Inhibitory associations between neutral stimuli in flavor-aversion conditioning

Abstract

In Experiments 1 and 2, rats were exposed to two compound flavors, AXand 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.In their theory of stimulus representation, McLaren, Kaye, and Mackintosh (1989) suggested that exposure to two compound stimuli (AX and BX), containing a common element X, would permit the establishment of two types of associations between the elements of each compound. The first would be excitatory associations between the common element X and each unique element, A and B. The existence of such associations is evident from studies such as that of Rescorla and Cunningham (1978), employing compound flavors. Their effect will presumably be to enhance generalization between the two compounds: if AX is paired with an unconditioned stimulus (US), the ability of X to retrieve a representation of B could result in excitatory conditioning to B as well as to A on this trial. Acquired equivalence effects of this sort have been demonstrated by Hall (1989, 1991).It is well established, however, that prolonged exposure to AX and BX will often facilitate subsequent discrimination, rather than increase generalization between them (Hall, 1991). McLaren et al. (1989) attribute this perceptual learning etTect, in part, to differences in the associability of the common (X) and unique (A and B) elements, which occur as a consequence of greater latent inhibition of the former than of the latter. Several studies have provided good evidence for such a suggestion (e.g., Mackintosh, Kaye, & Bennett, 1991; Rodrigo, Chamizo, McLaren, & Mackintosh, 1994) see Experiments 3 and 4) concluded that such differential latent inhibition of common and unique elements could not be the only explanation of perceptual learning. They suggested that one additional mechanism was provided by a second set of associations that are formed between the elements of AX and BX-~specifically, inhibitory associations between A and 8. The establishment of excitatory associations between common and unique elements may cause X to retrieve a representation of B on AX trials (and of A on BX trials), but the presence of A signals the absence of the otherwise expected B (just as the presence of B signals the absence of the otherwise expected A), and according to standard associative theory, this should lead to the eventual establishment of inhibitory associations between A and B...

Dissociation between the Espinet and perceptual learning effects in flavour aversion conditioning

“…This factor may have made the test procedure more sensitive to the ability of B to inhibit A. It still remains true, however, that, unlike Espinet et al (1995), we were unable to find evidence of the acquisition of inhibition by B after intermixed pre-exposure to AX and BX. Our experiments differed from theirs chiefly in the nature of the control group that was used-we used the blocked condition whereas they (in the study most closely comparable to ours) gave the control subjects pre-exposure to A and B in the absence of the common element, X.…”

“…It has been proposed (Espinet et al, 1995;McLaren and Mackintosh, 2000) that a common mechanism underlies the Espinet and the perceptual learning effects. In both procedures, animals in the critical experimental condition are given pre-exposure to intermixed presentations of the compounds AX and BX, an arrangement that might be expected, according to standard associative principles, to establish inhibitory links between the unique features A and B.…”

“…One line of evidence comes from a series of experiments by Espinet et al (1995). In these experiments, rats were given conditioning with A element alone after intermixed exposure to AX and BX, and the consequences for the B element were tested.…”

“…It should be noted, however, that evidence for the role of this inhibitory mechanism in the perceptual learning effect comes from comparison of a group given the intermixed pre-exposure schedule with that of groups given the blocked schedule. In the experiments by Espinet et al (1995), however, different control groups were used, for example, comparison was made between subjects given intermixed pre-exposure to AX and BX and control subjects pre-exposed to A and B alone in the absence of the common X element. It is important, therefore, to see if the Espinet effect can be demonstrated when the comparison is with the blocked control.…”

Excitatory and inhibitory learning with absent stimuli.

“…If we view the present data within that framework, then we must assume that an inhibitory association was formed between noise and tone in Phase 2, and that this association was expressed as inhibitory potential between noise and sucrose after tone-sucrose pairings in Phase 3. According to this account, the inhibition exhibited by the noise is a sort of sensorypreconditioned inhibition, like that observed by Espinet, Iraola, Bennett, and Mackintosh (1995) in a conditioned taste aversion preparation. In their experiments, alternating exposure of two flavor compounds that shared a common element (AX/BX) created inhibitory learning between the unique elements (A and B).…”