Visually induced reactivity in posterior cerebral artery blood flow

To evaluate visually induced reactivity (VIR) in the posterior cerebral artery (PCA), mean flow velocities in the PCA were measured bilaterally in 35 normal subjects and in 17 patients with PCA territory infarctions, by means of transcranial Doppler ultrasound. After the individual PCA baseline flow was estimated, different visual stimuli were applied: on-off light, colored light, complex scene, and visual imagery task, and the CO2 test was administered. A sampling rate of 20 Hz was used, and the raw data were transferred to a computer. The baseline flow and the maximum flow increase were calculated with a specially designed program. In control subjects, the on-off light stimulus induced a mean increase in PCA flow velocities of 21.5+/-6.4%, and colored light induced an increase of 22.3+/-6.3%. Complex scenes significantly elevated VIR more than light and colored light, with a mean increase of 28.8+/-6.8% (p < 0.05). Mental imagery had no significant effect on PCA flow velocities. There was no significant difference in flow between the right and left PCA in healthy subjects. In patients with PCA territory infarctions with homonymous hemianopsia or quadrantanopsia, there was a marked decrease of VIR and CO2 reactivity on the affected side corresponding to the extent of PCA territory infarction. Visual stimuli increased blood flow velocity bilaterally in the PCA, which supply the visual cortex and visual association area. This noninvasive test seems to be well suited to normal subjects and to patients with vascular disorders affecting the PCA.