Blood flow velocity changes in the middle cerebral artery induced by processing of hierarchical visual stimuli

Transcranial Doppler ultrasound was used to measure mean flow velocities in both middle cerebral arteries while 16 young subjects performed a visual task involving the processing of hierarchically structured stimuli. Specifically, large (global) letters composed of smaller (local) letters were presented, with the subjects' task being to attend either to the local or to the global level and press a button whenever a target on the designated level occurred. Each run was comprised of a 35-sec period of passive stimulation, followed by 65 sec of active task. A highly significant increase of blood flow was detected upon initiation of the active task, which was clearly present after ca. 4 sec. The flow velocity reached a maximum after 20 sec and remained stable for the remainder of the active condition. No hemispheric differences with respect to global or local conditions were observed.