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

Abstract

In most current vision prosthesis designs, head movement is the sole director of visual gaze and scanning due to the head-mounted nature of the camera. Study of this unnatural behaviour may provide insight into improved prosthesis designs and rehabilitation procedures. In this paper, we conducted a psychophysical study to investigate the characteristics of head movements of normally sighted subjects undergoing a visual acuity task in simulated prosthetic vision (SPV). In 12 naïve, untrained subjects, we recorded spontaneous changes in the amount of head movements during SPV sessions compared to control (normal vision) sessions. The observed behaviour continued to be refined until five or six sessions of practice. Increased head movement velocity was shown to be correlated to improved visual acuity performance, up to 0.3 logMAR, an equivalent of detecting details at half the physical size compared to complete deprivation of head movements. We postulate that visual scanning can as much as double the spatial frequency information in prosthetic vision. Increased head movement velocity observed when subjects were attempting smaller test items and for low-pass filtering schemes with higher cut-off frequencies may be further evidence that higher frequency content may be available through visual scanning, unconsciously driving subjects to increase head movement velocity.

Visual Acuity of Simulated Thalamic Visual Prostheses in Normally Sighted Humans

“…In our letter recognition task, optotypes are viewed for a short, fixed time (1,000 ms), whereas other experiments allowed longer periods from 2,000 to 15,000 ms [8], [7] or, more typically, an indefinite viewing period [9], [11], [12], [36], [37], [38], [39], [40]. While the viewing time employed in this study is considered sufficient for reading with natural sight (second-grade children require 720 ms to read a letter while adults require 320 ms [41]), it could be insufficient with artificial sight [38]. Thus, our results most likely reflect minimal visual acuities achievable through thalamic visual prostheses, and should be viewed as conservative estimates in comparison to other reports from the literature.…”

Semantic and structural image segmentation for prosthetic vision

“…Current retinal prosthetic devices are limited to hundreds of electrical receptors, which produce a very limited visual elicitation [10][11][12]. From the actual technologies for retinal implants [13], one of the most active line of research is based on implants with a micro camera that captures external stimuli and a processor that converts the visual information in microstimulations in the implant, as can be seen in Fig 1. Following the computer image paradigm, we can say that the visual information evoked by the implants has very low spatial resolution and very limited dynamic range (only few levels of stimulus intensity are perceived as different) [14][15][16]. Intuitively, from an information theory perspective, the process from the external sensor input to the implant stimuli is analogous to taking a high definition image and convert it to a low resolution, grayscale image with just a few grey levels.…”

Eye Movement Compensation and Spatial Updating in Visual Prosthetics: Mechanisms, Limitations and Future Directions

“…Visual scanning is necessary to fill in the lack of information. For example, although a static image of phosphenes may appear as discrete and disjointed dots, they are integrated into more coherent structural percepts, once the person initiates movements to scan the visual scene (Chen et al, 2006, 2007). This critical enhancement of visual acuity is why head movements have been reported to be essential for implanted patients, since they support visual exploration (Chen et al, 2006, 2007; Dobelle, 2000).…”