Are bacteria capable of undergoing habituation, and does it imply learning?

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Staph bacteria. Picture courtesy of Janice Carr/CDC via BBC.

Bacteria are also said to display habituation, which Di Primio, Muller and Lengeler (2000, p. 7) define as "the waning of a response to a regularly repeated stimulus". For example, following prolonged exposure to an attractant, bacteria change from a "run" to a "tumble" movement. This is because methyl-accepting chemically sensitive proteins (MCPs) in bacterial cells are progressively saturated with methyl radicals, which reduces their sensitivity to attractant molecules (Illingworth, 1999).

The explanation of bacteria's behaviour is important, because habituation is regarded by psychologists as a form of learning, defined broadly as a relatively permanent modification in an organism's behaviour as a result of experience. The Appendix contains a detailed discussion of how learning, habituation, sensitization and dishabituation are defined in psychology textbooks.

These definitions invite two questions - one empirical and the other philosophical. First, is the behaviour of bacteria a true case of "learning" as defined by psychologists, or is there some other, simpler explanation? That is, are bacteria capable of genuine habituation? Second, does the popular psychological definition of "learning" specify behaviour that requires a mentalistic explanation?

The first question is discussed at length in an Appendix. While the case is not closed, the available evidence points to the conclusion that bacteria are not capable of genuine habituation, but that their behaviour is better explained as an instance of sensory adaptation (a fixed pattern of behaviour).

This brings us to the philosophical question: does "learning", as defined by psychologists, require a mentalistic explanation? Putting it another way: even if bacteria prove to be capable of being properly habituated, and alternative explanations can be ruled out, does that mean that they possess minds?

Because the verb "learn" retains a mentalistic connotation in English, we should (following the principle I suggested above), first specify what its minimal mental content is. In another, related paper, Kilian and Muller discuss important ways in which the adaptive behaviour of bacteria falls short of even the most basic definition of learning:

In ethology learning is defined as a change in the individual behavior which leads to a better adaptation and which is influenced by amplification and experience. Proper learning shows a reproducible learning effect... [E]pigenetic learning is based on the ability to form the interplay between stimulus input, information transfer, memory, and behaviour in an individual and reproducible way (2001, pp. 1 - 2, italics mine).

The stipulation of a "reproducible learning effect" suggests some minimum methodological criteria that must be satisfied by an organism before we can attribute learning to it:

L.1 The existence of memory in an organism is a necessary but not a sufficient condition for learning.

L.2 Learning should not be attributed to an organism unless it displays a change in its pattern of behaviour which it is able to reproduce on a subsequent occasion.

Additionally, Kilian and Muller argue that because unicellular organisms have "genetically fixed ... networks of information transfer" (2001, p. 3), they are unable to modify their information transfer pathways and so cannot learn to respond to a new stimulus:

Individual learning in unicellulars has to be based on... individual shaping of the information transfer regulatory networks. Individuals gain individuality if the same stimulus can lead to new and different regulatory networks and thus causes different behavioral reactions. Unicellulars are not able to synthesize or modify cellular substances not already coded genetically in a fixed way. So, they cannot install new, individualized, ... reproducible information transfer paths within their life-time... The synthesis of new substances for new irreversible and reproducible information transfer paths as an answer to a new, formerly not identifiable stimulus is not possible in unicellulars. Therefore, unicellulars seem most certainly not able to learn, at least according to an ethological definition (2001, pp. 2-3, italics mine).

According to this definition, flexibility of response patterns is a requirement for "true" learning. If we accept this definition of learning, organisms undergoing habituation are not learning to respond in a new way, but are simply diminishing their innate response to a stimulus, after repeated exposure to it. Putting it mathematically, we can describe an organism's response to a stimulus in terms of a function F, whose inputs include not only the intensity of the stimulus but also the number of exposures to it (a historical parameter). During habituation, the function F does not change; all that changes is the value of one of the parameters (number of exposures). In other words, F qualifies as a "fixed pattern" according to the definition given above. Nothing in the definition precludes historical parameters from being input variables to functions which describe animal behaviour. If we regard the behaviour of an organism undergoing habituation as conforming to a fixed pattern, then we have no grounds for imputing mental states to it (Conclusion S.7).

L.3 The ability of an organism to undergo (non-associative) habituation and sensitization is not a sufficient condition for learning.

Although psychologists customarily refer to habituation as a kind of "learning", their definition of learning (a relatively permanent modification in an organism's behaviour as a result of experience) effectively rules out historical parameters (such as number of exposures) as inputs to behavioural functions; instead, historical changes are treated as generating a new behavioural function. Kilian and Muller (2001) employ a more restrictive definition of learning, in which "an animal does learn to do something new or better" (Abramson, 1994, p. 38). This corresponds to what psychologists call associative learning.

The question of whether associative learning has to be envisaged in mentalistic terms will be discussed in the sections on worms and insects. In the meantime, we may conclude that non-associative "learning" should not be envisaged in mentalistic terms, unless accompanied by other patterns of behaviour which qualify as truly flexible according to the definition given above.

It should be stressed that habituation is a phenomenon which admits of varying degrees of complexity. The above description does justice to the simplest cases only. In some animals, a variety of circumstances can cause the response to the original stimulus, attenuated by habituation, to re-appear: a change in some "dimension" of the stimulus (e.g. a change in the pitch or volume of a sound); the passage of time; the presentation of a new stimulus, like the original one (dishabituation); a new context for the stimulus; and fatigue (Balkenius, 2000). Balkenius (2000) regards habituation as a process where an organism learns what to expect in a certain situation or context. Context-dependent learning will be discussed later; what we are considering here is purely non-associative "learning".

S.8 The occurrence of non-associative habituation and sensitization in an organism does not provide a sufficient warrant for the ascription of mental states to it. (Corollary of Conclusion S.7.)

N.12 Flexible behaviour by an organism must be internally generated (i.e. the organism must be able to modify its patterns of information transfer, by means of an inbuilt mechanism) before it can be regarded as a manifestation of a cognitive mental state.

Kilian and Muller (2001, pp. 3 - 4) contrast the non-associative forms of behaviour modification found in unicellular organisms with that of multicellular organisms, where the cells involved in chemical information transfer retain their functional specificity, but this specificity is de-coupled from the "goal" of the behaviour. At the beginning of the individual's life, the goal is unspecified or "open-ended": the synaptic connections are not fixed. The same stimulus may be linked to any one of a multitude of responses. Different paths open up, but the final selection from the range of goal cells that come into play is made by the stimulus in individual's local external environment (e.g. in imprinting, the first object the individual happens to see), leading to an individualised, reproducible learning effect. Paths between sensors and effectors vary from one individual to another, and are not genetically determined. Kilian and Muller propose that the kind of information transfer functionality required for true learning is possible "only when several cells of information transfer functionality come into close contact spatially, i.e. in organisms with central nervous systems" (2001, p. 4, italics mine). Organisms with central nervous systems will be discussed in the section on worms, after the evidence for learning in protoctista, plants, simple animals and animals with neural nets (cnidaria) has been evaluated.

David Beisecker. Photo courtesy of University of Nevada, Las Vegas.

Drawing on the work of Dretske and Dennett, Beisecker (1999, p. 298) proposes a slightly more restrictive definition of learning: organisms are capable of learning if they have the "ability to tailor their own responsive dispositions to their particular surroundings". On this interpretation, even imprinting would not qualify as true learning, as the behavioural response, once formed, cannot be subsequently modified to fit different circumstances (e.g. the death of the "parent" imprinted on the newborn individual's memory).

The inability of bacteria to respond in a new way means that on even a minimal definition of learning, bacteria cannot learn. We may conclude, then, that habituation does not necessarily constitute true learning, and that sensitivity, memory, indirect stimulus-response coupling and habituation are not, by themselves, sufficient to establish the existence of cognitive mental processes in an organism (see Conclusions S.5, S.6, S.7 and S.8).

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