Wednesday, July 26, 2006

Snakes in Primate Vision

A tip from Daphne led me to read the following news story about an anthropologist, Lynne Isbell, who believes that snakes drove the evolution of the primate visual system, in particular binocular vision.

Fear of Snakes Drove Primate Evolution, Scientist Says

That caught my eye to say the least, considering my interest in both primates *and* vision. Stereopsis! Plus primates. Excellent.

So, I did some digging and located the primary source article, "Snakes as agents of evolutionary change in primate brains," published in the Journal of Human Evolution (2006).

is well over 30 pages of microprint loaded with neurological jargon. I am only 11 pages into it and already I have received a nice refresher on neuroanatomy. I've also gained a new perspective so that next time I teach the neuro and S & P sections of intro psych I will be able to contextualize how these systems are interconnected. And I will be able to do it using primates *and* evolution! Most excellent. I love it when I can integrate topics.

Without getting into the nitty gritty details, suffice it to say, Isbell has synthesized a mountain of
information into a new, compelling theory on the evolution of binocular vision, one that extends the two prevailing theories (visual predation and arboreal life) of why early primates' eyes gradually moved from the sides of their heads to the front. She proposes that the ability to visually detect snakes using binocular cues proved advantageous. So far, the most compelling evidence she provides concerns the relationship between orbit convergence (movement of the eyes from the sides to the front of the face) and the activity of parvocellular and magnocellular neurons in the LGN (lateral geniculate nucleus) of the primate thalamus. Frontal vision anatomy is associated with parvocellular development. This system provides "vision for perception" (such as being able to spot a venomous snake camouflaged in leaves) while the magnocellular neurons enable "vision for action" (such as being able to reach and grasp an insect to eat or a branch while leaping). Other pieces of compelling evidence come from research into the koniocellular pathway (a new one to me!) as well as V1, V2, and V4 areas of the visual cortex. V2 cells respond preferentially to small spots of color and also to oscillating rodlike objects, i.e. stimuli that resemble snakes. Thus, ancient primate vision evolved in response to the need to quickly perceive the presence of a deadly snake in the field of vision.

On page 10 Isbell spends quite a few paragraphs talking about Parkinson's disease (PD) which seemed odd at first, but then she presented a few interesting factoids that show the relevance of the condition to understanding how the visual system works. Apparently people with PD experience a loss of sensitivity to light in blue wavelengths due to loss of dopamine in certain retinal cells that are concentrated in peripheral vision. That's trivial it seems, but what isn't is that people with PD often freeze in doorways or places where there are obstacles present. Freezing is also a typical response of many mammals to the sudden appearance of a predator or other danger in the periphery.
This response appears to be controlled by a portion of the cortex called the substantia nigra. The unnecessary freezing of PD could be caused by damage to dopamine producing neurons in this area combined with an upset connection of this region to the superior colliculus, a portion of the cortex that attends to visual stimuli outside of awareness.

Did you know your brain sees things that you do not consciously see? Even when you don't realize it or even try, your eyes and brain take in vast quantities of stimuli that you are never consciously aware of but that influence you to act nonetheless. The superior colliculus may be the neurological hardware that permits us to be influenced subliminally. It just happens that people with PD react to the subliminal awareness of doors, changes in floor tiles, and boxes as fear-arousing stimuli whereas for the rest of us, it takes a snake.

Here's a great photo of a Japanese snow monkey displaying a classic play face. It looks like a human surprise face. The position of the monkey's eyebrows (up and together) suggests this surprise is mixed with fear. Some surprises are scarry after all, like snakes!

In addition to being a fan of vision science, I am also a fan of nonverbal behavior, in particular facial expressions. Paul Ekman has done some amazing work on the facial expression of emotion, one of Darwin's pet projects. Ekman extended Darwin's study of emotional expressions by showing that people around the world display and understand the same facial expressions. People have the most difficulty telling the difference between surprise and fear. Perhaps that's because they often co-occur.


Nutbuk Ug Bulpin said...

I don't like creepy snakes though I was born on the year of the snake. Weird thing is, mum told me that on the day I was born there were housesnakes on our grandparents' house as if welcoming my coming into this world...and when I was a bit older I saw the same type of snakes on their loo,too. Do you know if there's a term for snake phobia?

However, I do appreciate the snake vision you wrote. Quite interesting!

I've seen several snow monkeys when I was studying in Japan, and they sure do love indulging a hot pleasure in an open hotspring resort.

Holly said...

Herpetologists study snakes, so I'd venture a guess that the fear of snakes is "herpetophobia."

That is so cool that you saw snow monkeys in Japan.

I will be visiting Japan next year. The thing I look forward to the most is seeing a wild snow monkey, preferably soaking, swimming or playing in water! I've heard that people can actually sit with them in some hotsprings. That would be something!

Alasdair said...

I like snakes in principle, but like nutbuk ug bulpin, I find them to be creepy. We're obviously not alone, but I think it is interesting that snakeys are the ones being fingered for our binocular vision. As opposed to, say, spiders or scorpions (I guess you don't need binocular vision to pick those up...) or some kind of arboreal predator like huge birds of prey or big cats. Que curioso.

Holly said...

That's a good question about poisonous spiders & scorpions. I wondered about that as I read the article - what about mongoose and meerkat vision? Do they have visual sysems more like Old World Monkeys?

Isbell presents evidence that OWMs have 'grown up' around snakes in an evolutionary sense while NWMs and Malagasy prosimians haven't, thus the differences observed in neurological features and fear responses to snakes among those primates. Humans, coming from the OWM group, are prepared to fear snakes (in the sense that we like OWMs learn very easily to fear & avoid snakes) while lemurs and some NWMs seem to think they are neat to go inspect.

Regarding arial predators, a specialized stereoscopic visual system is unecessary for detecting them - that can be achieved easily enough via monocular cues to their distance.

With snakes, particularly those that are leaf, dirt, and ground colored *and* sit & wait for prey to come to them, then monocular cues to movement will be less useful than the ability to spearate 'figure' from 'ground' to break through the camouflage. In Isbell's theory, breaking through the camouflage is a huge benefit of having stereopsis.

My guess is that perhaps meerkats and mongooses use monocular cues (i.e. movement) to detect scorpions and snakes.

Sam said...

The term for fear of snakes is not herpetophobia, but Ophidiophobia, derived from "ophis," which is Greek for snake and is related to Ophion, the primordial snake-god in Pelasgian (the ancestors of the Greeks) mythology who was the consort of the all-powerful goddess Eurynome. An alternate phrase is Ophiophobia. Herpetology derives from the Greek word for creep, "herpeton."

Holly said...

Thanks, Sam!
Evidently I was not too far off the mark - fear of reptiles is herpetophobia. I'm sure anyone who is afraid of reptiles is afraid of snakes.

helena collins said...

Hi, I found your site while looking for info for part of an assignment - how over evolutionary time, in a move from sensation to perception, organisms learned to give value to perceived objects and react accordingly. Also how that response became hard wired. I was hoping for something that told of monkeys' hardwired response to seeing a snake. Is this an appropriate example, or can youplease suggest another?
Cheers, Helena

Holly said...

Hi Helena!

Thanks for your question. Isbell's research does NOT provide evidence for a hard-wired fear of snakes. It just shows that ability to perceive snakes probably gave primates a survival advantage.

I can suggest a better (more relevant) source. It demonstrates that monkeys do NOT have a hard-wired fear of snakes.

Mineka, Davidson, Cook & Keir (1984). Observational conditioning of snake fear in rhesus macaques. Journal of Abnormal Psychology, 93, 355-372.

Susan Mineka has published a number of studies that show monkeys learn to fear snakes; it isn't hard-wired.

Anonymous said...

that is really interesting and i never would have thought about it before i read your page of information but now i am aware!!