June 05 [Sun], 2011, 22:23
In-key chords elicited a large mean global field power (MGFP, a measure of the strength of an evoked field), present in all sub- jects at around 200 ms (relative to stimulus onset, Fig. 2a). (This mag-
netic effect will henceforth be referred to as the P2m.) Brain responses elicited from Neapolitan and in-key chords in the fifth position clearly differed (Fig. 2b). Neapolitan chords elicited a particular early magnetic field effect, which was, at any sensor, nearly uni-modal over time, and was largest around 200 ms (like the P2m). This effect (henceforth referred to as the mERAN) can best be seen in the difference waves of Fig. 2b. Virtually no magnetic effects were observable after around 350 ms, for Neapolitans or for in-key chords.
The field maps of both P2m and mERAN reveal a dipolar pattern over each hemisphere (Fig. 3a and b). In all subjects, the fields of the mERAN had virtually an inversed ‘polarity’ compared to the fields of the P2m. Moreover, the steepest field gradients of the mERAN are anterior to those of the P2m, indi- cating that the neural generators of the mERAN are anterior to those of the P2m.
Effects elicited by Neapolitan chords at the third and fifth position were very similar in distribution and time course; how- ever, the third-position effects were distinctly smaller (about half of the strength of fifth-position effects, Figs. 2c and 3c). The MGFP of the mERAN (in-key chord signals subtracted from Neapolitan chord signals, Fig. 4) elicited at the third position dif- fered significantly from the MGFP of the mERAN elicited at the fifth position (paired t-test; t = 5.69, p = 0.005). (MGFP was cal- culated for third and fifth position for each subject separately in the time window from 170–210 ms.)
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