Cochlear implant thresholds: comparison of middle latency responses with psychophysical and cortical-spike-activity thresholds.

The electrically evoked middle latency response (EMLR) is a potentially useful measure of activation of the auditory system by a cochlear prosthesis. The present study compared cochlear prosthesis thresholds determined using EMLR with thresholds determined for psychophysical detection and for spike activity in cortical neurons. In systemically deafened guinea pigs, the difference between EMLR and psychophysical threshold level varied, with differences ranging from -4.6 dB (EMLR threshold more sensitive) to +10.7 dB (psychophysical threshold more sensitive) across animals and phase durations. Threshold differences between EMLR and auditory cortex neural spike responses were similar in magnitude and range (-6 to +15 dB) to those seen for EMLR vs. psychophysical thresholds. These ranges are comparable to the behavioral operating range for a given condition. In 3 of 12 subjects, the EMLR was absent for some or all electrode configurations tested, even at levels well above the threshold for psychophysical detection or cortical neuronal response. These results suggest that neither the EMLR thresholds nor cortical neuronal spike thresholds are an adequate substitute for psychophysical measures of threshold. While not sufficient for use in place of psychophysical measures, EMLR threshold level is strongly correlated with psychophysical threshold level across subjects (R(2)=0.82). Interestingly, plots of thresholds vs. phase duration were roughly parallel for psychophysical and EMLR thresholds, in contrast to the divergence of psychophysical and more peripheral (e.g., electrically evoked auditory brainstem response) evoked neural threshold vs. phase duration functions.