Perceptual Indices of Performance: The Measurement of ‘Inspection Time’ and ‘Noise’ in the Visual System

Indices of human performance are less clear-cut than analogous measures in the physical sciences for several reasons. Variables which can alter performance include the noise upon which sensory signals are imposed, the bias shown by an observer towards one or another alternative, the use of an optional-stopping strategy for examining sensory input, and the accumulation of statistical information over time concerning the source of the input. The development of a theoretical appreciation of these variables is traced and, on this basis, a measure of the noise, which sets a limit to discriminative capacity, is suggested. The proposed index is simply the standard deviation of the best fitting normal ogive, calculated for the psychometric function obtained in a forced-choice discrimination task by the method of constant stimuli, the discriminanda being presented for 100 ms, in random order, and followed by appropriate backward masking. The index is thus closely related to traditional psychophysical measures, but is distinguished by the detailed specification of conditions under which it should be obtained. Analysis of data from previous experiments gives some indication of the order of magnitude that might be expected from a carefully controlled determination of the measure. In addition, three experiments were carried out to evaluate this suggestion, and to test its underlying rationale. In the first two, observers were required to discriminate, by pressing one of two keys, between two lines of markedly different length, exposed in random order for ten different durations. In the third, stimulus exposure was held constant, while ten different stimulus differences were presented in random order. Results from the first two experiments yielded estimates of minimum inspection time close to 100 ms, and were inconsistent with the view that observers can abandon an optional-stopping procedure of processing sensory information in favour of responding by a deadline. Measures of noise calculated in the third experiment were of the order expected on the proposed rationale, while response latencies were again inconsistent with the notion of deadline responding. Further analysis of the results of the three experiments suggests that measures of inspection time and noise, together with a third index related to the degree of caution exercised by an observer, appear to be stable and consistent descriptors of performance. The wider implications of the successful use of this kind of perceptual index of performance are discussed with reference to the measurement of visual acuity, as a means for detecting effects of environmental stress, and as a conceptual framework for the understanding of individual differences.