Neurodiversity

“I have found the midsagittal area of the corpus callosum to be larger in mixed and left handers, referred to as non-consistent-right-handers (nonCRH), than among CRH subjects (Witelson, 1985).  Hand preference is a rough index of the pattern of brain organization.  Left handers (by various definitions) have a higher prevalence of atypical right-hemisphere representation of speech and language functions than do right handers and, in general, show a greater degree of bihemispheric representation of verbal and spatial skills (for review, see Bryden, 1988).”  (Witelson, S. F. (1991) Neural sexual mosaicism:  Sexual differentiation of the human temporo-parietal region for functional asymmetry.  Psychoneuroendocrinology 16: 139)

There seems to me to be tantalizing answers to riddles in human evolution in the various papers discussing corpus callosum structure in different kinds of human beings.  There are papers that support the conclusion that larger corpus callosums, or corpus callosums with larger sections, appear in left-handed people, women, those with two cerebral hemispheres that are the same size, musicians, the autistic and those that stutter.

“Theoretical speculation in humans (S. F Witelson,  Psychoneuroendocrinology 16 (1991) 131-153) and empirical findings in animals (R. H. Fitch, P. E. Cowell, L. M. Schrott, V. H. Denenberg, Int. J. Dev. Neurosci. 9 (1991) 35-38) suggest that testosterone (T) may play a significant role in the development of the corpus callosum (CC).  However, there are currently no empirical studies directly relating T concentrations to callosal morphology in humans.  The purpose of the present study was to investigate the relationship between free T concentrations as determined by radioimmunoassay, and the mid-sagittal area of the corpus callosum, as determined by magnetic resonance imaging (MRI).  Subjects were 68 young adult (20-35 years), neurologically normal, right-handed males.  All subjects underwent MRI and provided two samples of saliva for radioimmunoassay of T and cortisol.  Anatomical regions of interest included total brain volume, left and right hemisphere volume and regional areas of the CC.  CC regions were defined using two different measurement techniques, each dividing the CC into six sub-sections.  Anatomical measurements were performed blind with respect to the hormone levels of subjects.  A significant positive correlation between T concentration and cross-sectional area of the posterior body of the CC was found.  This finding was consistent across the two measurement techniques and was not attributable to individual differences in total brain volume.  All correlations between cortisol and CC sub-regions were non-significant.  The results of this study are consistent with the notion that T, at an earlier stage in development, may play a significant role in modulating cortical/callosal architecture in humans.”  (Moffat, S. D, Hampson, E., Wickett, J. C., Vernon, P. A., Lee, D. H. (1997) Testosterone is correlated with regional morphology of the human corpus callosum.  Brain Res 767 (2):297)

I would be curious to know whether there is a difference in corpus callosum size between modern humans and matrifocal aboriginals, if musicians and artists have larger corpus callosums and if there is a general trend in growing corpus callosum size that would correlate with matrifocal trends in contemporary society.

“The size of the midsagittal area of the human corpus callosum obtained from postmortem measurement varied with tested hand preference.  The corpus callosum, the main fiber tract connecting the two cerebral hemispheres, was larger by about 0.75 square centimeters, or 11 percent, in left-handed and ambidextrous people than in those with consistent right-hand preference.  The difference was present in both the anterior and posterior halves, but not in the region of the splenium itself.  This callosal morphology, which varied with hand preference, may also be related to individual differences in the pattern of hemispheric functional specialization.  The greater bihemispheric representation of cognitive functions in left- and mixed-handers may be associated with greater anatomical connection between the hemispheres.  The naturally occurring regressive events in neurogenesis, such as neuronal cell death and axonal elimination, may be factors in the individual differences in brain morphology and in functional lateralization.  Specifically, right-handers may be those with more extensive early elimination of neural components.”  (Witelson, S.F. (1985) The brain connection:  the corpus callosum is larger in left-handers.  Science 229: p. 665)

I have a vague memory of a paper that suggested that after sampling several groups of children immersed in music, researchers found that those children playing and composing the most exhibited thicker corpus callosums.  It was implied that this brain structure could grow thicker through lives lived in specific ways.  I’m not sure I remember that right; it seems such a radical conclusion.  This is Your Brain on Music by Daniel Levitin notes studies concluding that musicians have larger corpus callosums (Wikipedia link http://en.wikipedia.org/wiki/Corpus_callosum#cite_note-Levitin-5).

“Mixed-handers showed significantly larger callosal areas for all measures except for posterior fifth…” (Witelson, S. F. (1985)  The brain connection:  the corpus callosum is larger in left-handers.  Science 229: p. 666)

Regarding autism, I’ve hypothesized that the autistic brain is an ancient brain primed for aesthetic manipulation/appreciation with a larger brain size and larger hemispheric bridge having evolved as a sexually selected device for wowing potential mates.  This is closely related to the Geoffrey Miller thesis, see The Mating Mind.  Lately I’ve been playing with the idea that in addition to a mother’s testosterone levels informing the maturation rates of her children, her estrogen levels in combination with her children’s estrogen levels may be informing the timing of testosterone surges that prune right hemispheric growth in infants and small children.  Not unlike how fat levels in a preteen girl can influence the timing of pubertal onset, perhaps similar factors are affecting the timing of infant cerebral lateralization.  I ask myself what might be influencing the size of corpus callosum development.  Assuming it is a combination of degrees of cerebral lateralization (with left and right hemispheres differing in size) and corpus callosum size that together are influenced by changes in maturation rates and timing, then what exactly are the levers of change that are responsible for their moderated forms?  How might the rate and timing of testosterone and estrogen be involved?

I have a related question.  Let’s assume an autistic brain is a healthy brain, a brain anachronistically located in modern times with perhaps inappropriate environmental conditions making it difficult to operate as it naturally would.  Would modulating the environment to nudge the autistic brain to acquire the features of the modern asymmetrical modern brain with a smaller corpus callosum be an appropriate intervention, if it worked?  This might be, for example, an intervention that lowers a mother’s testosterone levels while increasing the male infant’s estrogen levels, hypothetically accelerating his maturation rate while encouraging the beginning of synapse pruning.

Personally, this idea gives me the creeps.  We need to find out what the autistic brain demands and provide the appropriate environment.

Still, is the size of the corpus callosum influenced by estrogen levels?  Is the timing of its growth triggered by body fat or estrogen?

Does music or rhythm influence corpus callosum size, and if so, might music and rhythm prove integral to the autistic brain?

One more thing.

Sarah Blaffer Hrdy in Mothers and Others has hypothesized the current hunter gatherer societies exhibit several features that together heavily encourage theory of mind. She also suggests that without several of these features, particularly a mother having several female allies that are close intimates of her children, humans would never have evolved. Consider that current autistics require the kind of attentions we received while we evolved, attentions still utilized in matrilineal/matrifocal hunter gather societies today.

Music and rhythm are integral and ubiquitous in these hunter gatherer societies.

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Proceed to author’s FREE book download on this subject (The book is called Evolution, Autism and Social Change). 10 minute introductory video here.