In order to understand Gazzaniga and LeDoux’s results, it’s necessary to bear in mind two facts about human biology. The first is that the brain’s control of the human body is contralateral, which is to say, the left hemisphere of the brain controls the right side of the body, and the right hemisphere controls the left. The second is that speech is centered in the brain’s left hemisphere, so information that reaches only the right hemisphere of a split-brain subject literally cannot be talked about (though the right hemisphere can, for example, use the hand it controls to draw a picture of an image it can’t verbalize).

Gazzaniga and LeDoux’s experiments relied on this bifurcation. They asked split-brain subjects to look straight ahead while images were flashed quickly in their right and left fields of vision. Because the images appeared in non-overlapping areas of the visual field and vanished before the subject’s eyes could move, the scientists were able to send images directly to one hemisphere while preventing the other from receiving the image at all (because, of course, the hemispheres couldn’t communicate with each other). Here is what happened with a subject called P.S., as Gazzaniga describes it in Who’s in Charge? Free Will and the Science of the Brain:

We showed . . . two pictures: A chicken claw was shown to his right visual field, so the left hemisphere only saw the claw picture, and a snow scene was shown to the left visual field, so the right hemisphere only saw that. He was then asked to choose a picture from an array of pictures placed in full view in front of him, which both hemispheres could see. The left hand pointed to a shovel (which was the most appropriate answer for the snow scene) and the right hand pointed to a chicken (the most appropriate answer for the chicken claw).

Then we asked why he chose those items. His left-hemisphere speech center replied, “Oh, that’s simple. The chicken claw goes with the chicken,” easily explaining what it knew. . . . Then, looking down at his left hand pointing to the shovel, without missing a beat, he said, “And you need a shovel to clean out the chicken shed.”

As Gazzaniga goes on to observe, “What was interesting was that the left hemisphere did not say, ‘I don’t know,’ which truly was the correct answer. It made up a post hoc answer that fit the situation.” Or as Frost might put it, the left hemisphere realized what had happened—which road had been chosen—and took credit for it. It made up an impressively creative story to fit the reality that it perceived, one it might be telling “ages and ages hence.”