Localization and interaural time difference (ITD) thresholds for cochlear implant recipients with preserved acoustic hearing in the implanted ear

Abstract

The purpose of this study was to investigate horizontal plane localization and interaural time difference (ITD) thresholds for 14 adult cochlear implant recipients with hearing preservation in the implanted ear. Localization to broadband noise was assessed in an anechoic chamber with a 33-loudspeaker array extending from -90 to +90°. Three listening conditions were tested including bilateral hearing aids, bimodal (implant + contralateral hearing aid) and best aided (implant + bilateral hearing aids). ITD thresholds were assessed, under headphones, for low-frequency stimuli including a 250-Hz tone and bandpass noise (100-900 Hz). Localization, in overall rms error, was significantly poorer in the bimodal condition (mean: 60.2°) as compared to both bilateral hearing aids (mean: 46.1°) and the best-aided condition (mean: 43.4°). ITD thresholds were assessed for the same 14 adult implant recipients as well as 5 normal-hearing adults. ITD thresholds were highly variable across the implant recipients ranging from the range of normal to ITDs not present in real-world listening environments (range: 43 to over 1600 μs). ITD thresholds were significantly correlated with localization, the degree of interaural asymmetry in low-frequency hearing, and the degree of hearing preservation related benefit in the speech reception threshold (SRT). These data suggest that implant recipients with hearing preservation in the implanted ear have access to binaural cues and that the sensitivity to ITDs is significantly correlated with localization and degree of preserved hearing in the implanted ear.

Fig. 1

Individual and mean (large shaded circles) unaided audiometric thresholds, in dB HL, for the implanted ears (A) and the non-implanted ears (B). Error bars represent ±1 SEM.

Fig. 2

Loudspeaker arrangement in anechoic chamber. Filled symbols show the “active” loudspeakers; open symbols indicate “dummy” loudspeakers. Loudspeakers were labeled 1–33 (numbers 1, 17, and 33 are shown here).

Fig. 3

Individual and mean localization thresholds, in overall rms error, for the 14 hearing preservation CI subjects in the bimodal (CI + contra HA), bilateral HA, and best-aided (CI + bilateral HA) conditions. Error bars represent ±1 SEM.

Fig. 4

Localization responses for individual subjects in the bimodal (filled circles) and best-aided conditions (unfilled circles). Each individual panel specifies the participant number and the implanted ear.

Fig. 5

Individual and mean ITD thresholds, in microseconds, for (A) 250 Hz and (B) 100–900 Hz bandpass noise. ITD thresholds are also shown as a function of rms localization error in the best-aided condition, for (C) 250 Hz and (D) 100–900 Hz. For (A) and (B) means were calculated only for those participants who had a tracked threshold. Regression analysis did not include results for the listeners with normal hearing nor the data points representing those participants unable to track ITD thresholds.

Fig. 6

ITD thresholds for (A) 250 Hz and (B) bandpass noise (100–900 Hz) as a function of the magnitude of interaural asymmetry, in dB, for quiet thresholds.

Fig. 7

Hearing preservation (HP) related benefit in the speech reception threshold (SRT), in dB, as function of (A) ITD thresholds at 250 Hz, (B) ITD thresholds for 100–900 Hz bandpass noise, (C) localization in the best-aided condition, (D) asymmetry at 250 Hz, and (E) asymmetry for 100–900 Hz bandpass noise.