Experiment Module: What Split Brains Tell
Us About Language
Communication between the two hemispheres of the brain is made possible by the bundles of axons, or commissures, that connect them. The largest of these bundles, known as the corpus callosum, consists of about 200 million axons running from one hemisphere to the other.
In the 1950s, American neuroscientist Roger Sperry and his team discovered that curiously enough, severing the corpus callosum in the brain of a cat or monkey had no notable effects on the animal’s behaviour. Only some special experimental protocols revealed that these animals were actually sometimes behaving as if they had two brains.
This absence of major deficits in animals with a severed corpus callosum gave neurosurgeons the idea of performing this operation on certain patients whose frequent, severe epileptic attacks were ruining their lives. In some of these patients, the epileptic focus was located in only one hemisphere, so this operation could successfully prevent the attacks from propagating to the other hemisphere. Having had this operation, these “split-brain” individuals could go back to enjoying their lives; as with the animals in Sperry’s experiments, their day-to-day behaviour was practically unaffected by the separation of their brains into two halves.
The subjects could easily repeat numbers or words or describe images projected in their right visual field, because the left hemisphere, which received and processed this information, is the dominant hemisphere for language. Similarly, when asked to close their eyes and feel an object with their right hand, they could describe the object readily.
But when the visual stimuli were projected in the subjects’ left visual field or when they were asked to feel objects with their left hand, their performance was quite different: they could not describe the stimuli or objects concerned. In fact, for the visual stimuli, they even said that they hadn’t seen anything at all!
Though the right hemisphere does have some serious gaps in its language-processing abilities, it is not completely devoid of them. It can read and understand numbers, letters, and short statements, so long as the individual does not have to demonstrate this understanding verbally.
In another experiment, a photo of a naked man was presented to the right hemisphere of a female split-brain patient. When asked about the nature of the photo, she began to laugh and explained that she didn’t know why she was laughing, but that maybe it was because of the machine that was projecting the images.
But when the patient is asked to explain why his left hand is pointing at the shovel, his talking hemisphere—the left one—has no access to the information seen by the right, and so instead interprets his behaviour by responding that the reason is that you use a shovel to clean out the chicken house! Experiments like this show just how ready the brain is to provide language-based explanations for behaviour.
thus helped to demonstrate the lateralization of language as well as other functional
differences between the left and right hemispheres.