Mar 11 2011

Localizing Spacial Reasoning

This is a bit of a wonky neuroscience news item, but I thought it was interesting – and there are some general points to be raised. Researchers studied brain tumor patients to see which brain parts contribute to certain kinds of visual-spacial reasoning, specifically the ability to mentally rotate objects. (Sorry, the article itself is pay only.)  Right off I was intrigued because this is an old-school method of localization – studying pathology. Today we are far more likely to see fMRI studies, perhaps using transcranial magnetic stimulation, in healthy individuals. But prior to this technology neuroscientists studied patients with stroke and tumors to infer what the different parts of the brain did by what deficits were created when those brain sections were damaged. It’s nice to see this method is still valuable.

At issue here is the question of where, exactly, in the brain is the ability to mentally rotate objects located? This is part of a more general question of what are the different types of spacial reasoning. Neuroscientists have long recognized that people use two basic strategies of spacial reasoning: the first is categorical – reasoning about qualitative properties such as relative position and rotation. The second is quantitative, such as precise distance and angle. While driving, for example, you may know when to take a turn based upon the relative position of various landmarks (categorical reasoning), or you may know that it is the third left after the intersection (quantitative reasoning). Prior data suggested that categorical reasoning is carried out in the dominant (usually left) hemisphere and quantitative spacial reasoning in the non-dominant (usually right) hemisphere.

Of note, the non-dominant hemisphere is the primary side in which visuo-spacial processing occurs. However (and this is one of the main themes to draw from research like this) the brain is comprised of many modules and networks that simultaneously involve various brain regions and both hemisphere. This is partly why the “left brain, right brain” dichotomy is such nonsense. Many brain functions involve either networks or cooperation between different brain regions in both hemispheres – both hemispheres acting as one unified brain.

Getting back to localization, the new study found that patients with a tumor removed from the right parietal lobe had difficulty with mental rotation – a categorical form of spacial reasoning. This is contrary to prior beliefs that categorical reasoning was mainly left hemisphere.  Further, patients with tumors removed from the left prefrontal cortex made more spacial reasoning errors that patients with tumors removed from other areas, but not categorical errors of rotation.

The researchers interpret that latter result as indicating that the non-dominant prefrontal cortex is involved with planning a sequence of operations necessary to carry out a specific task. This demonstrates the complex cooperation among different brain regions to create the final outcome of specific cognitive functions.

This study also shows that, while research in this area is fairly mature, there is still a great deal of complexity to brain organization that we have yet to work out. It is a difficult puzzle, because we have to work out not only where the different specific processing functions are, but what they are (for example, that there is distinct categorical and quantitative reasoning), and how they interact. We are long past any simplistic notion of one brain structure carrying out one specific function.

Of interest  – the same issue of Cortex that carried this study also has a cover story about Phineas Gage. Gage was the railroad worker who, through an accident with explosives, had an iron spike fly through his skull, resulting in frontal lobe damage. As a result he underwent a significant personality change, what we would now call a disinhibitory syndrome. I should write a longer post about Gage, but in short – his case became famous and helped convince the neuroscientific community that specific brain functions were indeed localized to specific places in the brain. His accident helped lead to the current study, and others like it, that seek to localize these specific functions.

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