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Biomedical subjects

D R Shanks

Publications and source records attributed to D R Shanks.

6 recordsLinked to original sources
PubMed1992

Evidence for a distinction between judged and perceived causality.

Two experiments investigated Michotte's launch event, in which successive motion of two objects appears to evoke an immediate perception that the first motion caused the second, as in a collision. Launching was embedded in event sequences where a third event (a colour change of the second object) was established as a competing predictor of the second motion, in an attempt to see whether subjects' learning of alternative predictive relationships would influence their causal impressions of launch events. In Experiment 1 subjects saw launch events in which temporal contiguity at the point of impact was varied so that an impact itself did not reliably predict when the second object would move. Half of these scenes, however, contained a colour change of the second object which did reliably predict when it would move. In accordance with Michotte's theory, subjects' ratings of the degree of perceived causality were not affected by the colour change. In Experiment 2 subjects saw scenes that contained launch events with or without temporal contiguity and a colour change. These were interspersed with events in which a colour change alone did or did not precede the second motion. Thus, movement of the second object was either contingent on or independent of the impact. Subjects repeatedly (a) rated perceived causality in single launch events and (b) judged the necessity of collisions for movement in the overall set of events. These responses dissociated, in that ratings of type (a) showed only a substantial contiguity effect, whereas judgements of type (b) showed both a contingency and a much smaller contiguity effect. These results appear to support a distinction between judged and perceived causality and are discussed with respect to Michotte's theory of direct causal perception.

Adult↗
PubMed1991

Instrumental judgment and performance under variations in action-outcome contingency and contiguity.

The intentional theory of instrumental performance proposes that performance of an action is determined in part by a belief about its causal effectiveness in producing a desired outcome. At variance with this notion, previous implicit learning experiments appear to have yielded dissociations between subjects' performance and beliefs. In two experiments, subjects were given an opportunity to perform an action--pressing a key on a computer keyboard--which was associated with an outcome on the computer screen according to a free-operant contingency. The subjects in one group were asked to judge the effectiveness of the action in causing the outcome, while those in a second group were asked to maximize their points score under a payoff schedule. In the first study, the effect of varying the contingency between the action and outcome was examined by keeping the probability of an outcome contiguous with an action constant and varying the probability of an outcome in the absence of an action. Performance and judgments showed a comparable sensitivity to variations of the instrumental contingency. In the second study, the delay between the action and the resultant outcome was varied. Increasing the action-outcome delay from 0 sec up to 4 sec produced a systematic decline in both causal judgments and performance relative to noncontingent, control conditions. These results are in accord with the intentional theory of performance, but they present difficulties for the notion of implicit learning.

Adult↗
PubMed1990

On the cognitive theory of conditioning .

Furedy (1988) has advanced several arguments which, he considers, produce difficulties for a cognitive theory of Pavlovian conditioning. In this paper I first discuss some of the reasons why a cognitive theory must be couched in the language of intentionality. I then argue that it is extremely difficult to see how a cognitive or intentional theory of Pavlovian conditioning can be constructed. Instead, the evidence points to the view that Pavlovian conditioning consists of the learning of excitatory or inhibitory links between the mental images of the conditioned and unconditioned stimuli. Finally, Furedy introduces considerable confusion into his account by failing to preserve an absolutely crucial distinction: between the mechanism of learning and the nature of the knowledge that is acquired during learning.

Animals↗
PubMed1990

Connectionism and the learning of probabilistic concepts.

Three experiments examine the claim of Gluck and Bower (1986, 1988a, 1988b) that the learning of medical concepts can be simulated by a connectionist network in which the symptoms are the input and the diagnosis is the output. The first experiment replicates the main finding of Gluck and Bower. In this experiment, subjects were required to estimate the probability of each of two diseases, given a particular target symptom. In fact these two probabilities were identical, but because one illness was more common than the other, the target symptom was a better predictor of the rare disease than of the common disease. Contrary to a normative probability judgement account, subjects were biased in that they judged the probability of the rare disease given the target symptom to be greater than the probability of the common disease given the target symptom. Gluck and Bower argued that such a result was predicted by a connectionist network using the Rescorla-Wagner learning rule, but it is argued that Gluck and Bower's network simulation was not appropriate for the experiment they had performed. In fact, it appeared that the connectionist network failed to predict the bias in the subjects' probability estimates. However, this conclusion rests on an assumption that Gluck and Bower implicitly made. They arranged for P(rare disease/target symptom) and P(common disease/target symptom) to be identical across all trials on which the target symptom occurred, both on its own and with other symptoms present. Gluck and Bower assumed that the subjects were estimating these probabilities. But the results of the second experiment showed instead that the subjects were estimating the probability of each disease given only the target symptom. In the final experiment the design was changed so that this problem might be circumvented. In this experiment, again, the subjects were biased in their probability judgements exactly as the connectionist network predicted. Thus, finally, evidence was found which was compatible with the network model but not with a normative account, but this was true only if the network did not include a layer of hidden units.

Adult↗
PubMed1989

Selectional processes in causality judgment.

Two experiments illustrate the way in which competition between potential causes occurs when subjects are asked to judge the extent to which an action is the cause of an outcome. In the first experiment, it was found that introducing occurrences of the outcome in the absence of the action reduced causality judgments, but this effect was attenuated if these outcomes were signaled by another stimulus. In the second experiment, a delay between the action and the outcome reduced judgments, but this could be abolished by inserting a stimulus between the action and the outcome. The results are discussed in terms of a view of causality judgment that assumes that such judgments are based on associations between the mental representations of the action and the outcome.

Association Learning↗