Prosthetic Visual Acuity in Irregular Phosphene Arrays Under Two Down-Sampling Schemes: A Simulation Study

Abstract

To investigate the effects of phosphene array irregularity in visual prostheses, a model of phosphenes' positional uncertainty was devised and two different image down-sampling schemes, adapting and non-adapting, were proposed. Based on a simulation system, visual acuity tests were given to four normally sighted subjects under seven degrees of array irregularity and the two down-sampling schemes. With the irregularity index increasing from 0 to 0.6, visual acuity fell over a range of 0.22 logMAR (logarithm of minimum angular resolution) for the adapting scheme and 0.47 logMAR for the non-adapting scheme, both monotonously. Comparison between the two down-sampling schemes was made. For low array irregularity, non-adapting down-sampling afforded higher visual acuity, whereas for higher irregular the adapting approach did. Head movements were observed to play significant roles in the acuity tests.

A quantitative analysis of head movement behaviour during visual acuity assessment under prosthetic vision simulation

“…Cha et al [11] suggested that under such circumstances subjects directed their gaze so as to 'choose' the best viewing position in order to identify the orientation of the Snellen E, thus improving visual acuity scores. With irregular phosphene matrices, Cai et al [12] noted that subjects learned to select areas with denser phosphenes for scanning in order to maximize the spatial frequency content of the stimuli, supporting the self-directed maximization hypothesis. Head movements also allowed subjects to expand their effective fields of view and rudimentary depth perception through parallax [13].…”

“…Furthermore, though a static image of phosphenes may simply appear like discrete dots, when relative movement is induced via scanning, a more coherent structure of the stimulus emerges. The existing literature on simulated prosthetic vision tends to attribute this to visual memory and mental imagery [12,[14][15][16][17][18][19], filling-in the lack of visual information in the gaps between the discrete phosphenes by accumulating the visual information through temporal integration. This is believed to subserve the identification of spatial details beyond the sampling-theoretic limit [20] of the phosphene matrix [12,14,17,21].…”

“…The existing literature on simulated prosthetic vision tends to attribute this to visual memory and mental imagery [12,[14][15][16][17][18][19], filling-in the lack of visual information in the gaps between the discrete phosphenes by accumulating the visual information through temporal integration. This is believed to subserve the identification of spatial details beyond the sampling-theoretic limit [20] of the phosphene matrix [12,14,17,21]. For example, studies in our laboratory have demonstrated the ability to identify details 3.5-fold smaller than that suggested by the theoretical limit [17].…”

“…The SPV experimental paradigm has been used by other researchers to examine visual acuity [11,12,14], object recognition [14], face recognition [15], reading [14,18,19,[22][23][24][25], navigation [13] and hand-eye coordination tasks [14], as well as in our laboratories to study object tracking [26,27], visual acuity [16,17,21] and head movements [28,29]. This methodology allowed controlled examination of normally sighted subject response and task performance to a simulation of prosthetic vision.…”

Perceptual learning in a non-human primate model of artificial vision

“…Contemporary artificial vision devices are based on inducing small, discrete visual percepts, called phosphenes, generated with electrical stimulation12345678. Users of these devices face the task of grouping collections of discrete elements into perceptual wholes to recognize visual content.…”