The functional anatomy of the normal human auditory system: Responses to 0.5 and 4.0 kHz tones at varied intensities

Most functional imaging studies of the auditory system have employed comple x stimuli. We used positron emisssion tomography to map neural responses to 0.5 and 4.0 kHz sine-wave tones presented to the right ear at 30, 50, 70 a nd 90 dB HL and found activation in a complex neural network of elements tr aditionally associated with the auditory system as well as non-traditional sites such as the posterior cingulate cortex. Cingulate activity was maxima l at low stimulus intensities, suggesting that it may function as a gain co ntrol center. In the right temporal lobe, the location of the maximal respo nse varied with the intensity, but not with the frequency of the stimuli. I n the left temporal lobe, there was evidence for tonotopic organization: a site lateral to the left primary auditory cortex was activated equally by b oth tones while a second site in primary auditory cortex was more responsiv e to the higher frequency. Infratentorial activations were contralateral to the stimulated ear and included the lateral cerebellum, the lateral pontin e tegmentum, the midbrain and the medial geniculate. Contrary to prediction s based on cochlear membrane mechanics, at each intensity, 4.0 kHz stimuli were more potent activators of the brain than the 0.5 kHz stimuli.