Studying people who have undergone brain surgery to treat disorders has given researchers a better understanding of the conscious mind. Often this surgery has
FIGURE 4.6 The Range of
Consciousness (a) Terry Schiavo spent more than 15 years in a persistent vegetative state before she was taken off life support.
Her parents and their supporters believed she showed some awareness, despite brain scans that showed no brain activity. (b) Jan Grzebski was in a minimally conscious state for 19 years before he awoke and reported that he had in fact been aware of events around him.
(b) (a)
attempted to treat epilepsy by removing the part of the brain in which the seizures began. Another strategy, pioneered in the 1940s and still practiced on occasion when other interventions have failed, is to cut connections within the brain to try to isolate the site of seizure initiation, so a seizure that begins at that site will less likely spread throughout the cortex. (Chapter 7, “Attention and Memory,”
includes a discussion of H.M., one of psychology’s most famous case studies because of the memory loss he suffered as a result of brain surgery conducted to stop his severe seizures.)
The major connection between the hemispheres that may readily be cut with-out damaging the grey matter is the massive fibre bundle called the corpus callosum (FIGURE 4.7). Most epilepsy responds to treatment with modern medications, but in rare, extreme cases that do not respond to medications, the corpus callosum may be cut completely as a means of treatment.When the corpus callosum is severed, the brain’s halves are almost completely isolated from each other, a condition called split brain.This surgical procedure has provided many important insights into the basic organization and specialized functions of each brain hemisphere. But this procedure also raises an interesting question: “If you split the brain, do you split the mind?”
What is it like to have your brain split in half? Although you might think it would lead to dramatic personality changes, perhaps the most obvious thing about split-brain patients after their operations is how normal they are. Unlike patients following other types of brain surgery, split-brain patients have no immediately apparent major problems. In fact, some early investigations suggested the surgery had not affected the patients in any discernible way. They could walk normally, talk
FIGURE 4.7 The Corpus Callosum This MRI shows a patient’s brain after the cor-pus callosum was completely sectioned.
split brain A condition in which the corpus callosum is surgically cut and the two hemispheres of the brain do not receive information directly from each other.
Right hemisphere:
better with spatial relationships
Left hemisphere:
better with language FIGURE 4.8 Visual Input Images from the
left side go to the brain’s right hemisphere, and images from the right side go to the left hemisphere.
When a split-brain patient is asked what he sees, the left hemisphere sees the fork on the right side of the screen and can verbalize that.
The right hemisphere sees the left side of the screen, but cannot verbalize what is seen. However, the patient can pick up the correct object using the left hand.
“A fork”
FIGURE 4.9 Split-Brain Experiment: The Left Hemisphere versus the Right Hemisphere
normally, think clearly, and interact socially. In the 1960s, this book’s co-author Michael Gazzaniga, working with the Nobel laureate Roger Sperry, conducted a series of tests on the first split-brain participants.The results were stunning: Just as the brain had been split in two, so had the mind!
As discussed in Chapter 5, images from the visual field’s left side (left half of what you are looking at) go to the right hemisphere, and those from the right side go to the left; the left hemisphere also controls the right hand, and the right hemi-sphere controls the left hand. With a split-brain patient, these divisions allow researchers to provide information to and get information from a single hemisphere at a time (FIGURE 4.8).
Psychologists have long known that in most people the left hemisphere is dominant for language. If a split-brain patient sees two pictures flashed on a screen briefly and simultaneously—one to the visual field’s right side and one to the left side—the patient will report that only the picture on the right was shown.
Why is this? Because the left hemisphere, with its control over speech, sees only the picture on the right side, so it is the only picture a person with a split brain can talk about. The mute right hemisphere (or “right brain”), having seen the picture on the left, is unable to articulate a response. The right brain can act on its perception, however: If the picture on the left was, for example, of a spoon, the right hemisphere can easily pick out an actual spoon from a selection of objects, using the left hand (which is controlled by the right hemisphere). Still, the left hemisphere does not know what the right one saw. Splitting the brain, then, produces two half brains, each with its own perceptions, thoughts, and consciousness (FIGURE 4.9).
Further explorations have revealed much more about the division of labour within the brain. In all the split-brain patients studied, the left hemisphere was far more competent at language than the right, so much so that in many patients the right hemisphere had no discernable language capacity. In some patients, though, the right hemisphere displayed some rudimentary language comprehension, such as being able to read simple words. Interestingly, such right hemisphere language capabilities tend to improve in the years following the split-brain operation, pre-sumably as the right hemisphere attains communication skills unnecessary when that hemisphere was connected to the fluent left brain.
A split-brain participant watches as different images flash
simultaneously on the left and right.
1
Below the screen is a row of other images.
2
The patient is asked to point each hand at a bottom image most related to the image flashed on that side of the screen.
The right hemisphere points the left hand at a picture of a snow shovel.
The left hemisphere decides that the shovel is used to clean up after chickens. (It does not see the house.) 3
The left hemisphere points the right hand at a picture of a chicken head. The left hemisphere says that the chicken claw goes with the chicken head.
FIGURE 4.10 The Left Hemisphere Interpreter On the basis of limited informa-tion, the left hemisphere attempts to explain behaviour governed by the right hemisphere.
The right hemisphere, however, has its own competencies, which complement those of the left.The left brain is generally hopeless at spatial relationships. In one exper-iment, a split-brain participant is given a pile of blocks and a drawing of a simple arrangement in which to put them—for example, a square.When using the left hand, controlled by the right hemisphere, the participant arranges the blocks effortlessly.
When using the right hand, controlled by the left brain, the participant produces only an incompetent, meandering attempt. During this dismal performance, the right brain presumably grows frustrated, because it makes the left hand try to slip in and help!
THE INTERPRETER Another interesting dimension to the relationship between the brain’s hemispheres is how they work together to reconstruct our experiences.This collaboration can be demonstrated by asking a disconnected left hemisphere what it thinks about previous behaviour that has been produced by the right hemisphere. In one such experiment, the split-brain patient sees different images flash simultaneously on the left and right sides of a screen, while below those images is a row of other images.The patient is asked to point with each hand to a bottom image that is most related to the image flashed on that side of the screen above. In one such study, a picture of a chicken claw was flashed to the left hemisphere, and a picture of a snow scene to the right hemisphere. In response, the left hemisphere pointed the right hand at a picture of a chicken head, and the right hemisphere pointed the left hand at a picture of a snow shovel. The (speaking) left hemisphere could have no idea what the right hemisphere had seen. When the participant was asked why he pointed to those pictures, he (or, rather, his left hemisphere) calmly replied,
“Oh, that’s simple.The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” The left hemisphere evidently had interpreted the left hand’s response in a manner consistent with the left brain’s knowledge (FIGURE 4.10).
The left hemisphere’s propensity to construct a world that makes sense is called the interpreter, because the left hemisphere is interpreting what the right hemi-sphere has done (Gazzaniga, 2000). In this last example, the left hemihemi-sphere inter-preter created a ready way to explain the left hand’s action, which was controlled by the disconnected right hemisphere.The explanation was unrelated to the right hemisphere’s real reason for commanding that action.Yet to the patient, the move-ment seemed perfectly plausible once the action had been interpreted. Usually, the interpreter’s explanations come readily.To give another example: If the command Stand up is flashed to a split-brain patient’s right hemisphere, the patient will stand up. But when asked why he has stood up, the patient will not reply,“You just told me to,” because the command is not available to his (speaking) left hemisphere.
Instead, unaware of the command, he will say something like, “I just felt like get-ting a soda.” His left hemisphere is compelled to concoct a story that explains, or interprets, his action after it has occurred.
Such interpretation does not always happen instantly. Sometimes it takes the patient’s left hemisphere as long to figure out why the left hand is acting as it would take an outside observer. In one session, Gazzaniga and his colleagues presented the word phone to the right hemisphere of patient J.W. and asked him to verbalize what he saw. J.W. replied that he did not see anything. Of course, J.W. was speak-ing from his left hemisphere, which did not see the word phone, and his right hemi-sphere was mute. However, when a pen was placed in his left hand and he was asked to draw what he saw, J.W. immediately started drawing a phone. Outside observers who had not seen the word phone took some time to make out what J.W. was drawing. J.W.’s left hemisphere was in the same boat. Fortunately, J.W.
tended to articulate what he was thinking (a helpful trait in a research subject). He was initially confused by what he was drawing and started guessing about what it was. Not until the picture was almost complete did the outside observers, includ-ing J.W.’s left hemisphere, understand what his left hand was drawinclud-ing.At that point, J.W. exclaimed,“Duh, it’s a phone!”The communication between the hemispheres occurred on the paper and not within his head. J.W.’s right hemisphere drew what it saw; after viewing the drawing, his left hemisphere identified it as a phone. In the meantime, his interpreter struggled to guess what his hand was drawing.
THE INTERPRETER SPECULATES The interpreter strongly influences the way we view and remember the world. Shown a series of pictures that form a story and asked later to choose which of another group of pictures they had seen previously, normal participants have a strong tendency to falsely “recognize” pictures consis-tent with the theme of the original series and to reject those inconsisconsis-tent with the theme.The left brain, then, tends to “compress” its experiences into a comprehen-sible story and to reconstruct remembered details based on the gist of that story.
The right brain seems simply to experience the world and remember things in a manner less distorted by narrative interpretation.
Sometimes the left brain interpreter makes life more difficult than it needs to be. In one experiment, human or (nonhuman) animal participants must predict, on each trial, whether a red light or a green light will flash. A correct prediction produces some small reward. Both lights flash in a random sequence, but overall the red light flashes 70 percent of the time. Participants pretty quickly notice that the red light comes on more often. So to receive the most reward, what strategy do they follow? After doing this task a number of times, most animals simply choose the red light—the most probable response—100 percent of the time; by doing so, they receive rewards on 70 percent of the trials.This strategy makes great sense in terms of adaptiveness, in that it guarantees the animals receive the maximum
interpreter A left hemisphere process that attempts to make sense of events.
rewards, yet humans do something much different.They try to figure out patterns in the way the lights flash, and they choose the red light about 70 percent of the time. That is, overall their choices match the frequency of how often red flashes, but because the lights flash randomly, on any given trial humans may choose incorrectly. Indeed, when humans choose the red light 70 percent of the time, they generally receive rewards on only 58 percent of the trials.
Why do humans not follow the optimal strategy, which even rats can figure out? According to Dartmouth College’s George Wolford and colleagues (2000), the left hemisphere interpreter leads people to search for patterns that might not even exist.To test this idea, the researchers had two split-brain patients perform a version of the task described above. The patients’ right hemispheres tended to respond in the optimal way that animals did (choosing the same thing 100 percent of the time), whereas the patients’ left hemispheres chose red only 70 percent of the time.The left hemisphere interpreter’s tendency to seek relationships between things may be adaptive in some contexts, but it can produce less-than-optimal out-comes when such relationships (e.g., patterns) do not exist.
The split brain is a rare condition, of course, and nearly all people have two hemi-spheres that communicate and co-operate on the tasks of daily living. Maryse Lassonde, at the Université de Montréal, studies the rare condition in which the cor-pus callosum does not develop properly. Patients with this condi-tion do not show all the characteristics of those with surgical split brains, but they show impairments in tasks that require information flow between the hemispheres (Lassonde & Ouimet, 2010). The popular media have sometimes exaggerated this research’s findings, suggesting that certain people are “left brain” logical types and oth-ers are “right brain” artistic types. In reality, although the hemispheres are specialized for certain functions, such as language or spatial nav-igation, most cognitive processes involve both hemispheres’ coor-dinated efforts. Split-brain research provides a unique opportunity to study each hemisphere’s capacities.