Processing of tonal relatedness

In addition, the negativity observed between 280 and 400 ms on final tones tended to be larger in the right-anterior region for in-tune subdominants than for in-tune tonics, and this tendency was also observed between 300 and 420 ms on penultimate tones. This tendency is reminiscent of the RATN observed by Patel (1998), which was a larger right-anterior negativity for out-of-key chords than for in-key chords. It is also reminiscent of the ERAN that Koelsch et al. (2000, 2002, 2007, 2008) observed and interpreted as reflecting syntactic processes. This tendency suggests that two ERPs could have overlapped in the 280-400 ms range: a N2b related to the detection of pitch deviations, and a RATN/ERAN-like component that would index the processing of tonal relatedness. These two components might have been confounded in the ERPs studies that have used strong violations, which were both tonal and acoustic violations (Koelsch et al., 2000, 2002; Patel, 1998).

On penultimate tones, tonal relatedness was also associated with a larger positivity after 600 ms (larger positivity for tonally related tonics than for less tonally related subdominants). This positivity cannot be interpreted in the same way than the P300 elicited by the final tones since the N2b-P3 complex had a larger amplitude for subdominants than for tonics. A late positive component was found to be associated with memory processes in both visual and auditory modality. In the visual modality, larger late positivities have been observed in visual memory tasks for higher recallers than for lower recallers (Dunn BR, Dunn DA, Languis, & Andrews, 1998). In the auditory modality, a positivity between 300 and 500 ms was associated with online processing and maintenance of pitch (and location) in working memory (Alain, Arnott, Hevenor, Graham, & Grady, 2001). This might plead for the observed P600 to reflect working memory processes, with an advantage for tonics over subdominants. This memory interpretation is congruent with behavioral data in music cognition showing that the tonic is more stable in memory than tones of other tonal degrees (Krumhansl, 1979). The found laterality for this wave (larger amplitude for the right hemisphere) is congruent with imaging studies associating pitch memory with right cortical areas (Zatorre, Evans, & Meyer, 1994).

Memory processes may be also reflected in the observed larger right-anterior P2 for tonally related tonics than for less related subdominants. Studies in the visual modality suggest that, as the late positive component, the P2 may be associated with memory processes. Dunn et al. (1998) suggested that P2 amplitude might reflect retrieval of semantic information from long-term memory into working memory. Also, Chapman RM, McCrary, and Chapman JA (1978) observed in a letter or number comparison task that the amplitude of P250 predicted recall performance, and interpreted this wave as a short-term memory storage component. This suggests that the larger P2 observed for tonics in our study might reflect a facilitation of pitch encoding and storage in short-term memory (like the observed late positive component).

P2 components have also been associated with attentional processes. In a dichotic listening study, Woldorff and Hillyard (1991) asked participants to focus on fixed-pitch tones presented in one ear while ignoring tones of another pitch presented in the opposite ear. In both ears, tones could be either frequent “standard” tones or infrequent (9%) tones of lesser intensity. Attended infrequent tones were found to elicit a larger centroparietal P2 than unattended infrequent tones. This raises the possibility that the P2 observed in our study might also reflect the greater expectedness of tonics over subdominants.

The larger anterior P1 observed in the present study for tonics over subdominants is also reminiscent of the results of Woldorff and Hillyard (1991). In their study, an enhanced centrofrontal positivity between 20 and 50 ms was observed for attended tones over unattended tones. The authors links the occurrence of such an early attentional effect to the high attentional load of their experiment (fast presentation rate, difficult task) and the use of different pitch for both ears which made easy to focus attention on the requested ear. The finding of such an early attentional effect was interpreted as strong evidence for the theory of early selection, which hypotheses the existence of a filtering or gain mechanism allowing sensory input to be selected before the completion of perceptual analysis (Woldorff, 1999). Interestingly, modulation of an early P1 component was also found in a study on temporal attention in the visual modality (Correa, Lupiánez, Madrid, & Tudela, 2006), thus suggesting that early selection may be a cross-modal attentional mechanism triggered by different top-down processes of different modalities. The observed P1 in our study is in agreement with these findings and strongly suggests that top-down mechanisms elicited by listeners’ tonal knowledge modulates pitch processing at early attentional levels. Related and less-related penultimate tones were acoustically identical but, for related tones, the match between listener’s expected pitch and the actual pitch might have resulted in stronger early selection processes. Evidence for early attentional effects in perception of tonality has been provided by Krohn, Brattico, Välimäki and Tervaniemi (2006), who observed that the importance in tonal hierarchy of infrequent tones presented in an oddball paradigm (where the standard was the tonic) was reflected in the amplitude of N1. In our study, the comparison task may have highly focused listeners' attention on the pitch of penultimate tones, resulting in an even earlier attentional effect.

In conclusion, the present study deepens our previous finding suggesting that top-down processes linked to tonal relatedness influences pitch processing at perceptual (as opposed to decisional) levels (Marmel et al., 2008). In the present study, participants’ attention was directed to the pitch dimension with a pitch discrimination task, and tonal relatedness was found to modulate early processing of the penultimate tones’ pitch, possibly by enhancing early attentive selection and encoding in working memory for expected tones. The repeated “penultimate-final tones” design allowed us to investigate these influences independently of task-related decisional processes, which were requested on final tones only. These decisional processes elicited a N2b-P3 complex for deviant mistuned tones, but the negative component of this complex was modulated by tonal relatedness for the in-tune tones. This suggests that a ERAN–like component, linked to processing of tonal relatedness, may have overlapped on this time window with the N2b component.

This study is in line with recent electrophysiological studies that have highlighted the influence of musical expertise on pitch processing by showing that musicians had better pitch encoding than non-musicians at the brainstem level (Wong, Skoe, Russo, Dees, & Kraus, 2007; Musacchia, Sams, Skoe, & Kraus, 2007). However, the present study did not focus on the musical expertise differentiating musicians from non-musicians but on the implicit tonal knowledge shared by both musicians and non-musician that makes these both groups “musical experts” (Bigand & Poulin-Charronnat, 2006). The present study showed an influence of this unconscious expertise on early cortical processes, and thus raises the question of its possible influence on even earlier subcortical processes.