The rational crayfish, Procambarus economicus?

You might expect a paper whose title starts with “Neural control” to include neurons.

This new paper by Liden and collegues doesn’t. It’s straight behaviour paper in the style of classic neuroethology. It starts by explicitly trying to tie itself to a hot new field: neuroeconomics. Neuroeconomics is about value assessment and decision making in humans. In many cases, this means doing brains scans of people while they play with experimenter’s money.

Liden and company argue that humans are far too complicated (which is true), and the way to go about understanding how you make decisions is by looking at an escape system. Escape systems are reasonably simple in terms of neurons, and they definitely control a decision: Fight or flight? Or, in the case of crayfish, freeze or tailflip?

When crayfish see a large dark object looming over them, they can freeze, or perform an escape tailflip. The experimenters put crayfish in a tank, and passed shadows over them. They did record from the giant pair of escape neurons that trigger visual escape responses, but they way they used those recordings was as a marker for whether an escape tailflip occurred.

Not surprisingly, when you change the stimulus the crayfish gets, the behaviour changes. Speed the shadow up, and the crayfish is more likely to freeze. But if the crayfish does tailflip, it “decides” to do so in less time.

The big finding that has the authors tooting the neuroeconomics horn is that if you put a strong smell of food in the water, the animal is more likely to freeze than if there’s very little smell of food. This, the researchers argue, means that the crayfish moderating the use of its escape behaviour due to the presence of the food, because the escape would be costly in that it would take the animal away from the food it wants.

But. This effect happens only at one of two shadow speeds that they tested. The effect does not appear to be very robust.

There are just two two problems with integrating this sort of study into the fold of neuroeconomics:

1. The neurons.
2. The economics.

As for the neurons, the title of this paper implies that it’s found how these neurons make decisions, but this paper only suggests what to look for. It doesn’t give new discoveries at the neuronal level. There are no new neurons or synapses or neurotransmitters described here.

And the search isn’t going to be easy. The main neurons that make the escape decision in these cases are medial giant (MG) neurons. While the MG axons are well known (they run throughout the crayfish’s body), the bit where all the interesting integration is going on – the place where sensory neurons and interneurons are connecting with the MGs – are nearly a complete mystery, tucked away inside the crayfish’s brain. A while ago, I went looking in the scientific literature for any good picture or diagram of the MG cell bodies and dendrites in the brain – and came up empty handed.

I’m glad that the authors are working with the MG-mediated escape responses; they’re overdue for attention. But make no mistake, the low level of information about them is a bug, not a feature, for crayfish escape as a neuroeconomics model.

As for the economics, it seems difficult to assign values in this situation. Human economics is pretty easy: $20 is better than $10, but not as good as $50. Having a standardized interval scale makes some of the most interesting neuroeconomics research possible.

But what is the value of the smell of food? What is the cost of tailflipping? An escape tailflip is very brief, and it doesn’t take the animal very far from the food. The authors point out that crayfish who win fights gain access to food – which is true – to argue how important food resources are to crayfish. What they don’t mention is that will crayfish fight in the complete absence of food or any other resource. The advantages that crayfish gain through fighting are subtle enough that it took several decades of research before people figured out that there were any resource advantages gained by winning fights.

It’s also worth noting that these experiments are very similar to some theoretical models about the regulation of crayfish behaviour. Don Edwards (a former supervisor of senior author Jens Herberholz) wrote a paper discussing how a crayfish might choose between different behaviours (getting food, escaping a predator) back in 1991. The results of that computer model feel very much like the results presented here.

Indeed, thinking back to the 1990s, there were a lot of important papers on how crayfish escape responses were modulated via changes to the lateral giant (LG) interneuron circuit (e.g., Yeh et al. 1996). The changes were caused by social interactions, so this was pitched as a model for... human aggression!

All these little moments of “spin” in this paper seem to have carried through to how the paper has been promoted. This press release, from the interesting (albeit sometimes dodgy) Science Direct is a testament to good marketing. There’s another press release here. Here’s a snippet from one:

(A) new line of research that may help unravel the cellular brain activity involved in human decisions.

Yeah, and if I don’t clean out my fridge, I may help develop a new antibiotic.

This is good research. But to say that it’s going to help us understand human decision making? Not yet. Not by a long shot.

P.S. – I have to bust the authors and reviewers’ and editor’s chops for letting this get into the paper:

(D)ifferences were not statistically significant although only marginally(.)

A p value is either significant, or it is not. It is not legitimate to treat the p value as some sort of direct indication of the “reality” or the size of an effect; see Schmidt (2010) for more.

Shameless self-promotion! For those who might want more information on the crayfish escape system, I wrote a review on this, focusing on the evolution and diversity of the behaviour and the underlying neurons responsible for it (Faulkes 2008).

References

Edwards DH. 1991. Mutual inhibition among neural command systems as a possible mechanism for behavioral choice in crayfish. The Journal of Neuroscience 11: 1210-1223. http://www.jneurosci.org/cgi/content/abstract/11/5/1210

Faulkes Z. 2008. Turning loss into opportunity: The key deletion of an escape circuit in decapod crustaceans. Brain, Behavior and Evolution 72(4): 351-361. http://dx.doi.org/10.1159/000171488

Liden, William H., Phillips, Mary L., & Herberholz, Jens (2010). Neural control of behavioural choice in juvenile crayfish Proceedings of the Royal Society B: In press. 10.1098/rspb.2010.1000

Schmidt, F. (2010). Detecting and Correcting the Lies That Data Tell Perspectives on Psychological Science 5(3): 233-242. DOI: 10.1177/1745691610369339

Yeh, S., Fricke, R., & Edwards, D. (1996). The Effect of Social Experience on Serotonergic Modulation of the Escape Circuit of Crayfish Science 271(5247): 366-369. DOI: 10.1126/science.271.5247.366

Homo economicus by keystricken on Flickr; MG picture by rustyheaps on Flickr. Crayfish showdown picture by ratterrall on Flickr. All used under a Creative Commons license.