Information encoding in the temporal aspects of electromagnetic fields consequent to human cortical neuronal activation

This study investigates the temporal signature of neuronal responses to auditory stimuli and, specifically, encoding of pitch and timbre characteristics in the auditory evoked neuromagnetic field. Recordings are made with a 37-channel biomagnetometer with stimuli consisting of (a) sinusoidal waves, (b) square waves, (c) amplitude-modulated (AM) sine waves and (d) synthesized speech sounds. For (a) and (b), frequencies in the range 100–5000 Hz were examined. For the AM sounds, a carrier frequency of 500–1500 Hz was used with 100 Hz and 200 Hz modulation. Speech sounds of pseudo-male and pseudo-female utterance were used. All stimuli elicited neuromagnetic fields characterized by distinct peaks of coherent activity approximately 100 ms post-stimulus (M100). Source localization estimates indicated that all neuronal sources lay in the superior temporal gyrus. However, no tonotopic organization was resolvable. For sine-wave tones, frequency-dependence of the M100 latency was detected, being up to 30 ms longer at low frequencies, and largely invariant from 1000–5000 Hz. This effect was mimicked using square-wave tones, although the magnitude of latency shift was attenuated to 15 ms. AM and speech sounds demonstrated M100 latencies more characteristic of carrier frequency and formants rather than modulation frequency and fundamental, respectively. Information is encoded in the time domain of neuronal response to auditory stimulation. The frequency of a stimulus gives rise to a specific latency of neuronal coherence. Timbre is also encoded, since more complex sounds give rise to latencies reflecting their spectral energy distribution. High temporal resolution recording of electromagnetic activity allows new insights into the brain's functional encoding of stimulus attributes.