C. H. Bennett

Perceptual learning in flavour aversion conditioning

In each of four experiments, rats drank a solution of saline or of lemon and saline shortly before receiving an injection of lithium chloride, and the generalization of the resulting aversion to sucrose or to lemon and sucrose was measured. There was little generalization from saline alone to sucrose alone, and prior exposure to the two solutions had no effect on their discriminability. An aversion conditioned to lemon-saline, however, did generalize to lemon-sucrose, and the extent of this generalization was substantially reduced by prior exposure to the two compound solutions. This perceptual learning effect was partly, but not entirely, attributable to the latent inhibition of the common element, lemon, produced by exposure to the two compounds: animals pre-exposed to lemon alone discriminated between lemon-saline and lemon-sucrose better than animals pre-exposed to saline and sucrose alone; but exposure to the three elements in isolation was not as effective as exposure to the two compound solutions in enhancing their discriminability. The final experiment established that one critical feature of compound pre-exposure is that it involves experience of saline and sucrose in the presence of the same common element. According to an associative theory of perceptual learning, this would result in the establishment of inhibitory associations between saline and sucrose, thus reducing generalization between the two compound solutions.

Evidence for inhibitory associations between the unique elements of two compound flavours.

In each of two experiments, rats were pre-exposed to two flavoured solutions, saline-lemon and sucrose-lemon. For group ALT, trials with one solution alternated with trials with the other. Group BLK received all trials with one solution in a block, before any trials with the other. An associative theory suggests that the alternating, but not the blocked, schedule would establish an inhibitory association between sucrose and saline. To provide a retardation test of this inhibition, some animals in each group were then given a single pairing of saline and sucrose, experienced sodium depletion, and were finally tested for their consumption of sucrose. Sodium depletion increased consumption of sucrose more in group BLK than in group ALT. In groups given no saline-sucrose pairing, sodium depletion had only a small effect on sucrose consumption, which was the same in both groups. After multiple pairings of saline and sucrose, sodium depletion had an equally large effect on sucrose consumption in both ALT and BLK groups. These results imply that alternating pre-exposure to two compound solutions does establish an inhibitory association between their unique elements, and thus provide support for an associative theory of perceptual learning.

Inhibitory associations between neutral stimuli in flavor-aversion conditioning

In Experiments 1 and 2, rats were exposed to two compound flavors, AX and BX, containing one flavor in common (X). Following this exposure phase, an aversion was conditioned to A in the experimental group by pairing its consumption with an injection of lithium, while a control group drank A without being poisoned. The effect of this treatment was to establish B as a conditioned inhibitor. In Experiment 1, experimental animals were slower than controls to condition an aversion to B when its consumption was paired with lithium (a retardation test of conditioned inhibition). In Experiment 2, B alleviated the suppression of intake of another flavor previously paired with lithium (a summation test). Experiments 3 and 4 established that these effects depended upon prolonged prior exposure to AX and BX.

Comparison and contrast as a mechanism of perceptual learning

In a series of flavour aversion experiments, rats received different schedules of pre-exposure to two compound flavours (AX and BX). Discrimination between them was assessed by establishing an aversion to AX and measuring generalization of this aversion to BX. Experiment 1 demonstrated that alternating pre-exposure to AX and BX resulted in less generalization than did blocked exposure, where animals received all exposure to AX before exposure to BX (or vice versa). This difference was not accompanied by any difference in the strength of the aversion conditioned to the common X element. Varying the interval between exposure to AX and BX in the alternating condition from a minute or two to several hours had no effect on generalization. However, Experiments 2 and 3 showed that when the interval between exposure to AX and that to BX was reduced to zero sec, the alternating schedule increased generalization between AX and BX. In this case, the increase in generalization was accompanied by an increase in the strength of the aversion conditioned to X.

The role of inhibitory associations in perceptual learning

Preexposure to two compound flavors (AX and BX) typically enhances their discriminability: An aversion conditioned to AX will generalize less to BX, especially if the preexposure regime has involved alternated presentations of AX and BX rather than presenting all AX trials before BX trials (or vice versa). One possible explanation of this finding is that alternating preexposure establishes inhibitory associations between the two unique features A and B, thus counteracting the generalization produced by excitatory associations between X and A and between X and B, which might result in either the retrieval of B on a conditioning trial to AX, or the retrieval of A on a test trial to BX. Three experiments on flavor aversion conditioning in rats tested these predictions. Experiment 1 suggested that the more important of these excitatory associations was that which allowed X to retrieve A on the test trial to BX. Experiment 2 suggested that the more important inhibitory association was that which allowed B to inhibit the representation of A on this test trial. Experiment 3 provided direct evidence of the role of this inhibitory B⊣A association.

Reduced generalization following preexposure: latent inhibition of common elements or a difference in familiarity?

Rats injected with lithium chloride after drinking a saline-lemon solution also reduced their consumption of saline-lemon and sucrose-lemon, but a single prior exposure to sucrose-lemon decreased the generalized aversion. Experiment 1 showed that this decrease in the aversion to sucrose-lemon could not be explained by a decrease either in the aversion conditioned to saline-lemon or in neophobia to sucrose-lemon. Experiment 2 ruled out the possibility that generalization between saline-lemon and sucrose-lemon was based on shared novelty. Experiment 3 showed that generalization between the two solutions was related to the strength of the aversion to the lemon flavor they shared in common and that prior exposure to sucrose-lemon reduced generalization by causing latent inhibition of this common element.

Is the context specificity of latent inhibition a sufficient explanation of learned irrelevance

In three experiments, rats were pre-exposed either to uncorrelated presentations of a light and sucrose pellets (group CS/US) or to equivalent presentations of the light and pellets in separate sessions (control). In Experiment 1, subsequent conditioning to the light proceeded more slowly in group CS/US than in the control group, whether this conditioning was excitatory, with the light signalling the delivery of pellets, or inhibitory, with the light signalling their absence. Bonardi and Hall (1996) have argued that this learned irrelevance effect may be reducible to latent inhibition, which would be stronger in group CS/US because they are both pre-exposed and conditioned to the CS in the presence of traces of previous USs occurring in the same session. This analysis implies that group CS/US should have conditioned more rapidly to the CS than controls on the first trial of each session in Experiment 1, but this did not happen. It also implies that the learned irrelevance effect should be reversed if conditioning trials are given at a rate of one per day. Experiments 2 and 3 found no support for this prediction. We conclude that learned irrelevance effects cannot always be reduced to latent inhibition.