Systematic timing variations observed during music performance have usually been attributed to a musical expression hypothesis, related to relatively high-level processes, by which musicians emphasize certain events in order to transmit a particular musical interpretation to the listener. We propose, in addition, a perceptual hypothesis: related to lower-level professes, in which some observed variations would be related to functional constraints of the auditory system. (Some intervals would be heard shorter and thus played longer as a phenomenon of perceptual compensation.) We present a psychological model of temporal organization proposing two types of process (regularity extraction and segmentation into groups) operating parallel that allow listeners to parse complex auditory sequences such as music. Each type of process operates at both a low processing level (beat extraction and segmentation into basic groups) and a higher processing level (hierarchical metric organization and hierarchical segmentation organization). The analysis of musical and mechanical performances of Schumann's Traumerei demonstrated performance variations in relation to both hierarchical segmentation and hierarchical metric organizations, and to rhythmic groups. Variations were not systematically observed in relation to melodic groups. Regression analyses quantified these effects and demonstrated that hierarchical segmentation and rhythmic groups accounted for approximately 60% of the variance for musical and mechanical performances, leaving room for the description of other, as yet unidentified, processes. The percentage of variance explained by high-level processes (hierarchical segmentation) decreased from musical to mechanical performances, whereas the percentage of variance explained by lower-level processes (rhythmic groups)increased. We conclude that it is important to go beyond the traditional approach of describing performance variations in relation to musical structure and to adopt the approach of studying performance variations in relation to the psychological processes that allow the musician to perceive the musical structure. Finally, we adapt the psychological model of temporal organization to expressive timing: similar psychological processes operate at multiple hierarchical levels - namely, those of segmentation and grouping -, and these similar processes result in the same pattern of performance variations (an accelerando/ritardando profile).
