Suppression of spontaneous cortical activity during visual and verbal tasks
The magnetic field associated with spontaneous activity in the alpha bandwidth (8-12 Hz) reveals spatially selective suppression over the head, for various lengths of time, depending on the cognitive task. Magnetic recordings (MEG) from either occipital or fronto-temporal areas were made while subjects performed mental rotation and verbal tasks. In each task the subject had to make a decision about the relationship between two consecutive stimuli. In the mental rotation task, the second polygon was either rotated at various degrees or displayed as if reflected in a mirror relative to the first polygon; in the verbal task the second word was either orthographically similar to, or rhymed with the first word. The suppression of alpha does not appear as a global attentional effect since its onset time in the fronto-temporal area is about 200 msec after the onset time measured in the occipital region. This time delay reflects the time needed for communication between the cortical areas. For both tasks the stimuli are given visually, and therefore alpha is suppressed over the occipital area in both tasks, but its duration is markedly longer for the mental rotation task. Over the fronto-temporal areas an asymmetry of the duration of the suppression between the two hemispheres was found. For the verbal task, the duration is longer over the left temporal than over the right temporal and the reverse is true for the mental rotation task. This shows that the changes of alpha pattern, both in the domains of time and space, reflect the areas of the brain which are involved in performing cognitive tasks.
0989: Physiological psychology