Tuesday, June 20, 2006

Autism and the imprinted brain   posted by JP @ 6/20/2006 10:26:00 AM

Right on the heels of Razib's ten questions for David Haig comes this (free) paper proposing an extreme imprinted brain theory of autism.

The theory is based off the fact that paternally and maternally inherited alleles are expressed in different parts of the brain. Given a species where the father contributes less parental investment than the mother, we have a classic setup for genetic conflict: the father hedges his bets, so to speak, by wanting more resources for his offspring at the expense of the mother and her other children (who may have different fathers). This is achieved through the control of his "territory" in the brain: the limbic system and areas controlling basic drives and appetites.

The mother, on the other hand, looks out for herself and her other children (all of which are equally related to her). The authors put it like this:
Her maternally active genes will be expressed in all her children and should further the mother's interests by building a cortical brain capable of integrating mental activity in the greater interests of her whole family. Her genes will control the parts of the child's brain that can be educated by verbal instruction and practical example. She will be able to use the speech centres of the cortex to teach her child its mother tongue and the inhibitory and prioritizing functions of the frontal lobes to control behaviour in accordance with her commands and instructions. Here a top-down, contextual, holistic and empathic cognitive style might be particularly useful in influencing a child's social interaction with its siblings, peers and parents. This would make a child much more likely to see things from its mother's point of view and perhaps less likely to act impulsively on the promptings of its paternal brain.
Their description is obviously a little self-serving, as on reading it one wants to take the logical next step and hypothesize that autism is caused by an underperformance of this maternal part of the brain or an overperformance of the paternal brain. Which is exactly what the authors do. They think of the brain as a "social placenta" (their term, which I dig) in which alleles with different interests compete for resources.

This is an extention of Baron-Cohen's "extreme male brain" theory of autism, but the authors claim it does a better job explaining the data-- for example, the sex ratio in severe autism is not nearly as skewed as the sex ratio in "mild" autism, which is puzzling in the context of Baron-Cohen's theory: shouldn't the sex ratio become more skewed the more severe the symptoms? In the imprinting theory, a decreased activity of the maternal brain would cause severe autism, while an increase in the activity of the paternal brain would cause "autism spectrum disorders", the less severe cases. In this framework, one could argue that males are more succeptible to increases in paternal brain action, while both sexes could be equally affected by decreases in maternal brain action.

This is a hypothesis, of course, and they make three main predictions:

1. "The primary causes of autism should be alterations in imprinted genes, genes regulated by imprinted genes, and genes associated with the regulation of imprints, via their application and removal."

2. "Autism may be caused by diverse genetic, epigenetic and environmental factors that cause paternal–maternal brain imbalance."

3. "The behavioural changes involved in autism should reflect extreme manifestations of general, evolved mechanisms for mother-offspring and among-sibling competition over resources."

The rest of the paper presents their evidence; I'm no autism researcher, so I've got to take them at their word. The idea of the brain as a "social placenta" is what I particularly like; it wouldn't surprise me if other mental phenotypes were controlled in a similar manner.