Of snakes, the pulvinar, and fear

A new paper in PNAS suggests that “Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes” (from the title). I’m happy that more research is being done on the functions of the pulvinar, a structure that is fascinating. There are many interesting findings in the paper, and it’s certainly worth reading.

snake

The problem, as usual, is not with the results but with their interpretation.Establishing selectivity to visual stimuli is challenging at best (cf. all the disputes re. faces in ventral visual cortex). Some puzzling (and to me telling) aspects of the data that the authors barely discuss are:

  • Good responses were observed to high spatial frequency stimuli (!), not just low pass images. In fact, the effect of low vs. high pass had a small effect size (given a p value that was only < .1)
  • Latency to snake pictures was fast (around 55 ms on average) but how much faster than other stimuli it was not clear (but maybe I missed this).
  • The authors suggest that they recorded from the medial pulvinar (the “associational” sector). Talking to colleagues who are familiar with the intricacies of pulvinar anatomy in several species, the  figure shown by the authors does not make this point convincingly. The authors really need to demonstrate that this is not visual pulvinar (that is, from what is shown it is not clear that they were in the medial pulvinar as described in the literature).

These are issues that can be resolved with further research. My main concern is the evolutionary conclusion of the paper.  As phrased by the authors: “Our data provide unique neuronal evidence supporting the hypothesis that snakes provided a novel selective pressure that contributed to the evolution of the primate order by way of visual modification”. This is unfortunate; I’m not a comparative neuroscientist, but without studying multiple extant species, a claim like this is clearly over-reaching.

Reference: Van Le, Q., Isbell, L. A., Matsumoto, J., Nguyen, M., Hori, E., Maior, R. S., … & Nishijo, H. (2013). Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. Proceedings of the National Academy of Sciences, 110(47), 19000-19005.

Brain evolution: amygdala bigger than PFC??

This year I attended the pre-SFN meeting on Evolutionary Neuroscience by the J.B. Johnston Club. I enjoyed the meeting a lot (though was somewhat baffled by their obsession with isometric lines with slope 1…) and ended up bumping into a couple of comparative papers on the amygdala (that I should have known about).

Although fairly crude, one can gain insight into brain evolution by measuring volume or counting cells across brain regions and species. This has led to much debate, for instance, regarding the PFC and its possible “enlarged status” in humans. If you do that for different amygdala nuclei, you find that “the human amygdala is evolutionarily reorganized in relation to great ape amygdala”.

This quote is also quite revealing: “Neuron numbers in the human lateral nucleus were nearly 60% greater than predicted by allometric trends, a degree of magnitude rarely seen in comparative analyses of human brain evolution (Sherwood et al., 2012). For example, the volume of the human neocortex is 24% larger than expected for a primate of our brain size (Rilling and Insel, 1999), whereas the human frontal lobe, long assumed to be enlarged, is approximately the size expected for an ape of human brain size (Semendeferi et al., 2002; Semendeferi and Damasio, 2000).”

So much for such a highly conserved structure… Interesting also that the authors discuss “evolutionary specializations” of the amygdala in terms of the social brain, not “fear processing” (as for instance described in this previous post).

Reference: Barger, N., Stefanacci, L., Schumann, C. M., Sherwood, C. C., Annese, J., Allman, J. M., … & Semendeferi, K. (2012). Neuronal populations in the basolateral nuclei of the amygdala are differentially increased in humans compared with apes: a stereological study. Journal of Comparative Neurology, 520(13), 3035-3054.

The other reference is also interesting: Barger, N., Stefanacci, L., & Semendeferi, K. (2007). A comparative volumetric analysis of the amygdaloid complex and basolateral division in the human and ape brain. American journal of physical anthropology, 134(3), 392-403.