Oculomotor Behaviour and Perceptual Localisation in Retinal Prostheses

© 2020 Samuel Andrew TitchenerProsthetic vision is an emerging technology aiming to provide artificial vision to the profoundly blind. Present-day visual prostheses provide useful assistance in everyday life but the quality of vision is poor, with only fractional visual field coverage and limited resolution of detail. The retina, optic nerve, or primary visual cortex are electrically stimulated using implanted electrodes to elicit artificial visual percepts. Eye movements cause movement of the percept within the visual field, however, in many devices, electrode activity is modulated by images captured by a head-mounted camera that does not move in conjugate with eye movement. Percept locations are therefore dissociated from the real-world, leading to localisation errors. Users must be trained to use head movement, rather than eye movement, to control the camera, potentially increasing the difficulty of using the device for every-day activities. This thesis explores oculomotor behaviour in prosthetic vision and investigates the use of eye tracker feedback to redirect the video input in real time (‘gaze compensation’) to restore naturalistic control of gaze. The first study presented in the thesis investigated the effect of visual field loss on eye and head movement coordination in low-vision subjects with retinitis pigmentosa (RP), the current primary indication for retinal prostheses. Visual field loss was found to be associated with a habitually confined range of eye movement and a greater reliance on head movement. This has implications for training and rehabilitation in visual prostheses, as recipients with RP express atypical eye and head scanning behaviour. An investigation of the oculomotor behaviour of retinal prosthesis recipients in a forced-choice localisation task and a motion discrimination task is also presented. Although the participants were aware of the potential for eye movements to impair task performance, systematic eye movements were observed in response to the task stimuli. These were interpreted as reflexive eye movements made in response to the static and dynamic stimuli, as would be expected in normal vision, suggesting preserved oculomotor capacity. This is a promising indication for the naturalistic integration of artificially evoked percepts into the visual system, but the primary purpose of these eye movements, namely foveation, cannot be fulfilled without gaze compensation. Following the demonstration of naturalistic eye movement in retinal prosthesis users and the finding that suppression of eye movement was difficult or impossible, the thesis then examines the effect of eye movement on localisation and the possible benefits of gaze compensation. It was found that eye movement lead to localisation errors in a target localisation task in simulated prosthetic vision, but the introduction of gaze compensation resolved this. A subsequent pilot study in retinal prosthesis recipients is also presented, in which no benefit of gaze compensation for localisation was observed, possibly because the subjects had learned compensatory strategies. However, some methodological problems were identified, and similar studies from a different group do show a benefit of gaze compensation. Overall the thesis advances the understanding of the perceptual experience and oculomotor behaviour of visual prosthesis users and argues for the integration of gaze compensation in camera-based visual prostheses