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Archive for February, 2006

As a finale to the two previous posts about brain evolution, let me end by referring to this study by Mekel-Bobrov et al in Science. If you have followed blogs such as John Hawks, Gene expression or The Scientist you have probably heard about this story before.

The study “Ongoing Adaptive Evolution of ASPM, a Brain Size Determinantin Homo sapiens” has the following abstract:

“The gene ASPM (abnormal spindle-like microcephaly associated) is a specific regulator of brain size, and its evolution in the lineage leading to Homo sapiens was driven by strong positive selection. Here, we show that one genetic variant of ASPM in humans arose merely about 5800 years ago and has since swept to high frequency under strong positive selection. These findings, especially the remarkably young age of the positively selected variant, suggest that the human brain is still undergoing rapid adaptive evolution.”

So the brain has developed significantly during the past 6000 years or so? That is indeed an interestring finding. So what does the ASPM do? What is it related to? Let me recap a brief survey of Hubmed search for “ASPM and brain”.

  • It is related to microencephaly and seizures (1)
  • It is also related to cortical malformation (2)
  • The pathological changes are caused by deficient neurogenesis within the neurogenic epithelium (3)
  • It has been strongly positively selected since the divergence from our common ancestor to the chimp (4)

The list goes on and on, but these are maybe the most prevalent reports and topics.

But listen: we’re back to discussing brain size again, right? As I claimed in my first brain-evo post “brain size” can mean a lot of things. Since the brain does not evolve like an inflating balloon, it would be much more interesting to know what parts of the brain that increase in size. I’ll bet a dollar that we find the prefrontal cortex driving much of this evolution, but that should also mean that the PfC-connected areas would increase in size, too.

I’m diving more into this matter now, and immediately find some interesting and critical remarks by other blogs: again, John Hawks and Gene Expression share their view in the most eloquent way. Read and learn, and don’t forget to speculate about what that ASPM haplotype world map actually might mean.

References

(1) ASPM mutations identified in patients with primary microcephaly and seizures. Shen J, Eyaid W, Mochida GH, Al-Moayyad F, Bodell A, Woods CG, Walsh CA. J Med Genet. 2005 Sep ; 42(9): 725-9

(2) Cortical malformation and pediatric epilepsy: a molecular genetic approach. Mochida GH. J Child Neurol. 2005 Apr ; 20(4): 300-3

(3) Autosomal recessive primary microcephaly (MCPH): a review of clinical, molecular, and evolutionary findings. Woods CG, Bond J, Enard W Am J Hum Genet. 2005 May ; 76(5): 717-28(4) Adaptive evolution of ASPM, a major determinant of cerebral cortical size in humans. Evans PD, Anderson JR, Vallender EJ, Gilbert SL, Malcom CM, Dorus S, Lahn BT Hum Mol Genet. 2004 Mar 1; 13(5): 489-94

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Parts of a private email exchange between philosophers Dan Dennett and Michael Ruse have been published on ID doyen William Dembski’s blog. And apparently with the permission on Ruse! Several bloggers have commented on the issues raised by Dennett and Ruse*, but to me the real question is: Why on Earth is Ruse, an avowed Darwinist, forwarding his email to Dembski?

* See:

PZ Myers

Jason Rosenhouse

Chris Mooney

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While we are thinking about brain evolution, consider this study by Evans et al. in Science. They studied the gene Microencephalin (MCPH), which is known for its severe reduction in brain size coupled with mental retardation. Remarkably, despite this abnormality, there is an overall retention of normal brain structure and a lack of overt abnormalities outside of the nervous system. The MCPH function in healthy humans is less well known, and one can speculate whether it has specific brainy advantages to its carrier. As Evans et al conclude in their article:

“[There] could be several possibilities, including brain size, cognition, personality, motor control, or susceptibility to neurological and/or psychiatric diseases.”

What makes this study interesting is the finding that the MCPH has changed during the past ~37.000 years, and that the spread has been fast. In other words there has been a strong positive selection for this gene, indicating that the brain has continued to evolve even in more recent times. The MCPH is also known to be involved in the evolution of hominids, eventually leading to Homo sapiens. The new finding by Evans et al. demonstrates that this trend has been continuing until more recent times, and is there really any reason to think that the same evolutionary trend has stopped?

So evolutionary psychologists be aware — don’t ever say that today’s humans minds are the same as that of the stone-age man… Of well, to a large extent, it probably is, but this and other similar reports forcefully tells us that we need to unravel the relative contribution of recent evolutionary trajectories in man. It is also necessary to speculate and study the touchy subject on whether there have been local variations in brain size and function according to the recent brain evolutions that have occurred. After all, evolutionary developments about 40.000 years old indicate that there could be geographical variations in the prevalence of this mutation. I don’t think we’ll see yet another claim of the out-of-Africa pertaining to the past 40.000 years or so.

See also comments by John Hawks and Bob Scheid

Here is the abstract from that article:

Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans

Patrick D. Evans in Science, vol. 309, September 2005

The gene Microcephalin (MCPH1) regulates brain size and has evolved under strong positive selection in the human evolutionary lineage. We show that one genetic variant of Microcephalin in modern humans, which arose È37,000 years ago, increased in frequency too rapidly to be compatible with neutral drift. This indicates that it has spread under strong positive selection, although the exact nature of the selection is unknown. The finding that an important brain gene has continued to evolve adaptively in anatomically modern humans suggests the ongoing evolutionary plasticity of the human brain. It also makes Microcephalin an attractive candidate locus for studying the genetics of human variation in brain-related phenotypes.

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Brainy chromosome

The brain changes continually. Not only during development or ageing, but over generations, too. While the common popular notion is that the human brain of today is identical to that of the stone age man, recent studies of the genetics underlying brain development has shown that the human brain has changed significantly over a far shorter time — only a few thousand years.

A recent study by Nusbaum et al. in Nature (see full PDF) analysing the human chromosome 8 briefly mentions two regions called the major defensin gene cluster and MCPH1. They speculate that these regions have played a significant role in the expanded brain size that can be observed through hominid evolution.

At the end of the article, Nusbaum et al. open up a whole new field of study:

“(…) the majority of the genes in the region of high divergence in distal 8p play important roles in development or signalling in the nervous system. Notably, the extremely large CSMD1 gene, which lies at the peak of divergence and diversity, is widely expressed in brain tissues. High regional mutation rates and positive selection are generally assumed to be distinct, but it is possible that the former may facilitate the latter by increasing the rate of appearance of potentially advantageous single, or interacting, alleles. It is intriguing to speculate whether the accelerated divergence rate of this region has contributed to the rapid expansion and evolution of the primate brain.”

In other words, the study of chromosome 8 might open a whole new field of enquiry about what makes the human brain special. Knowing the genomic makeup of our closes evolutionary peers will also make it possible to study the relative contribution of this region to brain size and, not to forget, the underlying role this change has had for cognitive processes. Yes, brain size is interesting, but at our current standard, we need to know more than that. Increasing brain size is normally NOT thought to work like an inflating balloon where all areas increase equally. Rather, the evolution of areas occur within systems of modules or areas, often underlying one or a few cognitive functions.

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There is now an online-only published paper in PNAS from the Max Planck Institute on the evolution of language. What is surprising is that the researchers have used functional MRI to infer the evolutionary lineage from their results. Basically, what Angela Friederici and her colleagues have done is to compare language processing that is “simple” to processing that is “complex”. While simple processing activated left frontal operculum, a phylogenetically older region of the brain, more complex language processing also activated Broca’s area, which is thought to be a more recent development specific to humans. in addition, the researchers also studied the white matter connectivity of the two brain regions by using MR tractography. Here, they found that the two regions showed different structural connectivity signatures, further supporting the functional segregation of these two areas.

This makes the researchers conclude:
“Here we report findings pointing toward an evolutionary trajectory with respect to the computation of sequences, from processing simple probabilities to computing hierarchical structures, with the latter recruiting Broca’s area, a cortical region that is phylogenetically younger than the frontal operculum, the brain region dealing with the processing of transitional probabilities”I first found this through the Max Planck Society press release page. Just reflecting briefly on this, I think that despite the study is interesting itself in terms of functional segregation of language processes, I am not convinced about the argument about the phylogeny of the two regions. As we know from research on subcortical structures such as the “limbic system“, we cannot divide between the phylogenetic “old” and limbic brain and the “newer” cortical brain. It is today considered total gibberis, because evolution of “higher” areas in the cortical surface has had a dynamic and synergetic co-evolution of cortical and subcortical areas. In similar vein, I suspect that the evolutionary trajectories of the frontal operculum and Broca’s area share a lot, and that a clear-cut division between the two areas will prove hard to make.

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Every week there seem to be new papers out on some neuroeconomics or neuromarketing related topic. Let me briefly mention three new papers I’ve stumbled over during the last few days.

[1] In the latest issue of Science a group of researchers at University of Amsterdam report two psychological studies testing how we reach a decision as to what product to buy. Here’s a short description of the first test from Greg Miller’s accompanying news piece:

To test the idea, Dijksterhuis and colleagues asked volunteers to read brief descriptions of four hypothetical cars and pick the one they’d like to buy after mulling it over for 4 minutes. The researchers made the decision far simpler than it is in real life by limiting the descriptions to just four attributes such as good gas mileage or poor legroom. One of the cars had more plusses than the others, and most participants chose this car. But when the researchers made the decision more complex by listing 12 attributes for each car, people identified the best car only about 25% of the time–no better than chance. The real surprise came when the researchers distracted the participants with anagram puzzles for 4 minutes before asking for their choices. More than half picked the best car. The counterintuitive conclusion, Dijksterhuis says, is that complex decisions are best made without conscious attention to the problem at hand.

They then left the laboratory to further test this result in a more ecological setting:

To test the idea in a more natural setting, the researchers visited two stores: the international furniture store IKEA and a department store called Bijenkorf. A pilot study with volunteer subjects had suggested that shoppers weigh more attributes when buying furniture than when buying kitchen accessories and other simple products commonly purchased at Bijenkorf. The researchers quizzed shoppers at the two stores about how much time they’d spent thinking about their purchases and then called them a few weeks later to gauge their satisfaction. Bijenkorf shoppers who spent more time consciously deliberating their choices were more pleased with their purchases–evidence that conscious thought is good for simple decisions, Dijksterhuis says. But at IKEA, the reverse was true: Those who reported spending less time deliberating turned out to be the happiest.

Of course, these results square well with a host of other recent neuroeconomic experiments which have found that decision-making is not only a matter of pain-staking cognitive deliberations, but also involves automatic and unconscious emotional biases. Yet, I’m beginning to wonder if we are not now in a position where we need more experimental attention to the interplay of emotions and cognition.

[2] In forthcoming issue of NeuroImage there will appear a new neuromarketing study by a German team that suggest that brand knowledge is computed by parts of the prefrontal cortex. Here’s the abstract:

Brands have a high impact on people’s economic decisions. People may prefer products of brands even among almost identical products. Brands can be defined as cultural-based symbols, which promise certain advantages of a product. Recent studies suggest that the prefrontal cortex may be crucial for the processing of brand knowledge. The aim of this study was to examine the neural correlates of culturally based brands. We confronted subjects with logos of car manufactures during an fMRI session and instructed them to imagine and use a car of these companies. As a control condition, we used graphically comparable logos of car manufacturers that were unfamiliar to the culture of the subjects participating in this study. If they did not know the logo of the brand, they were told to imagine and use a generic car. Results showed activation of a single region in the medial prefrontal cortex related to the logos of the culturally familiar brands. We discuss the results as self-relevant processing induced by the imagined use of cars of familiar brands and suggest that the prefrontal cortex plays a crucial role for processing culturally based brands.

Again, if you have paid attention to the now rather huge literature on preferences, you will not be overly surprised by this result, although we may wonder why the Neuron brand study I mentioned last week found activation in lateral parts of the PFC, and this study activation in the medial parts. Two things, however. (1) First, a lot of studies are pointing to the medial OFC, or orbitofrontal cortex, as the locus of utility tracking, or the seat of the brain’s overall preference system. But what is this section of the brain actually doing. (2) What does the brain’s preference system more precisely have to do with brands? Is “a brand” just certain emotional response to some product or person? If so, how are such emotional preferences build?

[3] Finally, let me point you to a new review of the current status of the field of neuroeconomics which are set to appear in the next issue of Trends in Cognitive Science. The in press version can be found here. The authors are Alan Sanfey, George Loewenstein, Samuel McClure and Jon Cohen, four of the leaders of the field.

References

Dijksterhuis, A. et al. (2006): On Making the Right Choice: The Deliberation-Without-Attention Effect. Science 311: 1005-1007.

Schaefer, M. et al. (In press): Neural correlates of culturally familiar brands of car manufacturers. NeuroImage, to appear.

Sanfey, A. et al. (In press): Neuroeconomics: cross-currents in research on decision-making. Trends in Cognitive Science, to appear.

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Judy Illes, director of the Program in Neuroethics at the Stanford Center for Biomedical Ethics, has a new paper out in last Friday’s Science. Co-written with a large number of researchers, working with brain imaging techniques, the paper highlights the ethical issues raised by incidental findings in such studies. The basic problem is that, although a subject may appear healthy, and feel healthy, structural MRI’s and other types of imaging data may yet divulge unexpected brain abnormalities. It is something all non-clinical experiments from time to time are certain to experience – I have myself – and we therefore need a policy for dealing with such findings, especially since many PET and fMRI experiments these days are conducted by investigators who are not medically trained.

Also, we should bear in mind that what today is solely a question of incidental clinical findings may in the future expand to many other, non-clinical areas. You volunteer to partcipate in a language study, and your scanning data turn out to indicate that you have paedophelic tendencies. Should this finding be reported or not?

Reference

Illes, J. et al. (2006): Incidental findings in brain imaging research. Science 311: 783-784.

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