Multi-electrode stimulation evokes consistent spatial patterns of phosphenes and improves phosphene mapping in blind subjects

Abstract

Background: Visual cortical prostheses (VCPs) have the potential to restore visual function to patients with acquired blindness. Successful implementation of VCPs requires the ability to reliably map the location of the phosphene produced by stimulation of each implanted electrode. Objective: To evaluate the efficacy of different approaches to phosphene mapping and propose simple improvements to mapping strategy. Methods: We stimulated electrodes implanted in the visual cortex of five blind and fifteen sighted patients. We tested two fixation strategies, unimanual fixation, where subjects placed a single index finger on a tactile fixation point and bimanual fixation, where subjects overlaid their right index finger over their left on the tactile point. In addition, we compared absolute mapping in which a single electrode was stimulated on each trial, and relative mapping with sequences containing stimulation of three to five phosphenes on each trial. Trial-to-trial variability present in relative mapping sequences was quantified. Results: Phosphene mapping was less precise in blind subjects than in sighted subjects (2DRMS, 16 ± 2.9 vs. 1.9 ± 0.93 ; t (18) ¼ 18, p ¼ <0.001). Within blind subjects, bimanual fixation resulted in more consistent phosphene localization than unimanual fixation (BS1: 4.0 ± 2.6 vs. 19 ± 4.7 , t (79) ¼ 24, p < 0.001; BS2 4.1 ± 2.0 vs. 12 ± 2.7 , t (65) ¼ 19, p < 0.001). Multi-point relative mapping had similar baseline precision to absolute mapping (BS1: 4.7 ± 2.6 vs. 3.9 ± 2.0 ; BS2: 4.1 ± 2.0 vs. 3.2 ± 1.1 ) but improved significantly when trial-to-trial translational variability was removed. Although multi-point mapping methods did reveal more of the functional organization expected in early visual cortex, subjects tended to artificially regularize the spacing between phosphenes. We attempt to address this issue by fitting a standard logarithmic map to relative multi-point sequences. Conclusions: Relative mapping methods, combined with bimanual fixation, resulted in the most precise estimates of phosphene organization. These techniques, combined with use of a standard logarithmic model of visual cortex, may provide a practical way to improve the implementation of a VCP.

Sequential epiretinal stimulation improves discrimination in simple shape discrimination tasks only

“…To improve the clarity of images produced by stimulating through multiple electrodes, recent studies tested a stimulation paradigm that sequentially and rapidly stimulated individual electrodes placed on human visual cortex (Beauchamp et al 2020; Oswalt et al 2021). They found that sequential stimulation produced shapes that were recognized more quickly and accurately than traditional simultaneous stimulation of electrodes.…”

A narrative review of cortical visual prosthesis systems: the latest progress and significance of nanotechnology for the future

“…In this research, the following 2 fixation methods were employed: (I) unimanual fixation, in which a single index finger was placed on a tactile fixation point; (II) bimanual fixation, in which the right index finger on left was overlaid on the tactile point. Additionally, by comparing absolute mapping (the stimulation of a single electrode) and relative mapping (the stimulation of 3 to 5 phosphenes), researchers found that bimanual fixation related relative mapping strategies resulted in the most precise estimates of phosphene organization (76). In conclusion, the above-mentioned techniques, together with a standard logarithmic model of visual cortex, may provide a practical way to improve the implementation of a CVP.…”

Identification of novel coloboma candidate genes through conserved gene expression analyses across four vertebrate species

“…This means that some non-linear dynamics might be missed. Also, several studies reported that higher stimulation amplitudes may give rise to double phosphenes (18,19,23,77), or a reduction of phosphene brightness (23). Furthermore, interactions between simultaneous stimulation of multiple electrodes can have an effect on the phosphene size and sometimes lead to unexpected percepts (6).…”