Characterizing the Visual Cortical Hemodynamic Response in Retinal Prostheses Users

Abstract

Purpose : To investigate the visual cortical hemodynamic response (HDR) in the occipital area of patients wearing Argus II retinal prosthesis (Argus II) visual prostheses by using functional near-infrared spectroscopy (fNIRS).

Methods : Blind patients with end-stage retinitis pigmentosa who have the Argus II were invited to participate. Subjects sat in front of a computer screen with a chinrest and viewed patterns in two conditions: via the Argus II camera while both of their eyes were patched, and using their remaining bare light perception (BLP) vision without aid of Argus II. Visual stimuli were four horizontal black-and-white stripes on a uniform gray screen, and the black and white stripes switched locations at 1 Hz. The size of the pattern was adjusted to fit the field-of-view of the Argus II implant. Subjects’ HDR while viewing the visual stimuli or a blank gray screen were measured using the NIRScout system with a 22-channel montage centered over the occipital brain area. The duration of each trial was 50 seconds, including 10 seconds of stimuli presentation and 40 seconds of resting period. Each combination of visual condition (prosthetic/residual) × stimuli (pattern/blank) were recorded in 6 trials. Data were analyzed using the nirsLAB package.

Results : Initial results from one subject with Argus II implanted in the left eye showed an increase of oxyhemoglobin (HbO) concentration and decrease of deoxyhemoglobin (HbR) concentration with visual pattern presentation in most channels, indicating higher brain activity when viewing patterns. More importantly, when using prosthetic vision, pattern presentation resulted in larger decrease in HbR concentration compared to blank in mostly the right occipital and parietal-occipital sites (Statistical Parametric Mapping, t-statistics, p < 0.001). This corresponds to the fact that the perceived location of the pattern is centered in the left visual field for this subject. In contrast, when using the subject’s residual BLP vision, only one single channel in the left posterior parietal-occipital region showed more decrease in HbR concentration in response to pattern than to blank, whose origin was unclear.

Conclusions : fNIRS is a non-invasive neuroimaging modality that can be safely used when the visual implant is turned on. Our results demonstrated a hemodynamic signature that can possibly characterize and evaluate the percept generated by prosthetic vision.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.