindness, but often unconscious visual abilities are retained; this is known as ‘blindsight' and is characterised by responses to visual stimuli a patient denies seeing. Three types of blindsight have been proposed: action blindsight, attention blindsight and agnosopsia [1]. Here we study action blindsight — motor responses to unseen stimuli — via the influence on eye saccades of a visual stimulus presented in a blind area. One pathway that hypothetically enables action blindsight in humans, but that has never been formally proven to do so, is the primitive retino-tecto-reticular pathway. We demonstrate, in hemidecorticate patients with no available neural structures for vision on one side except for the primitive retino-tecto-reticular pathway, that saccades to their blind hemifield can be perturbed by an unseen visual probe according to visuo-motor interactions on the logarithmically-encoded motor map of their superior colliculus (SC) [2]. The primitive retino-tecto-reticular pathway thus appears to be functional in these patients.