Abstract
Vickers et al. (1972) suggested that a useful index of noise in the visual system should be provided by a measure sD, defined as ‘the standard deviation of the best fitting normal ogive calculated for the psychometric function obtained in a forced-choice discrimination task using the method of constant stimuli, the discriminanda being presented for 100 ms, in random order, and followed by appropriate backward masking’. The sensitivity of sD to anxiety produced by painful electric shocks, arriving at unpredictable times, is examined. Results suggest that, although increased anxiety does not produce consistent or significant changes in heart rate or in degree of caution, it does cause a marked increase in noise.

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