Silent Functional Magnetic Resonance Imaging (fMRI) of Tonotopicity and Stimulus Intensity Coding in Human Primary Auditory Cortex

Objectives. The aims of this study were to determine the feasibility of obtaining auditory cortex activation evoked by pure tones presented at threshold and suprathreshold hearing levels, to evaluate tonotopicity of the primary auditory cortex, and to determine the effect of stimulus intensity on auditory cortex activation using silent functional magnetic resonance imaging (fMRI). Methods: Sixteen subjects with normal hearing underwent silent fMRI. An audiometer was used to deliver pure tones of 1,000, 2,000, and 4,000 Hz to the left ear. Two levels of acoustic stimulation were used: 1) threshold, hearing level determined in the scanner room and 2) suprathreshold, 70 dB hearing loss (HL). Tonotopicity and stimulus intensity coding was assessed on the basis of the location, extent, and amount of the auditory cortex activation. Results: The localization of activation moved to more medial and posterior regions of the primary auditory cortex as the frequency of the pure tone increased. Compared with a threshold stimulus, a suprathreshold stimulus evoked the same regions with increased spatial extent. The average increase in the right auditory cortex activation in response to suprathreshold stimulus was 57% at 1,000, 51% at 2,000, and 45% at 4,000 Hz compared with that activated by the threshold stimulus. Conclusions: Silent fMRI can be used to evaluate auditory cortex activation using low-intensity stimuli. The level of stimulus intensity increases the amount of auditory cortex activation and influences the fMRI mapping of the tonotopic organization of the primary auditory cortex.