title: Chronic exposure to broadband noise at moderate sound pressure levels spatially shifts tone-evoked responses in the rat auditory midbrain.
abstract: Noise-induced hearing disorders are a significant public health concern. One cause of such disorders is exposure to high sound pressure levels (SPLs) above 85 dBA for eight hours/day. High SPL exposures occur in occupational and recreational settings and affect a substantial proportion of the population. However, an even larger proportion is exposed to more moderate SPLs for longer durations. Therefore, there is significant need to better understand the impact of chronic, moderate SPL exposures on auditory processing, especially in the absence of hearing loss. In this study, we applied functional magnetic resonance imaging (fMRI) with tonal acoustic stimulation on an established broadband rat exposure model (65 dB SPL, 30 kHz low-pass, 60 days). The auditory midbrain response of exposed subjects to 7 kHz stimulation (within exposure bandwidth) shifts dorsolaterally to regions that typically respond to lower stimulation frequencies. This shift is quantified by a region of interest analysis that shows that fMRI signals are higher in the dorsolateral midbrain of exposed subjects and in the ventromedial midbrain of control subjects (p<0.05). Also, the center of the responsive region in exposed subjects shifts dorsally relative to that of controls (p<0.05). A similar statistically significant shift (p<0.01) is observed using 40 kHz stimulation (above exposure bandwidth). The results suggest that high frequency midbrain regions above the exposure bandwidth spatially expand due to exposure. This expansion shifts lower frequency regions dorsolaterally. Similar observations have previously been made in the rat auditory cortex. Therefore, moderate SPL exposures affect auditory processing at multiple levels, from the auditory cortex to the midbrain.
authors.affiliation: Department of Physics and Materials Science, The City University of Hong Kong, Hong Kong, China. Electronic address: condon.lau@cityu.edu.hk.
authors.forename: Condon
authors.initials: C
authors.lastname: Lau
authors.affiliation: Osborne College of Audiology, Salus University, Elkins Park, PA, USA.
authors.forename: Martin
authors.initials: M
authors.lastname: Pienkowski
authors.affiliation: Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong, China; Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China.
authors.forename: Jevin W
authors.initials: JW
authors.lastname: Zhang
authors.affiliation: Division of Speech and Hearing Sciences, The University of Hong Kong, Hong Kong, China.
authors.forename: Bradley
authors.initials: B
authors.lastname: McPherson
authors.affiliation: Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong, China; Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China; Department of Anatomy, The University of Hong Kong, Hong Kong, China; Department of Medicine, The University of Hong Kong, Hong Kong, China.
authors.forename: Ed X
authors.initials: EX
authors.lastname: Wu
mesh_terms: D000161:Acoustic Stimulation; D000818:Animals; D001931:Brain Mapping; D005260:Female; D008279:Magnetic Resonance Imaging; D008636:Mesencephalon / Q000503:physiopathology* / Q000528:radiation effects; D009622:Noise / Q000009:adverse effects*; D011312:Pressure / Q000009:adverse effects*; D011833:Radiation Injuries, Experimental / Q000503:physiopathology*; D051381:Rats; D017207:Rats, Sprague-Dawley
keywords: Acoustic noise exposure; Auditory processing; Auditory system; Functional magnetic resonance imaging; Rat