Magnetic fields elicited by a tone onset time continuum in humans

Event-related Fields (ERFs) were recorded in eleven normal volunteers in response to four two-tone stimuli selected from a tone onset time (TOT) continuum. The latter has been used extensively in the past as an analog of voice onset-time, an important cue for making voicing distinctions (e.g., ¿pa¿ versus ¿ba¿). TOT values ranged from 0 to 60 ms in 20-ms steps. Three components of the ERF (P50m, N1m, and P2m) in the hemisphere contralateral to the stimulated ear were analyzed. The most significant finding was an abrupt reduction in the peak field amplitude of N1m (bilaterally) as TOT values increased from +20 to +40 ms. No systematic differences were noted between the 0 and +20 ms stimuli or between the +40 and +60 ms tokens. On the other hand, peak component latencies rose steadily with increasing TOT values. The discontinuity in the change of peak RMS as a function of TOT is in close agreement with behavioral evidence of categorical perception of similar non-speech as well as speech stimuli.