Electrically Evoked Compound Action Potential Thresholds Research Articles

Speech perception using maps based on neural response telemetry measures.

To determine how speech perception is affected for Nucleus cochlear implant users when their speech processor is programmed using neural response telemetry (NRT) measures rather than traditional behavioral estimates of threshold and maximum comfort level. Electrically evoked compound action potential (EAP) thresholds were measured for a group of 10 adult Nucleus cochlear implant users. These physiologic threshold estimates were used to create three SPEAK MAPs. One MAP ("+10/-20 MAP") was created using only the NRT data. The second MAP ("Combined MAP") was created using a combination of both EAP thresholds and a single behavioralmeasure of threshold and maximum comfort level from electrode 10. A third MAP ("Measured MAP") was created using standard programming techniques. Speech perception was then assessed using each of these three MAPs for either CUNY or HINT sentences presented at two different presentation levels (70 dB SPL and 55 dB SPL). On average, at the higher presentation level (70 dB SPL) subjects performed significantly better when using the Measured MAP than whenusing either of the NRT-based MAPs. No significant difference in speech perception was obtained for either of the three MAPs when the lower presentation level (55 dB SPL) was used. This study demonstrated that at relatively high presentation levels, speech perception scores obtained by subjects who use MAPs based solely or primarily on EAP thresholds ar eslightly lower than similar scores obtained using a more traditional MAP. This difference did not reach statistical significance at lower presentation levels. We interpret these findings in a positive light to suggest that although NRT-based MAPs may not be optimal, they are of sufficient quality to support reasonable levels of speech understanding. This is important to establish because these MAPs may prove to be most useful when very young children receive a cochlear implant.

A longitudinal study of electrode impedance, the electrically evoked compound action potential, and behavioral measures in nucleus 24 cochlear implant users.

The primary goal of this study was to examine changes that may occur in electrode impedance, electrically evoked compound action potential (EAP) threshold and slope of the EAP growth function, and behavioral measures of threshold T-level) and maximum comfort (C-level) over time in both adult and child cochlear implant users. Secondary goals were to determine whether changes in these measures are consistent between children and adults, and to determine whether behavioral measures (MAP T- and C-levels) and electrophysiologic measures (EAP thresholds) exhibit the same trends over time. Thirty-five children and 33 adults implanted with the Nucleus CI24M between November 1996 and August 1999 participated in this study. Subjects were included in this study if 1) they had used their implant for at least 1 yr after device connection, and 2) they had participated in the necessary data collection at a minimum number of the time intervals assessed in this study. EAP threshold, slope of the EAP growth function, and common ground electrode impedance measures were collected intraoperatively, at initial stimulation, and at several subsequent visits up to 2 yr post initial stimulation. MAP T- an d C-levels weremeasured at initial stimulation and at the same time intervals as described above. Changes in electrode impedance, EAP thresholds, and slope of the EAP growth function from measures made intraoperatively, at initial stimulation, and at 1 to 2 mo post initial stimulation were similar in both children and adults. Beyond the 1- to 2-mo visit, children exhibited significant increases in electrode impedance, EAP thresholds, slope, and MAP T-levels, whereas these samemeasures in adults remained relatively stable. EAP thresholds in children stabilized by the 3- to 8-mo visit, and electrode impedance stabilized by the 6- to 8-mo visit, while slope of the EAP growth function, MAP T-levels,and MAP C-levels werestable by 1 yr post initial stimulation. C-levels in adults increased up to 1 yr post initial stimulation; however, the amount of increase was much smaller than that seen in children. In both children and adults, longitudinal trends in EAP thresholds mirrored T-level more closely than C-level. The results of this study suggest that peripheral changes occur in many children that do not generally occur in adults within the first year of cochlear implant use. One implication of these results is that if EAP thresholds are to be used to assist in programming the speech processor for children, it is best to make those measures at the same time interval as device programming rather than using measures made intraoperatively or at the initial programming session to set MAP levels at later visits.

The neuronal response to electrical constant-amplitude pulse train stimulation: additive Gaussian noise

Experimental results from humans and animals show that electrically evoked compound action potential (EAP) responses to constant-amplitude pulse train stimulation can demonstrate an alternating pattern, due to the combined effects of highly synchronized responses to electrical stimulation and refractory effects (Wilson et al., 1994). One way to improve signal representation is to reduce the level of across-fiber synchrony and hence, the level of the amplitude alternation. To accomplish this goal, we have examined EAP responses in the presence of Gaussian noise added to the pulse train stimulus. Addition of Gaussian noise at a level approximately −30 dB relative to EAP threshold to the pulse trains decreased the amount of alternation, indicating that stochastic resonance may be induced in the auditory nerve. The use of some type of conditioning stimulus such as Gaussian noise may provide a more ‘normal’ neural response pattern.