Splitting the brain does not split the person

To try to get to the bottom of things, my team at the University of Amsterdam re-visited this fundamental issue by testing two split-brain patients, evaluating whether they could respond accurately to objects in the left visual field (perceived by the right brain) while also responding verbally or with the right hand (controlled by the left brain). Astonishingly, in these two patients, we found something completely different than Sperry and Gazzaniga before us. Both patients showed full awareness of presence and location of stimuli throughout the entire visual field – right and left, both. When stimuli appeared in the left visual field, they virtually never said (or indicated with the right hand) that they saw nothing. Rather, they would accurately indicate that something had appeared, and where.

But the split-brain patients we studied were still not completely normal. Stimuli could not be compared across the midline of the visual field. Moreover, when a stimulus appeared in the left visual field, the patient was better at indicating its visual properties (even when he responded with the right hand or verbally!), and when a stimulus appeared in the right visual field, he was better at verbally labelling it (even when he responded with the left hand).