Sperry (1968): split-brain study

This study is hard to recall accurately. Four hooks earn credit: 11 patients whose corpus callosum was cut for severe epilepsy; a central dot, one eye covered; a tenth-of-a-second exposure; and being asked to say, write or draw it, or find it by touch. A vague "the two sides differed" states no procedure. The count is worth a mark on its own — learn it as 11.

The most muddled rule. The left visual field goes to the right hemisphere; the right field to the left hemisphere; each hand obeys the opposite hemisphere. So the left hand is the right hemisphere's hand — why it finds an object the patient swears they never saw. Everything crosses: send the left field to the left hemisphere and the answer is wrong.

Zero-scoring candidates wrote about the corpus callosum as though it were another region of the brain with its own function. It is not: it has no job of its own the way the occipital lobe has vision. It is the bundle of nerve fibres joining the two hemispheres so information can pass — cut in Sperry's patients. Treat it as a connection, never a region.

A result is what was observed; a conclusion is what Sperry inferred. "Could not name a left-field picture" is a result; "the right hemisphere has no speech" is the conclusion. "The left hand found the object by touch" is a result; "the right hemisphere perceived it, so the failure was in speech not sight" is the conclusion. Give the meaning, not the observation.

"A weakness is only 11 patients" identifies but does not justify. Every strength or weakness needs point, why, consequence. Point: the sample is small and unusual. Why: all 11 had severe epilepsy and brain surgery. Consequence: the pattern may describe the disconnected surgical brain, not how an intact brain divides its work — limiting generalisability.

Asked for the difference between the two hemispheres, candidates write two definitions side by side, leaving the difference only implied — one mark of two. Use a connective (whereas, while) naming what they differ on: the left hemisphere has speech, whereas the right can show what it saw only through the left hand. A 2-mark describe also needs two unique points.

"The right hemisphere cannot see and is useless" — wrong. It saw it: the patient reports nothing, yet the left hand finds the object by touch, impossible unless it saw it; what it lacks is speech, not perception. Nor is it the weaker half — on spatial tasks the left hand beat the right. Everyday behaviour looked normal: odd effects appear only under the tight test.