Thursday, November 03, 2005

Are You Looking at Me?

Robert Barton shows that "like brain size and neocortex size, relative expansion of parvocellular lateral geniculate nucleus layers is correlated with both frugivory and social group size" in primates1. The parvocellular system is a part of the brain that specializes in processing colors, in contrast to the magnocellular system which processes motion2. The link between frugivory and color vision doesn't require much explanation; if you specialize in eating fruit, color vision helps you spot the fruit amidst the trees, and allows you to discern qualities such as ripeness. A competing hypothesis holds that large brain size is correlated with frugivory in the primates because of a selection for spatial memory. Barton's case for the primacy of color vision should lead to a reexamination of the emergence of spatial memory in primates.

But that's not the end of the story. How do we explain the correlation between parvocellular elaboration and large social groups? Barton argues that the answer lies in another aspect of the parvocellular system: the capacity for high-acuity vision.

The parvocellular system mediates a range of visual processes in the neocortex, particularly those that involve the perception of fine details. This kind of processing is critically involved in facial recognition, and perception of gaze direction and facial expression. Several areas of the neocortex whose main visual inputs are parvocellular, such as [the] inferotemporal cortex, are specialized for processing social information of this type. These kinds of complex visual cues must be processed and integrated to achieve what Brothers calls "the accurate perception of the dispositions and intentions of other individuals."3

Barton wants us to understand social cognition not as a higher cognitive function over and above basic sensory processing, but rather as a "large array of sensory-cognitive operations occuring in parallel"4. Okay. What are the implications of that idea?

Aporiai, thoughts along the way:

Lewontin's Triple Helix just arrived. Later.

1 Robert Barton, "The Evolutionary Ecology of the Primate Brain," IN Phyllis C. Lee, Comparative Primate Socioecology, Cambridge: Cambridge University Press, 1999, p. 183.

2 For more on the neurology of vision see the work of Peter Lennie.

3 Barton, Op. cit, pp.187-188. For ease of reading I have stripped Barton's citations from this paragraph. I'm assuming that his references are on target, but I don't know the relevant literature well enough to swear to it.

4 Ibid, pp.186-187.

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posted by Fido the Yak at 4:29 PM.


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