Tracing Causation

A colleague told me she’d found something puzzling about a recent article in Science News, and wondered what I might think of it. The article turned out to be quite interesting, so here’s a very brief summary, with my reactions.

The article is “Cerebral Delights” (Susan Gaidos, Science News, 2/26/11, v. 179(5), p. 22). It reports some research on the amygdala, a pair of almond-shaped structures symmetrically located in left and right sides of the brain. It has long been known that some cells in the amygdala increase their activity in circumstances we find fearful. The leading discovery reported in the article is that cells in these same structures also play a role in situations that provide something pleasant.

One line of research involved inserting electrodes in monkeys’ amygdalae and presenting them with either a rewarding sip of water or an annoying puff of air to the face. The finding was that different neurons increased their activation, depending on which of these stimuli were used.

Converging results came from another experiment in humans, who already had electrodes implanted in preparation for a medical procedure (necessary for reasons unrelated to the experiment). Assignments by these volunteers of different values to foods was correlated with differences in activation of some individual cells in their amygdalae.

A third line of approach again involved monkeys. Like people, normal monkeys will shift to a less desired food after having a chance to eat a preferred food for a while. Monkeys with damaged amygdalae, however, did not switch their food choices in this way.

Finally, a fourth study found that monkeys with damaged amygdalae made choices that led to larger rewards in preference to smaller ones, just as normal monkeys did. But they were different from normal monkeys on measures of pupil diameter and heart rate – measures typically indicative of emotional response.

These are important results that increase our understanding of our brains. I find them particularly interesting because they give a useful example of complexity of function in a limited region. They help us avoid the fallacious move from “Region R is involved in task T” to “What region R does is contribute to T” – fallacious, because there may be many tasks to which a given region may contribute.

So, why might my colleague have been puzzled? Our conversation was brief, so I’m not sure. But the likely answer lies in the many abilities that this article seems to attribute to the amygdala. The amygdala is said to “help shape behavior”, to “act as a spotlight, calling attention to sensory input that is new, exciting and important”. It is said to “evaluate information”, “assign value”, serve “as a kind of watchdog to identify potential threats and danger”, and “judge” the emotional value of stimuli. (Some of these phrases appear to have been introduced by the reporter, but others seem to have come from some of the researchers themselves.)

Phrases like these should make us pause. The evidence is that cells in the amygdala are activated by certain circumstances, and that behavior can be altered by interfering with its operations. That’s good reason to think that the amygdala is indeed a part of the causal chain leading from perception to behavior. But judging? Assigning value? The evidence does not show that such complex operations – operations we normally ascribe to whole persons – are done in, or by, the amygdala. *Maybe* they are done there, but the research doesn’t show that. It leaves open the question as to *how* a set of circumstances that is potentially dangerous gets to increase the activation of one cell in the amygdala rather than another, or how a situation that offers rewards gets to activate an amygdala cell that’s different from the ones activated by potential danger.

In short, as far as the reported study conclusions go, it could be that the actual sorting out of what is to be avoided from what is to be pursued is done elsewhere, and that what the amygdala does is to communicate the results quickly to the many brain regions that must be mobilized for appropriate action. Or it may be that evaluation results from interaction of cells in the amygdala with cells in other regions. In either case, attributing evaluation to the amygdala would be stopping too soon in tracing the causes of our behavior. Instead of stopping there, we should continue to press the causal question – in this case, What are the prior processes that cause one set of neurons rather than another in the amygdala to increase their activation?

[ Two papers reported in the article under discussion are  Morrison, S. E. & Salzman, C. D. (2010) “Re-valuing the amygdala”, Current Opinion in Neurobiology, 20:221-230; and Jenison, R. L., Rangel, A., Oya, H., Kawasaki, H. and Howard, M. A. (2011) “Value encoding in single neurons in the human amygdala duringdecision making”, Journal of Neuroscience, 31(1):331-338. In addition, conference presentations by S. Rhodes & E. Murray, and by P. Rudebeck are discussed.]

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