Forward-masked psychophysical spatial tuning curves (fmSTCs) were measured in twelve cochlearimplant subjects, six using bipolar stimulation (Nucleus devices)and six using monopolar stimulation (Clarion devices). fmSTCs were measured at several probe levels on a middle electrode using a fixedlevel probe stimulus and variable-level maskers. The average fmSTC slopes obtained in subjects using bipolar stimulation (3.7 dB/mm) were approximately three times steeper than average slopes obtained in subjects using monopolar stimulation (1.2 dB/mm). Average spatial bandwidths were about half as wide for subjects with bipolar stimulation (2.6 mm) than for subjects with monopolar stimulation (4.6 mm). None of the tuning curve characteristics changed significantly with probe level. fmSTCs replotted in terms of acoustic frequency, using Greenwood's [J. Acoust. Soc. Am. 33, 1344-1356 (1961)] frequency-to-place equation, were compared with forward-masked psychophysical tuning curves obtained previously from normal-hearing and hearing-impaired acoustic listeners. The average tuning characteristics of fmSTCs in electric hearing were similar to the broad tuning observed in normal-hearing and hearing-impaired acoustic listeners at high stimulus levels. This suggests that spatial tuning is not the primary factor limiting speech perception in many cochlear implant users.

Simultaneous or near-simultaneous activation of adjacent cochlear implant electrodes can produce pitch percepts intermediate to those produced by each electrode separately, thereby increasing the number of place-pitch steps available to cochlear implant listeners. To estimate how many distinct pitches could be generated with simultaneous dual-electrode stimulation, the present study measured place-pitch discrimination thresholds for single- versus dual-electrode stimuli in users of the Clarion CII device. Discrimination thresholds were expressed as the proportion of current directed to the secondary electrode of the dual-electrode pair. For 16 of 17 electrode pairs tested in six subjects, thresholds ranged from 0.11 to 0.64, suggesting that dual-electrode stimuli can produce 2-9 discriminable pitches between the pitches of single electrodes. Some subjects demonstrated a level effect, with better place-pitch discrimination at higher stimulus levels. Equal loudness was achieved with dual-electrode stimuli at net current levels that were similar to or slightly higher than those for single-electrode stimuli.

Spectral ripple discrimination thresholds were measured in 15 cochlear-implant users with broadband (350-5600 Hz) and octave-band noise stimuli. The results were compared with spatial tuning curve (STC) bandwidths previously obtained from the same subjects. Spatial tuning curve bandwidths did not correlate significantly with broadband spectral ripple discrimination thresholds but did correlate significantly with ripple discrimination thresholds when the rippled noise was confined to an octave-wide passband, centered on the STC's probe electrode frequency allocation. Ripple discrimination thresholds were also measured for octave-band stimuli in four contiguous octaves, with center frequencies from 500 Hz to 4000 Hz. Substantial variations in thresholds with center frequency were found in individuals, but no general trends of increasing or decreasing resolution from apex to base were observed in the pooled data. Neither ripple nor STC measures correlated consistently with speech measures in noise and quiet in the sample of subjects in this study. Overall, the results suggest that spectral ripple discrimination measures provide a reasonable measure of spectral resolution that correlates well with more direct, but more time-consuming, measures of spectral resolution, but that such measures do not always provide a clear and robust predictor of performance in speech perception tasks.

Under normal conditions, human speech is remarkably robust to degradation by noise and other distortions. However, people with hearing loss, including those with cochlear implants, often experience great difficulty in understanding speech in noisy environments. Recent work with normal-hearing listeners has shown that the amplitude fluctuations inherent in noise contribute strongly to the masking of speech. In contrast, this study shows that speech perception via a cochlear implant is unaffected by the inherent temporal fluctuations of noise. This qualitative difference between acoustic and electric auditory perception does not seem to be due to differences in underlying temporal acuity but can instead be explained by the poorer spectral resolution of cochlear implants, relative to the normally functioning ear, which leads to an effective smoothing of the inherent temporal-envelope fluctuations of noise. The outcome suggests an unexpected trade-off between the detrimental effects of poorer spectral resolution and the beneficial effects of a smoother noise temporal envelope. This trade-off provides an explanation for the long-standing puzzle of why strong correlations between speech understanding and spectral resolution have remained elusive. The results also provide a potential explanation for why cochlear-implant users and hearing-impaired listeners exhibit reduced or absent masking release when large and relatively slow temporal fluctuations are introduced in noise maskers. The multitone maskers used here may provide an effective new diagnostic tool for assessing functional hearing loss and reduced spectral resolution.

SignificanceSound consists of a dynamic stream of energy at different frequencies. Auditory processing of sound frequency is critical in determining our ability to interact and communicate in a complex acoustic world, yet fundamental gaps remain in our understanding of how this is achieved. Indeed, the resolving power of the system, how best to measure it, and the mechanisms that underlie it are all still debated. Here, we provide critical evidence demonstrating that humans can resolve the frequency components of competing sounds better than other commonly studied mammals. This finding raises important questions both for theories of auditory perception and for our understanding of the evolutionary relationships between the auditory system and acoustic communication, including speech.