Iain McGilchrist is a former psychiatrist living on the Isle of Skye, in Scotland. He wrote a book about the two cerebral hemispheres in 2009, titled “The Master and his Emissary“. In that book he describes the two ways of thinking of the two hemispheres as complementary and both necessary. He believes that the right hemisphere shows “the big picture”. If you have ever heard the saying “To miss the forest for the trees”, that would describe the left hemisphere.
I will list some differences of the left and right hemisphere that he lists.
- The right hemisphere has more “white matter”. This is explained by the axons (communication cables leading from each neuron) traveling greater distances, which makes sense for a “big picture” view.
- Neurochemically the hemispheres differ in their sensitivity to hormones (for example, the right hemisphere is more sensitive to testosterone). They depend on preponderantly different neurotransmitters (the left hemisphere is more reliant on dopamine and the right hemisphere on noradrenaline)
- Novel experience induces changes in the right hippocampus, but not the left. This squares with the idea that the right hemisphere is attuned to new experiences
- The right hemisphere outperforms the left when prediction is difficult. Also, as far as learning new skills, once skills are familiar, they shift to being the concern of the left hemisphere. “The left hemisphere prefers what it knows”, he writes.
- The right hemisphere is more capable of a frame shift; and not surprisingly is important for flexibility of thought. If the right frontal lobe is damaged, you get “perseveration”, a pathological inability to respond flexibly to changing situations. For example, having found an approach that works for one problem, subjects seem to get stuck, and will inappropriately apply it to a second problem that requires a different approach.
- In problem solving the right hemisphere presents an array of possible solutions, which remain live while alternatives are explored. The left hemisphere, by contrast, takes the single solution that seems best to fit what it already knows and latches onto it.
- V. S. Ramachandran’s studies of anosognosia reveal a tendency for the left hemisphere to deny discrepancies that do not fit its’ already generated schema of things. The right hemisphere, by contrast is actively watching for discrepancies, more like a devil’s advocate. These approaches are both needed, but pull in opposite directions.
- The right hemisphere takes whatever is said within its entire context. If someone says to you “it’s a bit hot in here today”, your right brain understands he is hinting you should open a window. Your left brain just thinks he has uttered a random observation about the temperature. As you might expect, the right hemisphere underpins the appreciation of humor, since context is important for jokes.
- Insight, the sort of problem solving that happens when were not concentrating on it, is associated with activation in the right hemisphere. Insight is also a perception of the previous incongruity of one’s assumptions, which leads it to the right hemisphere’s capacity for detecting an anomaly.
- The left hemisphere is the hemisphere of abstraction. Abstraction is an act of removing from context, and making a general concept.
- The left hemisphere classifies, while the right hemisphere identifies individuals.
- Functional imaging of the brain shows that the left hemisphere takes a “God’s eye” or invariant view of objects, while the right hemisphere uses stored “real world” views.
- It is the left hemisphere alone that codes for nonliving things while both hemispheres code for living things. If your right hemisphere is damaged, you can still use tools, but if your left hemisphere is damaged, you can’t even use a key in a lock, or a hammer with a nail.
- The right hemisphere plays an important role in what is known as ‘theory of mind’, a capacity to put oneself in another’s position. It reminds me of the supposed ancient Indian saying “Don’t judge a man, until you have walked a mile in his moccasins”.
- The right hemisphere identifies emotional expression of the face, and the tone of voice (vocal intonation or prosody). The left hemisphere reads emotions by reading the lower part of the face – the mouth, rather than the eyes. “A patient of mine with a right temporoparietal deficit asked me ‘Whats all this with the eyes?’. When I asked what she meant, she explained that she had noticed people apparently communicating coded messages with their eyes, but could not understand what they were.”
- The right frontal lobe is much more important in most emotional expression, but is not superior to the left in one emotion – anger.
- Left anterior lesions are associated with depression, right anterior lesions associated with ‘undue cheerfulness’. ‘anterior’ means front, in this case, and in truth, things are more complicated, because contradictory things happen as you move back in the lobes. Confirming this observation: if a patient with depression responds to antidepressants, the left anterior lobe starts functioning better.
The corpus Callosum is a cable of neurons that connect the two hemispheres, both with excitatory connections and inhibitory connections.
The Corpus Callosum both inhibits and excites, and Iain explains that as follows. Co-operation requires difference, not more of the same. “It is not cooperation for the surgeon and the assistant both to try to make the incision…Think of the two hands of a pianist – they must cooperate, but they must also be independent.” Or think of two singers harmonizing together.
It is interesting that there are children who only have one hemisphere, and grow up to be normal. Relevant to that might be this fact:
The left hemisphere inhibits the right hemisphere on some tasks. The inhibition signal is sent via the Corpus Callosum . If that inhibition goes down (perhaps due to a distracted left hemisphere), we find the right hemisphere can do things that we normally expect the left to be better at, for instance, understand abstract words.
V. S. Ramachandran (he is the director of the Center for Brain and Cognition at UC San Diego) has used the notion of layered belief — the idea that some part of the brain can believe something and some other part of the brain can believe the opposite (or deny that belief) — to help explain anosognosia. In a 1996 paper, he speculated that the left and right hemispheres react differently when they are confronted with unexpected information. The left brain seeks to maintain continuity of belief, using denial, rationalization, confabulation and other tricks to keep one’s mental model of the world intact; the right brain, the “anomaly detector” or “devil’s advocate,” picks up on inconsistencies and challenges the left brain’s model in turn. When the right brain’s ability to detect anomalies and challenge the left is somehow damaged or lost (e.g., from a stroke), anosognosia results. for instance Ramachandran tells this story:
I saw a lady, not long ago, in India, and she had complete paralysis on her left side… I said, “Can you move your left arm?” She said, “Yes.” “Can you touch my nose?” “Yes, I can touch your nose, sir.” “Can you see it?” “Yes, it’s almost there.” The usual thing, O.K.? So far, nothing new. Her left arm is lying limp in her lap; it’s not moving at all; it’s on her lap, on her left side, O.K.? I left the room, waited for a few minutes, then I went back to the room and said, “Can you use your right arm?” She said, “Yes.” Then I grabbed her left arm and raised it towards her nose and I said, “Whose arm is this?” She said, “That’s my mother’s arm.” Again, typical, right? And I said, “Well, if that’s your mother’s arm, where’s your mother?” And she looks around, completely perplexed, and she said, “Well, she’s hiding under the table.” So this sort of confabulatory thing is very common, but it’s just a very striking manifestation of it. No normal person would dream of making up a story like that.
Bloggers opinion: After reading the chapter, I think of the mindset of ideologues – they come across as very self-righteous, very convinced of a narrow belief system, rejecting large contradictions or small anomalies. They are not good at detecting double standards. They don’t appreciate certain types of humor, at least not the kind that points up their own contradictions. It is as if their right hemisphere has gone AWOL in some respects. It is interesting that several comedians don’t want to perform on our increasingly politicized American campuses anymore.
The Master and his Emissary – Iain McGilchrist, 2010 paperback edition Yale University Press.