Biomedical subjects

I P McLaren

Publications and source records attributed to I P McLaren.

9 recordsLinked to original sources

PubMed2001

Retrospective revaluation in humans: learning or memory?

The phenomenon of retrospective revaluation has posed considerable problems for many associative learning theories as it involves a change in the associative strength of a cue on trials on which that cue is absent. The present series of experiments pursues this idea of changes in associative strength between evoked representations of cues, in an effort to establish, de novo, an excitatory connection between two cues simultaneously activated in memory. Given the finding of Dwyer, Mackintosh, and Boakes (1998) that simultaneous activation of absent cues in the memory of rats resulted in learning comparable to that seen in retrospective revaluation, we expected that if retrospective revaluation was found in humans, then excitatory learning due to simultaneous activation would also be seen. This was not the case. The implications of our results are discussed in terms of Dickinson and Burke's (1996) modified SOP model and a version of McLaren's (1993) APECS network. We conclude that many of the effects attributed to learning in retrospective revaluation studies are better thought of as due to changes in the retrievability of items in memory.

Adult↗

PubMed2000

Tests of the ratio rule in categorization.

Many theories of learning and memory (e.g., connectionist, associative, rational, exemplar based) produce psychological magnitude terms as output (i.e., numbers representing the momentary level of some subjective property). Many theories assume that these numbers may be translated into choice probabilities via the ratio rule, also known as the choice axiom (Luce, 1959) or the constant-ratio rule (Clarke, 1957). We present two categorization experiments employing artificial, visual, prototype-structured stimuli constructed from 12 symbols positioned on a grid. The ratio rule is shown to be incorrect for these experiments, given the assumption that the magnitude terms for each category are univariate functions of the number of category-appropriate symbols contained in the presented stimulus. A connectionist winner-take-all model of categorical decision (Wills & McLaren, 1997) is shown to account for our data given the same assumption. The central feature underlying the success of this model is the assumption that categorical decisions are based on a Thurstonian choice process (Thurstone, 1927, Case V) whose noise distribution is not double exponential in form.

Adult↗

PubMed1998

Perceptual categorization: connectionist modelling and decision rules.

Although it is currently popular to model human associative learning using connectionist networks, the mechanism by which their output activations are converted to probabilities of response has received relatively little attention. Several possible models of this decision process are considered here, including a simple ratio rule, a simple difference rule, their exponential versions, and a winner-take-all network. Two categorization experiments that attempt to dissociate these models are reported. Analogues of the experiments were presented to a single-layer, feed-forward, delta-rule network. Only the exponential ratio rule and the winner-take-all architecture, acting on the networks' output activations that corresponded to responses available on test, were capable of fully predicting the mean response results. In addition, unlike the exponential ratio rule, the winner-take-all model has the potential to predict latencies. Further studies will be required to determine whether latencies produced under more stringent conditions conform to the model's predictions.

Adolescent↗

PubMed1998

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 groups in any way that seems reasonable or sensible to them. 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.

Adolescent↗

PubMed1997

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.

Animals↗

PubMed1997

Categorization and perceptual learning: an analogue of the face inversion effect.

This paper reports two experiments that investigate the extent to which it is plausible to suppose that an associatively based mechanism for perceptual learning acts as the basis for the effects of inversion on identification, recognition, matching and discrimination of faces (and certain other stimuli rendered familiar by expertise, e.g. gundogs). In the first experiment, an inversion effect that is contingent both on familiarity with a category and on the category possessing prototypical structure is demonstrated using discrimination learning of chequerboard stimuli. The second experiment demonstrates that the inversion effect found in Experiment 1 can generalize to a recognition paradigm as well. These results are discussed within the framework provided by associative learning theory, and a parallel is drawn with models employing a norm-based coding in similarity space. The conclusion is that it would be remarkable if the inversion effects demonstrated with the abstract categories used in the experiments reported here were not implicated in the inversion effects found with other classes of stimuli, whilst conceding that the analogy is not complete, particularly in the case of faces.

Adolescent↗

PubMed1994

Latent inhibition, context specificity, and context familiarity.

In two appetitive licking experiments, thirsty rats were pre-exposed on two stimuli prior to conditioning sessions in which one or both of these stimuli were paired with water. Consistent with other results, latent inhibition was disrupted when conditioning took place in a context different from that in which stimulus pre-exposure had occurred. But in animals given prior exposure to the context of stimulus pre-exposure before the start of stimulus pre-exposure, substantial and equivalent latent inhibition was evident whether or not there was a change of context between stimulus pre-exposure and conditioning. These results are discussed in terms of current theories of latent inhibition.

Animals↗

PubMed1994

Effects of pre-exposure to the same or different pattern of extra-maze cues on subsequent extra-maze discrimination.

In two experiments, rats learned a spatial discrimination between maze arms defined by their relationship to a variety of extra-maze cues. Prior exposure to the actual arms between which animals were required to discriminate tended to retard subsequent learning (by comparison with a control group either given no pre-exposure to the extra-maze cues or exposed only to arms pointing in the opposite direction), whereas prior exposure to arms intermediate between those used in discrimination training tended to facilitate subsequent learning. These results are consistent with the suggestion that pre-exposure will facilitate discrimination learning when it reduces the associability of features or elements common to the stimuli between which animals are required to discriminate, more than it reduces the associability of the features or elements unique to each.

Animals↗

PubMed1990

The conditioning connection.

In 1948 Konorski argued that conditioning reflects the strengthening of a connection between elements representing the signal and the reinforcer, as a result of the coincidence of activity in the signal element with a rise in activity in the reinforcer element. This Konorskian process represents one way of implementing an error-correcting learning rule and thus, unlike a simple Hebbian process, anticipates selective conditioning, such as that observed in Kamin's blocking procedure. However, the Konorskian process, in common with other error-correcting learning rules, fails to explain why blocking is attenuated by 'surprising' changes in reinforcement conditions that should not augment activity in the reinforcer element. Rather, the reinforcer-specificity of such unblocking suggests the operation of an associability process by which stored information about the past predictive history of the signal is expressed at the connection between signal and reinforcer elements to modulate changes in the connection weight.

Animals↗