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Abstract
This study investigated learning, in normal-hearing adults, associated with training (i.e. repeated practice) on the discrimination of ongoing interaural time difference (ITD). Specifically, the study addressed an apparent disparity in the conclusions of previous studies, which reported training-induced learning at high frequencies but not at low frequencies. Twenty normal-hearing adults were trained with either low- or high-frequency stimuli, associated with comparable asymptotic thresholds, or served as untrained controls. Overall, trained listeners learnt more than controls and over multiple sessions. The magnitudes and time-courses of learning with the low- and high-frequency stimuli were similar. While this is inconsistent with the conclusion of a previous study with low-frequency ITD, this previous conclusion may not be justified by the results reported. Generalization of learning across frequency was found, although more detailed investigations of stimulus-specific learning are warranted. Overall, the results are consistent with the notion that ongoing ITD processing is functionally uniform across frequency. These results may have implications for clinical populations, such as users of bilateral cochlear implants.
| Acronyms | ||
| AM | = | Amplitude modulated |
| ANOVA | = | Analysis of variance |
| BMLD | = | Binaural masking level difference |
| CI | = | Confidence interval |
| ILD | = | Interaural level difference |
| IPD | = | Interaural phase difference |
| ITD | = | Interaural time difference |
| SAM | = | Sinusoidal amplitude modulated |
| SPL | = | Sound pressure level |
| Acronyms | ||
| AM | = | Amplitude modulated |
| ANOVA | = | Analysis of variance |
| BMLD | = | Binaural masking level difference |
| CI | = | Confidence interval |
| ILD | = | Interaural level difference |
| IPD | = | Interaural phase difference |
| ITD | = | Interaural time difference |
| SAM | = | Sinusoidal amplitude modulated |
| SPL | = | Sound pressure level |
Sumario
Este estudio investigó el aprendizaje en adultos normoyentes, asociado al entrenamiento (p.ej. práctica repetida) de la discriminación de diferencia interaural de tiempo (ITD) en curso. Específicamente en este estudio se señala una aparente disparidad con las conclusiones de estudios previos que reportaban un aprendizaje inducido por entrenamiento en las frecuencias altas, pero no en las frecuencias bajas. Veinte adultos normoyentes fueron entrenados ya fuera con estímulos de altas o bajas frecuencias, asociados con umbrales asintóticos comparables o bien como sujetos control, no entrenados. En general los sujetos entrenados aprendieron más que los sujetos control y en múltiples sesiones. La magnitud y la progresión del aprendizaje con estímulos de frecuencias bajas y altas fue similar. Al ser esta conclusión inconsistente con la de un estudio previo de ITD en frecuencia baja, puede ser que dicha conclusión previa no esté justificada por los resultados reportados. Se encontró generalización del aprendizaje a través de las frecuencias, sin embargo son necesarias más investigaciones sobre el aprendizaje de estímulos específicos. En general los resultados son consistentes con la noción de que el procesamiento de ITD en curso es funcionalmente uniforme a través de las frecuencias. Estos resultados pueden tener implicaciones en la población clínica como en los usuarios de implante coclear bilateral.
Notes
1. A general expression for an AM tone, x(t), is:
where m(t) is the modulator, t is time (s), ωc is the angular carrier frequency (rad s−1), and φc is the starting phase of the carrier (rad). The modulator of the transposed tone, mtrans(t), consists of a half-wave rectified tone. The Fourier series of this is (Hartmann, Citation1998, p. 106):
where ωm is the angular modulation rate (rad s−1) and φm is the starting phase of the modulator (rad); φm was manipulated in the present study to produce an envelope-based ongoing ITD. The equivalent of low-pass filtering the modulator can be achieved by selecting an appropriate value of n. To restrict the rectified tone to the DC component and the first two harmonics, as in the present study, one would set n=2, giving:
Substituting this into Equation (1) and expanding gives an expression for the transposed tone, xtrans(t):
The Fourier series of the transposed tone can be obtained using the following trigonometric identity:
giving:
This analytical approach permits straightforward control over the number of spectral components and the amplitudes and phases of the components.
2. The target coupler sound pressure level (SPL) corresponding to a given loudness level at a given frequency was derived as follows. The difference, in dB, between the appropriate equal-loudness contour and the minimal audible field (for loudspeakers) at the given frequency was determined from ISO 226: Citation2003. This was then added to the reference equivalent threshold SPL (for insert earphones) at same frequency from ISO 389-2: Citation1994 to provide the target coupler SPL. Data from 125 Hz were used for 128 Hz. No correction was made for the use of AM at 4000 Hz as Moore et al (Citation1998) found that the SPLs of unmodulated and AM tones at equal loudness are within 1–2 dB when using modulation rates comparable to that used here.