Brain hemispheres and eyes

I have read this part of my psychology book dozens of times. I have looked at the diagram. I still can’t figure out how this works. Can someone please explain?

“The left half of each eye is connected directly to the left hemisphere, but not to the right hemisphere; similarly, the right half of each eye is connected directly, and only, to the right hemisphere. Thus, if you stare straight ahead, objects to the left are seen first by the right brain, and those to the right are seen by the left brain.”

Why? Both eyes are connected to both hemispheres. If the optic nerves and corpus callosum are intact, shouldn’t both hemispheres see the object at the same time?

I can give it a shot.

Here is an image which clearly illustrates what your book is telling you.

First lets define some words. Medial or nasal means towards the middle, or closer to your nose. Lateral or temporal means towards the sides or closer to your temporal bone.

Lets consider this situations. A doctor shines a light in your eyes on your left side. Light from the light bulb travels to your eyes and hits both of your retinas. Because its coming from the left, it doesnt hit the entire retina on either side. In your left eye, the light hits your medial, or nasal, retina. In your right eye, the light hits your lateral, or temporal retina.

If you look on that diagram, you see that the nerve fibers from both medial retinas cross at the optic chiasm, and continue in the opposite side of the brain. Because of this, information from the left side of both eyes travels to the right brain. Similarly, information from the right side of both eyes travels to the left brain.

So while yes, both eyes are connected to both hemispheres, information from the left and right sides of your visual field go to the right and left hemispheres, respectively.

At the occipital lobe, the rearmost part of the brain, the information received from the retinas is processed, and your brain attributes “what”, “where”, and “how do I feel about this” to the things that you see. Without the occipital lobe, your brain can’t understand the information it receives. Losing the occipital lobe is sort of like trying to stream a video with the wrong codec. You get all the raw information, but you have no way to interpret it.

Interestingly, lesions of the occipital lobe can cause a phenomena known as cortical blindness. Patients will swear up and down that they can see just fine, but will be unable to tell you what they are seeing. You can predict what areas of their visual field will be affected by the extent of the lesion.

So yeah, that diagram explains it all pretty well. The corpus callosum is not involved with vision until after it has been processed by the occipital love.

Oh my god, I could kiss you. I totally understood that. No matter how many times I looked at the diagram in the book, I never thought that the retina would receive and send the information differently depending on where the information came from. I thought that the signal would come in, process everywhere and go both ways equally.

I’ve always thought of the retina as being a solid piece but it’s not like that at all.

I am not entirely sure I understand what your problem is, but perhaps it would help if you took account of the fact that all talk of the brain, let alone parts of the brain, seeing anything at all is merely loose and metaphorical. Brains do not see anything, nor does conscious visual experience arise solely because visual information from the eyes arrives at some location in the brain. (How conscious visual experience actually does arise is still a mysterious and very controversial matter, but one thing we do know is that it is not just a question of some part of the brain being activated, or receiving information.) People and other animals (whole organisms) see things. Their brains play a large and essential role in the process of seeing, but brains themselves (let alone hemispheres, visual cortices, or whatever) do not see. (Much the same could be said of eyes.)

The passage you quote is really talking about what parts of the brain (and, specifically, what parts of the primary visual cortex, a.k.a. V1 or area 17) the visual information from the different eyes and different parts of the eyes is first delivered to. It tries to make that simpler and more vivid by talking about the different parts of the brain seeing different parts of the visual field, but, unfortunately, it seems that it has only confused you by talking in this inaccurate way. When you oversimplify things too much, it is the smarter students who get confused! What it should have said is that these different parts of the brain are where the information from different parts of the visual field first arrive.

Even that is oversimplifying the actual situation greatly, as visual information first goes to the lateral geniculate nuclei (LGN - part of the thalamus), not the primary visual cortex, and although most of the information arriving at the LGN is passed on directly to V1, some of it goes to other parts of the brain. Not all visual information is routed through the primary visual cortex. Neither is visual processing complete when the information arrives in V1. There is a lot more to be done in other brain areas further on down the line. (And this is just the bottom-up, feedforward aspect of visual processing. There is a large top-down, feedback component too, with information from “higher” brain areas modulating the information as it passes through the LGN, V1, and other visual processing areas, and, indeed, modulating how the eyes themselves behave. It is extremely complicated and very far from being fully understood.)

There is no need to delve into the more subtle complexities of vision. While it is true that current theories which describe it are probably oversimplifications, they do quite well in correlating lesion localization to neurological deficits. The OP just wanted a brief explanation of the neurophysiology behind hemianopsias, which is simple enough.

Well I am glad if you could read her mind better than I could. As the passage she quoted seemed to say straightforwardly enough where the nerve tracts were going, and as she said her puzzlement related to why “shouldn’t both hemispheres see the object at the same time?”, I thought she was confused by the deeper ambiguities in the passage.

Her post does not mention hemianopsias, either under that name or in any less technical way. It appears to be about the neurophysiology of vision in general, and it does appear to be concerned with where “seeing” takes place.

It was just a misunderstanding about the retina itself.

njtt: Thank you for the answer. It was a lot more specific than I needed but, as you said, how could you know exactly what I needed? I appreciate the different explanations for the same problem. You never know what will make sense to one person and not another.


It’s safe to say that most of the people in this class have never taken an Anatomy class before. I have but it was about 13 years ago. I kept trying to picture the eye I dissected and I kept closing one eye and looking at things from the side, trying to figure out why only half the eye would receive the image from the side.

In this case, something as simple as the words medial and later in reference to the retina whipped my brain back on track.