Sequential Expectancies and Decision Making in a Changing Environment: An Electro Physiological Approach

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Event‐related potentials, P300, Slow Wave, Subjective probability, Decision making

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This experiment addressed two questions. First, we sought to determine how sequential expectancies, as indexed by the amplitude of P300, are formulated when subjects are presented with a series in which the event probabilities are continuously changing. Second, we wanted to determine the effect on P300 amplitude of seeking information prior to a decision when the decisions are required at random intervals. Eight subjects were presented with a Bernoulli series of high and low tones in which the event probabilities reversed at random intervals. Thus, while the event probabilities were either .33 or .67 over short segments of the series, they were .50 over each trial block. The subject's task was to count the number of low‐pitched tones. In two separate conditions (“Unknown” and “Known”) the subjects received different information about the series. In the Unknown condition, the subjects were not informed of the reversals in stimulus probability and were only told that the events were equally probable. In the Known condition, the subjects were informed of the probability reversals and, in addition to the counting task, were asked to detect the probability reversals.

The trials were divided into two categories: “transition” and “stable.” The transition trials were those that occurred between the actual probability reversal and the subject's reported detections. The stable trials were all those not included in the transition category. The data from the stable trials revealed that, in most cases, subjects generated their expectancies, as indexed by the amplitude of P300, in an identical manner during both conditions. Accurate knowledge of the stimulus probabilities did, however, influence the amplitude of P300 for events that were preceded by sequences of stimuli that occur only rarely at each level of probability. These data suggest that the mechanism responsible for generating sequential expectancies is largely automatic. Certain aspects of this mechanism are, however, modulated by the subject's knowledge of the parameters of the series. The data from the transition trials revealed that, in the Unknown condition, the subjects’ assignments of subjective probabilities, as indexed by the amplitude of P300, adapted rapidly to meet the new sequence‐generating rules following the probability transitions. In contrast, during the Known condition, the amplitude of the P300 and Slow Wave components steadily increased, as the decision point approached, to levels well above those in the Unknown condition. By the first trial following the decision, this enhancement in P300 and Slow Wave activity was no longer evident. These results are discussed in terms of a descriptive model of the subjects’ decision making behavior.

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Citation / Publisher Attribution

Psychophysiology, v. 19, issue 2, p. 183-200