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Abstract
The advent of bilateral cochlear implants (CIs) has increased interest in learning on binaural tasks, and studies in normal-hearing listeners provide important background information. However, few studies have considered learning with discrimination of interaural time difference (ITD). Here, learning with ITD was explored using stimuli that are more relevant to bilateral CIs than used previously. Inexperienced listeners were trained with envelope-based ITD using high-frequency amplitude-modulated tones with or without an interaural carrier frequency difference (IFD), the former to simulate asymmetrical bilateral CI insertions. All were tested with and without IFD before and after training. In most listeners, ITD thresholds improved substantially with training, not necessarily reaching asymptote after 3000 trials. In these, the magnitude and time-course of learning was larger than anticipated from a previous study with low-frequency ITD. Learning generalized across IFD and the effect of IFD on ITD thresholds at post-test was smaller than reported previously. These results have implications for studies of bilateral CIs, such as the need to provide extensive training to avoid over-estimating any apparent ‘impairment’.
Sumario
El advenimiento de los implantes cocleares bilaterales (CI) ha aumentado el interés en aprender sobre tareas binauriculares, y los estudios en sujetos oyentes normales han aportado importante información pertinente. Sin embargo, pocos estudios han considerado el estudio del aprendizaje con base en la discriminación de diferencias interauriculares de tiempo (ITD). Aquí, el aprendizaje con ITD fue explorado utilizando estímulos más relevantes para los CI bilaterales que los usados previamente. Se entrenó a oyentes sin experiencia con una ITD con base en envolventes que comprendían tonos de alta frecuencia de amplitud modulada, con o sin diferencia interauricular de la frecuencia portadora (IFD), esta última para simular inserciones bilaterales asimétricas de CI. Todos fueron evaluados con y sin IFD antes y después del entrenamiento, sin alcanzar necesariamente la asíntota después de 3000 intentos. En ellos, la magnitud y el tiempo del aprendizaje fue mayor de lo anticipado en un estudio con una ITD de baja frecuencia. El aprendizaje en general en todas las IFD y el efecto de la IFD sobre los umbrales de ITD después de la prueba fue similar a los reportes previos. Estos resultados tienen implicaciones en los estudios de CI bilaterales, tales como la necesidad de proveer amplio entrenamiento para evitar la sobre-estimación de cualquier ‘impedimento’ aparente.
| Acronyms | ||
| AM | = | Amplitude-modulated |
| CI | = | Cochlear implant |
| IFD | = | Interaural carrier frequency difference |
| ILD | = | Interaural level difference |
| ITD | = | Interaural time difference |
| SAM | = | Sinusoidal amplitude-modulated |
| Acronyms | ||
| AM | = | Amplitude-modulated |
| CI | = | Cochlear implant |
| IFD | = | Interaural carrier frequency difference |
| ILD | = | Interaural level difference |
| ITD | = | Interaural time difference |
| SAM | = | Sinusoidal amplitude-modulated |
Notes
1. Unrelated-samples analysis (also known as independent-samples analysis) as opposed to related-samples analysis was conducted on the pre- and post-test scores for each individual separately, for the following reason. The statistical comparison of the mean measurements across sessions for any one listener assumes that the six measurements within each session differ only because of random variations. Thus, the measurements obtained in the two sessions would be expected to be independent, and the unrelated sample analysis would seem the most appropriate choice.
2. Bandwidth depends on the type of amplitude modulation and the modulation rate. SAM tones have half the bandwidth of the transposed tones used here for the same modulation rate. The bandwidth is directly proportional to the modulation rate, such that doubling the modulation rate doubles the bandwidth.