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 :)

Patient DF uses haptics, not intact visual perception-for-action to reach for objects

Before functional neuroimaging techniques like PET and fMRI became common, what we knew about which parts of the brain did what came from neuropsychology. This is the study of patients with specific injuries to the brain, and the basic logic of the field is that if you have a patient with a lesion in area A who can't do task 1, then area A is involved in performing task 1. It gets a little more complicated than this, as you search for double dissociations, etc, but this is essentially it.

A surprising amount of what we think we know about the brain comes from neuropsychology; famous case studies such as HM have informed theories of memory so that they include short and long term storage, which are separable, and so on. These case studies can have a profound effect on research; my favourite story, though, was about a memory researcher who had a skiing accident and temporarily developed retrograde amnesia - he couldn't remember anything except that there was this guy in Connecticut (HM) who couldn't remember things either!

I always enjoyed classes in neuropsychology; the case studies are always fascinating. But they are deeply limited in what they can actually tell us about the brain. First, they are typically single patient case studies, which restricts how general the conclusions are. Second, they are data from damaged brains; the fairly linear assumption that some localised function has been subtracted out is simply not true, and the damage will have had complex effects on distributed functional networks.Third, the damage is never straight-forward, because these almost all come from accidents or strokes (HM's surgery being a rare example of more detail being known). This has not stopped the field being very excited by these cases, though, and from basing a lot of theory on these patterns of deficits.

In movement research, the most famous neuropsychology case study is Patient DF She suffered bilateral damage along the ventral stream of visual processing (James et al, 2003). The effect was visual form agnosia: she is able to control her actions with respect to objects, but cannot describe or recognise these objects verbally. Crucially, her accident did not damage her parietal lobe; specifically, the dorsal stream of visual processing was left intact. These two streams are well defined anatomical pathways leading out of primary visual cortex, and were first described by Ungerleider & Mishkin, 1982). DF's pattern of deficits led Mel Goodale and David Milner (Goodale & Milner, 1992) to suggest functional roles for these streams. The ventral stream, they suggested, was for perception - things like object and scene recognition. The dorsal stream, in contrast, was for perception-for-action, and used visual information for the online control of action. This perception-action hypothesis has been hugely dominant in the field, and the theory rests heavily on DF's shoulders.

Recently, Thomas Schenk (2012a) published some data which claims to show that DF's visually guided reaching is not normal if she doesn't have access to haptic feedback about the object. His data suggests that the only reason she succeeds at reaching while failing judgment tasks is that haptic information is only normally available in the former case. If correct, this is actually quite a shot across the bow of the perception vs perception-for-action work; naturally Goodale and Milner don't buy it, and have published a reply to which Schenk has then replied.

An invitation
I like seeing these arguments happen in the literature; but to be honest, the time scale is too slow. Schenk publishes, then Milner et al get to reply and Schenk gets right of reply to that. They may or may not iterate again and it's always left as 'we agree to disagree'. But these critiques have answers, and I think a blog comment feed might be the right place to work through the various cycles of suggestions and rebuttals until the obviously wrong things have been weeded out. It would also provide a place for other interested parties to weigh in. So if Schenk, Milner and Goodale (and anyone else!) feel like using the comments for this post or another made to purpose to bang around ideas until an obvious experiment or analysis pops out, please feel free!

Did language emerge from the neural systems supporting aimed throwing?

Aimed throwing is surprisingly uncommon in the animal kingdom. Humans do it par excellence, and otherwise it only shows up occasionally, even in our closest relatives. Chimpanzees will throw things (often faeces) but unlike humans don't throw things when hunting or trying to get food; when non-human animals throw things, it's usually part of a social encounter.

Throwing is a fascinating task for many reasons; I hope to blog some about the perception-action aspects of this task in the future as I prepare a couple of papers on the topic with my colleagues Qin Zhu and Geoff Bingham (who have previously done some excellent work on throwing to a maximum distance and the size-weight illusion; various papers available here). There are many fascinating questions about the perception of the affordances of throwing and distances to targets which we're starting to tackle empirically.

Biomechanically, throwing an object accurately over any distance requires the precise transmission of force from the large trunk muscles along a kinetic chain formed by the segments of the arm. The large trunk muscles generate forces the arm cannot, and this force is then transmitted by the motion of the arm; each segment weighs progressively less and so the force accelerates each one faster than the last. The end result is a hand moving at high speed. This requires careful timing; if the motion of the segments aren't coordinated carefully you will waste energy moving the limbs in ways that aren't helping the throw.

There has been some speculation for a while now that the neural mechanisms that help support this fine tuned coordination and control for throwing might also be just the kind of resources that could support the development of spoken language. Speech is a complex action that requires exquisite control over the coordination and timing of numerous elements, just like throwing. One hypothesis is that our ancestors began to develop the ability to throw long distances (this being favoured by natural selection processes because it enabled us to hunt and kill huge prey with much less physical risk to ourselves; e.g. Calvin, 1983). Evolution selected for neural resources that supported this activity, and this then opened the door to the possibility of complex spoken language. So do we speak the way we do because we throw the way we do?