Modulation of Spontaneous Brain Activity during Mental Imagery

Magnetic measurements of average power of human alpha and beta activity over the occipital and parietal areas of the scalp reveal spatially selective suppression of the activity of the occipital cortex when abstract figures are briefly presented visually and subjects simply indicate that they saw the figure. However, the duration of the suppression increases markedly when subjects must indicate whether or not they had previously seen the figure. The reaction time is similarly prolonged during the search of visual memory, and is commensurate with the duration of selective suppression of brain activity. It is also demonstrated that alpha activity is not replaced by beta activity during this suppression, but that power in the beta band is also diminished during memory search. Low correlations between the scalp distributions of power in the beta and alpha bands indicate that partly different neuronal populations give rise to activity of these different frequency bands. Since magnetic fields are negligibly affected by intervening bone tissues, dramatic asymmetries in the distribution of alpha activity across the scalps of individuals and the differences in distribution between individuals cannot be ascribed to differences in skull thickness but are due instead to differences in underlying brain anatomy or function. Nevertheless, a common pattern of suppression of alpha activity is observed across subjects during well-controlled cognitive tasks. This implies that the visual system is involved in mental imagery.