Abstract
Purpose :
This study addresses the unanticipated need to construct a detailed map of a dense phosphene cluster elicited by a cortical prosthesis. The inaugural ICVP implantee received 25 wireless floating multielectrode arrays (WFMAs) in February 2022. Despite wide distribution across the right superior occipital cortex, phosphenes elicited by transcranial stimulation of electrodes within these WFMAs, while located in the lower left visual quadrant, as expected, do not present an even distribution throughout this quadrant. Rather, 10 WFMAs (color coded green in the accompanying figure) produce phosphenes in a dense cluster at ~4° from fixation, while 4 additional ones produced phosphenes in two larger, more peripheral clusters.
Methods :
Absolute mapping techniques are inadequate to locate phosphenes in the central cluster with the precision required to map images from a camera. A relative mapping method was developed in which the subject was shown a reference phosphene for 4 s, paired with a test phosphene blinking twice. The subject then drew a vector representing the direction from the reference to the test phosphene. To test this approach, four electrodes (a “quad”) within each WFMA were selected to pilot test the method, and selected WFMA pairs were presented as both reference and test, with 3 repetitions each.
Results :
The relative method worked well for most pairs, with direction vectors providing standard deviations at or below 10°. However, multiple pairs were reported to be too close to reliably draw a direction. A triangulation procedure was applied, in which the inverse of the angular dispersion was used as a surrogate for the presumed distance between phosphenes. Based on our current results an estimated map was constructed (see accompanying figure), in which 8 of the phosphenes in the central cluster remain unresolved.
Conclusions :
To overcome the problem of coincident phosphenes, we will next explore the use of single and paired electrodes, and expect that the inherently smaller phosphenes they produce will allow better relative localization across WFMAs. As a next stage relative mapping within WFMAs will be attempted; results of those tests will likely be affected by the columnar structure of the visual cortex, which is on the order of the 400 µm inter-electrode spacing of the WFMAs.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.