Separate Speech and Nonspeech Processing in Dichotic Listening

Abstract

Temporal order judgment (TOJ) in dichotic listening can be a difficult task. Previous experiments that used two speech stimuli on each trial (S/S) obtained sizable error rates when subjects were required to report which ear led (TOJ-by-ear). When subjects were required to identify the leading stimulus (TO J-by-stimulus), the error rate increased substantially. Apparently, the two speech stimuli were competing for analysis by the same processor, and so were overloading it. The present experiment used the same TOJ tasks, but presented a speech and a nonspeech stimulus on each trial (S/NS). The error rate was comparable to that of S/S for TO J-by-ear, but did not increase for TO J-by-stimulus. This would be expected if the speech and nonspeech stimuli are being sent to different processors, each of which performs its analysis without interference from the other. The interpretation of the data given here is consistent with the results of standard identification experiments reported elsewhere: when asked to identify both stimuli on each dichotic trial, subjects made many errors on S/S, while performance was virtually error-free on S/NS.