This study provides hard empirical evidence for the hypothesis that psychosis (and schizophrenia in particular) represents a costly "byproduct" of complex human (social) brain evolution. Interestingly, the activation paradigms in the fMRI study (N-back and emotional face-matching tasks) are both testing social cognition. And the demonstrated changes in frontostriatal connectivity support the hypothesis that schizophrenia is a disorder of evolved intrahemispheric circuits comprising the Social Brain in our species.

I would suggest that further candidates (conferring vulnerability to psychosis) should be sought from amongst those genes known to have played a significant role in human brain evolution.

References:

Burns J. (2007) The Descent of Madness: Evolutionary Origins of Psychosis and the Social Brain. Routledge Press: Hove, Sussex.

Burns J. The social brain hypothesis of schizophrenia. World Psychiatry. 2006 Jun;5(2):77-81. Abstract

Burns JK. Psychosis: a costly by-product of social brain evolution in Homo sapiens. Prog Neuropsychopharmacol Biol Psychiatry. 2006 Jul;30(5):797-814. Epub 2006 Mar 3. Review. Abstract

Burns JK. An evolutionary theory of schizophrenia: cortical connectivity, metarepresentation, and the social brain. Behav Brain Sci. 2004 Dec;27(6):831-55; discussion 855-85. Review. Abstract

View all comments by Jonathan Burns

  I recommend the Primary Papers

The phosphoprotein DARPP-32 occupies a central position in the dopamine-regulated intracellular cascades of cortical and striatal neurons (Greengard et al., 1999). It is a point of convergence for multiple signaling pathways, is differentially affected by D1- vs. D2-class receptor activation, and mainly through inhibition of protein-phosphotase-1 mediates or contributes to a number of the dopaminergic effects on voltage- and ligand-gated ion channels. These, in turn, by regulating intracellular Ca2+ levels, themselves influence phosphorylation of DARPP-32 and thereby interact with dopamine-induced processes.

Given its central, vital role in dopamine-regulated signaling pathways, it is quite surprising that (to my knowledge) only a few studies exist on the implications of DARPP-32 variations for cognitive functions and brain activity. Therefore, this comprehensive series of studies by Meyer-Lindenberg et al. combining human genetics, structural and functional MRI, and behavioral testing represents an important milestone. Meyer-Lindenberg et al. identified different functionally relevant DARPP haplotypes, associated with differential DARPP mRNA activity in postmortem studies, and found that these were linked to significant differences on a number of cognitive tests probing “executive functions,” as well as to differences in putamen volume and activity, and structural and functional covariation between striatal and prefrontal cortical areas. Thereby, they paved the way for detailed investigations of the role of DARPP-32 in human cognition.

Since DARPP-32 is so intricately interwoven into so many intracellular and physiological feedback loops, as with dopamine itself (Durstewitz and Seamans, 2002), mechanistic accounts for the functional involvement of DARPP-32 variations in neural network dynamics may be hard to obtain. “Linear” causal thinking usually breaks down in such complex functional networks constituted of so many interacting positive and negative feedback loops on different time scales. Thus it may still be a while until we gain a deeper, biophysically based understanding of the neural processes that mediate the influence of DARPP variations on cognition, and integrative computational approaches may be required to help resolving these issues. Given the complexity of DARPP-regulated networks, I also would expect that fine-grained behavioral testing and analysis of error types of human subjects on different cognitive tasks may ultimately reveal quite subtle and differential effects of DARPP polymorphisms. Moreover, the effects on neural network dynamics may be such (e.g., changing the temporal organization of spiking patterns) that they may not always be detectable by current neuroimaging methods, meaning that while the most dramatic effects were found on activation and volume of striatum, where DARPP-32 is most abundantly expressed, a significant contribution of other brain areas in DARPP-associated cognitive differences may not be ruled out. Regardless of these difficulties in unraveling the underlying neural mechanisms, the work by Meyer-Lindenberg et al. allows us to tackle the question of how the balance in dopamine-regulated intracellular networks relates to cognition in humans, and points toward the neural structures and interactions most interesting to look at.

View all comments by Daniel Durstewitz