The Piéron function in the threshold region.

The Piéron function (Piéron, 1914, 1920, 1952) describes the decay of reaction time (RT) when the intensity of the stimulus is increased. It is generally demonstrated within a suprathreshold range of intensities. However, in some studies, for the lowest range of intensities, the exponent of the function is clearly greater than that for the upper ranges of intensities. Such an increase in the exponent for the lowest intensities is assumed to result from a combined effect of stimulus intensity and of stimulis uncertainty in detection. Our first experiment used luminance levels that covered all the scotopic range and a spatial two-alternative forced-choice task in which both accuracy and RT were measured. It demonstrated a drastic increase in the exponent in the Piéron function when the intensities reached the threshold region. Since the estimates of the threshold region may have been biased by the use of a much larger range of luminances, a second experiment was conducted using luminances that covered only the threshold region. This experiment confirmed the previous estimates for the threshold region.