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1 Institute of Hearing Research, MRC, Nottingham, United Kingdom
* To whom correspondence should be addressed. E-mail: Markw{at}ihr.mrc.ac.uk.
In the auditory system some ascending pathways preserve the precise timing information present in a temporal code of frequency. This can be measured by studying responses that are phase-locked to the stimulus waveform. At each stage along a pathway there is a reduction in the upper frequency limit of the phase-locking and an increase in the steady state latency. In the guinea pig, phase-locked responses to pure tones have been described at various levels from auditory nerve to neocortex, but not in the inferior colliculus (IC). Therefore we made recordings from 161 single units in guinea pig IC. Of these single units 68% (110/161) showed phase-locked responses. Cells that phase-locked were mainly located in the central nucleus but also occurred in the dorsal cortex and external nucleus. The upper limiting frequency of phase-locking varied greatly between units (80 -1034 Hz) and between anatomical divisions. The upper limits in the three divisions were: central nucleus - over 1000 Hz; dorsal cortex - 700 Hz; external nucleus - 320 Hz. The mean latencies also varied and were: central nucleus - 8.8 ms (SD 3); dorsal cortex - 16.5 ms (SD 3.4); external nucleus - 13.4 ms (SD 0.9). We conclude that many cells in the central nucleus receive direct inputs from the brainstem whereas cells in the external and dorsal divisions receive input from other structures that may include the forebrain.
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