Which organisms are capable of associative learning?

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Case study 1: Are protoctista capable of associative learning?

Image of an amoeba. Courtesy of Michael Davidson and the Florida State University.

The kingdom of protoctista includes organisms such as amoebae, algae, seaweeds, slime moulds, ciliates, diatoms, paramecia and forams. Unlike bacteria, protoctista are eukaryotes - organisms whose cells have a nucleus. They are described in more detail in an Appendix.

It has been claimed that paramecia possess a capacity for learning through classical conditioning. The original experiment was reported in a study by Hennessey, Rucker and McDiarmid (1979) and is still widely quoted (e.g. by Martin and Gordon, 2001). However, as far as I have been able to ascertain, no-one has replicated, or even attempted to replicate, this result. In accordance with the conservative methodological principles laid down earlier, I shall not consider this evidence. For the time being, it would be unwise to ascribe associative learning to paramecia.

Hinkle and Wood (1994) dispute the existence of associative learning in protozoa. In their study, they examined a claim that the protozoan Stentor exhibited a form of instrumental conditioning while learning to escape from a capillary tube over repeated trials. In a recent personal email communication, Wood (personal email, 18 June 2003) writes:

Our results indicated this phenomenon is not a real example of instrumental conditioning (Hinkle and Wood, 1994).

I also know of no other well substantiated cases of associative conditioning in ciliates.

It should be noted that The experiment reported by Hennessey, Rucker and McDiarmid (1979) was conducted on Paramecium (a quite different type of ciliate). Clearly, further investigation is warranted.

Case study 2: Are plants capable of associative learning?

Image of a mimosa plant. Courtesy of The Nature Conservancy, University of California, Davis.

The question of what makes a plant is dicussed at further length in an Appendix.

Research into the possibility of classical conditioning in Mimosa has produced negative or conflicting results, and the methodology of studies which found conditioning has been criticised (ibid., pp. 175-176). Attempts to condition Mimosa with light touch as the conditioned stimulus (CS) and electrical or mechanical shock as the unconditioned stimulus (US) failed. Other attempts, using light as the CS and touch as the US yielded positive results in two (arguably flawed) studies, and negative results in another study. In keeping with my methodological constraints, these studies will be ignored here.

A more refined plant learning experiment by Abramson, Garrado, Lawson, Browne and Thomas (2002, pp. 173 - 185) is described in an Appendix. The study used a new method: different groups of Philodendron cordatum plants were exposed to a six-hour training period of light only, darkness only, or alternating one-minute periods of light and darkness, and subsequently exposed to a ten-minute testing period in darkness, when their bioelectrical potentials were recorded, using EEG equipment. The researchers looked for differences in the amplitudes recorded for plants in the last group, corresponding to the switching of the light in the training period. The results were negative. There are several possible explanations for this failure: the intervals of light and darkness may have been too short for the plants to adjust to changes, or the plants may have been poor at timing (like most invertebrate animals), or it may not be possible for true learning to occur in the absence of a nervous system. Follow-up research with longer intervals is recommended by the authors.

Summary: protoctista and plants

Although protoctista and plants are highly adaptive, multicellular, eukaryotic organisms which are capable of being habituated (unlike bacteria), there is no good evidence that they are capable of associative learning. Habituation, as we have seen, does not need to be explained in terms of mental states (see Conclusion L.4). After reviewing the evidence, my conclusion (a tentative one, bearing in mind the dearth of learning studies) is they do not appear to be capable of having cognitive mental states. Further research into the alleged learning abilities of these organisms is warranted.

Case study 3: Are worms capable of associative learning?

The roundworm Caenorhabditis elegans. Courtesy of The Sanger Institute.

The relationships between the different groups (or phyla) of worms, and other animals, are discussed in further detail in an Appendix. Of special interest here are flatworms (believed to be the most "primitive" group) and roundworms (or nematodes), the group to which the well-studied C. elegans belongs.

Even the simplest flatworms have been credited with the capacity for associative learning. However, the attribution of associative learning to even the simplest flatworms is controversial, as different authorities use definitions of "associative learning" and some of the effects of associative learning can be mimicked by phenomena that can be explained in non-mentalistic terms.

The issue to be addressed here is: how can we know that an animal is "doing something new" - the hallmark of associative learning (Abramson, 1994, p. 38)?

Evidence for associative learning in worms

(a)Classical conditioning

It is no easy matter for scientists to verify that an animal has undergone classical and/or instrumental conditioning, as these processes are easily confused with other behavioural processes in animals which do not involve learning - for example, pseudo-conditioning and sensitization. The problems involved in verifying scientifically that an animal has undergone classical and/or instrumental conditioning, and the history of previous mis-identifications of conditioning in worms, are discussed in an Appendix.

I have not been able to locate any studies to date showing unequivocally that flatworms (platyhelminthes) are capable of classical conditioning. However, recent research on another worm, the well-studied Caenorhabditis elegans, has demonstrated that even worms with very "simple" nervous systems are capable of associative learning - in particular, classical conditioning. C. elegans belongs to the phylum Nematoda (roundworms) and is a favourite of scientists studying the genetic and molecular bases of learning, because it has a fully mapped nervous system with only 302 neurons and a small, almost completely sequenced genome. Although roundworms, like flatworms, are protostomes (animals with one opening that serves both as a mouth and an anus), they are not closely related to flatworms.

The evidence that C.elegans is truly capable of undergoing classical conditioning is discussed in an Appendix. In particular, recent studies have shown that C. elegans worms can actually be conditioned to radically alter their preferences: they will avoid a stimulus they had formerly been attracted to, after it has been paired with an aversive stimulus. This change of preference cannot be explained away as "conditioned sensitization" because the old response is not re-awakened. The worms are actually learning to do something new: they are changing their pattern of response to a stimulus.

Additionally, Catharine Rankin, who specialises in learning mechanisms in C. elegans, claims (personal email, 31 May 2003) that recent work has established that it can indeed associate a CS with a US. We can now formulate the following conclusion regarding the range of organisms capable of associative learning:

L.8 Associative learning appears to be confined to organisms with central nervous systems. It is found in most but possibly not all phyla of animals with central nervous systems. (Flatworms may not be capable of associative learning, but many other phyla of worms are.)

(b)Instrumental and operant conditioning

Charles Abramson. Picture courtesy of the Public Information Office, Oklahoma State University.

The evidence to date for instrumental conditioning in worms is very limited, and its interpretation is open to debate. Additionally, classical conditioning may also be confused with instrumental or operant conditioning. Examples are given in an Appendix.

The other key reason for the current uncertainty regarding operant conditioning is the lack of an agreed definition. Some psychologists define operant behaviour as "behaviour controlled by its consequences". Abramson prefers to call this kind of behaviour "instrumental conditioning", and reserves the term "operant behaviour" for special cases. He observes that if we adopt the commonly-used definition of operant behaviour, then

...such behavior is present in all animal groups. However, if operant behavior is defined in terms of its functional influence on the environment and the ability to use an arbitrary response, then... [it] is limited to vertebrates and perhaps some species of mollusks, crustaceans, and insects. A rule of thumb I have found useful... is that in operant behavior, an animal must demonstrate the ability to operate some device - and know how to use it, that is, make an arbitrary response to obtain reinforcement. (1994, p. 151, italics mine).

Abramson provides an illustration of behaviour conforming to his more restrictive definition:

For example, we know that rats can be taught to press a lever in various directions and with various degrees of force. They can also be trained to run down an alley with speeds selected by the experimenter... I would be more convinced that an invertebrate has operant responses if they can adjust their, for example, swimming speed to fit the contingencies. These studies have not been performed (personal email, 2 February 2003, italics mine).

Abramson's distinction between instrumental and operant conditioning recalls Anscombe's dictum (1957, p. 68) that "the primitive sign of wanting is trying to get". If an organism's behavioural repertoire in the presence of a stimulus is very limited (e.g. if it does nothing more than move towards or away from a stimulus), then we may reasonably doubt that it is really "trying" to acquire it or avoid it. But if an organism can adjust its behaviour in a graduated fashion to obtain an attractive stimulus or avoid a noxious one, then we have something that looks like an instance of genuine trying. In fact, operant behaviour is popularly referred to as "trial-and-error learning". The mental requirements (if any) of operant behaviour will be discussed below.

Two studies were recently conducted which suggest that C. elegans worms may indeed be capable of modifying their responses to obtain a reinforcement. These studies are outlined in an Appendix. Caution is advised, as the studies produced some conflicting results, for reasons that are not altogether clear. It would be premature to draw any conclusions from this research, but at least it suggests how one might proceed in attempting to verify operant behaviour - as defined by Abramson - in a species of animal.


The evidence indicates that many (probably most) phyla of worms are capable of undergoing at least one form of associative learning: classical conditioning. That leaves two philosophical questions unanswered. First, should philosophers call this learning? Second, do we need to explain it in terms of mental states?

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