*** Appendix: Are bacteria capable of genuine habituation?

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Bacteria. Courtesy of DiscoverySchool.com.

Abramson (1994) makes a distinction between habituation and sensory adaptation, which is one of several factors that can complicate habituation experiments. An organism's response to a stimulus may decrease because it adjusts to the repeated presentation of the stimulus. This is habituation, properly speaking. Alternatively, the response may diminish because the organism's sensory organs no longer detect the stimulus - a phenomenon known as sensory adaptation (not to be confused with sensitization). Effector fatigue is another possible reason for the decline in an organism's response: the bodily mechanisms (effectors) that respond to stimulation may tire over the course of time. Finally, some organismic responses diminish naturally over time, even without stimulation, so the (underlying) base rate of responding needs to be established before experiments are performed.

These distinctions are philosophically significant: ignoring an irrelevant stimulus is a very different thing from failing to detect it, or being too tired to respond to it, or simply slowing down over the course of time. First, the ability to ignore a stimulus serves an important biological function, as described above. Second, while ignoring a stimulus could be considered as a self-initiated activity, sensor failure, fatigue and natural slow-down can hardly be said to qualify as activities.

Franco di Primio. Photo courtesy of Fraunhofer Institute for Autonomous Intelligent Systems.

Di Primio, Muller and Lengeler (2000, p. 7) claim that "[w]ith bacteria, it is impossible to draw a dividing line between habituation and [sensory] adaptation", and the Encyclopedia Britannica article (1989, Vol. 1, p. 872) agrees that "these distinctions make rather little sense in the case of a single-celled animal." However, according to Abramson (1994, pp. 108 - 109) there are two commonly used procedures for differentiating between habituation and sensory adaptation: make the time between two successive presentations of the habituation stimulus long enough for adaptation to wear off, or perform follow-up testing of the effects of habituation, some time after the organism has been trained to diminish its response. Effector fatigue can be distinguished from habituation in the same ways, or by presenting the organism with a second stimulus from a different sensory modality (e.g. vibration instead of light) to see if the same response (e.g. withdrawal) can be elicited. If it can, then fatigue can be ruled out.

There is no reason why these procedures could not be applied to bacteria. As far as I have been able to ascertain, the relevant tests have not been performed to date. The question of whether bacteria are capable of "learning", as defined by psychologists (i.e. habituation), remains open.

However, if we examine the chemical basis for so-called "habituation" in bacteria, it appears to be a case of sensory adaptation, rather than habituation. As Illingworth puts it:

With increasing attractant concentrations the MCPs are progressively converted into the fully methylated state with a low affinity for the attractants (1999).

In other words, at high concentrations, bacterial receptors become less sensitive. This effect wears off when methylation of bacterial sensors is reversed (by removal or dilution of the attractant). If we follow Abramson's suggestion that time trials can be used to distinguish habituation from sensory adaptation, then the hypothesis that the observed waning of responsiveness in bacteria is explicable in terms of sensory adaptation entails that the removal of a stimulus (e.g. an attractant), followed by its presentation after an interval of time, should cause a bacterium to respond in the same way as it usually does when exposed to an attractant. As far as I have been able to ascertain, this is indeed the case. (If the decrease in response were due to habituation instead, it should persist even when the attractant is removed, thereby de-methylating the bacterium's sensors.)

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