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J. Z. Xie, G.-J. Wang, L. Yow, M. S. Humayun, J. D. Weiland, G. Chader, H. Jadvar, S. H. Tsang, J. S. Fowler; PET Study of Light and Transcorneal Electrical Stimulation in Humans. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6215.
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Transcorneal electrical stimulation (TcES) has been used to assess the neural remodeling process following visual loss. Here we use positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG) to evaluate the connection between retina and visual cortex using TcES and compare it with light stimulation in normal sighted controls. The study can help us gain insight into cortical reorganization and serve as a precursor for evaluating retinal degenerative process.
Three FDG PET scans were performed on each of 5 right-handed, normal sighted controls (3 males, 2 females, 36.8±12.4 year of age). All subjects were dark-adapted for 30 minutes prior to each scan. For the baseline scan, subjects were blindfolded during the 30-minute FDG uptake. For the light stimulation scan, the subjects’ right eye was exposed to light flashes from a computer monitor for 30 minutes. The visual stimulus, which subtended a 10° field of view on the retina, was a repeating sequence (30-second duration) of a flashing white square with temporal frequency increasing incrementally from 2.5 to 30 Hz. During the TcES scan, a clinical grade neurostimulator (DS7A, Digitimer, LTD) was connected to a DTL-Plus microfiber corneal electrode (Diagnosys, LLC) that was placed on the subjects’ right eye. A continuous train of rectangular, biphasic pulses (2Hz, 2msec pulse width, 150% of phosphene threshold current amplitude) was delivered to the electrode during the 30-minute FDG uptake. Each PET scan was acquired for 20 minutes. Converted metabolic images were analyzed using SPM5 to identify changes in cortical activity.
Comparing light stimulation to baseline, significant activation (p<0.05) was seen in primary visual cortex (V1) and association visual areas V2 and V3. Comparing TcES to baseline, significant activation (p<0.05) was seen in association visual areas V2 and V3. Light stimulation resulted in activation of visual cortex that mapped to the central visual field, while TcES resulted in upper right visual field stimulation and corresponding visual cortex activation below the calcarine sulcus that mapped to the lower nasal retina.
Light stimulation and TcES in our normal sighted group demonstrated similar cortical activation pattern. We will now extend the experiment to patients with retinitis pigmentosa to study the effect of cortical reorganization as a result of retinal degeneration.
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