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APPENDIX: PILOT EXPERIMENTS TO DETERMINE SIGNAL-TO-NOISE RATIOS
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Global SNR for the Acoustic Stimuli
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As discussed in the main text, the SNR for the A stimulus was held fixed at a single value
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for all listeners, motivated by a planned future study aimed at identifying psychophysical
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correlates of the bimodal benefit for individual listeners. To determine the global SNR for the A
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stimulus to be used for all listeners and conditions in the study, two initial listeners (S1 and S2)
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were brought in for pilot testing. S1 and S2 were selected because their clinical records showed
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the highest and lowest word recognition performances, respectively, with their acoustic hearing
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ear, amongst the pool of subjects available for recruitment when the study commenced. These
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listeners were presented with test blocks over a range of SNRs in the A and AV conditions to
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determine the minimum SNR at which S2 would receive some speech information in the A
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condition, and at which S1 would be below ceiling in the A and AV conditions. This SNR was
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determined to be 6 dB.
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Baseline Testing and Training
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Prior to participating in experiments 1 and 2, each listener participated in an initial
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baseline experiment conducted to determine the range of performance to be expected in the
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various modalities. Each listener was tested in a subset of the main experimental conditions
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where the A and E stimuli were not combined (E, EV, V, AV, and A). This baseline testing also
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served as training in the consonant-identification task. The A and E stimuli were both presented
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in speech-spectrum-shaped noise at an SNR of 6 dB (i.e., the global SNR selected for all A
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stimuli). For the V condition, the noise was presented at the same level as in the other conditions
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(6 dB below the speech level specified for each ear), but no auditory speech signal was
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presented. Two blocks of 56 trials were presented for each condition. One block each was
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presented for the E, EV, V, AV, and A conditions, in that order, and then a second block for each
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condition was presented in the reverse order.
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Figure 2 shows mean percentage-correct consonant-recognition scores for each listener
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and the group mean for each of the five baseline/training conditions. The range of performance
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for the implant only condition (E) varied greatly across listeners (13-86% correct, mean 51.1%).
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Performance in the EV conditions was considerably better than in the E condition, reflecting the
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benefit of the added visual cues (53- 100% correct, mean 83.3%). EV performance across
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listeners was also somewhat less varied than in the E condition, perhaps because performance
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was at or near ceiling (above 90% correct) for many of the listeners. For the visual-only
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condition (V), performance was much more consistent across listeners, ranging from 27 to 39%
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with a mean of 33.8%. Performance for the acoustic ear stimulus with visual cues (AV) ranged
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from 47-90% correct (mean of 74.2%), whereas performance for the acoustic only condition (A)
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was lower and had greater variability (8-65% correct, mean 29.8%). Note that although
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performance was near chance (7% correct) in the A condition for two of the lowest-performing
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listeners (S2 and S7), performance was better in the AV than in the V condition for all listeners,
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indicating that some speech information was provided by the A stimulus.
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Overall, these results demonstrate the need for using different SNRs for the E and A
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stimuli in the main experiment to avoid floor and ceiling effects. The results show that an SNR
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less than 6 dB is required to avoid ceiling effects in the EV condition, whereas reducing the SNR
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below 6 dB for the A condition would likely eliminate most of the speech information available
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for the lowest-performing listeners in the A modality. Furthermore, there were large differences
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in performance between the E and EV conditions when the same SNR was used in both
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conditions. This would make it difficult to make accurate inferences about the relative benefit
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provided by the additional acoustic signal in each condition (E and EV) because of the fact that
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performance increases would be measured from two different performance reference points.
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Therefore, an attempt was made to identify two different E stimulus SNRs applied for each
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listener – one SNR for the E conditions and another lower SNR for the EV condition – to
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equalize CI performance at the 50% correct level.
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Individualized SNRs for the Cochlear Implant Stimuli
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To determine the SNR required for 50% correct consonant-identification performance for
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each listener, three blocks of trials each were tested for the E and EV conditions with the SNR
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varying on every trial using a one-up-one-down psychophysical tracking procedure (Levitt,
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1971). Consonant tokens were selected at random on each trial. The first token was played at an
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SNR of 18 dB. The SNR was then lowered after each correct response or raised after each
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incorrect response. The step size was 6 dB for the first two reversals, 4 dB for the next two
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reversals, and 2 dB for the remaining 20 reversals. An initial estimate of the SNR required for
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50% correct performance (SNR50) was calculated to be the mean of the SNRs across the last
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eight reversal points of the three runs for a given condition (24 points total). Further testing was
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then conducted with each listener to refine the SNR50 estimates via trial and error. This was
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accomplished by testing at least one block each (56 trials) for the E and EV conditions at the
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initial SNR50 estimate for each condition. Additionally, at least one block of trials was presented
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for the E condition, but at the generally lower SNR required to reach 50% correct for the EV
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condition (SNREV50). This latter test was included to ensure that listeners were obtaining at least
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some auditory speech information at this lower SNR and that they were not relying solely on
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visual cues to achieve 50% correct performance in the EV condition. If these criteria were not
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reached for a given listener, the estimates of the SNRE50 and SNREV50 were adjusted, and testing
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continued iteratively until these performance criteria were reached. These final values of SNRE50
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and SNREV50 were then used in the testing phase.
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The resulting estimates of the SNRE50 and SNREV50 are listed for each listener in Table 2. The
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SNRE50 ranged across listeners from -3.5 dB for S5 to infinity (i.e., no noise) for S2, S4, and S7.
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Note that S8 was unable to reach 50% consonant recognition even in quiet, thus an SNRE50 could
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not be determined for this listener. The SNREV50 ranged across listeners from -9 dB for S1 to
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infinity (i.e., no noise) for S8.
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Reference
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Levitt, H. (1971). Transformed up-down methods in psychoacoustics. J Acoust Soc Am, 49 Suppl
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2, 467–477.
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