N. J. Mackintosh

An elemental model of associative learning: I. Latent inhibition and perceptual learning

This paper presents a brief, informal outline followed by a formal statement of an elemental associative learning model first described by McLaren, Kaye, and Mackintosh (1989). The model assumes representation of stimuli by sets of elements (i.e., microfeatures) and a set of associative algorithms that incorporate the following: real-time simulation of learning; an error-correcting learning rule; weight decay that distinguishes between transient and permanent associations; and modulation of associative learning that gives high salience to and, hence, promotes rapid learning with novel, unpredicted stimuli and reduces the salience for a stimulus as its error term declines. The model is applied in outline fashion to some of the basic phenomena of simple conditioning and, in greater detail, to the phenomena of latent inhibition and perceptual learning. A detailed account of generalization and discrimination will be provided in a later paper.

Overshadowing and stimulus intensity

Two experiments on conditioned suppression in rats examined overshadowing between visual and auditory components of a compound conditioned stimulus. In the first experiment, when one component was markedly more salient than the other, the more salient overshadowed the less salient, but the latter, although acquiring significant associative strength, did not overshadow the former. When the two components were of approximately equal salience, each overshadowed the other. In the second experiment, reciprocal overshadowing was again observed between two equally salient stimuli, but only when their absolute intensities were relatively low. The failure to observe reciprocal overshadowing under all conditions raises problems for those theories of stimulus selection which assume that stimuli compete for some strictly limited resource. It was suggested, instead, that overshadowing might occur when animals fail to learn to attend to, or actually learn to ignore, stimuli that are not uniquely successful predictors of reinforcement.

Implicit learning as an ability

The ability to automatically and implicitly detect complex and noisy regularities in the environment is a fundamental aspect of human cognition. Despite considerable interest in implicit processes, few researchers have conceptualized implicit learning as an ability with meaningful individual differences. Instead, various researchers (e.g., Reber, 1993; Stanovich, 2009) have suggested that individual differences in implicit learning are minimal relative to individual differences in explicit learning. In the current study of English 16-17year old students, we investigated the association of individual differences in implicit learning with a variety of cognitive and personality variables. Consistent with prior research and theorizing, implicit learning, as measured by a probabilistic sequence learning task, was more weakly related to psychometric intelligence than was explicit associative learning, and was unrelated to working memory. Structural equation modeling revealed that implicit learning was independently related to two components of psychometric intelligence: verbal analogical reasoning and processing speed. Implicit learning was also independently related to academic performance on two foreign language exams (French, German). Further, implicit learning was significantly associated with aspects of self-reported personality, including intuition, Openness to Experience, and impulsivity. We discuss the implications of implicit learning as an ability for dual-process theories of cognition, intelligence, personality, skill learning, complex cognition, and language acquisition.

Associative learning and elemental representation: II. Generalization and discrimination

This paper follows on from an earlier companion paper (McLaren & Mackintosh, 2000), in which we further developed the elemental associative theory put forward in McLaren, Kaye, and Mackintosh (1989). Here, we begin by explicating the idea that stimuli can be represented as patterns of activation distributed across a set of units and that different stimuli activate partially overlapping sets (the degree of overlap being proportional to the similarity of the stimuli). A consequence of this view is that the overall level of activity of some of the units representing a stimulus may be dependent on the nature of the other stimuli present at the same time. This allows an elemental analysis in which provision for the representation of configurations of stimuli is made. A selective review of studies of generalization and discrimination learning, including peak shift, transfer along a continuum, configural discrimination, and summation, suggests that the principles embodied in this class of theory deserve careful consideration and will form part of any successful model of associative learning in humans or animals. There are some phenomena that require an elemental/associative explanation.

Blocking in the Spatial Domain

An initial series of experiments with rats in a swimming pool established that they could find a hidden platform the location of which was defined in terms of 3 or 4 landmarks and that, when trained with all 4, any subset of 3 (or even, after a sufficient number of swimming trials, 2) landmarks was sufficient to produce accurate performance. When only one landmark was present during testing, however, performance fell to chance. Two additional experiments demonstrated a significant blocking effect: If rats were first trained to locate the platform with 3 landmarks, they did not learn to use a 4th landmark added to their initial set of 3.

Transfer of Relational Rules in Matching and Oddity Learning by Pigeons and Corvids

Three experiments compared the performance of pigeons and corvids when they were given the opportunity to transfer the relational rule underlying matching or oddity discriminations to new sets of stimuli. In the first, pigeons and jackdaws were initially trained either on a matching or on a non-relational conditional discrimination and then transferred to a new matching discrimination. In the second, pigeons and jays were trained on a series of three matching (or oddity) discriminations with three different pairs of colours and finally tested, either with the same or the reversed rule, on matching or oddity to line orientations. In the third, pigeons and rooks were trained to perform one response when two coloured panels were the same and a different response when the two colours were different and then transferred, either with the same or the reversed rule, to a new set of colour stimuli. All three experiments produced the same result: no evidence of transfer of the relational rule by pigeons, but substantial and significant transfer by corvids.

Successful overshadowing and blocking in hippocampectomized rats

Overshadowing (Experiment 1) and blocking (Experiment 2) were investigated using a conditioned suppression paradigm in rats. Neither hippocampectomy nor cortical control lesions affected the extent to which a salient stimulus overshadowed a less salient one. Nor did the lesions affect the extent to which a stimulus that was highly correlated with shock overshadow a stimulus that was less well correlated with shock. Finally, the lesions did not alter the extent to which a previously reinforced stimulus blocked conditioning to another stimulus when both were presented as a reinforced compound stimulus. It is thus possible for hippocampectomized rats to show apparently normal overshadowing and blocking, at least under some testing conditions.

Excitatory and inhibitory conditioning following uncorrelated presentations of CS and UCS

In two experiments on conditioned licking in rats, we examined the effects of preexposure to a tone alone or to uncorrelated presentations of the tone and water on subsequent conditioning. Both treatments retarded excitatory conditioning in Experiment 1 and inhibitory conditioning in Experiment 2. In both cases, the uncorrelated procedure caused more severe retardation.

An analysis of overshadowing and blocking

In classical aversive conditioning experiments, rats do not always learn about all aspects of a compound stimulus predicting shock. A strong stimulus may overshadow a weaker one; and pretraining on one component may block learning about a second component. These results have been explained either by appealing to a notion of selective attention, or by assuming that learning about one component is a function of prior response strength to the entire compound of which it forms a part. In Experiment I, overshadowing was demonstrated on the first trial of conditioning, i.e. before either component had acquired any response strength. In Experiment II, pretraining on one component resulted in complete failure to learn about a second component during compound training, but did not prevent additional learning about the first component. Both results were interpreted as supporting an attentional analysis of blocking and overshadowing.

Neurobiology, psychology and habituation

Abstract Habituation, the decline in response to a repeatedly presented stimulus, is found in a very wide variety of animals, and recent neurobiological analysis by Kandel and his colleagues has started to reveal the cellular changes responsible for these changes in behaviour, at least in invertebrates. But such analysis rests on a psychological theory which attributes habituation to a decline in the efficiency of transmission at some point in the pathway between stimulus and response. Such a theory seems to contrast with a comparator theory of habituation, first proposed by Sokolov and now accepted by many psychologists, according to which incoming stimuli are compared to the contents of memory and if they match are denied access to some further point in the system. The most detailed version of this theory is that proposed by Wagner, but the behavioural evidence thought to support Wagners theory is found inadequate to do so, and a number of experiments directly contradict it. The only consistent support to be found for Wagners account is that long-term habituation to biologically significant or potent stimuli (the development of tolerance to drugs is the best example) depends on an association between such a stimulus and the context in which it is repeatedly presented. This set of findings is best explained by supposing that long-term habituation to such potent stimuli, but not other instances of habituation, involves the conditioning to contextual stimuli of an opponent process or compensatory reaction which counteracts the initial responses elicited by the stimulus. This proposal is incorporated by Wagner into later versions of his theory, but is logically distinct from comparator theory. There is no good evidence to support a comparator theory of habituation, either in its general form or in Wagners earlier version of it, and the only unique prediction which might be derived from this general class of theory is that habituation involves the ability to discriminate novel from familiar stimuli. Since the prediction is surely disconfirmed, comparator theory is either incorrect or no more than a redescription, in more fanciful language, of the stimulus-response theory of habituation.