Browsing by Subject "Psychophysics"

It is well established that cochlear implants (CIs) are able to provide many users with excellent speech recognition ability in quiet conditions; however, the ability to correctly identify speech in noisy conditions or appreciate music is generally poor for implant users with respect to normal-hearing listeners. This discrepancy has been hypothesized to be in part a function of the relative decrease in spectral information available to implant users (Rubinstein and Turner, 2003; Wilson et al., 2004). One method that has been proposed for increasing the amount of spectral information available to CI users is to include time-varying stimulation rate in addition to changes in the place of stimulation. However, previous implementations of multi-rate strategies have failed to result in an improvement in speech recognition over the clinically available, fixed-rate strategies (Fearn, 2001; Nobbe, 2004). It has been hypothesized that this lack of success was due to a failure to consider the underlying perceptual responses to multi-rate stimulation.

In this work, psychophysical experiments were implemented with the goal of achieving a better understanding of the interaction of place and rate of stimulation and the effects of duration and context on CI listeners' ability to detect changes in stimulation rate. Results from those experiments were utilized in the implementation of a tuned multi-rate sound processing strategy for implant users in order to potentially ``tune" multi-rate strategies and improve speech recognition performance.

In an acute study with quiet conditions, speech recognition performance with a tuned multi-rate implementation was better than performance with a clinically available, fixed-rate strategy, although the difference was not statistically significant. These results suggest that utilizing time-varying pulse rates in a subject-specific implementation of a multi-rate algorithm may offer improvements in speech recognition over clinically available strategies. A longitudinal study was also performed to investigate the potential benefit from training to speech recognition. General improvements in speech recognition ability were observed as a function of time; however, final scores with the tuned multi-rate algorithm never surpassed performance with the fixed-rate algorithm for noisy conditions.

The ability to improve upon speech recognition scores for quiet conditions with respect to the fixed-rate algorithm suggests that using time-varying stimulation rates potentially provides additional, usable information to listeners. However, performance with the fixed-rate algorithm proved to be more robust to noise, even after three weeks of training. This lack of robustness to noise may be in part a result of the frequency estimation technique used in the multi-rate strategy, and thus more sophisticated techniques for real-time frequency estimation should be explored in the future.

Cochlear implant listeners are presented with a time and frequency-quantized version of speech signals. In the frequency domain, resolution is limited by the number of electrodes in each listener's array. Current cochlear implant speech processing strategies implicitly assume that the information presented to each one of these electrodes is perceived as unique and independent. However, previous research suggests that stimuli presented on different electrodes can be indiscriminable (e.g. Zwolan et al., 1997; Throckmorton and Collins, 1999; Henry et al., 2000) . Additional studies suggest that stimuli presented on one electrode can influence the perception of stimuli on neighboring electrodes (e.g. Shannon, 1990; Chatterjee and Shannon, 1998; Boëx et al., 2003). Removing this redundant or occluded information could cause more distinct or perceivable information to be presented to the listener and possibly result in improved speech recognition.

Previous studies have used psychophysical data to identify the electrodes with the highest potential to confound speech recognition (Zwolan et al., 1997, Boëx et al., 2003, and Garadat et al., 2012). In order to minimize electrode interactions and maximize the amount of perceivable information, each of these studies used a single psychophysical metric to deactivate the electrodes across all time windows of the speech processing strategy. For some listeners, these reduced electrode sets resulted in improved speech recognition over using the of the electrodes in their array. These studies did not compare the results of using different psychophysical metrics to exclude electrodes for a group of listeners nor did they investigate speech recognition performance as a function of the number of electrodes excluded from the array.

In this work, three different psychophysical metrics were used to obtain a multidimensional estimate of the potential "usefulness'' of each electrode. These results were then used to inform two different methods of psychophysics-motivated electrode selection. The first method incorporated individual data into each listener's energy-driven speech processing strategy. For each time window, the electrodes with the highest energy that were also most likely to be perceived, according to the psychophysical data, were selected for stimulation. The second method sequentially excluded the electrodes with the highest potential to confound from the array across all time windows, resulting in a group of psychophysics-motivated electrode sets for each metric. Evaluating each of these electrode sets exhaustively would require a prohibitive amount of experimental time. To mitigate this problem, an adaptive procedure was developed to estimate performance as a function of cochlear implant parameters in a time-efficient manner. For each metric, the procedure estimated the set with the highest estimated probability of correct phoneme identification. Listeners' speech recognition performance using this electrode set was then compared to their performance using their full electrode array. For both electrode selection methods, listeners' speech recognition scores were generally comparable to those obtained in the clinical condition. This finding supports the hypothesis that listeners were not perceiving all the information presented to them using their clinical speech processing strategy and their complete set of electrodes. Additionally, these results suggest that improvements to the proposed electrode selection strategies should be in investigated in order to increase the amount of perceivable information presented to cochlear implant listeners.