Perceptual learning in flavor-aversion learning: Alternating and blocked exposure to a compound of flavors and to an element of that compound

Rats received different schedules of pre-exposure to a compound flavor (AX) and to one element of that compound (X). In Group ALT, exposure consisted of alternating trials with AX and X; Group BLK received a block with all AX trials before a separate block with all X trials (or vice versa). Discrimination between AX and X was assessed then by establishing an aversion to X and measuring the generalization of this aversion to AX. In Experiments 1A and 1B, generalization was less in Group ALT than in Group BLK. In Experiment 2, this latter result was confirmed and furthermore only Group ALT, and not Group BLK, showed less generalization than a group that received exposure to X alone. These results are discussed in terms of their implications for theories of perceptual learning.     

Representation-mediated Inhibitory Learning in the Conditioned-suppression Procedure

In three experiments, rats in group I (for intermixed) were given non-reinforced exposure to two compound stimuli, AX and BX, where A and B represent different auditory cues, and X represents a visual cue. AX and BX were presented in alternation. Group B (blocked) received similar exposure except that subjects experienced a block of AX trials and then a block of BX trials. Subsequent shock reinforcement of A was found to endow B with inhibitory strength in group I, as assessed by retardation (Experiments 1 and 2) and summation tests (Experiment 3). This outcome confirms and extends the results reported by Espinet, Iraola, Bennett, and Mackintosh (1995) and constitutes a further example of mediated learning in which the associative strength of a stimulus is found to be modified as a consequence of training given to some other event with which that stimulus is associated.     

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.     

Mediated configural learning in rats

Three experiments investigated mediated configural learning in male rats. In Experiment 1, after exposure to audio-visual compounds AX and BY, rats received trials where X was paired with shock, and Y was not. The idea that conditioning with X enables the evoked configural representation of AX to be linked to shock received support from the facts that while AX provoked more fear than BX, there was no difference between BY and AY. Similarly, Experiment 2 showed that after exposure to AX and BY, separate pairings of X and Y with shock resulted in more fear to AX and BY than AY and BX. In Experiment 3, rats in group consistent received separate exposures to A and X in Context C, and B and Y in D, while those in group inconsistent received A and X (and B and Y) in both C and D. After rats had received shocks in both C and D, rats in group consistent showed more fear to AX and BY than to BX and AY, but this was not the case in group inconsistent. These results indicate that configural representations, formed either by presenting auditory and visual stimuli as parts of a compound or in a shared context, are subject to a process of mediated learning.     

Salience modulation in serial preexposure: implications for perceptual learning

In three experiments rats were given serial preexposure to two flavor stimuli. In Experiment 1, some animals were given exposure to AX followed by the presentation of BX, a forward schedule; the others were given backward preexposure (BX→AX). Conditioning and test trials with the A element showed that salience or effectiveness of A was better protected in the forward than in the backward condition. Experiments 2 and 3 assessed the relevance of this salience modulation mechanism for perceptual learning. In these experiments, generalization of a conditioned aversion from AX to BX was reduced in the forward (but not in the backward) condition only after prolonged exposure, indicating that the establishment of an inhibitory link from B to A is required for successful discrimination. However, generalization to a novel compound stimulus, NX, was reduced in the forward group both after short and long preexposure, suggesting the existence of salience modulation processes that work in parallel with associative inhibition. These results seem to support the existence of a salience modulation mechanism that seems to be beyond the scope of current theories of perceptual learning.     

Perceptual learning in flavor aversion: evidence for learned changes in stimulus effectiveness

Rats were exposed to the compound flavors AX and BX, presented in alternation, and to CX on a separate block of trials. Generalization to BX after aversion conditioning with AX was less than to CX. An equivalent effect was found when the nature of the common element was changed after preexposure but not when the common element was omitted during preexposure, during conditioning and test, or both. Rats conditioned with X alone again showed less aversion to BX than to CX; similarly, rats conditioned with a novel flavor (Y) showed less aversion to BY than to CY. These effects support the proposal that intermixed preexposure to AX and BX enhances the perceptual effectiveness of their unique features, A and B.     

The influence of word shading and word length on eye movements during reading

In three experiments rats were given short or long preexposure (4 or 10 sessions) to two compound flavours, AX and BX, according to an intermixed or a blocked schedule. Following preexposure, aversion conditioning trials were given with AX as the conditioned stimulus (CS). In Experiments 1 and 2, retardation and summation tests were then carried out to assess the inhibitory properties of B (an Espinet procedure). In Experiment 3, test trials evaluated generalization from AX to BX (the standard perceptual learning procedure). The results showed that B performed as an inhibitor of the unconditioned stimulus (US; an Espinet effect) only after long intermixed preexposure, whereas a reliable perceptual learning effect was observed both after short and after long preeexposure. The observation that B had no detectable inhibitory properties after short preexposure casts doubt on the suggestion that inhibitory learning is responsible for perceptual learning after brief exposure to AX and BX.     

The Espinet and the perceptual learning effects in flavour aversion conditioning: do they depend on a common inhibitory mechanism?

In three experiments rats were given short or long preexposure (4 or 10 sessions) to two compound flavours, AX and BX, according to an intermixed or a blocked schedule. Following preexposure, aversion conditioning trials were given with AX as the conditioned stimulus (CS). In Experiments 1 and 2, retardation and summation tests were then carried out to assess the inhibitory properties of B (an Espinet procedure). In Experiment 3, test trials evaluated generalization from AX to BX (the standard perceptual learning procedure). The results showed that B performed as an inhibitor of the unconditioned stimulus (US; an Espinet effect) only after long intermixed preexposure, whereas a reliable perceptual learning effect was observed both after short and after long preeexposure. The observation that B had no detectable inhibitory properties after short preexposure casts doubt on the suggestion that inhibitory learning is responsible for perceptual learning after brief exposure to AX and BX.     

Memory for, and salience of, the unique features of similar stimuli in perceptual learning

In two experiments, participants received exposure to complex checkerboards (e.g., AX and BX) that consisted of small distinctive features (A and B) superimposed on a larger common background (X). Subsequent discrimination between AX and BX, assessed by a same-different task, was facilitated when the stimuli were presented on alternate trials in preexposure--a perceptual learning effect (Experiment 1). The hypothesis that this form of exposure results in more accurate representations of the unique features was supported in Experiment 1, which showed that participants were well able to match the color of the feature with its shape. Experiment 2 showed that exposure to A and B in isolation, intermixed with presentations of AX and BX, enhanced the perceptual learning effect, which confirmed that the better encoding of the unique features during intermixed preexposure is a direct cause of the enhanced discrimination observed following preexposure on this schedule.     

Enhanced discrimination between flavor stimuli: roles of salience modulation and inhibition

Rats were given intermixed preexposure to the compound flavors AX and BX and to the compound CX in a separate block of trials (4 presentations of each compound). In Experiment 1, rats showed less generalization of conditioned aversion from AX to BX than from CX to BX, a perceptual learning effect. Experiment 2 showed that the formation of an excitatory association proceeded more readily between A and B than between C and B, suggesting that intermixed preexposure maintains the effective salience of A and B and does not establish inhibition between them, a process that would require prolonged preexposure. According to this analysis, salience modulation and associative inhibition may contribute to perceptual learning at different stages of preexposure.     

Attention and relative novelty in human perceptual learning

Four experiments examined the role of attention in human perceptual learning. In Experiment 1, participants were preexposed to a pair of visual (checkerboard) stimuli AX and BX, with common elements X and unique features A and B. A same-different task was then used to assess discrimination of AX and BX and a pair of control stimuli, CY and DY. In addition, participants' eye movements were recorded to assess the role of attentional processes. The results showed that preexposure enhanced discrimination between AX and BX. Furthermore, participants showed greater attention to the preexposed unique features A and B than to the novel unique features C and D, as measured by the eye gaze monitor. Experiments 2 and 3 examined the prediction that perceptual learning is due to the relative familiarity of the common and unique stimulus features. Experiment 4 replicated the intermixed-blocked effect and showed that the way in which AX and BX are presented is also important for perceptual learning. The results generally support the idea that intermixed preexposure to AX and BX increases attention to the unique stimulus features A and B. Some aspects of the results are consistent with a relative novelty account, whereas others implicate a high-level attentional process that is not driven by stimulus novelty.     

Perceptual learning in humans: Roles of preexposure schedule, feedback, and discrimination assay

In three experiments, humans received preexposure to two compound flavours (AX and BX: saline-lemon and sucrose-lemon) that were presented either in an intermixed (e.g., AX, BX, . . . BX, AX, . . .) or a blocked (e.g., AX, AX, . . . BX, BX. . .) fashion. Subsequently, AXwas paired with an unpleasant bitter taste, and the discriminability of AX and BX was assessed using the accuracy of same/different judgements and by the extent to which any learned dislike of AX generalized to BX. When participants received feedback about the accuracy of their same/different judgements during preexposure those given intermixed preexposure were more accurate in making these judgements during the test than those given blocked preexposure (Experiments 1 and 2A), however, there was no evidence of any learned dislike in these experiments. In Experiment 2B, in which participants did not receive feedback about the accuracy of their judgements, there was no effect of the preexposure regime on same/different judgements, but there was a learned dislike of AX, and this generalized less to BX in participants given intermixed than in those given blocked preexposure. The beneficial effects of intermixed preexposure are consistent with results from other species (chicks and rats), but the differences created by the presence or absence of feedback place constraints on the analysis of these effects.     

Analysis of the role of associative inhibition in perceptual learning by means of the same-different task

In Experiment 1a, participants were exposed, over a series of trials, to separate presentations of 2 similar checkerboard stimuli, AX and BX (where X represents a common background). In one group, AX and BX were presented on alternating trials (intermixed), in another, they were presented in separate blocks of trials (blocked). The intermixed group performed to a higher standard than the blocked group on a same-different test. A superiority of intermixed over blocked exposure was also evident in a within-subject design (Experiment 1b) and when the test required discrimination between a preexposed stimulus and the background (e.g., AX vs. X), even if the background changed between preexposure and test (AY vs. Y) (Experiment 2). In Experiment 3, the intermixed/blocked effect was observed when, in preexposure, stimulus presentations were alternated with the background alone (e.g., AX/X). This suggests that the perceptual learning effect is not the consequence of inhibitory associations between unique features but to increased salience of those features. Experiment 4 confirmed this finding and also ruled out an account of the effect in terms of trial spacing.     

Associative activation of stimulus representations restores lost salience: implications for perceptual learning

In 3 experiments, rats received preexposure to presentations of a compound flavor BX. The effective salience of B was then tested by assessing its ability to interfere with the aversion controlled by another flavor or the tendency to drink a saline solution after the induction of a salt need. It was found that the effective salience of B was maintained when during preexposure, presentations of BX alternated with presentations of X alone. This was true both when BX was presented as a simultaneous compound (Experiment 1) and as a serial compound (X-->B; Experiments 2 and 3); salience was not maintained when the serial compound took the form B-->X (Experiments 2 and 3a). It was argued that the salience of B declines during preexposure but is restored when presentations of X are able to activate the representation of B by way of the associative X-B link.     

Stimulus Preexposure, Comparison, and Changes in the Associability of Common Stimulus Features

In four experiments, rats received preexposure either to both of two compound flavours (AX and BX), or to just one (BX). Experiment 1 demonstrated a perceptual learning effect, showing that, for animals given preexposure to both flavours, an aversion conditioned to AX generalized only poorly to BX. Subsequent experiments assessed the properties of the common feature, X. Experiment 3 showed that the two preexposure treatments did not differ in the extent to which they produced habituation of the neophobia evoked by X. Experiment 2 showed that conditioning to X proceeded more rapidly in subjects given preexposure to both AX and BX than in subjects preexposed to BX alone. In Experiment 4, a similar effect was found when the elements of the compounds were presented serially. It is concluded that the perceptual learning effect of Experiment 1 occurs in spite of the fact that preexposure to two stimuli tends to maintain the associability of their common elements.     

The Effect of Stimulus Preexposure on Responding during a Compound Stimulus

In two experiments with rats, three stimuli, A, B, and X, were used for an AX+ BX+ Xo discrimination in which food was presented after the simultaneous presentation of A and X, or B and X, but never after X by itself. For one group in each experiment, X was repeatedly presented by itself before the start of discrimination training. This manipulation weakened responding on test trials in which the three stimuli were presented together. The results are interpreted in terms of a configural theory of conditioning that assumes that preexposure to a stimulus will reduce its capacity to promote generalization among different configurations to which it belongs.     

Associative changes in elements and compounds when the other is reinforced.

Four experiments investigated the symmetry of associative changes in stimulus compounds and elements in a Pavlovian conditioned magazine-approach situation with rats. Experiment 1 used multiple groups to examine the associative changes in a conditioned element (A) as a result of its subsequent reinforcement in compound with a neutral X. These were compared with the changes in an AX compound when one of its elements (A) was subsequently reinforced alone. Although reinforcing A enhanced responding to the AX compound, compared with a control compound, reinforcing AX failed to enhance responding to A, compared with a control element. Experiments 2 and 3 made similar comparisons in a fully within-subject design, finding greater changes in a previously trained AX compound when A was subsequently conditioned than in a B element when BY was subsequently conditioned. Experiment 4 found associative decrements in A when it was reinforced in the presence of a moderately conditioned X. The results observed in each of these experiments are more consistent with an elemental model of conditioning than with a recently proposed configural model.     

Effects of preexposure on stimulus discrimination: an investigation of the mechanisms responsible for human perceptual learning

The effect of preexposure on human perceptual learning was investigated in four experiments. In Experiments 1a and 1b, participants were preexposed to one pair of visual stimuli on an intermixed schedule (AX/BX) and one on a blocked schedule (CX_DX). The ability to discriminate between AX and BX and between CX and DX was then assessed by examining the extent to which key presses assigned to each member of a stimulus pair generalized to the other member (Experiment 1a) and by looking at the accuracy of same-different responses (Experiment 1b). Stimuli were more easily discriminated following intermixed than following blocked preexposure on both the generalization and same-different tasks. This suggests that two stimuli are more perceptually distinct after intermixed preexposure. Experiments 2a and 2b investigated the mechanisms responsible for perceptual learning using same-different tasks. The results support the suggestion that the enhanced discrimination observed after intermixed preexposure is due to increases in the salience of the unique elements.     

Inhibitory sensory preconditioning detected with a sodium depletion procedure

In each of two experiments, two groups of rats were exposed to three flavoured solutions: A (citric acid), B (salt), and AX (a compound of citric acid and saccharin). Both experiments used a between-subjects design in which a paired group received presentations of A followed by B, alternating with presentations of AX (i.e., A → B/AX), and an unpaired group received alternating presentations of A, B, and AX (i.e., A/B/AX). This arrangement was expected to establish X as an inhibitor of B in group paired but not in group unpaired. In Experiment 1, after preexposure all subjects received a single presentation of an XB compound, then experienced sodium depletion, and were tested for their consumption of X, which was greater in group unpaired than in group paired. In Experiment 2, after preexposure, all subjects received four presentations of a new flavour, C, in compound with B and subsequently, under sodium depletion, were tested for consumption of XC. Intake of the XC compound was less in group paired than in group unpaired. These results suggest that, in group paired, X acquired an inhibitory relationship with B both retarding the acquisition of an excitatory association with B (retardation test, Experiment 1) and reducing the response to a new stimulus, C, strongly associated to B (summation test, Experiment 2). These results provide direct evidence of inhibition between two neutral stimuli and, therefore, of inhibitory sensory preconditioning.     

Inhibitory sensory preconditioning detected with a sodium depletion procedure

In each of two experiments, two groups of rats were exposed to three flavoured solutions: A (citric acid), B (salt), and AX (a compound of citric acid and saccharin). Both experiments used a between-subjects design in which a paired group received presentations of A followed by B, alternating with presentations of AX (i.e., A → B/AX), and an unpaired group received alternating presentations of A, B, and AX (i.e., A/B/AX). This arrangement was expected to establish X as an inhibitor of B in group paired but not in group unpaired. In Experiment 1, after preexposure all subjects received a single presentation of an XB compound, then experienced sodium depletion, and were tested for their consumption of X, which was greater in group unpaired than in group paired. In Experiment 2, after preexposure, all subjects received four presentations of a new flavour, C, in compound with B and subsequently, under sodium depletion, were tested for consumption of XC. Intake of the XC compound was less in group paired than in group unpaired. These results suggest that, in group paired, X acquired an inhibitory relationship with B both retarding the acquisition of an excitatory association with B (retardation test, Experiment 1) and reducing the response to a new stimulus, C, strongly associated to B (summation test, Experiment 2). These results provide direct evidence of inhibition between two neutral stimuli and, therefore, of inhibitory sensory preconditioning.