Oculomotor behavior of blind patients seeing with subretinal visual implant Alpha IMS

Abstract

 

Purpose

 

The CE-approved Retina Implant Alpha IMS senses light through the eye’s optics and subsequently stimulates retinal bipolar cells via ~1500 independent pixels. Because it is directly implanted beneath the fovea, it can harness the benefit of eye movements in vision. However, so far, oculomotor strategies of patients with light sensitive subretinal implants have not been characterized.

 

Methods

 

Eye movements were tracked in two patients after presentation of white geometric shapes (eg. squares, circles) on a dark background non-invasively using a high-speed, high-resolution video-based system. Patients reported seeing them by pressing a button.

 

Results

 

1. Once patients visually localized shapes, they fixated well and exhibited classic oculomotor fixational strategies, including generating microsaccades and drifts (Fig. 1). 2. Saccade/microsaccade frequency varied during periods of visibility of a stimulus such that these eye movements tended to be fewer right before losing a percept compared to when first acquiring one 3. Gaze location corresponded to the location of the stimulus, and stimulus shape/size were reflected in saccade direction and size.

 

Conclusions

 

There is a clear relation between perception and ability to fixate a target via a subretinal implant. The impact of saccades/microsaccades on "refreshing" images can be strong at retinal level. Our results pave the way for eye tracking in subretinal implant patients, not only to understand their oculomotor behavior, but also to design oculomotor training strategies that can improve their quality of life.  

 

Sample trial (A) Horizontal bars indicate periods in which the patient reported seeing/not seeing the stimulus. Eye movements were large (arrows) when the patient could not see the stimulus but fixation became stable during a percept. (B) Magnified representation of stable fixation during a percept. Classic fixational drifts and microsaccades (example-arrows) occurred. (C) Eye position with two visibility intervals separated by several seconds without. During intervals, eye position showed repeatable behavior. (D) Eye position trajectories for the two percept intervals in (C). In both intervals, percept onset/offset occurred at specific eye positions, which presumably brought the implant onto/away from the stimulus. Dashed circle shows stimulus shape/size, and provides a reference for the amount of drift needed before the stimulus was lost.