Brain Infection and Aversion Reversals

Christof Koch recently reported on a grim little organism, Toxoplasma gondii. This one celled protozoan reproduces sexually, but only in cat intestines. Mammals that encounter cat feces can become infected with the offspring. The eventual result is an attack on the host’s brain that reduces its aversion to the smell of cats. That’s bad news if you’re a small rodent. From T. gondii’s point of view, however, this is a great strategy for getting itself into a cat intestine, where the cycle can begin again.

There are several lines of reflection that knowledge of this organism may trigger. The one that intrigues me the most is that it may provide evidence for resolving a conundrum that seems at first sight to be irresolvable.

The conundrum can be introduced through a fact commonly reported by beer lovers – namely, that they did not like beer at all when they first tasted it. Many readers will be able to think of parallel examples with other drinks or foods. The reversal can go in the opposite direction, too. I used to like licorice, and also lobster, so much that I overdosed on them, with strikingly unpleasant results. Immediately afterward, I detested these foods, and I avoid them to the present day.

The conundrum is a question about what happens during this kind of reversal. Is it (A) or (B), or, perhaps, some combination of them?

(A) What happens after consuming the item (alcoholic euphoria in one case, nausea in another) causes changes in your brain that makes the item taste differently. For example, you still don’t like the taste that beer produced in you when you first had it in your mouth, but now beer in your mouth no longer produces that taste. It produces a different one that you like better.

(B) What happens after consuming the item causes changes in your brain that alter your evaluation of the taste. For example, beer in your mouth produces exactly the same taste it did the first time you sipped it. It’s just that now you like that taste, whereas before you didn’t.

The problem is that it seems that any case of reversal could be equally well accounted for by either of these views. So, it seems that they are different views, but that nothing could count as supporting one more than it supports the other.

That situation is deeply troubling to some philosophers. A view that was very popular in the mid-Twentieth Century, and still not completely dead, held that there could not even be a difference in what two claims meant, unless it was understood how some conceivable evidence could count as supporting one of them better than the other. If that view were right, and (A) and (B) are equally good at accounting for all possible evidence, then the appearance that they are different accounts must somehow be an illusion.

T. gondii helps us untie this knot. That’s because of three facts that Koch reports. First, infected rodents do not show abnormalities in their sense of smell of other things. More importantly,

     “the density of cysts [housing T. gondii] in the amygdala is almost double that in other brain structures involved in odor perception. Parts of the amygdala have been linked to anxiety and the sensation of fear.”


     “the genome of T. gondii contains two genes related to mammalian genes involved in the regulation of dopamine, the molecule associated with reward and pleasure signals in the brain, including ours.”

While these facts do not rule out some difference in how cat urine smells to infected versus uninfected rodents, they offer more support for a view like (B), i.e., for the idea that infected rodents may smell cat urine just as before, but no longer react to it with fear (and may even find it somewhat pleasant). So there is, after all, a case in which some evidence counts more in favor of one of these views rather than the other.

By the way, many humans are also infected with T. gondii. Effects are not definitively established, but there are suggestive connections between such infection and psychiatric diseases and risky behavior. Hence Koch’s title: “Protozoa Could Be Controlling Your Brain”.

[Koch’s article is at . Reversals have been discussed in D. C. Dennett, Consciousness Explained (Boston: Little, Brown & Co., 1991) and by me in Understanding Phenomenal Consciousness (Cambridge: Cambridge University Press, 2004).]

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