THE DETECTION OF ANISOCHRONY IN MONAURAL AND INTERAURAL SOUND SEQUENCES

Nakao and Axelrod (1976) and van Noorden (1975) showed that the thresh old for discriminating an anisochronous duple rhythm (a series of clic ks with a temporal offset on every other one) from an isochronous rhyt hm (no offset) is poorer when the clicks are presented alternately to the two ears than when they are presented to the same ears. Van Noorde n reported that the difference between the thresholds in the alternati ng and nonalternating conditions varied with the tempo of the sequence . Nakao and Axelrod found invariance of this threshold difference with sequence speed. According to our quantification of temporal processin g of interaural sequences, the latter result should be expected. We ca rried out five psychophysical experiments to establish interaural and monaural discrimination between isochronous and anisochronous rhythms. Across experiments, base time intervals of 60-720 msec were spanned. The main result was that we replicated the poorer discrimination for i nteraural sequences. This deterioration in discrimination was the same for all sequence speeds. It was also the case that the thresholds wer e almost constant up to a sound repetition rate of about 3 per second, but increased linearly with slower rates. This result supports eviden ce in the literature that temporal processing of sequences faster than about 3-4 sounds per second differs from temporal processing of slowe r sequences.