Temporal brightness enhancement studied with a large sample of observers: Evidence for individual differences in brightness perception

Brightness vs. duration relations were measured for 80 naive observers using a method in which a short pulse (10–500 msec) was compared to a pulse 500 msec longer in duration at the same luminance. Pulses were presented under two conditions of pulse asynchrony: simultaneous onset and simultaneous offset. Three classes of observers were found: Type A observers (57% of all observers) showed temporal brightness enhancement (the Broca-Sulzer effect) for both simultaneous onset and offset conditions. Type B observers (33% of the sample) showed brightness enhancement for simultaneous offset but not onset. Type C observers (10% of the sample) did not exhibit brightness enchancement under either asynchrony condition. The distribution of these classes was not affected by the specificity of instructions concerning the brightness judgment or by the sequencing (ordered vs. random) of stimuli. We analyze how these classes of observers might be explained either on the basis of differing neurophysiological mechanisms or responses or on the basis of differing psychophysical criteria. Further, we present a model of the data which illustrates how behaviorally distinct types of observers could be generated by continuous distributions within the general population of either perceptual criteria or neurophysiological responses.