Are worms capable of learning?

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The roundworm Caenorhabditis elegans. Courtesy of The Sanger Institute.

Can "simple" worms acquire new behaviour patterns?

Even the simplest flatworms have been credited with the capacity for associative learning, defined by Abramson as "a form of behavior modification involving the association of two or more events, such as between two stimuli, or between a stimulus and a response. In associative learning, an animal does learn to do something new or better" (1994, p. 38, italics mine). Classical conditioning and instrumental and operative conditioning are well-researched categories of 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.

There are three issues - one scientific and two philosophical - to be addressed here. First, how can we know that an animal is "doing something new" - the hallmark of associative learning? Second, if an animal acquires the capacity to do something new, should philosophers call this "learning"? Third, even if an animal can learn to do something new or better, do we have to explain this in mentalistic terms, or is there an alternative, mind-neutral explanation?

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 are protostomes, 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.

R.9 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 Department of Psychology, Oklahoma State University.

The evidence to date for instrumental conditioning 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.

Alternatively, Abramson's description of operant behaviour could be expressed using the notion of control. If an animal can adjust and fine-tune its responses to a variable stimulus, then we could say that it has control over its responses, and is able to engage in operant behaviour. If it cannot fine-tune its responses to the stimulus, then it lacks control over them and is only capable of instrumental behaviour. The word "control", like "try", has mentalistic overtones and is amenable to an agent-centred intentional stance. However, it has not yet been shown that such a stance is required to account for operant behaviour.

Mental states such as beliefs and desires are primarily identified through the performance of intentional acts, which presuppose the notions of trying and control. Since these can only be manifested in organisms that are capable of fine-tuning their bodily movements, we can formulate the following requirement:

N.15 An organism must be capable of fine-tuning its bodily movements before it can be said to have cognitive mental states.

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.

First, it is important to construct a complete pictorial catalogue of the behavioural patterns of that species - i.e. an ethogram. Second, it is essential to learn what attracts them and what repels them. Third, the animals have to be able to control (or fine-tune) one of their patterns of behaviour, in order to obtain a "reward" or avoid a "punishment".

One might ask: how do we identify "control" over patterns of behaviour? For example, is an increase in a worm's innate tendency to turn when it becomes too hot or cold an instance of controlled behaviour, or is it an aversive reaction to change? The answer is not always clear-cut. However, if the worm were able to adjust, say, the speed of its movement in an arbitrary fashion as the temperature rose or fell, in order to obtain a more "comfortable" temperature setting, this would be a bona fide instance of "controlled behaviour". I will discuss this issue further below, in connection with Dretske's account of belief.


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 questions unanswered. First, should philosophers call this learning? Second, do we need to explain it in terms of mental states?

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