I just came out of my midyear psych exam at school and one of the questions referred to the split brain surgery that used to be administered to severe epilepsy sufferers involving the severing of their corpus colossum.
The question was something along the lines of
“If visual information was presented in the left visual field of a person who had undergone split brain surgery, and they were asked to describe what they saw, what would the outcome be?”
I would have thought that because the corpus colussom had been split, the information entering the eye would go straight to the left occipital lobe and be processed there. However, in a lot of the text books they explain that if a picture is shown to the left eye, the person will not be able to describe what they see as what is processed in the right hemisphere will not be able to cross back over to the left to be processed for language.
This is confusing because there is no way for the information to get to the right hemisphere in the first place!
Can anyone clear this up for me? Sorry if it’s a little confusing.
There was a piece on one of the TV magazine shows last night about a girl that had this type operation. She seemed to be able to see with both eyes and there was no mention of her not being able to.
I also remember seeing another show that talked about the disconnect with seeing something with the left eye and being able to name it.
In neither case do I remember them answering your question which once you think about it is very logical. You are the one taking the class: Where does the nerve connect to the brain? Could it possibly be to a part that isn’t removed?
I’m not big on anatomy, but I seem to remember (prolly wrongly) that what the left eye percieves is processed by the right side of the brain. Am I just mistaken? That’s the only way I could explain this.
Dirty Dingus McGhee
DDMcG got it right. The left hemisphere of the brain controls the right side of the body, and vice-versa. So a stimulus presented only to the left visual field in a split-brain patient would be processed by the right hemisphere of the brain. Since language is generally controlled by the left hemisphere, most sbp’s would not be able to describe the object with words, though they might for example be able to act out its function with their left hands.
kniz has nailed it… I was after HOW the info is able to get from the left visual field to the right hemisphere if there is no means for it to get there by, ie, the C.C. has been severed.
In all the diagrams i’ve seen, the optic nerve appears to cross over through the C.C as there seems to be no other way. Is kniz perhaps right in suggesting it isn’t removed? Or does the visual info simply get processed in its corresponding hemisphere?
Maybe my textbooks are just screwed up 
I’m too tired to remember the experiments, but this page should get you off to a good start.
Descriptions of split-brain experiments show up in Julian Jaynes’ **The Origin of Consciousness in the Breakdown of the Bicameral Mind **. This is, despite the intimidating title, a “popular” book. But read it with a grain of salt.
Each hemisphere controls the opposite side of the body. This switch is done before it ever gets to the brain. Thus the right eye signal goes directly to the left hemisphere. It never goes to the right hemispere if the corpus colasum is cut.
IIRC, visual information is sent to both sides of the brain at the optic chiasm, before the optic nerves even enter the brain. To have complete optic sensation unlaterality then requires that not only the corpus callosum be severed, but the optic chiasm divided down the middle also. This was done in a number of the split brain patients, but not all. Again, all this is from memory of the topic, and not hard data.
Qadgop and DDMcG are correct. You, silverJ, should take another gander at your textbook. You are commingling the optic chiasm with the corpus callosum. Those are two different cross-overs.
It is important to understand the neuroanatomy of the visual system in binocular organisms with optic chiasma and bihemispheric brains (such as primates). Any good modern neuroanatomy textbook will clarify the circuitry.
The main point to appreciate (and this is often not appreciated) is that the site of crossing (left/right flip-flop) occurs at the pupil. Because of the optical properties of the pupil the left extraorganismal hemispace is projected to the right hemiretinae (and vice versa). The right hemiretinae (containing visual information from the left extraorganismal hemispace) project via the optic chiasm to the right hemisphere. The left hemiretinae project to the left hemisphere.
The chiasm thus is not the site of the crossing of right/left visual sensory experience. What the chiasm does is combine the visual impressions of the two hemiretinae into one “cyclopian eye” located across both primary visual cortices. The chiasm is thus an operational necessity of having two eyes and a bihemispheric brain. If we had one eye and a unispheric brain a chiasm would be unnecessary.
Splitting the chiasm down the middle would not lead to left or right visual hemifield loss. It would only prevent the nasal hemiretinal fields from reaching the contralateral occipital lobes. The deficit could be revealed by special visual testing, but would be hard to detect otherwise.
The deficits apparent in “split brains” are all due to lesions of the corpus collosum that isolate the visual information projected to the left and right visual cortices from the rest of the left and right hemispheres. These deficits are apparent because in humans the left and right hemispheres also subserve specific cognitive functions, mainly language and praxis. Deficits in split brain non-human primates have been documented, but there is little evidence that non-primate vertebrates demonstrate these.
As a neurologist I have been very interested in this stuff. I even wrote a couple of articles on the subject. If anyone is interested, these are: Why are Vertebrate Nervous Systems Crossed? Med Hypotheses (1995)45, 471-475; and, Why do we speak with the left hemisphere? Med Hypotheses (1999) 52(6), 497-503.
Nick Capozzoli, MD
Neurodoc is even more correctly correct, and now that he’s mentioned it, I vaguely recall learning this in med school. Thanks for the definitive word on our visual paths concerns!
Neurodoc, you want to run that again for the non medically trained?
This appears to be saying that the light from the left pupil is projected onto the right retina. ??? How can it go from one eyeball to the other?
Not to speak for neurodoc, but as a physician, I read that as saying that the visual information from the left field of vision is presented to the right side of the retina, via the mechanism of the lens, which not only inverts the light image, but reverses it from left to right also. This jibes with what I recall from med school. I wish I had my old neuroanatomy text in front of me; it was a difficult concept for me to grasp even with visual aids.
I didn’t mention it specifically in my earlier post, but the optical reversal at the pupil provides a compelling explanation for the fact that the right hemisphere of the brain subserves sensorimotor control of the left extraorganismal hemispace, and vice versa. I.e., this explains the previously poorly explained fact that vertebrate nervous systems show contralateral control.
Damn.
I was just typing that.
I saw this anomaly better explained by people picking up objects through a screen, with the left hand. They could then pick up the same object left handed, but could not say what the object was even though they were distinct shapes.