Measuring implanted patient response to tone pips.

BACKGROUND: An electrical potential not previously reported-electrical cochlear response (ECR)-observed only in implanted patients is described. Its amplitude and growth slope are a measurement of the stimulation achieved by a tone pip on the auditory nerve. The stimulation and recording system constructed for this purpose, the features of this potential obtained in a group of 43 children, and its possible clinical use are described. The ECR is obtained by averaging the EEG epochs acquired each time the cochlear implant (CI) processes a tone pip of known frequency and intensity when the patient is sleeping and using the CI in everyday mode. The ECR is sensitive to tone pip intensity level, microphone sensitivity, sound processor gain, dynamic range of electrical current, and responsiveness to electrical current of the auditory nerve portion involved with the electrode under test. It allows individual evaluation of intracochlear electrodes by choosing, one at the time, the central frequency of the electrode as the test tone pip frequency, so the ECR measurement due to a variable intensity tone pip allows to establish the suitability of the dynamic range of the electrode current. RESULTS: There is a difference in ECR measurements when patients are grouped based on their auditory behavior. The ECR slope and amplitude for the Sensitive group is 0.2 µV/dBHL and 10 µV at 50 dBHL compared with 0.04 µV/dBHL and 3 µV at 50dBHL for the Inconsistent group. The clinical cases show that adjusting the dynamic range of current based on the ECR improved the patient's auditory behavior. CONCLUSIONS: ECR can be recorded regardless of the artifact due to the electromyographic activity of the patient and the functioning of the CI. Its amplitude and growth slope versus the intensity of the stimulus differs between electrodes. The relationship between minimum ECR detection intensity level and auditory threshold suggests the possibility of estimating patient auditory thresholds this way. ECR does not depend on the subject's age, cooperation, or health status. It can be obtained at any time after implant surgery and the test procedure is the same regardless of device manufacturer.

Quantification of LLAEP interhemispheric symmetry by the intraclass correlation coefficient as a measure of cortical reorganization after cochlear implantation.

OBJECTIVE: Electrical stimulation by a cochlear implant (CI) induces maturation of the auditory system and reorganization of the auditory cortex in deaf children. Cortical reorganization produces an interhemispheric asymmetry in auditory evoked brain potentials associated with sound stimulation after unilateral implantation. To objectively determine the onset of this phenomenon and follow this process over time, the interhemispheric symmetry needs to be quantified. In this paper, the intraclass correlation coefficient (ICC) between mean global field powers (MGFPs) of each hemisphere is proposed to quantify long latency auditory evoked potential (LLAEP) interhemispheric symmetries as a measure of auditory cortex reorganization in CI recipients. DESIGN: An LLAEP, in response to a simple tone, was recorded in 5 juvenile unilateral CI recipients at less and at more than two years post-implantation and the ICC between MGFPs was calculated for both recordings. The cross correlation coefficients (CC) between MGFPs of each hemisphere were also calculated and compared with the ICC. RESULTS: The experience-related visually observed increases in amplitude and shape asymmetries of the LLAEP topographic map (around the LLAEP P(1) peak), were reflected in a considerable reduction of ICC values (on average 41.4%), at more than two years post-implantation surgery. In contrast, CC values only showed much smaller decreases (on average 20.0%), at more than two years post-implantation. CONCLUSIONS: The ICC is a better descriptor of symmetry than the CC because it reflects both shape and amplitude similarity between left and right LLAEP MGFPs instead of only shape similarity. The decrease in ICC values at more than two years post-implantation is likely associated with a lateralization of the auditory response as a result of cortical reorganization. Our results show that the ICC between the MGFPs for each hemisphere can be useful to objectively determine the auditory cortex reorganization process and also to evaluate the performance of cochlear implant users without the necessity to use expensive technologies such as high density EEG recordings and/or fMRI scans.