We know that when people make responses which they did not intend they can discover this by monitoring kinaesthetic and visual feedback. It is less clear whether they can also correct perceptual errors which occur when they mistake one signal for another. It was argued that, if they can sometimes do this,extra errors which occur when discriminations become more difficult may be detected and corrected. Experiment I compared the ability of young fit subjects to detect errors made during easy and during difficult discriminations between tone signals. There was no evidence that any additional errors made to difficult discriminations were detected. Fast errors were detected, slower errors were not. The results were consistent with the idea that subjects can detect fast motor errors by monitoring feedback, but that they cannot detect perceptual mistakes.

In Experiment II subjects made easy and difficult line length discriminations. Displays lasted for 100 ms, 200 ms or 500ms and were followed by random masks. In this case, fast errors were again corrected more frequently than slow errors but eight aspects of the data suggest that subjects could, and did, correct perceptual as well as motor mistakes and that they managed to do this by continuing to process a display after the moment at which they may have initiated an impulsive response to it. The data are interpreted in terms of a “Committee Decision” model for extended perceptual processing previously applied to other, similar data by Rabbitt and Rodgers (1977) and Rabbitt, Cumming and Vyas (1978).