Investigation of the processing of ambiguous melodies in the brain by behavioral and neurophysiological experiments

Abstract

Melody recognition is not a mere succession of pitch and time perception but must be an active process to a make a meaningful line of tones. To investigate this process, we devised a form of illusory melody resembling but different from the ones used in the studies of streaming. A4 and E5 tones are continuously presented while another tone M (middle tone, for example, C5 or C#5) between the two continuous tones is presented intermittently (0.25 s presentation and 0.25 s rest and repeating). One may hear either an illusory continuous melody of repeated ME5 or MA4 depending on the preceding unambiguous phrase of repetition of ME5 or MA4. Assuming that this illusion reflects the active process of melody formation, we investigated 1) the transition time from the primary state (either one of the melody forms) fixed by the leading phrase behaviorally, and 2) possible neural correlates by measuring the MEG (magnetoencephalography) responses to the continuous tones separately by amplitude modulating them with different frequencies. MEG signal was filtered for the two modulation frequencies and the ASSR (auditory steady state responses) were obtained for different experimental conditions. The results so far showed wide variance among subjects but seem promising.