Abstracts

The baboon ([italic]Papio hamadryas[/italic] spp) offers a natural primate animal model of photosensitive generalized epilepsy. This study compared changes in cerebral blood flow (CBF) during intermittent light stimulation (ILS) between photosensitive and asymptomatic baboons., Six photosensitive and four asymptomatic baboons, matched for age, gender and weight, were selected based upon previous scalp EEG evaluation. Continuous intravenous ketamine (5-13 mg/kg) was used for sedation. Subjects underwent five sequential blood flow PET studies within 60 minutes using 20 mCi ¹⁵O-labelled water. Images were acquired in 3D mode (CTI/Siemens HR+ scanner, 63 contiguous slices, 2.4mm thickness). Three resting scans were alternated with two activation scans. ILS was performed at 25 Hz for 60 seconds before to 60 seconds after the start of an activation scan. PET images were coregistered with MRI (3T Siemens Trio, T1-weighted 3D sequence, TE=6, TR=14, TI=800msec, flip angle 19[ordm], NEX=6). PET scans were reviewed and corrected for motion artifact. Resting scans were contrasted with activation scans and averaged independently for both groups., The controls showed greatest ILS-induced activation in the left middle frontal and inferior temporal gyri, left brainstem structures and right postcentral gyrus, and bilaterally in the occipital lobes, posterior cingulate gyrus and cerebellum. In contrast, the photosensitive animals showed strongest ILS activation in the right anterior cingulate and medial orbital gyri, amygdala, globus pallidum, left inferior and superior temporal gyri, and the pineal gland. A striking finding was the absence of occipital activation during ILS in the photosensitive animals. Deactivations were noted in the right orbitofrontal and anterior cingulate cortices in the controls, and in the posterior cingulate gyrus, brainstem and cerebellum of the photosensitive animals., The patterns of ILS-induced CBF changes differed between control and photosensitive groups. These differences of activations and inhibitions suggest involvement of specific cortical-subcortical networks in photosensitivity., (Supported by National Institutes of Health (P51 RR13986 through the SFBR to CAS, 1 R01 NS047755 to JTW, and by facilities constructed with support from Research Facilities Improvement Program Grants C06 RR15456 and C06 RR014578 from the National Center for Research Resources).)