Shared neural resources for throwing and language: a whacky idea for an experiment

For some time now, there has been an hypothesis floating around in evolutionary biology that the human capacity for language emerged, in part, from the development of our ability to throw long distances with high speed and accuracy. There are a few reasons to think this, mostly correlational, inferential kinds of reasons, but they are accumulating.

We were chatting one day about how to test this hypothesis a bit more directly, and we came up with a whacky experiment. We'd like advice from neuroscientists with experience in brain stimulation techniques about whether this sort of thing is feasible. We'd also like to brainstorm the logic of this experiment and see if we can come up with a practical design that stands a chance of finding something. We then need collaborators; I can handle the throwing side (analysis, measurement, etc) but we don't know anything about TMS and would need an expert on board.

There are many other reasons why this might fail, though - I still need to do a detailed lit review on the throwing/language references I have. Our main problem is that we don't know the kind of obvious difficulties in doing TMS in this kind of context. We'd like to assemble a) an experiment and b) a research team to do the experiment if we can get it to make sense, and if it works we will submit the hell out of this to Nature :)


Throwing
Throwing for long distances with accuracy requires the ability to produce very precisely timed and coordinated action, spread across multiple limb segments.

Figure 1. The basics of a throwing action

High release speed requires a lot of force; the muscles in the hand and wrist are too small to generate the required forces, and so instead we use the large, slow muscles in the torso to develop force during the wind up. This force must then be transmitted along the link-segment system of the arm in a kinetic chain; done correctly, the force remains high and it therefore accelerates the lighter and lighter segments to faster and faster speeds. This force is finally transmitted to the projectile, which goes on it's merry way.

This only works if the timing of the segments taking up the force is controlled precisely. If the segment begins working too soon or too late, some of the force is lost. In addition, if you need accuracy as well, then the final release of the projectile has to occur at just the right time so that it heads in the right direction. There is some evidence that throwing was Homo sapiens' major advantage over competing species. Our brains are smaller than Neanderthals, for example, but we have larger cerebellums and posterior parietal regions, all implicated in timing.

Throwing & Language
The hypothesised relationship between throwing and language is based in this issue of coordination. If we came to develop throwing and this was selected for because of it's advantages in hunting, we will also have developed the neural resources required to support this complex perception-action skill (brains may not be everything, but they are critical resources for any embodied organism). These resources were then available to support other skilled action that requires complex and precise coordination across multiple body parts, specifically spoken language. Speech is a remarkable feat of skilled movement, and the hypothesis is that evolving the ability to throw made the capacity to speak an option, which Homo sapiens then took.

Common neural underpinnings
I recently reviewed a paper that examined throwing and communication in chimps. They performed structural scans on chimps who could throw and those who couldn't, and found chimps who throw have a higher proportion of white matter in inferior frontal gyrus (the chimp homologue of Broca's area) and areas of precentral gyrus compared to chimps who do not throw. This effect was lateralised to the left hemisphere for right handed chimps, while lefties were less lateralised. Finally, the two groups of chimps (throwers vs non-throwers) were only different in one cognitive measure; throwers out-scored non-throwers on a measure of communication skill from the modified Primate Cognition Test Battery. This is completely correlational and the relationship between the communication test and the timing stuff is weak, but it's suggestive and it's a start.

The whacky hypothesis
We started thinking about how to test for this connection in people. It's hard because language skill and throwing skill are mediated by some many varied factors in modern life, there's no particular reason to expect them to co-exist behaviourally. However, the hypothesis is about common neural machinery, so we thought of the following idea.

What if you could knock out, say, Broca's area or something else that's typically thought of as supporting speech. Then, what if, when you tested people's throwing, they were impaired in a way you wouldn't expect unless the two systems overlapped in important ways?

The basic design would be this:

1) Use transcranial magnetic stimulation to temporarily knock out Broca's area or something else suitable.
2) Immediately have the person throw multiple times to a target from 5m, 10m or 15m.
3) Profit!

Movement data analysis: I study throwing and we typically film the throw from the side using a high speed camera. The person has reflective markers attached to each joint (wrist, elbow, shoulder, hip, knee and ankle) and we then digitise the video to extract the kinematics of the markers. We use these position time series as sensitive measures of throwing performance (we can measure how the joint angles relate to one another and the timing along the segments, for example). We can also compute release angle and velocity, the two key parameters the thrower is trying to control; in experts, these vary systematically with target distance and height as the expert tunes their throw to current task demands.

In short, if we had expert throwers (who can produce stable throws) we could detect subtle changes in the throw kinematics after TMS, if there are any.

Here's where we need help
There are numerous questions about TMS and whether this design makes sense we need advice on from anyone with the relevant expertise,so please comment away! We have no particular ego invested in this basic design, it's just our starting point for a whacky test of something kind of cool, so feel free to go to town.

1. TMS: According to Twitter, hitting Broca's area is possible but can hurt, as it triggers facial twitches. What are the ethical and practical issues around getting people to agree to this -is it hard? This is particularly complicated as we need decent throwers - novice performance is already too variable.

2. Apparently continuous theta burst stimulation should knock out function for 10-15 mins. Is this complicated? How well can you target it? What do you need to target it?

3. We're predicting that knocking out a language related part of the brain will have detectable effects on throwing because these tasks rest on shared neural resources for coordination of precisely timed actions. We're starting with Broca's area because of the chimp study. Are there other areas worth thinking about?

4. Would anyone be surprised if we got motor effects with this kind of stimulation? I'm thinking we might need a dissociation task, where we do TMS, find it affects throwing but not some other fine motor task. Thoughts?

Thoughts and comments welcome!