Abstract
Purpose:
People with central field loss (CFL) can learn to use a Preferred Retinal Locus (PRL) in peripheral vision as a substitute fovea. The location of the PRL is generally measured monocularly, however people with CFL typically have asymmetric monocular scotomata forming a compound binocular scotoma. We simulate CFL with independent monocular scotomata in each eye of normally sighted observers to examine binocular visual function and oculomotor control with asymmetric vision loss.
Methods:
Gaze contingent binocular scotomata were simulated in 11 normally sighted participants using an Eyelink 1000Hz eye tracker and 3D stereoscopic shutter glasses. A Gaussian (σ=1°, 2°, 4° or 8° in different sessions) windowed patch of 100% contrast pink noise was positioned at the point of gaze of each eye on a 144Hz computer display (Lmean=25 cd/m2). In each session, observers were instructed to freely-view the screen and to identify a sequence of 50 black (0.1 cd/m2) SLOAN letters presented at the screen centre. Each letter was presented until the observer responded and its size was under control of an adaptive staircase. Binocular eye positions were recorded throughout each session so that retinal position of the target, fixation stability and vergence could be determined.
Results:
As the scotoma size increased, fixation shifted to a greater eccentricity (p<0.05) and fixation instability increased (p<0.05), in line with previous studies of monocular PRL behaviour, however visual acuity did not significantly worsen with scotoma size (p=0.07). In addition, spontaneous binocular fixation patterns differed for different scotoma sizes: for the 2 smaller scotomata, observers made vergence eye movements that brought the target onto a ‘disjunctive PRL’ (p<0.05); for the 2 larger scotomata, observers made conjugate eye movements that brought the target onto a ‘conjunctive PRL’.
Conclusions:
Simulation of independent binocular CFL reveals strategic oculomotor behaviour that cannot be predicted from studies of real or simulated monocular CFL.<br /> Observers spontaneously adopt a PRL that under some conditions is in a correlated location in each eye - conjunctive PRL. However, for small scotomata the PRL may be in uncorrelated locations in each eye - disjunctive PRL - which suggests that they may use binocular vergence eye movements to bring a target into a sighted area of their 3D visual field.