Functional importance of alpha-activity in the visual cortex during recognition of images and movement.

Twenty-seven studies were carried out on the recognition of the shapes of geometrical figures of different sizes by healthy adults, on the recognition of the direction of movement of a light spot within the field of vision, and of visual illusions produced by rhythmic visual stimulation. Tachystoscopic presentation of figures and the onset of movement were synchronized with different phases of the EEG alpha-rhythm in the occipital region. In controls, stimuli were presented without a shift in the alpha-rhythm. Recognition improved significantly when small figures were presented at relatively late phases of the alpha-wave and when large figures (up to 9 degrees) were presented at relatively early phases. Recognition of the side and direction of apparent movement (in the left or right halves of the visual field and centrifugal or centripetal) depended on the phase of the alpha-wave only for nonuniform (accelerating or decelerating, depending on direction) movement, allowing for the cortical magnification factor. Centrifugal movements in experiments were recognized better than in controls, while centripetal movements were recognized worse, and elicited a relatively long-latency movement response. Diffuse rhythmic light stimulation at the alpha-rhythm frequency produced the illusory percept of a ring or circle in 11 of 12 subjects. The optimal stimulation frequency for this was tightly connected with the dominant alpha-rhythm frequency, with a correlation coefficient of 0.86. The link between these effects and the propagation of the wave process through the visual cortex, as reflected by the EEG alpha-rhythm, is discussed. The data support the hypothesis of Pitts and McCulloch [29], which proposes scanning of the visual cortex by a wave process operating at the frequency of the alpha-rhythm, which reads information from the visual cortex.