Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains

By October 4, 2011

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Alana E. Kirby John C. Middlebrooks
J Association for Research in Otolaryngology Chronic Guinea Pig right hemispheres of six adult albino guinea pigs. Animals were of either sex and weighed 340– 490 g Cortex Auditory Penetrating Electrode a single silicon substrate shank with 16 iridium-plated recording sites at 100-μm intervals. The probe was 15 μm thick and 100 μm wide, tapering to the tip; a monolithic silicon substrate ribbon cable

Cochlear implant listeners receive auditory stimulation
through amplitude-modulated electric pulse trains.
Auditory nerve studies in animals demonstrate qualitatively
different patterns of firing elicited by low versus
high pulse rates, suggesting that stimulus pulse rate
might influence the transmission of temporal information
through the auditory pathway. We tested in awake
guinea pigs the temporal acuity of auditory cortical
neurons for gaps in cochlear implant pulse trains.
Consistent with results using anesthetized conditions,
temporal acuity improved with increasing pulse rates.
Unlike the anesthetized condition, however, cortical
neurons responded in the awake state to multiple
distinct features of the gap-containing pulse trains, with
the dominant features varying with stimulus pulse rate.
Responses to the onset of the trailing pulse train (Trail-
ON) provided the most sensitive gap detection at 1,017
and 4,069 pulse-per-second (pps) rates, particularly for
short (25 ms) leading pulse trains. In contrast, under
conditions of 254 pps rate and long (200 ms) leading
pulse trains, a sizeable fraction of units demonstrated
greater temporal acuity in the form of robust responses
to the offsets of the leading pulse train (Lead-OFF).
Finally, TONIC responses exhibited decrements in firing
rate during gaps, but were rarely the most sensitive
feature. Unlike results from anesthetized conditions,
temporal acuity of the most sensitive units was nearly as
sharp for brief as for long leading bursts. The differences
in stimulus coding across pulse rates likely originate from
pulse rate-dependent variations in adaptation in the
auditory nerve. Two marked differences from responses
to acoustic stimulation were: first, Trail-ON responses to
4,069 pps trains encoded substantially shorter gaps than
have been observed with acoustic stimuli; and second,
the Lead-OFF gap coding seen for G15 ms gaps in
254 pps stimuli is not seen in responses to sounds. The
current results may help to explain why moderate pulse
rates around 1,000 pps are favored by many cochlear
implant listeners.

Kirby, Alana E, and John C Middlebrooks. "Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains." Journal of the Association for Research in Otolaryngology JARO 80.1 (2011) : 67-80.

http://www.ncbi.nlm.nih.gov/pubmed/21969022 October 4, 2011 Department of Otolaryngology, University of California, Medical Sciences E, Irvine