The purpose of this study was to examine auditory-nerve temporal response
properties and their relation to psychophysical threshold for electrical pulse trains of
varying rates (“rate integration”). The primary hypothesis was that better
rate integration (steeper slope) would be correlated with smaller decrements in ECAP
amplitude as a function of stimulation rate (shallower slope of the amplitude-rate
function), reflecting a larger percentage of the neural population contributing more
synchronously to each pulse in the train. Data were obtained for 26 ears in 23
cochlear-implant recipients. Electrically evoked compound action potential (ECAP)
amplitudes were measured in response to each of 21 pulses in a pulse train for the
following rates: 900, 1200, 1800, 2400, and 3500 pps. Psychophysical thresholds were
obtained using a 3-interval, forced-choice adaptive procedure for 300-ms pulse trains of
the same rates as used for the ECAP measures, which formed the rate-integration function.
For each electrode, the slope of the psychophysical rate-integration function was compared
to the following ECAP measures: (1) slope of the function comparing average normalized
ECAP amplitude across pulses versus stimulation rate (“adaptation”), (2)
the rate that produced the maximum alternation depth across the pulse train, and (3) rate
at which the alternating pattern ceased (stochastic rate). Results showed no significant
relations between the slope of the rate-integration function and any of the ECAP measures
when data were collapsed across subjects. However, group data showed that both threshold
and average ECAP amplitude decreased with increased stimulus rate, and within-subject
analyses showed significant positive correlations between psychophysical thresholds and
mean ECAP response amplitudes across the pulse train. These data suggest that ECAP
temporal response patterns are complex and further study is required to better understand
the relative contributions of adaptation, desynchronization, and firing probabilities of
individual neurons that contribute to the aggregate ECAP response.