Tonotopic organisation of the auditory cortex in sloping sensorineural hearing loss

Although the tonotopic organisation of the human primary auditory cortex (PAC) has already been studied, the question how its responses are affected in sensorineural hearing loss remains open. Twenty six patients (aged 38.1 ± 9.1 years; 12 men) with symmetrical sloping sensorineural hearing loss (SNHL) and 32 age- and gender-matched controls (NH) participated in an fMRI study using a sparse protocol. The stimuli were binaural 8s complex tones with central frequencies of 400 Hz_CF, 800 Hz_CF, 1600 Hz_CF, 3200 Hz_CF, or 6400 Hz_CF, presented at 80 dB(C). In NH responses to all frequency ranges were found in bilateral auditory cortices. The outcomes of a winnermap approach, showing a relative arrangement of active frequency-specific areas, was in line with the existing literature and revealed a V-shape high-frequency gradient surrounding areas that responded to low frequencies in the auditory cortex. In SNHL frequency-specific auditory cortex responses were observed only for sounds from 400 Hz_CF to 1600 Hz_CF, due to the severe or profound hearing loss in higher frequency ranges. Using a stringent statistical threshold (p < 0.05; FWE) significant differences between NH and SNHL were only revealed for mid and high-frequency sounds. At a more lenient statistical threshold (p < 0.001, FDRc), however, the size of activation induced by 400 Hz_CF in PAC was found statistically larger in patients with a prelingual, as compared to a postlingual onset of hearing loss. In addition, this low-frequency range was more extensively represented in the auditory cortex when outcomes obtained in all patients were contrasted with those revealed in normal hearing individuals (although statistically significant only for the secondary auditory cortex). The outcomes of the study suggest preserved patterns of large-scale tonotopic organisation in SNHL which can be further refined following auditory experience, especially when the hearing loss occurs prelingually. SNHL can induce both enlargement and reduction of the extent of responses in the topically organized auditory cortex.