Abstracts

Rationale: Asymmetrical cerebral blood flow (CBF) changes due to intermittent light stimulation (ILS) extend beyond the occipital lobe in photosensitive (PS) baboons. As CBF changes in one brain region can be correlated with CBF changes in functionally connected brain regions, covariance analyses can demonstrate connectivity within a network. Covariance analyses were performed to evaluate whether extrastriatal CBF changes in PS baboons correlate with occipital CBF changes. Methods: Eight PS and 8 control baboons, matched for age, gender and weight, were selected based upon previous scalp EEG evaluations. Continuous intravenous ketamine (5-13 mg/kg) was used for sedation during the scans and the baboons were monitored for the level of sedation and epileptic discharges using scalp EEG. Five consecutive H215O-PET studies were acquired within 60 minutes, alternating between activation (ILS at 25 Hz) and resting scans. PET images were acquired in 3D mode (CTI/Siemens HR+ scanner, 63 contiguous slices, 2.4mm thick) and co-registered with MRI (3T Siemens Trio, T1-weighted 3D Turboflash sequence, TE/TR/TI = 3.04/2100/785 msec, flip angle=13 degrees). Inter-regional connectivity was computed voxel-wise for the whole brain as covariance with a seed voxel centered on the ILS-induced CBF response in each occipital lobe. Results: Extrastriate coactivations were noted with left more than right occipital lobe activation in both PS and CTL baboons (Figure 1). Left occipital lobe CBF increases were associated with activation of parietal lobe association cortices and primary motor areas, and deactivation in the medial frontal lobes in PS baboons. Left occipital lobe CBF increases were not associated with any CBF change in the right occipital lobe in controls, but with bilateral coactivation of the lateral geniculate nuclei. Conclusions: These covariance analyses not only confirmed differences in extrastriatal CBF changes between PS and CTL baboons, but also demonstrated differences in coactivation depending upon the side of occipital lobe activation. The more diffuse cortical coactivation with left occipital CBF changes in PS baboons, and bilateral coactivation of the lateral geniculate nuclei in CTL animals, underscores asymmetric visual processing of ILS in the baboon.