Auditory Cortical Temporal Processing and Hemispheric Asymmetry Revealed by N1 Dipole Source Activity in Adult Cochlear Implant Users

Lee, Jihyun; Han, Ji Hye; Lee, Hyo Jeong

Abstract

BACKGROUND AND OBJECTIVES
Cochlear implant (CI) changes the way that sound inputs are processed in the brain, possibly causing brain reorganization. Given that the pattern and degree of reorganization are known to be related to speech perception skill, information regarding hemispheric asymmetry can be used to predict behavioral performances in CI users. The current study investigated the hemispheric asymmetry of N1 dipole cortical activity in response to the temporally varied speech sounds and its relationship to speech perception abilities in adult CI users.
SUBJECTS AND METHOD
The cortical activity was recorded from 64 scalp electrodes in 10 CI users and 11 normal-hearing controls. Speech stimuli were synthesized consonant-vowels, the /ba/-/pa/ continuum that ranged from 0 to 50 ms with a 6-step voice onset time (VOT). N1 dipole amplitudes, latencies, and locations were analyzed as a function of VOT and the direction of implantation. Also investigated was the relationship between N1 dipole lateralization and speech perception.
RESULTS
For the N1 dipole location, significant location differences between CI and normal hearing groups were seen at 40 ms VOT in the anterior-posterior direction. For hemispheric asymmetry, the N1 dipole activity in good CI performers was higher in the auditory cortex contralateral to the stimulated ear, while poor CI performers showed greater ipsilateral activity. In addition, a lateralization index at 20 ms VOT showed significant correlation with the /ba/-/pa/ consonant perception scores in noise.
CONCLUSION
The results suggest that the hemispheric asymmetry of N1 dipole activity in response to stimuli of temporally varied speech has a substantial clinical value, and that this can be used to estimate CI speech perception.