Responsiveness of human cortical activity to rhythmical stimulation: a three-modality, whole-cortex neuromagnetic investigation.

We developed numerical indicators to quantify stimulus-related changes in cortical magnetic signals recorded from nine healthy subjects while they received 1- to 2.5-s trains of 15 stimuli (somatosensory, visual, or auditory in separate runs) at rates from 6 to 14 Hz, intermingled with 1.6-s pauses. A locking index (L) was introduced to quantify how well the responses are time locked to the stimuli and a global change factor (GC) to indicate changes in the whole-cortex oscillatory activity in the 5- to 25-Hz frequency range. The responses were visualized with color-coded images illustrating cortical reactivity for all stimulus rates simultaneously. These color maps clearly showed that the modality-specific cortical signals were enhanced at frequencies corresponding to the stimulus rate during the trains. For somatosensory stimulation the activity in the vicinity of the somatosensory hand area was enhanced at most stimulus rates, suggesting mainly superposition of evoked responses. In individuals with strong posterior resting rhythm, visual stimuli typically entrained activity in the parietooccipital sulcus at stimulus rates close to the main frequency of the spontaneous activity, probably reflecting driving of the intrinsic cortical rhythm, whereas in subjects with little spontaneous parietooccipital rhythm the cortical signal appeared to be composed mainly of visual evoked responses. No modality-specific enhancement was observed during auditory stimulation. During the pauses between the trains, the cortical signals were significantly suppressed compared with the resting condition: The peak activity (7-13 Hz) was modulated within, but also outside, the modality-specific areas, and the signals outside the frequency peaks of maximum power were consistently and reproducibly suppressed over the whole cortex by all stimuli.