April 2, 2012 | 1
Cocker et al. “Sensitivity to Cognitive Effort Mediates Psychostimulant Effects on a Novel Rodent Cost/Benefit Decision-Making Task” Neuropsychopharmacology, 2012.
One of the interesting things about this paper is that it highlights to me how different the perspectives on a specific data set can be when you’re coming from two different fields. These scientists are coming from the field of cognition and decision making, whereas I come from a field of pharmacology and drug abuse. While I think the data are equally interesting, it makes me think of very different things than they talked about in their discussion. To me, that’s a sign of just how different our readings in the field can be.
The authors of this article were coming at this study from the perspective of studies in choice and decision making. They were interested in finding a model of choice and decision making in rats that better mimics the way humans make decisions. In particular, they are interested in the amount of effort that it takes to achieve a goal. This has important clinical implications for changes that take place in depression or traumatic brain injury, where you sometimes see dramatic changes in motivation in humans.
To look at this, they had rats perform a modified cognitive task. In the task, the rats were given a choice between an easy and a hard task, and they could press a lever indicating which task they wanted. After the lever press, they had to watch a series of 5 lights. One of the lights would light up. If the rat correctly pressed his nose into the area under the lit up light, he got a sugar pellet. In the easy task, the light was on for a little while, but in the hard task, it was on for a short time, so the rat had to pay closer attention. But if he chose the hard task and completed it successfully, the rat got double the reward.
They then ended up with two groups of rats. The rats they dubbed “worker” rats usually chose the harder task for double the reward, while the rats the dubbed “slacker” rats usually chose the easier task. (An important caveat. ALL rats tended to choose the harder task, the “slackers” chose it 60% of the time, and the “workers” chose it 80% of the time). The animals were all equally GOOD at both the hard and the easy tasks, so it’s not a matter of task aptitude.
They then wanted to see how different drugs affected the responses of these two rats groups. First they gave amphetamine.
You can see here that the worker rats are on the top, and the slacker rats are on the bottom (this is in % of the time they pick the harder task). When the “worker” rats are given amphetamine, they begin to choose the hard task less (especially at the higher doses). When the “slacker” rats are given amphetamine, however, the opposite happens, and they choose the hard task more often.
When you give them caffeine, on the other hand, the worker rats show the same decrease, while the slacker rats show no effect.
This shows that drugs can differentially affect performance in a cognitive task, depending on how the animals responded in the task in the first place. The authors hypothesize that this means that amphetamine and caffeine may affect senses of effort in cognitive tasks.
And this is where I thought of something different. Because to me the results of this study remind me a great deal of something in drug abuse research called “rate dependency”. This is the idea of drugs having different effects on food-responding based on the rate at which the animals were responding in the first place. For example, if you have a high rate of responding (say the rat has to respond ten times for a food pellet), amphetamine will decrease response rates. If you have a low rate of responding (say the rat has to respond once per food pellet), amphetamine will increase response rates. The drug affects locomotor stimulation in both cases. This is something that the drug abuse field has been publishing on at least since the 1970s, and it appears to be mediated by the neurotransmitter dopamine, which is how amphetamine exerts its psychostimulant effects, and also a mechanism for some of the actions of caffeine.
And I wonder if this rate dependency effect could be happening here. While the responding here isn’t “rate dependent”, it could be that the mechanisms underlying rate dependency and the choices used in this task are similar. And given that dopamine underlies a lot of motivated behavior, this wouldn’t surprise me at all. But it also makes me wonder, what is the difference between the worker rats and the slacker rats? Do they differ in their underlying neurobiology? The rats they used were a strain called “Long Evans” which are “outbred”, meaning they don’t all having the same DNA (unlike many other strains of rats and mice which are “inbred”, to the point where they are all genetically identical). So it’s very possible that there could be underlying genetic differences which change the neurobiology of the rats, producing changes in the way rats make their choices in this task.
As to whether or not caffeine and amphetamine make hard working humans into slackers? Well I don’t think it’s that simple. After all, the tasks that you are going to have to do in a day are a lot more varied than those given the rats, and you will have varying motivations for them. Not only that, the types of tasks you are set could vary a lot based on your profession, and the many decisions you made or what happened to place you there many years before. And finally, the way that humans react to drugs that they take voluntarily may be very different from the way rats react to drugs that are given to them. We have expectations of what drugs will do to our performance that rats (presumably) don’t have. But this paper has interesting implications for future studies on attention and decision making, and what’s really going on in those rats’ heads.
Cocker, P., Hosking, J., Benoit, J., & Winstanley, C. (2012). Sensitivity to Cognitive Effort Mediates Psychostimulant Effects on a Novel Rodent Cost/Benefit Decision-Making Task Neuropsychopharmacology DOI: 10.1038/npp.2012.30