Assessing the utility of visual acuity measures in visual prostheses

Abstract

There are presently several ongoing clinical trials to provide usable sight to profoundly visually impaired patients by means of electrical stimulation of the retina. Some of the blind patients implanted with retinal prosthesis reported un-patterned perception and yet benefit from the device in many activities of daily living, seemingly because they adopt active scanning strategies. The aim of the present work is to evaluate if and under what conditions a measured visual acuity level is truly an indication that the brain perceived a patterned image from the electrical stimulation of the visual prosthesis. Sighted subjects used a pixelized simulator in which they perceived either a low resolution sub-sampling of the original image ("normal mode"--patterned vision) or an image that was solely a function of the brightness and size of the original image ("brightness mode"--no patterned vision). Results show that subjects were able to adopt a head scanning strategy that enabled acuity beyond the resolution set by a static view of the stimulus. In brightness mode, i.e. without patterned vision, most subjects achieved a measurable acuity level better than the limit set by the geometrical resolution of the entire array but worse than the limit set by the distance between neighboring simulated pixels. In normal mode all subject achieved acuity level that is better than the geometrical resolution of the simulated pixels. Thus, visual acuity levels comparable with the electrodes/pixels resolution implies that the patient perceives an image with spatial patterns.

Identification of Characters and Localization of Images Using Direct Multiple-Electrode Stimulation With a Suprachoroidal Retinal Prosthesis

“…Firstly, we were able to eliminate the contributions of head scanning to localization, as head scanning can increase the acuity provided by the device over that of the physical limit imposed by the geometric spacing between electrodes. 10 We were also able to eliminate other factors such as nonvisual information that can influence performance with prosthetic vision. 11 Lastly, performance using different stimulation strategies could be directly compared with a high degree of stimulus repeatability.…”

Word recognition: re-thinking prosthetic vision evaluation

“…Given the prostheses' limited field of view, users may learn to use head movements to scan along or trace across the contours of objects. Indeed, Caspi and Zivotofsky (2015) found that subjects were able to discriminate low resolution, non-patterned images of the four alternative Landolt C acuity targets using head scanning or tracing. Motor tracing may also be used to produce correct word recognition responses, since the use of large font sizes (da Cruz et al 2013a, Grant et al 2016) or the use of the prosthesis camera's zoom capability (Nau et al 2014a, Grant et al 2016 allows motor tracing along the high contrast letter strokes (da Cruz et al 2013a(da Cruz et al , 2013b.…”

“…The primary purpose of evaluation is to determine if a device provides pattern vision (Caspi and Zivotofsky 2015) with similar spatial and temporal characteristics to the normal human visual system. A normal visual system is capable of learning spatial patterns and it can generalize from training to novel stimuli and contexts.…”

“…Test objects were also typically presented over a clutter-free and high-contrast background, such as a banana on top of a black velvet cloth (Ahuja and Behrend 2013, Nau et al 2014a, 2014b, Stingl et al 2015, 2017, Edwards et al 2018. Under this paradigm, users of prosthetic systems can learn to develop strategies such as head motor scanning to achieve successful task performance, even in the explicit absence of visual spatial pattern data (Caspi et al 2009, Dorn et al 2013, Caspi and Zivotofsky 2015. These studies have demonstrated an ability to discriminate a few objects (e.g Ahuja and Behrend (2013) and Nau et al (2014a)) or word images (e.g Grant et al (2016)) after extensive training, but it remains unclear if the demonstrated capabilities transfer to novel stimuli or contexts.…”

“…It would be more appropriate to refer to performance on such repeated forced-choice tasks as discrimination. 'Identifying' a few highly trained items could be achieved through distinguishing low-level features, as directly demonstrated by the brightness-only mode in Caspi and Zivotofsky (2015), where the only available information was the brightness and size of the stimulation (no spatial details, such as orientation, were provided). Training discrimination may not generalize, even for the same objects in new contexts.…”