S J Jones1, M Vaz Pato, L Sprague. 1. The National Hospital for Neurology and Neurosurgery, London, UK. sjjones@ion.ucl.ac.uk
Abstract
OBJECTIVES: To examine whether two cortical processes concerned with spectro-temporal analysis of complex tones, a 'C-process' generating CN1 and CP2 potentials at cf. 100 and 180 ms after sudden change of pitch or timbre, and an 'M-process' generating MN1 and MP2 potentials of similar latency at the sudden cessation of repeated changes, are dependent on accumulation of a sound image in the long auditory store. METHODS: The durations of steady (440 Hz) and rapidly oscillating (440-494 Hz, 16 changes/s) pitch of a synthesized 'clarinet' tone were reciprocally varied between 0.5 and 4.5 s within a duty cycle of 5 s. Potentials were recorded at the beginning and end of the period of oscillation in 10 non-attending normal subjects. RESULTS: The CN1 at the beginning of pitch oscillation and the MN1 at the end were both strongly influenced by the duration of the immediately preceding stimulus pattern, mean amplitudes being 3-4 times larger after 4.5 s as compared with 0.5 s. CONCLUSIONS: The processes responsible for both CN1 and MN1 are influenced by the duration of the preceding sound pattern over a period comparable to that of the 'echoic memory' or long auditory store. The store therefore appears to occupy a key position in spectro-temporal sound analysis. The C-process is concerned with the spectral structure of complex sounds, and may therefore reflect the 'grouping' of frequency components underlying auditory stream segregation. The M-process (mismatch negativity) is concerned with the temporal sound structure, and may play an important role in the extraction of information from sequential sounds.
OBJECTIVES: To examine whether two cortical processes concerned with spectro-temporal analysis of complex tones, a 'C-process' generating CN1 and CP2 potentials at cf. 100 and 180 ms after sudden change of pitch or timbre, and an 'M-process' generating MN1 and MP2 potentials of similar latency at the sudden cessation of repeated changes, are dependent on accumulation of a sound image in the long auditory store. METHODS: The durations of steady (440 Hz) and rapidly oscillating (440-494 Hz, 16 changes/s) pitch of a synthesized 'clarinet' tone were reciprocally varied between 0.5 and 4.5 s within a duty cycle of 5 s. Potentials were recorded at the beginning and end of the period of oscillation in 10 non-attending normal subjects. RESULTS: The CN1 at the beginning of pitch oscillation and the MN1 at the end were both strongly influenced by the duration of the immediately preceding stimulus pattern, mean amplitudes being 3-4 times larger after 4.5 s as compared with 0.5 s. CONCLUSIONS: The processes responsible for both CN1 and MN1 are influenced by the duration of the preceding sound pattern over a period comparable to that of the 'echoic memory' or long auditory store. The store therefore appears to occupy a key position in spectro-temporal sound analysis. The C-process is concerned with the spectral structure of complex sounds, and may therefore reflect the 'grouping' of frequency components underlying auditory stream segregation. The M-process (mismatch negativity) is concerned with the temporal sound structure, and may play an important role in the extraction of information from sequential sounds.