Psychophysical studies with two binaural cochlear implant subjects.

Psychophysical studies have been completed with two binaural cochlear implant patients. In our earlier studies [van Hoesel et al., J. Acoust. Soc. Am. 94, 3187-3189 (1993); R. J. M. van Hoesel and G. M. Clark, Ann. Otol. Rhinol. Laryngol. Suppl. 106 104, 233-235 (1995)], lateralization experiments showed good sensitivity to interaural amplitudes but poor sensitivity to interaural time delays when compared with normal hearing subjects. In the studies presented here, both temporal and binaural intensity interactions were further explored. Interaural time delay (ITD) perception was investigated using direct measurement of the just-noticeable difference (jnd) in ITD. Both rate and place of stimulation were varied. Binaural rate discrimination was measured and compared with monaural rate perception. Binaural intensity interaction was explored for matched and unmatched place conditions by means of loudness summation and central masking studies. Results showed that ITDs for interaural time delays were large when compared to normal hearing, even when place of stimulation on each of the two sides was carefully matched. The jnds in ITD were similar for stimulation rates from 50 to 200 pps, and increased at 300 pps. Rate difference limens experiments showed similar results for diotic and monaural stimuli, but improved jnds for dichotic presentation at stimulation rates below 150-200 pps. Binaural intensity interactions showed loudness summation effects with both patients, for matched as well as unmatched place conditions. Central masking was also observed with both subjects, although it was not found to be place dependent.