Intervening stimulus effects on category judgments of duration

Auditory pulses (1,000 Hz) occurring between the 500-Hz bounding markers of durations being judged were varied in their position and number over three experiments which examined the effects of these factors on measures of amount of filled-duration illusion (FDI) and interduration discriminability. The results establish that the locus of the FDI is postattentive, since some of the incoming sensory information, selected on the basis of simple physical cues, can be excluded from further processing. FDI decreases both when the intervening stimuli come from a different source (earphone channel) to the marker stimuli and when they appear as part of an ongoing sequence of background pulses. Discrimination of durations containing intervening stimuli is enhanced if the filler events are placed such that repetitions of a given subinterval result. Thomas and Brown’s (1974) “chunking” model can account for the observed effects with the additional assumption that the error associated with encoding a given subinterval is sequentially modified as a function of number of recent encodings.