Psychophysical recovery from single-pulse forward masking in electric hearing

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Abstract
Psychophysical single-pulse forward-masking (SPFM) recovery functions were measured for three electrodes in each of eight subjects with the nucleus mini-22 cochlear implant. Masker and probe stimuli were single 200-μs/phase biphasic current pulses. Recovery functions were measured at several masker levels spanning the electric dynamic range of electrodes chosen from the apical, middle, and basal regions of each subject’s electrode array. Recovery functions were described by an exponential process in which threshold shift (in μA) decreased exponentially with increasing time delay between the masker and probe pulses. Two recovery processes were observed: An initial, rapid-recovery process with an average time constant of 5.5 ms was complete by about 10 ms. A second, slow-recovery process involved less masking than the rapid-recovery process but encompassed much longer time delays, sometimes as long as several hundred milliseconds. Growth-of-masking slopes for the rapid process depended upon time delay, as expected in an exponential recovery process. Unity slopes were observed at a time delay of 0 ms, whereas progressively shallower slopes were observed at time delays of 2 ms and 5 ms. Many recovery functions demonstrated nonmonotonicities or “facilitation” at very short masker-probe delays (1–2 ms). Such nonmonotonicities were usually most pronounced at low masker levels. Time constants for the rapid-recovery process did not vary systematically with masker level or with electrode location along the implanted array. Most subjects demonstrated rapid-recovery time constants less than 7 ms; however, the subject with the longest duration of deafness prior to implantation exhibited clearly prolonged time constants (9–24 ms). Time constants obtained on basal electrodes were inversely related to word recognition scores.