The Frequency Extent of Two-tone Facilitation in Onset Units in the Ventral
Cochlear Nucleus.
JIANG, DAN, ALAN R. PALMER AND IAN M. WINTER.
MRC Institute of Hearing Research, University Park, University of
Nottingham, Nottingham, NG7 2RD, UK.
APStracts 2:0269N, 1995.
SUMMARY AND CONCLUSIONS
1. The frequency threshold curves (FTCs) of 91 single units in the cochlear
nucleus of the anaesthetised guinea pig have been measured using a
conventional single-tone paradigm and a two-tone paradigm designed to
elucidate the frequency extent of two-tone facilitation in onset units. Units
were classified according to existing classification schemes into primary-like
(n = 3), chopper (n = 23) and three onset groups: OnI (n = 12), OnC (n = 29)
and OnL (n = 24). Histological reconstructions show onset units to be widely
distributed within the ventral cochlear nucleus in a manner generally
consistent with its tonotopic organization. 2.. The FTCs of onset units
differed in their minimum thresholds, the steepness of their high- and low-
frequency cut-offs and their sharpness of tuning as quantified by the Q10dB
values. There was considerable overlap in the sharpness of tuning between
onset units and auditory-nerve fibres as indicated by the distribution of
Q10dB values in the octave around 10 kHz: onset units had Q10dBs of 3.56 +1.38
(mean + SD) compared to 6.3 +2.48 for auditory nerve fibres. The tuning of
chopper units was similar to that of auditory-nerve fibres 5.52 + 1.46. 3.
Seventy-five percent of onset units showed some degree of facilitation (a
threshold reduction) when their FTCs were measured in the presence of best-
frequency (BF) tones 4 dB below BF threshold. The frequency extent of such
facilitation was variable with a maximum of 6 octaves around the BF. In
extreme cases facilitation could be measured when the BF tone was as low as 30
dB below BF threshold. 4. In 17% of onset units suppressive effects were
evident as shown by non-contiguous frequency regions of facilitation. These
suppressive effects might either be a reflection of suppression in the
auditory-nerve input or of a direct inhibitory input to the onset units. The
strength of this effect suggests that inhibition is a likely explanation,
consistent with the finding in previous morphological studies of profuse
synapses with pleomorphic vesicles on multipolar cells. 5. FTCs of chopper and
primary-like units measured in the presence of BF tones showed little
facilitation. The facilitation which was observed in chopper units was
confined to a narrow region around BF and disappeared on lowering the
facilitatory tone to 4 dB below BF threshold. 6. These data support the
hypothesis that onset units, but not chopper or primary-like units, receive
excitatory inputs from auditory-nerve fibres with a wide range of BFs.
However, the frequency range of facilitation and the magnitude of the
threshold facilitation varied from unit to unit, suggesting that the off-BF
inputs from auditory-nerve fibres are not evenly distributed or equally
effective in all units.
Received 22 August 1995; accepted in final form 31 May 1995.
APS Manuscript Number J354-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.