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.

Substantial evidence suggests that sensitivity to the difference between the major vs minor musical scales may be bimodally distributed. Much of this evidence comes from experiments using the “3-task.” On each trial in the 3-task, the listener hears a rapid, random sequence of tones containing equal numbers of notes of either a G major or G minor triad and strives (with feedback) to judge which type of “tone-scramble” it was. This study asks whether the bimodal distribution in 3-task performance is due to variation (across listeners) in sensitivity to differences in pitch. On each trial in a “pitch-difference task,” the listener hears two tones and judges whether the second tone is higher or lower than the first. When the first tone is roved (rather than fixed throughout the task), performance varies dramatically across listeners with median threshold approximately equal to a quarter-tone. Strikingly, nearly all listeners with thresholds higher than a quarter-tone performed near chance in the 3-task. Across listeners with thresholds below a quarter-tone, 3-task performance was uniformly distributed from chance to ceiling; thus, the large, lower mode of the distribution in 3-task performance is produced mainly by listeners with roved pitch-difference thresholds greater than a quarter-tone.