Maria A. Pastor, 1 Julio Artieda, 1 Javier Arbizu, 2 Josep M. Marti-Climent, 2 Ivan Pen ˜uelas, 2 and
Jose C. Masdeu 1
Departments of 1 Neurology and 2 Nuclear Medicine, University of Navarra School of Medicine, 31080 Pamplona, Spain
We used functional brain imaging with positron emission to-mography (PET)-H 2 15 O to study a remarkable neurophysiological finding in the normal brain. Auditory stimulation at various frequencies in the gamma range elicits a steady-state scalp electroencephalographic (EEG) response that peaks in amplitude at 40 Hz, with smaller amplitudes at lower and higher stimulation frequencies. We confirmed this finding in 28 healthy subjects, each studied with monaural trains of stimuli at 12 different stimulation rates (12, 20, 30, 32, 35, 37.5, 40, 42.5, 45, 47.5, 50, and 60 Hz). There is disagreement as to whether the peak in the amplitude of the EEG response at 40 Hz corresponds simply to a superimposition of middle latency auditory evoked potentials, neuronal synchronization, or increased cortical synaptic activity at this stimulation frequency. To clarify this issue, we measured regional cerebral blood flow (rCBF) with PET-H 2
15 O in nine normal subjects at rest and during auditory stimulation at four different frequencies (12, 32, 40, and 47 Hz) and analyzed the results with statistical parametric mapping. The behavior of the rCBF response was similar to the steady-state EEG response, reaching a peak at 40 Hz. This finding suggests that the steady-state amplitude peak is related to increased cortical synaptic activity. Additionally, we found that, compared with other stimulation frequencies, 40 Hz selectively activated the auditory region of the pontocerebellum, a brain structure with important roles in cortical inhibition and timing.
Key words: steady-state auditory evoked potentials; gamma oscillatory activity; regional cerebral blood flow; positron emission tomography; cerebellum; auditory cortex