How rests and cyclic sequences influence performance in tone-scramble tasks

When classifying major versus minor tone-scrambles (random sequences of pure tones), most listeners (70%) perform at chance while the remaining listeners perform nearly perfectly. The current study investigated whether inserting rests and cyclic sequences into the stimuli could heighten sensitivity in such tasks. In separate blocks, listeners classified tone-scramble variants as major versus minor ("3" task) or fourth versus tritone ("4" task). In three "Fast" variants, tones were played at 65 ms/tone as a continuous, random stream ("FR"), or with a rest after every fourth tone ("FRwR"), or as a repeating sequence of four tones with a rest after every fourth tone ("FCwR"). In the "Slow" variant, tones were played at 325 ms/tone in random order. In both the 3 and 4 tasks, performance was ordered from best to worst as follows: FRwR > FR > FCwR > Slow. Post hoc analysis revealed that performance was suppressed in the Slow and FCwR task-variants due to a powerful bias inclining listeners to respond "major" or "fourth" ("minor" or "tritone") if the 4-note sequence defining the stimulus ended on a high (low) note. Overall, the results indicate that inserting regular rests into random tone sequences heightens sensitivity to musical mode.