Perceptual Learning and Free Classification

Two experiments are reported that investigate the effects of stimulus preexposure on discrimination performance in a free classification task, using adult humans as subjects. In free classification subjects are asked to put stimuli into gruops in any way that seems reasonable or sensible tothem. Experiment 1 shows that the effect of preexposure is contingent on stimulus structure. Experiment 1b is the first demonstration of a retardation in learning as a consequence of simple preexposure in adult human subjects (previous demonstrations have relied on incidental or masked preexposure). Experiment 2 further supports the conclusions of Experiment 1 and extends them with the demonstration that stimulus similarity is a crucial factor. Taken together, these experiments rule out a class of attention-based explanations of the phenomena reported here. The experiments also provide novel information about the effects of preexposure. Preexposure can change the actual classifications subjects form in addition to altering the rate at which they are formed. Implications of these results for current theories of category formation and perceptual learning are considered.     

Latent Inhibition of Contextual Stimuli Reduces the US Preexposure Effect in Rats

Two experiments are reported which utilized latent inhibition of contextual stimuli prior to administering unsignaled presentations of the Us in an attempt to further assess the role of contextual stimuli in the US preexposure effect. Specifically in Experiment 1, rats received either 0-, 5-, 10-, or 15-minute exposure to the context in which the unsignaled Uss were to occur (latent inhibition). Following preexposure to the US, animals were trained in a CER paradigm with a tone CS. Measures of suppression to the tone indicated that the greatest US preexposure effect occurred in the 0 group and that no US preexposure effect was evident in the 15 group. Experiment 2 includec two important control groups which were omitted in Experiment 1 (no US preexposures) and an additional dependent variable (time to initiate licking) to measure fear to contextual stimuli. These results are discussed in terms of the role context conditioning may play in US preexposure.

     

Category effects on estimates of stimuli: perception or reconstruction?

The present study examined a common category effect that has been reported in the literature: the tendency for estimates of individual stimuli to be biased toward the central value of the presented set of stimuli. Both encoding and reconstruction accounts of this central-tendency effect are considered. Plain vertical lines and vertical lines embedded in the Müller-Lyer illusion were estimated while still in view or from memory. Although bias due to the Müller-Lyer illusion remained constant across the two conditions, bias due to the context set (category) occurred only when stimuli were estimated from memory. The results suggest that the category bias occurs at a later stage of processing that the Müller-Lyer effect and offer support for a reconstruction account of category effects on stimulus estimation.     

Category variability, exemplar similarity, and perceptual classification

Experiments were conducted in which observers learned to classify simple perceptual stimuli into low-variability and high-variability categories. Similarities between objects were measured in independent psychological-scaling tasks. The results showed that observers classified transfer stimuli into the high-variability categories with greater probability than was predicted by a baseline version of an exemplar-similarity model. Qualitative evidence for the role of category variability on perceptual classification, which could not be explained in terms of the baseline exemplar-similarity model, was obtained as well. Possible accounts of the effects of category variability are considered in the General Discussion section.     

The US preexposure phenomenon in the conditioned suppression paradigm: A role for conditioned situational stimuli

Four experiments evaluated possible associative and nonassociative accounts of the retardation in the acquisition of conditioned suppression produced by repeated prior exposure to an electric shock US. Associative interference resulting from conditioning of situational stimuli during preexposure to shock was suggested by the findings that signaling the occurrence of high-intensity shock with a discrete nontarget CS during the preexposure phase reduced the magnitude of the retardation effect compared to an unsignaled shock preexposure treatment (Experiments 1 and 4), nonreinforced presentations of putatively conditioned situational stimuli prior to conditioned suppression training reduced the magnitude of the retardation effect (Experiment 2), and the magnitude of the retardation effect was directly related to the intensity of preexposure shock (Experiment 3). Nonassociative interference was suggested by the finding that signaling the occurrence of low-intensity shock with a discrete nontarget CS during the preexposure phase did not reduce the magnitude of the retardation effect compared to an unsignaled shock preexposure treatment (Experiment 4). It was suggested that associative and nonassociative mechanisms govern the US preexposure phenomenon obtained in the conditioned suppression paradigm, and their relative contribution depends upon the intensity of shock.     

Interference with ingestional aversion learning produced by preexposure to the unconditioned stimulus: Associative and nonassociative aspects

Preconditioning exposure to a lithium chloride unconditioned stimulus (US) interferes with the subsequent conditioning of a taste aversion. Multiple US preexposures produce a long-lasting or durable interference with aversion learning, whereas a single US exposure produces a transient interference effect. The present experiments demonstrate that the durable US preexposure effect is substantially reduced if the method of drug treatment (infusion versus injection of the drug into the peritoneal cavity) or the spatial cues present during drug treatment (home cage versus a distinctive environment) are changed between the preexposure and taste conditioning phases of the experiment. In contrast, these manipulations do not attenuate the proximal/transient US preexposure effect. These findings indicate that different mechanisms are responsible for the two US preexposure effects. The results are consistent with previous suggestions that the durable effects of lithium preexposure are due to associative interference produced by the exteroceptive stimuli that accompany drug administrations and indicate that the proximal/transient US preexposure effect is mediated by nonassociative mechanisms.     

Drug-preexposure effects in flavor-aversion learning: associative interference by conditioned environmental stimuli

Previous research has documented that exposure to a drug reduces the ability of the drug to support subsequent flavor-aversion learning. The four experiments reported here examined the hypothesis that this drug-preexposure effect is due to associative interference from environmental stimuli associated with the drug effects during preexposure. When distinctive environmental stimuli (confinement in a black compartment) were present during drug preexposure, these stimuli significantly disrupted subsequent flavor-aversion learning. Furthermore, flavor conditioning was not significantly disrupted when drug preexposure occurred in the absence of salient environmental stimuli or when the previously conditioned environmental stimuli were extinguished prior to flavor conditioning. It is significant, and in contrast to other published research, that flavor conditioning was not disrupted when the distinctive cues paired with the drug during preexposure were absent at the time of the flavor-drug pairing. These results are thus consistent with results from conventional studies of stimulus blocking and suggest that associative processes can play a major role in the drug-preexposure effect.     

The influence of flavor preexposure and test interval on conditioned taste aversions in the rat

The effects of flavor preexposure and test interval on conditioned taste aversions were examined in four experiments. In the first three experiments, prior experience with a flavor different from that used as a conditioned-stimulus (CS) produced attenuated aversions when testing occurred after a 1-day interval but not after a 21-day interval. Preexposure to the same stimulus used as a CS produced attenuated aversions at both 1- and 21-day intervals. In Experiment 4, a delay interval between flavor preexposure and conditioning eliminated the attenuating effect of preexposure, but only when different stimuli were used for preexposure and conditioning. These data could not be easily accounted for by contemporary interpretations of preexposure as an event that interferes with subsequent acquisition of a conditioned aversion. An alternative retrieval interference hypothesis was outlined.     

Transfer of configural learning between the components of a preexposed stimulus compound: implications for elemental and configural models of learning

In 2 experiments, rats received preexposure to 2 compound contexts: AB and CD for the congruent group and AC and BD for the incongruent group. Subsequently, all rats received a configural discrimination in which separate placement in contexts A or B indicated that presentations of stimulus X would be followed by food and presentations of Y would not, and separate placement in contexts C and D indicated that Y would be followed by food and X would not. In both experiments, rats in the congruent group acquired the conditional discrimination more rapidly than those in the incongruent group. These results are inconsistent with conventional associative accounts of either stimulus preexposure effects or configural learning and instead provide support for a connectionist account.     

Disrupting the conditioned stimulus preexposure effect in flavor-aversion learning: effects of interoceptive distractor manipulations

Rats exposed to a flavor prior to a conditioning trial involving that stimulus learn a significantly diminished flavor aversion relative to nonpreexposed control animals. A series of four experiments investigated the ability of the conditioned stimulus (CS) preexposure effect to be disrupted by the introduction of a distractor flavor stimulus between the preexposure and conditioning episodes. Experiment 1 demonstrated that the preexposure effect could be reduced by a distractor presented immediately following the preexposure. In Experiment 2, it was discovered that a novel distractor was more effective than a familiar distractor, even though both stimuli were sensorily equivalent. Experiment 3 further analyzed the distractor effect and demonstrated that the magnitude of disruption was more pronounced with immediate than with delayed (3 hr) distractor manipulations. Finally, Experiment 4 assessed the effects of the distractor in the absence of CS preexposure. The relation of the results from these experiments to general information theory is discussed.     

Hippocampal lesions in pigeons (Columba livia) disrupt reinforced preexposure but not overshadowing or blocking

Three experiments examined the effects of hippocampal lesions in pigeons on overshadowing, blocking, and a latent inhibition treatment (non-differential reinforced preexposure) using a simultaneous visual discrimination paradigm. The results showed that hippocampal damage did not influence overshadowing (Experiment 1) or blocking (Experiment 2) but did attenuate the retardation in conditioning associated with non-differential reinforced preexposure to to-be-discriminated stimuli (Experiment 3). Hippocampal birds also displayed impaired autoshaping (Experiments 1, 2, and 3). The correspondence between the behavioural effects of avian and mammalian hippocampal lesions is discussed, and the implications of the present pattern of results for avian hippocampal function are considered.     

Latent inhibition and conditioning after preexposure to the training context

Experiment 1, using the conditioned suppression technique with rats, showed that the retardation of learning produced by prior exposure to a stimulus (latent inhibition) was more marked in subjects given an initial phase of preexposure to the training context. This effect was confirmed and extended in Experiment 2 in which an appetitive conditioning procedure was used. Experiments 3 and 4, again using conditioned suppression, found no effect of preexposure to the context on the acquisition of suppression when training was given with a novel stimulus, either immediately after preexposure or after a delay; but context preexposure was again found to be effective when exposure to the to-be-conditioned stimulus was given in the delay interval between context preexposure and conditioning. The implications of these findings for accounts of the role of contextual factors in latent inhibition are discussed.     

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.     

Target preexposure eliminates the effect of distraction on event-based prospective memory

Prospective memory is critical to everyday functioning and can be vulnerable to distraction. We conducted an experiment to explore whether we could buffer prospective memory against distraction. For half the participants, we preexposed stimuli that were later designated as prospective memory targets. Then, all participants performed an ongoing task (in which the prospective memory task was embedded) under standard and high attentional demand (i.e., under full and divided attention). Target preexposure improved prospective memory and eliminated the significant divided attention effect. Thus, target preexposure seems to buffer prospective memory against the disruptive effect of dividing attention. Moreover, target preexposure seemed to help participants to respond with the correct intended action. This result implies that preexposure to the target stimuli facilitated the encoding of an association between the target stimuli and the intended action, perhaps promoting relatively more reflexive retrieval and thereby buffering prospective memory against distraction.     

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.     

Event-related potentials as brain correlates of item specific proportion congruent effects

The item-specific proportion congruency (ISPC) effect is consistent with the idea that control processes can be applied rapidly in accord with previously experienced conflict for a particular category. An alternative account of this effect is that it reflects item-specific learning processes unrelated to control at the level of the category. The accounts predict the same behaviour but differ in terms of electrophysiological predictions. Two experiments examined the ISPC effect with a particular focus on neural correlates that might reveal whether, and how early in processing, high and low proportion congruent items are treated as distinct classes of stimuli. For both tasks, the proportion congruency category was distinguished prior to the congruence of the specific stimulus, as early as 100 ms post-stimulus onset for the global/local identification task (Experiment 1) and 150 ms for the Stroop task (Experiment 2). The results support an on-line control account of ISPC effects.     

Contextual and sequential effects on judgments of sweetness intensity

Presenting stimuli from skewed concentration distributions affects mean responses on category scales. However, if the number of categories on the response scale is increased, the degree of separation between the mean responses obtained for a positively as opposed to a negatively skewed concentration distribution diminishes. The present study investigates the effect of skewed concentration distributions upon ratings on a line scale and compares it to the context effect found for a 7-point category scale. In addition, sequential dependencies between consecutive stimuli and responses are investigated in order to assess their relevance in tasteintensity scaling studies. The context effects are similar for the 7-point category scale and for the line scale. The analyses of sequential effects show that both preceding responses and preceding stimuli affect current responses. However, since these two factors work in opposite directions, only a small contrast effect from the previous stimulus is significant in an overall analysis. The present study shows that even though the overall sequential effects between consecutive stimuli and responses are small, the effect of experimental context may be considerable. Since subjective context is established at the beginning of a session and sequential dependencies operate throughout the whole session, it is argued that contextual and sequential effects are only indirectly related.     

The Importance of Context in the US Preexposure Effect in CTA: Novel versus Latently Inhibited Contextual Stimuli

Three experiments were conducted to examine the role of novel contextual stimuli in producing the unconditioned stimulus (US) preexposure effect. Experiment 1 demonstrated that novel contextual stimuli produce a significantly stronger US preexposure effect than familiar or “latently inhibited” contextual stimuli. Moreover, subjects preexposed in the presence of latently inhibited contextual cues failed to show a significant US preexposure effect. Experiments 2 and 3 attempted to provide evidence that the addition of a single novel stimulus to the latently inhibited context would result in a significantly stronger US preexposure effect than when no such novel cue was present. Experiment 3 was able to demonstrate this effect. Results are consistent with the Rescorla–Wagner (1972) model of conditioning.     

Contextual and sequential effects on judgments of sweetness intensity.

Presenting stimuli from skewed concentration distributions affects mean responses on category scales. However, if the number of categories on the response scale is increased, the degree of separation between the mean responses obtained for a positively as opposed to a negatively skewed concentration distribution diminishes. The present study investigates the effect of skewed concentration distributions upon ratings on a line scale and compares it to the context effect found for a 7-point category scale. In addition, sequential dependencies between consecutive stimuli and responses are investigated in order to assess their relevance in taste-intensity scaling studies. The context effects are similar for the 7-point category scale and for the line scale. The analyses of sequential effects show that both preceding responses and preceding stimuli affect current responses. However, since these two factors work in opposite directions, only a small contrast effect from the previous stimulus is significant in an overall analysis. The present study shows that even though the overall sequential effects between consecutive stimuli and responses are small, the effect of experimental context may be considerable. Since subjective context is established at the beginning of a session and sequential dependencies operate throughout the whole session, it is argued that contextual and sequential effects are only indirectly related.     

Neural correlates of acquired color category effects

Category training can induce category effects, whereby color discrimination of stimuli spanning a newly learned category boundary is enhanced relative to equivalently spaced stimuli from within the newly learned category (e.g., categorical perception). However, the underlying mechanisms of these acquired category effects are not fully understood. In the current study, Event-Related Potentials (ERPs) were recorded during a visual oddball task where standard and deviant colored stimuli from the same or different novel categories were presented. ERPs were recorded for a test group who were trained on these novel categories, and for an untrained control group. Category effects were only found for the test group on the trained region of color space, and only occurred during post-perceptual stages of processing. These findings provide new evidence for the involvement of cognitive mechanisms in acquired category effects and suggest that category effects of this kind can exist independent of early perceptual processes.