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J. Z. Xie, G.-J. Wang, H. Jadvar, L. Yow, P. Vaska, Y. Ma, M. Humayun, M. McMahon; PET Assessment in Simulated Retinal Implant Tests. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1189.
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Objective measures that demonstrate activation of the retina, primary visual cortex, and possibly higher association areas will become increasingly more important to understand and hence improve the functionality of retinal prostheses. One such measure is cortical functional imaging using positron emission tomography (PET) with [18F]fluoro-deoxyglucose (18FDG). By simulating the cortical effects of retinal implant in normal controls using visual stimuli that are matched in size, shape, and brightness to electrical stimuli delivered by a retinal prosthesis in blind subjects, we hope to establish a baseline for comparison to the effect of electrical stimulation on brain function in retinal implant patients.
To date, two 18FDG PET scans were performed on each of 6 normal controls on separate days to establish a baseline for comparison to blind subjects. During one scan, subjects were blindfolded followed by 18FDG injection; during the other scan, only subjects’ right eyes were exposed to light flashes from a computer monitor during the 30-minute 18FDG uptake phase. The visual stimulus, which subtended 10° field of view on the retina to mimic the area of the retina covered by the retinal prosthesis, was a flickering square with temporal frequency increasing from 2.5 to 30 Hz. Each PET scan (CTI/Siemens Model 962 Scanner) was acquired for 20 minutes. Corrected metabolic images were analyzed using Statistical Parametric Mapping software (SPM99 from Wellcome Department of Cognitive Neurology, London, UK) to identify regions of cortical activation.
Statistical parametric analysis between visual stimulation and baseline demonstrated significant activation (p=0.05) bilaterally in the primary visual cortex (V1) and extrastriate visual areas (V2, V3). At higher levels of statistical significance (p≤0.01), activation is lateralized to the left occipital cortex with an additional strong activation corresponding to the left middle temporal area (MT).
Visual stimulation in the normal control group demonstrated the expected bilateral activation in the primary visual cortex. Activation in higher visual areas, including V2, V3 and MT, is possibly due to the presence of shape, orientation and temporal-frequency content of the stimulus. The experiment will now be extended to retinal implant patients to study the effect of retinal electrical stimulation on cortical function and for comparison to normal controls.
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