MODULATION OF THE PARIETOOCCIPITAL ALPHA-RHYTHM DURING OBJECT DETECTION

Changes in the human neuromagnetic alpha rhythm were monitored during an object detection task to study the effects of visual shape processi ng on the parieto-occipital activity. Pictures of coherent meaningful objects, which the observers had to detect, and of disorganized meanin gless non-objects were presented briefly between masks. The non-object s were systematically followed by a higher. level of alpha than the ob jects, the difference emerging on average 400 msec after the stimulus, with a median delay of 130 msec after evoked response onsets in the o ccipital, temporal, and parietal cortices. Without attention to visual shape, the alpha levels did not differ between objects and non-object s. The alpha level was higher after non-objects than missed objects, a nd higher after missed than correctly detected objects, suggesting tha t the alpha level is inversely related to saliency or familiarity of t he object and does not directly reflect visual awareness. The reactive alpha rhythm was generated in the parietooccipital sulcus, which in s everal primate species includes areas belonging to the dorsal visual p athway. According to current views, the parietal cortex produces atten tional signals that filter out irrelevant information in the ventral v isual stream. Our results reinforce the idea of bidirectional interact ion: information derived from visual shape can rapidly modify activity in the parieto-occipital region. The synchronized alpha oscillations may reflect attenuation of occipito-parietal information transfer and disengagement of parietal cortex from object selection.