Participants were exposed to a temporal generalization task where the duration of a small visual stimulus was judged. People received a 600ms standard duration, then had to judge whether other durations (longer than, shorter than, or equal to the standard) were or were not the standard (making a YES or NO response). In different experimental conditions, the spacing of non-standard durations around the standard was 150ms (Easy condition), or 75ms (Difficult condition), so the two conditions involved some judgements made with the same stimuli (450, 600, and 750ms). The experiment thus compared judgements of the same physical stimuli, when the basis of the judgement was the same, thus avoiding some problems of control that have been present in earlier electrophysiological studies of time judgements. As in previous work, fewer YES responses occurred in the Difficult condition and the 450ms duration was less confused with the 600ms standard than the 750ms one was. Computer modelling suggested that this (fewer YES responses) was due to a decrease in the decision threshold for the YES judgement. The electrophysiological results showed a distinction between the Easy and Difficult conditions observable by a change in the LCPt (Late Positive Component of Timing) measured after the stimulus presentations and by a change in the P1, the CNV (Contingent Negative Variation), and its positive counterpart during the presentation of the stimulus, which were larger when the discrimination was difficult. Our results therefore suggest that the increase in the difficulty of the generalization task not only changes decision processes but also alters attentional mechanisms. They also reveal that the decision does not seem to involve a unitary mechanism but depends on a group of sub-processes, notably attentional mechanisms which are altered from the moment of presentation of the stimulus.
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