Scalp potentials to pitch change in rapid tone sequences - A correlate of sequential stream segregation

The object of the study was to look for a neurophysiological substrate of s equential auditory stream segregation. When a sequence of tones alternates rapidly between pitches separated by more than a few semitones, there is a tendency for it to be perceived as two independent "streams". We examined t he scalp potentials evoked when the pitch interval abruptly changes, to see whether there are response parameters which might be correlated with sudde n stream segregation and/or integration. For 3 s a continuous synthesized t one of "clarinet" timbre oscillated between pitches of F4 and F#4 (one semi tone higher) at 16 notes/s, perceived as an integrated stream. The upper no te was then raised to E5 (11 semitones above F4, perceived as segregated st reams) for a further 3 s and the cycle was repeated 40 times. In a second c ondition also starting with oscillation between F4 and F#4, the upper note was lowered to E4 (one semitone below F4, still perceived as a single strea m). Further conditions examined the changes between oscillations of 1 and 1 1 semitones down from E5, 1 and 23 semitones up from F4, and 10 and 11 semi tones up from F4. Virtually no potentials were detectable during the period s of unchanging oscillation, but an N1/P2 complex was evoked on each change in the pitch interval. The N1 was termed "MN1" on account of its arguable relatedness to the mismatch negativity, recorded in a separate experiment u sing discontinuous tones at a much slower rate. The mean peak latency of th e MN1 varied between 96 and 123 ms, the shortest latencies being recorded, not to the largest changes of pitch interval but to the widest pitch interv als between the new tone and the immediately preceding one. Therefore, alth ough a causal relationship with streaming cannot necessarily be inferred, t he MN1 latency appears to mark the degree of pitch contrast between consecu tive tones, in correlation with the streaming effect.