Tuesday, May 13, 2008

asymmetric tail-wagging

This is old news by today's standards... but lately I've been thinking about it some more, after watching Fenris wag his tail when Aili or I come home or when he meets another dog.

In the April 24, 2007 New York Times, there was a Science Times article on asymmetric tail-wagging. Some Italian scientists hypothesized that dogs may possess hemispheric asymmetries in the neural regulation of their emotional/motivational responses. The valence hypothesis argues that the left hemisphere of the brain is more specialized for processing positive emotions (e.g. joy, safety, attachment), while the right hemisphere regulates more negative emotions (e.g. fear, anger).

I should note at the outset that I have never been a particularly big fan of hemispheric asymmetries - or rather, how they become distorted and exaggerated for public consumption. By and large, neuroscientists don't have a firm understanding of the various differences between the hemispheres. There are some "known" lateralizations:

  1. language processing is biased towards the left hemisphere
  2. spatial ability is biased towards the right
  3. the left hemisphere regulates somatosensation (touch) and motor control on the right side of the body, and vice-versa: the principle of contralateral control
But I cringe when I hear people say things like, "she's a right brain person - so emotional and artistic." First off, this assumes that because there are functional and structural differences between the hemispheres, there will also be significant differences between individual people in hemispheric dominance. This may or may not be the case. It is possible that Larry has better spatial ability than Moe because his right hemispheric spatial centers are more broadly developed (or distributed), due to either genetic or environmental influences. But this is a big assumption. Correlating individual differences in behavior & personality with individual differences in brain structure (a la phrenology) is not something that most neuroscientists focus their attention on.

Second, there's this thing called the corpus callosum, a dense collection of axons that connect the two hemispheres and allow them to fluidly communicate with one another. This means that most complex cognitive functions are mediated by both hemispheres. Even when it comes to language processing, the right hemisphere makes significant contributions (especially if you're left-handed and/or female). "Left brain" and "right brain" are misnomers. Such terms are really only applicable when referring to those wacky split-brain patients.

All this is not to say that hemispheric asymmetries don't exist. There are hundreds of empirical demonstrations, perhaps the most convincing being the aforementioned split-brain studies conducted by Gazzaniga and his students. I recall learning about these experiments during my junior year in college, at the same time I was taking a course in metaphysics from Derek Parfit. He had us read all these crazy thought experiments on "identity"... So, you're on Earth and some engineers first scan and then destroy your entire brain and body. They recreate you, cell by cell, on the Moon using that scanned blue-print. Is this Moon version still "you"? The emerging materialist in me would scream (silently, from the back of the lecture hall) "Yes!" And then Derek would throw that mind-blowing loop in: what if the engineers fucked up and didn't destroy your Earth copy but still created your Moon copy? Which version is actually "you"? Are they both "you"? Is that possible, given how personal identity is? Which individual's consciousness would you occupy? When you opened your eyes after the operation, would you see Earth's green fields or the desolate lunar landscape?

So here I am going through some weird existential identity crisis, and then my cognitive neuroscience professor starts telling me that if you cut your corpus callosum, you will actually have two identities within yourself. One hemisphere might have you reach out your right arm to hug your wife, while the other hemisphere is having your left hand grab a knife. Or something less dramatic, but you get the point. Why would our brain be designed this way?

Of course, there is a strong engineering advantage to this kind of specialization: division of labor. It is likely that lateralization evolved because individuals with more specialized hemispheres were better able to juggle multiple cognitive tasks at once, be more likely to survive and eventually reproduce. Turns out that hominids aren't the only ones with these kinds of design characteristics. On to the actual experiment...

Quaranta A, Siniscalchi M, Vallortigara G (2007). Asymmetric tail-wagging responses by dogs to different emotive stimuli. Current Biology, 17: R199-201.

The experimenters placed test dogs in an apparatus that allowed them to present controlled stimuli and concurrently measure tail-wagging responses. The stimuli used were: the dog's owner, an unknown person, an unfamiliar dog, and a cat. The specific tail-wagging variable the authors were interested in was wagging angle. This was assessed using the following method:

"Tail wagging scores associated with the different stimuli were analyzed from video-recordings. Positions of the tail were scored every 10 seconds by superimposition on the computer screen of a cursor on the long axis of the body: the maximum extents of the particular tail wag occurring at each 10 second interval was recorded. Using single frames from video recording two angles were identified with respect to the maximum excursion of the tail to the right and to the left side of the dog's body (Figure 1). Tail wagging angles were obtained with reference to the axes formed by the midline of the dog's pelvis — the segment extending lengthwise through the dog's hips, drawn from the largest points as seen from above; dotted line in Figure 1 — and the axes perpendicular to it. Tail wagging angles were evaluated by the segment that extended lengthwise through the base and the tip of the tail, considering the tip of the sacral spine as 180° and the base of the tail as 0° (as in Figure 1). Minimal movements of the tail, within a range of maximum 3° overall, which were plausibly not correlated to wagging, were discarded."

The results?
  • When dogs were shown their owner or an unknown person, they exhibited a right-sided bias in amplitude. In other words, their tail swung farther and more extremely to the right than to the left (as shown in the image above: compare an 85 degree swing to the right versus a 70 degree swing to the left). This more intense wag to the right in indicative of greater left hemisphere involvement. The left hemisphere, according to the valence hypothesis, is associated with more positive emotions. So, when a dog wags more intensely to the right, they are expressing joy/attachment/love/etc.
  • In contrast, when subjects were shown an unfamiliar dog, they exhibited a significant left-sided bias in tail-wagging. This is indicative of right hemisphere dominance and negative emotional valence. A dog wagging its tail more intensely to the left is expressing anxiety/threat assessment/etc.
  • The cat condition provoked minimal tail-wagging, but interestingly enough, a slight right-sided bias.
What are we to make of this? Well, before I swallow this whole, I'd really like to see a detailed explanation of how the canine brain regulates muscular control in the generation of a tail-wag. The issue here, as Dr. Davidson in the NY Times piece points out, is that the dog tail lies along the midline of the body. How does contralateral control of musculature apply to a limb that isn't asymmetric?

Hmmm... google search in progress...

"The canine tail usually consists of between six and 23 highly mobile vertebrae.These vertebrae are enclosed by a versatile musculature that make the various segments, especially the tip, capable of finely graded movements that lift the tail, move it from side to side, or draw it down toward the anus or between the hind legs. The caudal muscles lie on the lumbar vertebrae, sacrum (in the lower back region) and tail vertebrae. The muscles insert on the tail/caudal vertebrae exclusively. The muscles are attached to the tail vertebrae by tendons. The most posterior tendons attach to the last tail vertebrae. Part of the musculature is formed from muscles associated with the rectum, the anus and the pelvic diaphragm. Four to seven paired nerves serve the tail muscles. These muscles have many tendons that insert from the fifth or sixth caudal vertebra, then onto the next vertebra, and so on to the end of the tail."

So I suppose that the left hemisphere (motor cortex) controls muscles on the right side of the tail that can pull it in that direction. And the right hemisphere controls muscles on the left side of the tail that pull it in the other direction.

A final note: there is also evidence that our facial musculature exhibits a slight lateralization when we make emotional expressions. Happier emotions, exemplified by smiling, are expressed more strongly in the right side of our face. Negative emotions show a left-bias. It is possible that when we meet someone we have conflicted feelings about (we enjoy their company but find them slightly annoying), these emotions play out in a lateralized fashion on our face.

Might all this be partly responsible for the notion of left as sinister?

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