Is the clear cut dichotomy true?

April 22, 2008 5:50 AM

Jill Bolte Taylor uses a very clear cut dichotomy when explaining the respective roles of the left and right hemisphere: one is oriented towards parrallel processing of sense data, the other towards serial, conscious, target-oriented processing. Is this reflective of the modern state of the art neurological schientific knowledge? It's an intuitively attractive explanation. But is it true? It sounds simplistic.

I'm telling her theory from memory since I can't watch the video right now

posted by jouke to science & nature (13 comments total)

We don't know if it is "true" - any theory having to do with brain cognition is really talking about a black box.
posted by k8t at 5:57 AM on April 22, 2008

One of the leaders in this field is Vilayanur Ramachandran-- he focuses on patients with very narrow examples of brain-damage. By testing those patients, he can begin to map and understand how the different areas of the brain function and combine to create what we know as the human mind. I'm not sure if he directly answers the right equals logical mind, but I believe he does go into physical seperation of right and left lobes.

His Wikipedia entry has a good overview of his career and to both his excellent series of Reith lectures and his Ted Talks presentation.
posted by Static Vagabond at 7:43 AM on April 22, 2008

There are hardly ever any hard and fast rules like that. You never get to look at EEG data and see the left hemisphere lit up by a task, while the right hemisphere is inactive. There are biases towards areas, and there are areas "implicated" in certain processes. There are impairments and there are tendencies.

Where a task involves parallel processing or "gist" information, you might expect to see more activity in the right hemisphere if you look at EEG information. If someone is impaired or have damage that you know is mainly limited to the right hemisphere, you might expect them to be impaired at "gist" tasks. and vice versa.

When you compare right-handed and left handed people, you tend to find increases in artistic ability and what are thought of as "right brain" skills. When you look at comparisons in brain activity what you find are greater tendencies to the opposite side of the brain than you would expect to find activity for the type of task you're looking at. but by no means does this activity completely flip sides, and by no means can you identify one side of the brain as just for one type of activity. but there are generalised rules and they help to understand what's going on.

god, i miss teaching psychology.
posted by galactain at 7:43 AM on April 22, 2008

static vagabond is absolutely right. split-brain patients, people who've had their corpus collosum cut are an absolutely fascinating way to think about the dichotomy of the left and right brain. i don't have any links but if ramachandran has looked at it, then thats where to start. Different experiments demonstrate the different tactics and methods the different sides use to function and inform the other side, using their respective abilities, without a direct link of communication.
posted by galactain at 7:58 AM on April 22, 2008

Thanks for your explanations. Seems my scepsis was appropriate.
posted by jouke at 8:08 AM on April 22, 2008

Just a comment -- people who are impaired in some way (blind, deaf, missing a chunk of brain, etc.) will often find that their brain has compensated for their impairment somehow, usually by adapting a region of the brain not usually associated with activity x to fit activity x. There is a fair amount of plasticity in the brain. Something to keep in mind when looking at studies of people whose brains are impaired or atypical.
posted by prefpara at 8:16 AM on April 22, 2008

You're certainly right to be skeptical - even well-informed neurologists and cognitive scientists tend to overstate lateralization in talks and the like for the sake of clarity and time constraints - or that's what I've always suspect.

It's not really considered a crime against science (from what I can remember from the cognitive science dept. at Leiden at least), and I certainly wouldn't hold it against Taylor. The distinction, from what I can tell, is basically rooted in widely accepted knowledge in the field; there's discussion, but I don't think there's a noteworthy movement that seriously doubts the lateralization of at least some brain functions as a general phenomenon.

The problem arises from the many assumptions the simple version of the statement ("Left brains dance like THIS, right brains dance like THIS") rests upon, and this is why some researchers and teachers treat the topic with more restraint.

To name just one, handedness is a determining factor in certain aspects of lateralization, and much of the oft-cited effects assume right-handedness.

An excellent book on this precise topic is Left Brain, Right Brain by Springer & Deutsch.

Some Wikipedia pointers for general browsing:

Lateralization of brain function
Split-brain
Corpus Callosum

Also:

Some relevant Mefi posts (last one mine).
posted by goodnewsfortheinsane at 8:39 AM on April 22, 2008 [1 favorite]

*that's what I've always suspected*
posted by goodnewsfortheinsane at 8:40 AM on April 22, 2008

As many before have pointed out, Jill Boalt Taylor may have overstated the case a bit with regard to hemispheric lateralization. Some cognitive functions seem to be very strongly lateralized, others very weakly so, and many show no lateral specificity whatsoever.

I can think of a few domains in which cognitive functionality is highly lateralized. In language, for example, lesions in the posterior parts of the left inferior and middle frontal gyrus (Broca's area) can cause non-fluent aphasia--this is a language deficit where the person has very halting speech, and cant find the words to express themselves. Lesions to the left angular gyrus (Wernicke's area) can cause fluent aphasia, where the person will talk expressively and fluently, but not make a lot of sense. Their words may have extra syllables, or be entirely made up, and grammar will be almost completely non-evident.

In very few people, these two areas will be on the other side of the brain. People who have this left-right flip in their brain lateralization will almost always be left-handed, but most left-handed people do NOT have this flip.

The prosodic aspects of language are also lateralized enough to the other side of the brain so that (in the right hemisphere, for most people) a lesion can impair the ability of the person to modulate the tone of their voice, or place emphasis correctly within a word or sentence.

It has also been known for a long time that lesions to the right parietal cortex will cause hemispatial neglect of the left side of space, but lesions to the left parietal cortex do not. With hemispatial neglect, a person is impaired in noticing things on the left side of space. When asked to draw a scene, they may draw only the right sides of objects, or when eating food they may eat only the food on the left side of their plate. Ive even heard of patients who, after eating all the food on the right side of their plate, will turn around to the right a full turn until the left side of their plate enters their field of view from the right, and then proceed to eat the other half of their meal.

The reason for this is that the left parietal cortex seems to handle the right side of space, and the right parietal cortex seems to cover BOTH sides of space. Going back to the Jill Boalt talk, this is a very reliable and well-observed feature of brain damage to the right parietal cortex that does lead to a cut-and-dried clinical observation that is not seen in the same pattern of damage on the left, and to me, seems to be related to the somewhat weaker and more abstract notion that the right hemisphere is more global in its scope.

A somewhat different example is in the use of rTMS to treat depression (this is experimental in the US, but approved in Canada). rTMS involves applying a strong, oscillating magnetic field through the scalp and into the brain. The typical coil used to generate the field will stimulate around 1 square inch of cortex. Usually, for depression, the area that is stimulated is the left dorsolateral prefrontal cortex, and it is stimulated in such a manner as to increase its overall level of activity.

I don't know why the studies originally chose the left hemisphere, but it seems to fit into the general pattern outlined- in depression, sufferers will often attribute bad outcomes to fixed, eternal causes than proximal or contingent causes. (eg, "I failed the test because I am stupid" vs "I failed the test because I didn't study hard enough this time") The right hemisphere, if it is actually a globally-oriented and holistic hemisphere, seems to me to be more likely to furnish the first explanation, and the left hemisphere as a logical-thinking cause-effect oriented hemisphere seems to be more likely to furnish the second explanation.
posted by Maxwell_Smart at 9:57 AM on April 22, 2008

Half-Brain Fables and Figs in Paradise by Jacques M. Chevalier (Vol 1 of his 3-D Mind series) contains an interesting overview (2002) of lateralization studies, in which he observes that left-brain (sequential) thinking tends to characterize the research itself.

Interhemispheric communication and neuroplasticity is the other side of the story:

"While brain lateralization cannot be denied, hemispheres communicate directly with one another and show some degree of plasticity. The principle of neuroplasticity applies not only to each hemisphere but also to mechanisms of interhemispheric communication." (p.46)
posted by psyche7 at 10:04 AM on April 22, 2008

The generalities are just that, generalities. There are fascinating case studies based on people who have had a full hemispherectomy (the surgical removal of an entire brain hemisphere, usually due to severe epilepsy); some such patients recover most function--e.g. after a left hemispherectomy, one patient lost but then recovered language within a few months, and is now pretty much fluent.

Two points: there is lots of cross-hemisphere redundancy in the brain, and there is also a large degree of plasticity allowing brain regions to take over new functions with enough training.

So I'd be cautious about the simplistic dichotomies, though they do adequately capture some basic patterns of findings...certainly the entire brain is massively parallel though, so trying to chalk up parallel processing to one hemisphere seems highly questionable!

Here's a nice book focusing on one boy who had a hemispherectomy (link)
posted by slipperynirvana at 10:55 AM on April 22, 2008

I know it's a total tangent, but I'm compelled to bring up one of my favorite quasi-crackpot theories, Julian Jaynes' book The Origin of Consciousness in the Breakdown of the Bicameral Mind.

His basic premise is that the neurological connection between the two hemispheres of the human brain was much more tenuous, as recently as 3,000 years ago. He takes that to mean that everyone was basically almost schizophrenic, with the idea that one hemisphere theoretically interprets signals from the other as external "voices".

According to his theory, when the ancient Greeks talked about hearing "the voices of the gods", they literally were hearing voices, but it was really the two hemispheres of their brain communicating.

A stretch, I know, and I'm not trying to advance it as a real theory, but I've always found it an interesting way to think about the brain.
posted by LairBob at 11:12 PM on April 22, 2008

Wow. Some more great additions. Thanks people. I learned a lot.
posted by jouke at 5:39 AM on April 23, 2008