Localizing human visual gamma-band activity in frequency, time and space
Neuronal gamma-band (30 -100 Hz) synchronization subserves fundamental functions in neuronal processing. However, different experimental approaches differ widely in their success in finding gamma-band activity. We aimed at linking animal and human studies of gamma-band activity and at preparing optimized methods for an in-depth investigation of the mechanisms and functions of gamma-band activity and gamma-band coherence in humans. In a first step described here, we maximized the signal-to-noise
... ratio with which we can observe visually induced gamma-band activity in human magnetoencephalographic recordings. We used a stimulus-and task-design that evoked strong gamma-band activity in animals and combined it with multi-taper methods for spectral analysis and adaptive spatial filtering for source analysis. With this approach, we found human visual gamma-band activity very reliably across subjects and across multiple recording sessions of a given subject. While increases in gamma-band activity are typically accompanied by decreases in alpha-and beta-band activity, the gamma-band enhancement was often the spectral component with the highest signalto-noise ratio. Furthermore, some subjects demonstrated two clearly separate visually induced gamma-bands, one around 40 Hz and another between 70 and 80 Hz. Gamma-band activity was sustained for the entire stimulation period, which was up to three seconds. The sources of gamma-band activity were in the calcarine sulcus in all subjects. The results localize human visual gamma-band activity in frequency, time and space and the described methods allow its further investigation with great sensitivity. 12 Gross, J., Kujala, J., Hamalainen, M., Timmermann, L., Schnitzler, A., Salmelin, R., 2001.