For pair stimulation, this was the only alignment operation that was utilized.
#Primary auditory cortex trial
First, data from each simultaneous multi-electrode stimulation trial were adjusted to have the same center of mass that was observed for the phosphenes associated with individual stimulation of the same set of electrodes. We performed three manipulations to adjust for the trial-to-trial variability in phosphene locations associated with multi-electrode stimulation. Whenever multiple trials were used, we report the mean phosphene location across trials. In subject YAO, we also tested additional triplets where the subject reported only the number of phosphenes perceived and only one trial was performed. In subject YBH, we tested every possible pair available based on the implanted electrodes but had the subject report only the number of phosphenes perceived and only 1 trial was used for each pair tested. In subjects YAU and YAY, we tested more multi-electrode groups, the subjects drew each of the phosphenes they perceived, and we typically conducted only 1 trial for each individual electrode or group of electrodes tested. In subjects YAB, YAF, YAH, YAN, and YAO, the subjects drew each of the phosphenes they perceived, and we typically conducted multiple trials for each individual electrode (2–8 trials, median 3), pair (3–7 trials, median 3), or triplet (1–5 trials, median 3) tested. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)Ī variable sampling strategy was used with different subjects and with different blocks of testing in order to optimize the testing we were able to perform in the limited time available with each subject. After stimulation the subject was allowed as much time as necessary to draw the perceived phosphene. Biphasic pulse trains at 200 Hz were delivered for 200–300 ms. Subjects received a warning tone 1 s prior to stimulation and another tone at the onset of stimulation. The subject maintained fixation while electrical stimulation was delivered to of one or more electrodes and then drew the location of the perceived phosphenes on the touchscreen. Subjects performed a letter detection task at a central fixation point while checkerboard patterns were flashed in various locations on the screen. For pairs of electrodes pairs lying within one gyrus (green and blue pair), distance was calculated as the nominal separation on the electrode strip, while for electrodes lying on opposite sides of sulcus (blue and red pair) distance along the cortical surface was calculated in AFNI/SUMA. B) Cross section through occipital cortex near the calcarine fissure illustrating determination of electrode separations. Dashed line indicates location of calcarine fissure. Colored regions here and in all figures indicate predicted location of V1 (white) and V2 (light green) based on standard atlas. A) Surface model of the occipital lobe of one subject showing typical electrode placement for one of our hybrid electrode strips containing both clinical electrodes (large circles, 2 or 3 mm) and research electrodes (small circles, 0.5 mm). Materials and methods 2.1 Subjectsįig. 1 Methods. These results in sighted subjects validate the idea that subsets of the map of visual space can be directly stimulated to robustly convey a visual pattern, but also point out the challenges in assessing the results of multi-electrode stimulation in sighted subjects and indicate the complexity of trying to convey patterns to blind subjects using a VCP. Furthermore, subjects sometimes failed to report at least one of the phosphenes associated with a particular pattern, and in general had difficulty accurately reporting on the pattern of phosphenes perceived with stimulation of more than three electrodes on single trials. However, significant shifts, rotation, and scaling of this pattern are possible on each trial.
In addition, simultaneous stimulation of three or more electrodes typically resulted in a consistent spatial pattern of phosphenes. We found that simultaneous stimulation of surface electrodes separated by distances of less than 4 mm tended to produce perception of a single phosphene while stimulation of those separated by greater distances tended to result in perception of two phosphenes. Here we present the first systematic investigation of the rules governing perception of multiple phosphenes when groups of electrodes over visual cortex are stimulated simultaneously.
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