Biomedical subjects

Chris J Mitchell

Publications and source records attributed to Chris J Mitchell.

5 recordsLinked to original sources

PubMed2006

Forward blocking in human learning sometimes reflects the failure to encode a cue-outcome relationship.

In two "allergist" causal judgement experiments, participants were trained with a blocking design (A+|AB+). The procedure allowed different food cues to be paired with different fictitious allergic reactions. On test, participants were asked to rate the causal efficacy of the target cues and to recall the particular allergic reaction (outcome) that had followed each cue during training. Forward blocking was observed on the causal judgement measure and on the outcome recall measure in both Experiment 1 and Experiment 2. A backward blocking contingency was also trained in Experiment 2 (AB+|A+). Backward blocking was not observed either on the causal judgement or on the outcome recall measure. The evidence from the recall measure suggests that forward blocking in this task results from a failure to encode the B-outcome relationship during training. Associative and nonassociative mechanisms of forward blocking are discussed.

Association↗

PubMed2005

Inference-based retrospective revaluation in human causal judgments requires knowledge of within-compound relationships.

In an allergist causal-judgment task, food compounds were followed by an allergic reaction (e.g., AB+), and then 1 cue (A) was revalued. Experiment 1, in which participants who were instructed that whatever was true about one element of a causal compound was also true of the other, showed a reverse of the standard retrospective revaluation effect. That is, ratings of B were higher when A was causal (A+) than when A was safe (A-). This effect was taken to reflect inferential reasoning, not an associative mechanism. In Experiment 2, within-compound associations were found to be necessary to produce this inference-based revaluation. Therefore, evidence that within-compound associations are necessary for retrospective revaluation is consistent with the inferential account of causal judgments.

Allergens↗

PubMed2005

A dissociation between causal judgment and outcome recall.

It has been suggested that causal learning in humans is similar to Pavlovian conditioning in animals. According to this view, judgments of cause reflect the degree to which an association exists between the cause and the effect. Inferential accounts, by contrast, suggest that causal judgments are reasoning based rather than associative in nature. We used a direct measure of associative strength, identification of the outcome with which a cause was paired (cued recall), to see whether associative strength translated directly into causal ratings. Causal compounds AB+ and CD+ were intermixed withA+ and C- training. Cued-recall performance was better for cue B than for cue D; thus, associative strength was inherited by cue B from the strongly associated cue A (augmentation). However, the reverse was observed on the causal judgment measure: Cue B was judged to be less causal than D (cue competition). These results support an inferential over an associative account of causal judgments.

Association↗

PubMed2003

Forward and backward blocking of causal judgment is enhanced by additivity of effect magnitude.

When two causes for a given effect are simultaneously presented, it is natural to expect an effect of greater magnitude. However many laboratory tasks preclude such an additivity rule by imposing a ceiling on effect magnitude-for example, by using a binary outcome. Under these conditions, a compound of two causal cues cannot be distinguished from a compound of one causal cue and one noncausal cue. Two experiments tested the effect of additivity on cue competition. Significant but weak forward blocking and no backward blocking were observed in a conventional "allergy" causal judgment task. Explicit pretraining of magnitude additivity produced strong and significant forward and backward blocking. Additivity pretraining was found to be unnecessary for another cue competition effect, release from overshadowing, which does not logically depend on additivity. The results confirm that blocking is constrained when effect magnitude is constrained and provide support for an inferential account of cue competition.

Adult↗

PubMed2002

Backward and forward blocking in human electrodermal conditioning: blocking requires an assumption of outcome additivity.

Blocking was observed in two human Pavlovian conditioning studies in which colour cues signalled shock. Both forward (Experiment 1) and backward (Experiment 2) blocking was demonstrated, but only when prior verbal and written instructions suggested that if two signals of shock (A+ and B+) were presented together, a double shock would result (AB++). In this case, participants could assume that the outcome magnitude was additive. Participants given non-additivity instructions (A+ and B+ combined would result in the same outcome, a single shock) failed to show blocking. Modifications required for associative models of learning, and normative statistical accounts of causal induction, to account for the impact of additivity instructions on the blocking effect, are discussed. It is argued that the blocking shown in the present experiments resulted from the operation, not of an error-correction learning rule, nor of a simple contingency detection mechanism, but of a more complex inferential process based on propositional knowledge. Consistent with the present data, blocking is a logical outcome of an A+/AB+ design only if participants can assume that outcomes will be additive.

Acoustic Stimulation↗