Snakes on a Brain

In October, 2013 ScienceNews put out a press release describing an article published by researchers from UC Davis and U Toyama (Japan) advancing the “Snake Detection Theory.” They measured the responses of pulvinar neurons to visual stimuli, including a hand, face, geometric shape, and a snake. They report that they found a specific response to the snake as a visual stimulus, and that it supports the notion that the primate brain evolved under evolutionary pressure from living alongside snakes as competitors and predators.

The pulvinar neurons are located in the thalamus (in humans, the pulvinar nucleus, or cluster or neurons, is the largest in this structure). The thalamus is a structure that is deep in the vertebrate brain and it relays sensory and motor signals to the cerebral cortex, it regulates consciousness, sleep, alertness. This group of neurons receives input from crossed optic fibers (that is, the right part receives input from the left eye), which then relays information to the cortex of the occipital lobe. They also communicate with other regions, but that is the path of visual stimuli.

I hadn’t previously heard of the “Snake Detection Theory,” but the lead author published her theory in 2006, and in a book in 2009. This particular article in PNAS seems to rather heavily self-cited and is kind of relying on a circular body of evidence to support the theory.

I have to say that I am highly skeptical of the major claims of the paper, including the title. To summarize their findings, they measured responses to visual stimuli from 745 pulvinar neurons, 105 of them responded to at least one of the stimuli, and 91 of them were tested with all stimuli. They found that particular neurons responded “preferentially” to only one of the stimuli (40% of them to the snakes — and to the snakes — with a higher magnitude response). I do not think this provides, as stated in the title of the paper, “Neurobiological evidence of past selection for rapid detection of snakes.”

It simply shows that pulvinar neurons are encoded to respond to specific stimuli of a particular shape. Other neurons may respond also to shapes like fruit, tree  branches, other predators. Probably specific pulvinar neurons are encoded to respond to changes in light/dark as well, like shade, sunrise, and sunset. I think the more likely explanation for the expanded visual processing centers of the primate brain are due to the fact that primates evolved after its ancestor mammals had started to climb into trees, and in order to navigate the three dimensions of movement, grasping branches, holding on for dear life, climbing up and down, jumping. Probably we will find that any range of possibilities of shapes will excite these neurons (e.g., “novel shape! do something!”), which ignites a cognitive and emotional response, “have i seen similar shapes before? did it hurt me? was it edible? is it truly novel? what’s it doing now? …. etc etc.”

Since they only made measurements in one species, it doesn’t follow that they could make any conclusions about selection pressures. They should at least look at another species that had been under different selection pressures and seen if they observed similar or different results.

In the Intro of this paper, they go into the rich history of serpents and snakes in human culture. Yes, serpent imagery has a very long history in human culture (for thousands of years), for obvious reasons. But other things do, too. Particularly: geocentrism and past devastating floods. This is actually what a meme is. A unit of culture, or idea, which is selected for and transmitted across generations. (not the way it is used on the internet). This part of the paper, I think, is a way of riling up its importance or sexiness, but the data don’t go there. At all.

Links to:

  1. The Science News Article
  2. The article in PNAS, “Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes.”
  3. The book by the lead author, The Fruit, the Tree, and the Serpent: Why We See so Well.
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