Cortical auditory evoked potentials and hemispheric specialization of speech in individuals with learning disability and healthy controls: A preliminary study

Left Lateralization of the Cortical Auditory-Evoked Potential Reflects Aided Processing and Speech-in-Noise Performance of Older Listeners With a Hearing Loss

OBJECTIVES

We analyzed the lateralization of the cortical auditory-evoked potential recorded previously from aided hearing-impaired listeners as part of a study on noise-mitigating hearing aid technologies. Specifically, we asked whether the degree of leftward lateralization in the magnitudes and latencies of these components was reduced by noise and, conversely, enhanced/restored by hearing aid technology. We further explored if individual differences in lateralization could predict speech-in-noise abilities in listeners when tested in the aided mode.

DESIGN

The study followed a double-blind within-subjects design. Nineteen older adults (8 females; mean age = 73.6 years, range = 56 to 86 years) with moderate to severe hearing loss participated. The cortical auditory-evoked potential was measured over 400 presentations of a synthetic /da/ stimulus which was delivered binaurally in a simulated aided mode using shielded ear-insert transducers. Sequences of the /da/ syllable were presented from the front at 75 dB SPL-C with continuous speech-shaped noise presented from the back at signal-to-noise ratios of 0, 5, and 10 dB. Four hearing aid conditions were tested: (1) omnidirectional microphone (OM) with noise reduction (NR) disabled, (2) OM with NR enabled, (3) directional microphone (DM) with NR disabled, and (4) DM with NR enabled. Lateralization of the P1 component and N1P2 complex was quantified across electrodes spanning the mid-coronal plane. Subsequently, listener speech-in-noise performance was assessed using the Repeat-Recall Test at the same signal-to-noise ratios and hearing aid conditions used to measure cortical activity.

RESULTS

As expected, both the P1 component and the N1P2 complex were of greater magnitude in electrodes over the left compared to the right hemisphere. In addition, N1 and P2 peaks tended to occur earlier over the left hemisphere, although the effect was mediated by an interaction of signal-to-noise ratio and hearing aid technology. At a group level, degrees of lateralization for the P1 component and the N1P2 complex were enhanced in the DM relative to the OM mode. Moreover, linear mixed-effects models suggested that the degree of leftward lateralization in the N1P2 complex, but not the P1 component, accounted for a significant portion of variability in speech-in-noise performance that was not related to age, hearing loss, hearing aid processing, or signal-to-noise ratio.

CONCLUSIONS

A robust leftward lateralization of cortical potentials was observed in older listeners when tested in the aided mode. Moreover, the degree of lateralization was enhanced by hearing aid technologies that improve the signal-to-noise ratio for speech. Accounting for the effects of signal-to-noise ratio, hearing aid technology, semantic context, and audiometric thresholds, individual differences in left-lateralized speech-evoked cortical activity were found to predict listeners' speech-in-noise abilities. Quantifying cortical auditory-evoked potential component lateralization may then be useful for profiling listeners' likelihood of communication success following clinical amplification.

Development and validation of an automated dichotic double word test in Indian English using MATLAB

OBJECTIVE

Dichotic listening is an experimental paradigm where two different stimuli are presented to the right and left ear simultaneously. Currently, there are no clinically validated full version (Forced recall condition) of dichotic word test in Indian language with established normative. Hence the study involves the development of a MATLAB based Dichotic listening test in Indian English including free recall and forced recall conditions.

METHOD

This study was carried out in two phases. Phase 1 involved development and validation of an automated dichotic listening test using MATLAB software. Phase 2 involved data collection and constructing a normative data on 70 healthy adults (18-50 yrs) and 70 children (10-15 yrs) also establishing test retest reliability on 25 participants for the newly developed test.

RESULTS

Right ear advantage was observed for free recall condition similar to that of classical dichotic effect. Also, there was no significant difference between the right and left ear scores in the two forced recall condition. The test showed moderate to good test-retest reliability for both the conditions.

CONCLUSION

The results showed ideal retention of original dichotic effects for both free recall and forced recall conditions and hence can be used in clinical set to tests disordered population like Central auditory population disorder and Learning disability.