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The Journal of Neurophysiology Vol. 82 No. 3 September 1999, pp. 1363-1380
Copyright ©1999 by the American Physiological Society
Human Communications Research Centre, Department of Otolaryngology, The University of Melbourne, Parkville, Victoria 3052, Australia
Shepherd, Robert K.,
Jagir H. Baxi, and
Natalie A. Hardie.
Response of Inferior Colliculus Neurons to Electrical
Stimulation of the Auditory Nerve in Neonatally Deafened
Cats. J. Neurophysiol. 82: 1363-1380, 1999. Response
properties of neurons in the inferior colliculus (IC) were examined in
control and profoundly deafened animals to electrical stimulation of
the auditory nerve. Seven adult cats were used: two controls; four
neonatally deafened (2 bilaterally, 2 unilaterally); and one long-term
bilaterally deaf cat. All control cochleae were deafened immediately
before recording to avoid electrophonic activation of hair cells.
Histological analysis of neonatally deafened cochleae showed no
evidence of hair cells and a moderate to severe spiral ganglion cell
loss, whereas the long-term deaf animal had only 1-2% ganglion cell
survival. Under barbiturate anesthesia, scala tympani electrodes were
implanted bilaterally and the auditory nerve electrically stimulated
using 100 µs/phase biphasic current pulses. Single-unit
(n = 419) recordings were made through the lateral
(LN) and central (ICC) nuclei of the IC; responses could be elicited
readily in all animals. Approximately 80% of cells responded to
contralateral stimulation, whereas nearly 75% showed an excitatory
response to ipsilateral stimulation. Most units showed a monotonic
increase in spike probability and reduction in latency and jitter with
increasing current. Nonmonotonic activity was seen in 15% of units
regardless of hearing status. Neurons in the LN exhibited longer
latencies (10-25 ms) compared with those in the ICC (5-8 ms). There
was a deafness-induced increase in latency, jitter, and dynamic range;
the extent of these changes was related to duration of deafness. The
ICC maintained a rudimentary cochleotopic organization in all
neonatally deafened animals, suggesting that this organization is laid
down during development in the absence of normal afferent input.
Temporal resolution of IC neurons was reduced significantly in neonatal
bilaterally deafened animals compared with acutely deafened controls,
whereas neonatal unilaterally deafened animals showed no reduction. It
would appear that monaural afferent input is sufficient to maintain
normal levels of temporal resolution in auditory midbrain neurons.
These experiments have shown that many of the basic response properties are similar across animals with a wide range of auditory experience. However, important differences were identified, including increased response latencies and temporal jitter, and reduced levels of temporal resolution.
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