In humans, the left eye is NOT linked to the right hemisphere (and vice-versa). Each retina is split down the middle. You can think of it as temporal retinas and nasal retinas. The temporal retinas (the halves nearest our temples) send info to the side of the brain that they sit in front of (i.e., left side of left retina is connected to left hemisphere). The nasal retinas connect to the opposite hemisphere — the pathway crosses over at the optic chiasm.

To make matters even more confusing, you can’t forget that the visual image that lands on the retina is upside-down and left-right reversed. The end result is that things on the right side of your visual world are processed by the left hemisphere, and vice-versa.

Thanks for the great article. I teach a biopsychology class at my college and I’m thinking of referring my students to it. I did have a comment about the following sentence from your piece:

“In fact, if people view a face only through their left eye (which is linked to the brain’s right hemisphere), they will do a better job of recognizing it than if they use only their right eye.”

My understanding is that in humans, both eyes (left and right) send connections to both sides of the brain. You may mean that faces presented in the left visual field go first to the right side of the brain (and vice versa).

Thanks for the fantastic blog you maintain and your excellent books. Take care!

Carl: Thanks for pointing out that ambiguity, Don. I should have been more clear about how each side of the brain “sees” the world.

I have to wonder, if his ability to recognize faces is in his right hemisphere, how he can name the people that he meets (since facial recognition is so different from other types of object recognition)? Does his left hemisphere compensate in some other way?

Mind boggling, that you can just amputate half a brain. Nothing like watching people tinker with the machinery to make you question your sense of self!

A few questions and issues from a neuroscientist:

1. a geometric question: how does a developing embryo know its right from its left? After it develops bilateral symmetry, what is the signal that, for example, has it lie on its right, as opposed to its left, side?

2. some of the differences in the two hemispheres are built in, like the side of motor control (left hemisphere: right upper and lower extremities), visual fields (left half of visual fields, right hemisphere) and somatic sensory inputs (again crossed). In children who have a hemisphere removed there can be dramatic recovery. How much of this apparently hard-wired processing recovers? How much motor control is there contralateral to the hemisphere that is removed, etc?

3. There are suggestions of plasticity in the corpus callosum. The midpart of the cc of musicians appears to be thicker than non-musicians, and this appears to be due to practice. Also, I was surprised to learn recently that 40% of the axons in the corpus callosum are unmyelinated.

What if you cut the communication between them but did not remove anyone of them (and, im not sure if this can be done, made both brain halves “survive”). Would you then have split the persons “me” into two “me’s”?

Carl: Yes! Watch this for more.