DISEÑO DE TANQUES AGITADOS

Conditional discrimination training has been adopted in experiments that have investigated a trained tendency to imitate using laboratory rats as subjects (e.g.. Miller & Dollard, 1941), resulting in an effect which have become known as matched dependent behaviour. Hungry naïve follower rats were given the opportunity to follow a leader rat that had earlier been trained to solve a simultaneous visual discrimination problem on an elevated T maze. The arm on which food would be presented to the leader, and which leaders consistently approached, was signalled by a large coloured card (S+). A card of a different colour (S-) was located on the other, unbaited, arm, which the leader avoided. Half the follower rats (Group SAME) were trained to follow their leader: they received a reward when they chose the same arm as their leader, and a sharp tap on the nose if they chose the opposite arm to their leader. The remainder of the followers (Group DIFF) were trained to choose the opposite arm to their leader. Although there was no evidence of an initial tendency to follow the leader for the whole sample of rats {N = 16, first trial 50% followed), after 7 trials had

been administered in each o f 12 days of training, rats assigned to Group SAME had learned to follow their leader, while those assigned to Group DIFF had learned to choose the opposite arm. Subsequent transfer tests revealed that removal of the cue cards did not disrupt performance and that the conditional discrimination was maintained when the leaders w ere o f a different strain, with a different physical appearance. These results, and similar findings (e.g.. Church, 1957a, 1957b; Solomon & Coles, 1954; Stimbert, 1970), suggest that, in some circumstances, rats may use the movements of its leader as a conditional cue indicating which response will be reinforced.

These experiments are currently rarely considered in discussions o f imitation (but see Howard & Keenan, 1993, for an exception) because they are now conventionally interpreted as not being a social learning effect (Galef, 1988; Heyes, 1994b). Galef noted that new behaviours are not added to an observer's repertoire, because "once the leader (the discriminative stimulus) departs, those aspects of a follower's behavior dependent on the presence o f the leader are lost" (1988, p. 19), and Heyes (1994b) suggested that it follows that these effects have no place within a categorisation of social learning. Excluding these effects from theoretical treatments of imitation on the basis that they do not support the ' social transmission o f new behaviour is understandable when the research objective is to investigate the function o f imitation. However, when the mechanism of imitation is o f primary interest, the relevance of such effects depends upon the extent to which they can be shown to be accounted for by non-imitative processes, i.e., processes that do not involve the mapping problem.

In fact, experiments that have investigated the training of a tendency for rats to make the same response as a leader have not ruled out a role for imitation in producing following effects, and have produced some results which could be interpreted as providing indirect evidence for an imitative account. The orthodox, matched dependent behaviour, account of the findings of these experiments would contend that leaders' directional movements at the choice point had simply become discriminative stimuli for the elicitation of directional responding. Consequently, rats are not considered to be sensitive to the equivalence between the observed discriminative stimulus and their own movements; it has been argued (e.g., Galef, 1988; Zentall, 1996) that the discriminative stimulus for, say, left turning could just as easily be a rat turning to the right or an inanimate object. An alternative, imitative,

Figure 2.1: Miller and Dollard's (1941) results. These data appear to indicate an initial facilitation of training to follow a leader (Group SAME) relative to training to make the opposite response as a leader (Group DIFF). Data were recoded as percentage reinforced responses, having been graphically estimated from Social learning and imitation. (Chapter 7, Experiment 1, Figure 4), by N.E. Miller and J. Dollard, 1941, New Haven: Yale University Press. 100 9 0 1 • D IFF 30 SA M E 3 5 6 7 9 10 11 1 2 4 8 12 0 Days

account would be that the animals were sensitive to the correspondence between the observed movements of a leader and their own movements: Followers may have used the behaviour o f their leader as a conditional cue, but could have re-represented it into a format specific to the execution of the movements observed. On a theoretical level, it could be argued that if the members of a species possess a capacity for imitation, they might automatically represent any observed behaviour as the movements of another agent. On an empirical level, some indirect support for the imitative account comes from the findings that suggest that the movements of a conspecific both are a more effective conditional cue than inanimate objects (Stimbert, 1970), and tend to act as the conditional cue even when inanimate cues could also fulfil this role (Miller & Dollard, 1941, Experiment 1). Closer inspection o f the acquisition performance of the rats in Miller & Dollard's experiment, illustrated in Figure 2.1, also indicates that the rate at which the conditional discrimination was acquired tended to be faster for Group SAME than for Group DIFF. Although Miller & Dollard commented on this difference, they did not report whether it was statistically reliable.

M ore direct evidence that a behavioural concordance effect found after observers were provided with conditional discrimination training to reproduce observed responses were due to an imitative process could come from the application o f well selected transfer tests. Transfer tests, following conditional discrimination training, have been used to good effect in studies o f animal cognition (for a recent evaluation o f this methodology, see Heyes, 1993b). Tw o experiments that have followed the training of rats to reproduce observed responses with transfer tests involving a new target movement in a novel apparatus have provided inconclusive evidence for a generalised tendency to imitate (Miller & Dollard, 1941; Solomon & Coles, 1954). A marginal transfer effect was found in one experiment (Miller & Dollard, 1941, Experiment 4) and no evidence of transfer in the other (Solomon & Coles, 1954). However, the potential utility of transfer tests after training to imitate should not be dismissed on the basis of these mixed results. Experiments that have been considered as attempts to develop an imitation set (Hayes & Hayes, 1952; Custance, Whiten & Bard, 1995) provide some support for the idea of a trained tendency to imitate, and the use of a transfer methodology to test this. In these experiments (see Sections 2.1.4 and 2.1.5), chimpanzees were found to be likely to match novel probe movements after earlier training to do as I do.

effects. Human children with learning difficulties have also been found to reproduce observed movements after training to imitate (Baer, Peterson & Sherman, 1967).

The findings of studies involving conditional discrimination training appear to have been dismissed prematurely as being due to matched dependent behaviour. Although experiments employing this methodology have not provided conclusive evidence of imitation, training studies of imitation could be a valuable tool for the investigation of the mechanism of imitation by providing a sensitive assessment of a competence to imitate, rather than a method of measuring a spontaneous tendency to imitate (Howard & Keenan, 1993; Whiten & Custance, 1996).

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In the experiments reviewed in this section, like those in the previous section, investigators attempted to establish that visual exposure to the movements o f another agent caused a change in the probability with which the same movement is executed by manipulating the visual information presented to observers. However, the present experiments differ from those reviewed in the previous section in that independent groups of animals were presented with different types o f visual information; the critical controls were thus between subject manipulations.

Three types of between subjects tests are considered in turn. Single action tests, are concerned with observers' acquisition of a single, arbitrary, instrumental response, such as bar pressing in rats or key pecking in pigeons. Two action tests typically use two target movements, and are similar in rationale to within subjects designs involving numerous target movements (Section 2.1), and the cross-target procedure that has been used to investigate infant imitation (e.g, Meltzoff & Moore, 1977; see Chapter 1, Section 2.2.1). They have also been called "pattern control" procedures (e.g., Galef, 1988). Bidirectional control^ tests are

* Bidirectional control procedures have also been used within the investigation of asocial learning processes to distinguish Pavlovian conditioning from instmmental learning (e.g., Dickinson, Campos, Varga & Baileine, 1996; Grindley, 1932). In this thesis, the term bidirectional control will be used to refer to the social, rather than asocial, type of experimental design.

a type o f directional control test. Like other two action tests, a bidirectional control experiment involve two target movements directed at a single manipulandum, and utilises a measure o f observers' tendency to make the same type o f movement as had their demonstrator as an index o f imitation.