Effect of conductive hearing loss on the activation pattern of the primary auditory cortex as assessed by functional MRI

Objectives.  To investigate the activation pattern of the human auditory cortex to monaural sound stimuli using functional magnetic resonance imaging (fMRI) in volunteers with normal hearing or unilateral conductive hearing loss. Method.  Ten normal hearing volunteers and seven volunteers with unilateral conductive hearing loss were subjected to a random sequence of monaural sound stimuli and silence, whilst undergoing fMRI in a 3 Tesla scanner. The sound stimuli consisted of narrowband noise (250 Hz and 4000 Hz), as well as wideband noise (250–4000 Hz). A signal averaging algorithm was used to detect the response in the primary auditory cortex. A laterality index was calculated and compared between the study groups. Results.  In the normal hearing group the auditory areas of both hemispheres showed activity in response to monaural stimuli, but the responses were consistently greater in the contralateral primary auditory cortex. Wideband stimuli produced the greatest degree of contralaterality and the largest response across the auditory cortex. The responses to high and low frequency stimuli were found to be conform with established tonotopic organization of the primary auditory cortex. There was no difference in the laterality index between the study groups. Conclusions.  We have developed a non-invasive way to investigate the response pattern of the primary auditory cortex to sound stimuli using fMRI. A similar activation pattern of the auditory cortex was found in both study groups suggesting a preservation of the organisation of the ascending auditory pathways and auditory cortex in unilateral conductive hearing loss.