How can a message be sent specifically to one of the hemispheres of the brain?
Quite simply, anything presented to the right visual field - in this case, the banana -
falls on the left half of the retina in both eyes and is passed to the left hemisphere.
Inversely, anything presented to the left visual field - the beach ball - is passed to the right hemisphere.
Of course, if you focused your eyes directly on the beach ball, the image, the image of the ball would go to both hemispheres.
To compensate for this, images can be flashed briefly and then removed.
The person doesn't have time to move the eyes, so the information from each visual field will go to only one hemisphere.