The latency operating characteristic. II. Effects of visual stimulus intensity on choice reaction time.

Abstract

The purpose of this experiment was to study the effect of stimulus intensity on the operating (LOC), which is the trade-off relation between speed and accuracy. The stimuli were presented at three intensities that differed by equal intervals on a logarithmic scale. Both the slope and intercept of a linear relation between reaction time (RT) and discrimination accuracy (&') were found to vary with stimulus intensity: As intensity increased, the slope of the LOC increased and the intercept decreased. The greatest differences were between the lower two intensities. The findings contribute discriminant validity to the LOC as a measure of a perceptual process. The latency operating characteristic (LOC) refers to the functional relation between RT and discrimination accuracy. The usefulness of this measure lies in its potential as a measure of the perceptual process that remains invariant under changes in decision strategies that determine when the response to a given percept will be emitted. Our initial report (Lappin & Disch, 1972) provided evidence for the convergent validity of the LOC by showing its invariance under changes in stimulus and response probability. The LOC was described by the slope and intercept of a straight line relating reaction time (RT) and d' (or one of several alternative measures of discrimination accuracy). The purpose of the present experiment is to study the discriminant validity of the LOC—by determining the effects of stimulus intensity upon the slope and intercept of the LOC. If the LOC is in fact a measure of the perceptual process, then the parameters of the LOC should vary as a function of stimulus intensity. An inverse relation between stimulus intensity and RT has been repeatedly demonstrated, but the details of the relationship are uncertain: Decisional processes appear