Neurophysiology of cochlear implant users I-II

These two articles present a comprehensive study of neurophysiological measures in cochlear implant users. Several measures were obtained from a group of eleven subjects. The physiological measures included amplitude and latency of peaks in the electrical auditory brainstem response (EABR), electrical auditory middle latency response (EMLR) and the electrical late auditory response (ELAR). Behavioral measures included threshold and maximum comfortable stimulation level at different stimulation electrodes, as well as word and sentence recognition tests in quite and in noise. The first article systematically explores the effect of implanted electrode site and stimulus current level on latency, amplitude and threshold of the various physiological responses to electrical stimulation. In addition to their value as a normative reference, data from this study reveal important similarities and differences between electrically- and acoustically-evoked auditory potentials. The second article explores relations between physiological and behavioral measures. Among other results, listeners without open-set speech recognition demonstrated poorly formed or absent evoked potential responses and lack of change in the size of the dynamic range with a change in stimulus rate. These results support the hypothesis that adult recipients of cochlear implants who have open-set speech perception and those recipients with no open-set speech perception differ in their
neurophysiologic responses recorded at one or more levels of the auditory pathway.

A sensitive period for the development of the central auditory system in children with cochlear implants: Implications for age of implantation.
Anu Sharma, Michael F. Dorman, Anthony Spahr

Because Pl latencies of the cortical auditory evoked potential vary as a function of chronological age, they can be used to infer the maturational status of auditory pathways and the human central auditory system. This measure was used to assess the consequences of deafness and of cochlear implantation at different ages on auditory development. The development of P1 response latencies was measured in 104 congenitally deaf people who had been fit with cochlear implants at ages ranging from 1.3 years to adulthood. A comparison of Pl latencies in implanted children with those of age-matched normal-hearing peers revealed that implanted children with the longest period of auditory deprivation before implantation - 7 or more years -had abnormal cortical response latencies to speech. Implanted children with no more than 3.5 years of auditory deprivation evidenced age-appropriate latency responses within 6 months after the onset of electrical stimulation. These data suggest that in the absence of normal stimulation there is a sensitive period of about 3.5 years during which the human central auditory system remains maximally plastic.

Speech Understanding in Noise with a Med-El COMBI 40+ Cochlear Implant Using Reduced Channel Sets.
Carolyn Garnham, Martin O’Driscoll, Richard Ramsden, Shakeel Saeed

This study aimed to determine the number of stimulation channels needed for cochlear implant users to achieve asymptotic performance in the recognition of speech in noise. Performance in speech tests was measured for patients using the Med-El implementation of the CIS stimulation strategy with 2,3,4,6,8 and 10 channels out of a possible maximum of 12. Vowel, consonant and sentence identification tests were presented against a background of pink noise to eleven subjects. Using moderate signal to noise ratios, asymptotic performance was achieved using 4 channels for consonants, 6 channels for vowels and 8 channels for sentences. In another experiment, understanding of monosyllabic words reached a maximum value at a similar number of channels both in quite and in pink noise, and the mean increase in test score between 6 and 11 channels (although statistically significant) was only 7%. The data suggest that 12 frequency channels are more than adequate for cochlear implant users to achieve asymptotic performance levels in clinical speech tests applied in the presence of wideband noise at moderate signal to noise ratios.