Decoding ASPM: Part II

In my last post, I reviewed the debate over ASPM, a gene implicated in regulation of brain growth. A new ASPM variant arose some 6,000 years ago in our species, apparently somewhere in the Middle East. It then spread outward, becoming much more prevalent in the Middle East and Europe than in East Asia. This temporal and spatial spread seems to match that of alphabetical writing, specifically the emergence of literate, scribal classes who had to process alphabetical script under premodern conditions (continuous text with little or no punctuation, real-time stenography, absence of automated assistance for publishing or copying, etc.). Since this subpopulation enjoyed prestige and apparently high reproductive success, it may have been a vector for the new ASPM variant (Frost, 2007).

All of this assumes the existence of a heritable cognitive ability that is specific to reading and writing of alphabetical script. This assumption runs counter to the view, held by many psychologists, that human cognition shows heritable variation only for general intelligence (commonly referred to as g). This view was key to ending debate over human variation in ASPM. As Philippe Rushton and other psychologists have shown, the new ASPM variant does not improve performance on standard IQ tests. Nor does it correlate with increased brain size.

Recently, however, it has been found that ASPM variants do not correlate with brain size in other primate species. Instead, they seem to regulate the growth of specific brain tissues, especially within the cerebral cortex. Parallel to this is another finding that alphabetical script processing is localized within a specific region of the brain, the ‘Visual Word Form Area (VWFA):

Brain imaging studies reliably localize a region of visual cortex that is especially responsive to visual words. This brain specialization is essential to rapid reading ability because it enhances perception of words by becoming specifically tuned to recurring properties of a writing system. The origin of this specialization poses a challenge for evolutionary accounts involving innate mechanisms for functional brain organization. (McCandliss et al., 2003).

Psychological, neuropsychological, and neuroimaging data converge to suggest that the human brain of literate subjects contains specialized mechanisms for visual word recognition (functional specialization), which map in a systematic way onto the properties of a cortical subregion of the left posterior occipitotemporal sulcus (reproducible localization).

… Such observations predict the existence of highly specialized but patchy and distributed neuronal populations coding for alphabetic stimuli at the single-neuron level. The intermingling of such neurons with others coding for objects or faces would translate into a partial regional selectivity at the single-voxel level, which is all that we can presently measure with PET or fMRI. (Cohen & Dehaene, 2004)

One puzzling issue remains: why is there a reproducible cortical site responsive to visual words? Reading is a recent cultural activity of the human species. The 5400 years that have elapsed since its invention are too short to permit the evolution of dedicated biological mechanisms for learning to read. (Cohen & Dehaene, 2004).

Indeed, how could the VWFA have arisen so recently and over such a short time? This is a puzzle only if we assume that evolution creates new features from scratch. But this is not how evolution usually works. Typically, new features evolve through a process of tinkering with old features that may have served some other purpose. And such tinkering may occur over less than a dozen generations. As Harpending and Cochran (2002) note, domestic dog breeds display much diversity in cognitive and behavioral characteristics, yet this diversity has arisen largely within the time span of human history.

A second puzzle is of the chicken and egg sort. If the VWFA is crucial for reading and writing, how did humans first learn to read and write? Some light has been shed on this puzzle by lesion studies, particularly one where part of the VWFA was surgically removed:

… our patient presented a clear-cut reading impairment following surgery, while his performance remained flawless in object recognition and naming, face processing, and general language abilities.

… Furthermore, the deficit was still present 6 months after surgery, albeit with some degree of functional compensation. This confirms that the VWFA is indeed indispensable for expert reading.

(Gaillard et al., 2006)

So a person can learn to read and write without a VWFA. This brain area did not arise to make reading and writing possible. It simply arose to make these tasks easier.

But what about human populations that have never used alphabetical script? This is notably true for the Chinese, who have long had a logographic script:

As the most widely used logographic script, Chinese characters have thousands of diverse word forms and differ markedly from alphabetic scripts in orthography. In Chinese characters, there is a distinctive square-combined configuration within each character and no obvious letter-sound correspondence. … Although some phonological information is encoded in some characters, this information is not consistent and is not at a level of correspondences between phonemes and letters. … In alphabetic stimuli, it is clear that each individual letter is the basic unit of words, so how different letters are combined is critical in defining orthographic regularities. In Chinese, however, it is still not clear what the basic processing units really are. (Liu et al., 2008)

When the brains of Chinese subjects were studied by functional MRI, Chinese characters seemed to be processed in the same region of the brain (the VWFA) that other populations use to process alphabetical characters. The Chinese subjects, however, seemed to be using other regions as well:

These results indicated that in addition to the VWFA being located in the left middle fusiform gyrus (BA 37), the left middle frontal cortex (BA 9) might also be an indispensable area for orthographic processing of Chinese characters, as opposed to alphabetic orthographies. (Liu et al., 2008)

For human populations that use alphabetical scripts, the VWFA seems to be much more of a bottleneck for text processing. This finding seems to dovetail with other evidence suggesting that logographic script evokes meaning more directly and does not impose the same set of cognitive demands (Frost, 2007).

References

Cohen, L., & Dehaene, S. (2004). Specialization within the ventral stream: the case for the visual word form area. Letter to the Editor. NeuroImage, 22, 466-476.

Frost, P. 2007. "The spread of alphabetical writing may have favored the latest variant of the ASPM gene", Medical Hypotheses, 70, 17-20.

Gaillard, R., Naccache, L., Pinel, P., Clémenceau, S., Volle, E., Hasboun, D., Dupont, S., Baulac, M., Dehaene, S., Adam, C., & Cohen, L. (2006). Direct intracranial, fMRI, and lesion evidence for the causal role of left inferotemporal cortex in reading. Neuron, 50, 191-204.

Harpending, H. and G. Cochran. 2002. "In our genes", Proceedings of the National Academy of Sciences, 99(1), 10-12.

Liu, C., Zhang, W-T., Tang, Y-Y., Mai, X-Q., Chen, H-C., Tardif, T., & Luo, Y-J. (2008). The visual word form area: evidence from an fMRI study of implicit processing of Chinese characters. NeuroImage, 40, 1350-1361.

McCandliss, B.D., Cohen, L., and Dehaen, S. (2003). The visual word form area: expertise for reading in the fusiform gyrus. Trends in Cognitive Sciences, 7, 293-299.