Archive for July, 2010

mysteries of the brain

July 9, 2010

Brain localization, as a topic of inquiry, has certainly had its ups and downs, and the topic of “lateralization”, in particular, has been subjected to so much gross generalization that even the wikipedia page on this topic devotes a section to “Pseudoscientific exaggeration of the research”. Most of all, I would like to suggest that we need to find a sufficient degree of sophistication in talking about these matters so that we can recognize both the significant predisposition to brain lateralization and the conversely marvelous plasticity of the brain.

That said, it seems undeniable that most people show preferences for certain sides on different tasks. However, questions of lateralization become more complex when we ask whether such preferences correlate with other tendencies and/or whether such preferences are genetic. Wikipedia also claims “In more than 95% of right-handed men, and more than 90% of right-handed women, language and speech are subserved by the brain’s left hemisphere. In left-handed people, the incidence of left-hemisphere language dominance has been reported as 73% and 61%” (see wiki for the multiple citations).

There is also a growing body of interesting research which attempts to explain lateralization from an evolutionary perspective. That is to say that there are a growing number of studies which attempt to show that lateralization provides certain advantages for survival (though a priori the converse just as well might seem true). For instance, “Proc. R. Soc. Lond. B (Suppl.) 271, S420–S422 (2004)” shows that lateralized chicks were better at “finding food and being vigilant for predators”. That paper also cites numerous studies indicating that lateralization derives from light exposure during egg incubation, and could be suppressed by incubating the chick eggs in the dark. Careful geneticists will also note that separated twin studies may not properly consider the significance of long-term prepartum developmental effects.

Stanislas Dehaene’s “Reading in the Brain” also makes much of the left hemisphere region usually dedicated to reading (and the puzzle of its evolution in the face of the brief existence of reading on an evolutionary scale– leading to the conclusion of what he calls “recycling”, but which I believe SJ Gould called “recruitment”*), but ultimately mentions briefly a young girl’s normal reading development, regardless of the necessary early removal of this region from her brain (her brain activation during reading focused on the mirrored location in her right hemisphere).

In any case, what mystifies me even more than lateralization is what I will call “transposition”: that is, the fact that the left hemisphere focuses on right side tasks and vice versa. I have yet to encounter any “explanations” for this phenomenon, though part of my problem may be not even knowing how to technically refer to it.

It strikes me as somewhat reminiscent of the fact that the image projected on our retinas is “upside-down”, but the basic physical properties that necessitate that situation are hardly analogous to the anatomical conditions that insure transposition. More similarly, I also find it surprising that the visual cortex is located far to the rear of the brain, while our eyes are located far forward.

For the moment, I’ll just have to keep wondering about it, and can at least keep my eyes out for any relevant, legitimate research.

*”Therefore, a large component of evolvability must be attributed to inherent structural properties of features that originated by natural selection for one reason, but also manifest a capacity for subsequent recruitment (with minimal change) to substantially different and novel functions”.
-The structure of evolutionary theory, Stephen Jay Gould, p. 1228
Perhaps that sentence mixes a bit the “spandrel” issue of “inherent structural properties” and “natural selection”, and maybe the ‘original’ functions of the reading region of the brain weren’t even that “substantially different” from their reading function, but I still think “recruit” sounds more appropriate than “recycle”; in fact that other term of his, “exadaption”, is probably the least prone to confusion, but may or may not also evoke other irrelevant associations.