Integrating oculomotor and perceptual training to induce a pseudofovea: A model system for studying central vision loss

Abstract

People with a central scotoma often adopt an eccentric retinal location (Preferred Retinal Locus, PRL) for fixation. Here, we proposed a novel training paradigm as a model system to study the nature of the PRL formation and its impacts on visual function. The training paradigm was designed to effectively induce a PRL at any intended retinal location by integrating oculomotor control and pattern recognition. Using a gaze-contingent display, a simulated central scotoma was induced in eight normally sighted subjects. A subject's entire peripheral visual field was blurred, except for a small circular aperture with location randomly assigned to each subject (to the left, right, above, or below the scotoma). Under this viewing condition, subjects performed a demanding oculomotor and visual recognition task. Various visual functions were tested before and after training at both PRL and nonPRL locations. After 6-10 hr of the training, all subjects formed their PRL within the clear window. Both oculomotor control and visual recognition performance significantly improved. Moreover, there was considerable improvement at PRL location in high-level function, such as trigram letter-recognition, reading, and spatial attention, but not in low-level function, such as acuity and contrast sensitivity. Our results demonstrated that within a relatively short time, a PRL could be induced at any intended retinal location in normally-sighted subjects with a simulated scotoma. Our training paradigm might not only hold promise as a model system to study the dynamic nature of the PRL formation, but also serve as a rehabilitation regimen for individuals with central vision loss.

Figure 1

Schematic diagram of study design. The sequence of experimental conditions, duration of each session and a brief description of each session are provided.

Figure 2

Illustration of the gaze-contingent display and task procedure. (a) The circular shaped scotoma (12° of visual angle in diameter) was rendered as a uniform gray patch while the rest of the visual field except a small circular window (5° diameter) was blurred. This clear window appeared at one of the four predetermined locations tangent to the scotoma (i.e., to the right 0°, to the left 180°, above 90°, and below 270° of the scotoma). The location of the clear window was randomly assigned to each subject. (b) Task sequence of the oculomotor and perceptual training (OPT). The OPT consisted of three kinds of visual recognition tasks: (i) face recognition, (ii) object recognition and (iii) word recognition. Each task was tested in a subblock comprised of 30 trials. One block contained three subblocks (one subblock for each task) and took approximately 45 min to 1 hr to complete. Each task followed the same sequence. This sequence shown here is an example of one trial. Each trial was comprised of three phases: Phase 1, target following and recognition, recognizing target as it changes its identity and position (a total of six changes per trial); Phase 2, gaze centering; Phase 3, visual search, searching for the last target object presented in the Phase 1. Subjects were instructed to report whether the target is present or absent amidst either a black solid background (face recognition task) or clutter background (object or word recognition task) with an array of nontarget distracters. (c) Illustration of the no blur experiment. The task procedure was identical to that of the free exploration experiment except that no blur was presented in the periphery. For ease of visibility in the figure, target objects and the clear window are rendered at two times their sizes used in the experiment, relative to the rest of the displayed elements.

Figure 3

Illustrations of the task procedure for pre- and posttests. Changes between pre- and posttests were compared for each task at both trained (the clear window) and untrained locations (on the opposite side with equal eccentricity, 8.5°). In order to ensure our measurements were made at the intended retinal location, all the testing stimuli were displayed on the screen in a gaze-contingent manner using the eye-tracker. (a) Measuring visual acuity. Visual acuity was measured using the psychophysical method of constant stimuli to determine the ability to resolve a gap in the Landolt C.A subjects' task to report the orientation of the Landolt C gap by pressing one of four keys. (b) Measuring contrast discrimination threshold. Contrast discrimination thresholds were measured with a temporal, two-alternative, forced-choice (2AFC) staircase procedure. The two stimulus intervals only differed in stimulus contrast; otherwise they were identical. The subject's task was to judge which stimulus interval contained the higher contrast by pressing one of two keys. (c) Measuring letter recognition. Letter recognition was measured with the trigrams method described in detailed by Kwon and Legge (2012). Trigrams (random strings of three letters) were presented at five letter positions for 200 ms. Subjects were asked to read the letters from left to right as accurately as possible without a time limit. (d) Measuring RSVP reading speed. Oral reading speed was measured with Rapid Serial Visual Presentation (RSVP). The sentences were presented sequentially one word at a time at the same screen location for a given stimulus duration. Subjects were instructed to read the sentences aloud as accurately as possible. (e) Spatial attention task. The effect of spatial attention was measured as the ability to deploy attention to a particular location under an uncued condition. The Landolt Cs were simultaneously presented in each of the four locations 0°, 90°, 180°, and 270° at an eccentricity of 8.5° for 150 ms. For a given location, the direction of rotation (up, down, left, or right) was determined at random in each trial. The Landolt Cs were followed by a postcue interval during which a target location (a red line) was given. Subject's task was to judge the orientation of the Landolt C gap in the target location by pressing one of four keys.

Figure 4

Preferred retinal locus (PRL) for fixation. (a) Probability density maps of the retinal positions of a target object at fixation are shown for all eight subjects (S1–S8). Rows, from left to right: the orientations of the clear window chosen for each two subjects are 0°, 90°, 180°, and 270° in the radial direction. Columns, from top to bottom: a density map estimated from the last (second) block of the free-exploration period, the last block of the explicit-training period, the no blur condition, and the fovea condition. Each block took approximately 1 hr to complete. Each polar plot represents the visual field. The gray patch depicts the central scotoma (12° in diameter). The orange dashed circle represents the region corresponding to the clear window. The red dot marks the location of peak density, which we took as the estimated location of the fixational PRL. The color bar shows the colors corresponding to different probability density values. (b) The mean variance of the fixational PRL as a function of block number. Variance was defined as the bivariate contour ellipse area (BCEA) that encompassed 68% of fixations around the mean. The solid orange lines indicate the average variance value across subjects. Error bounds are ±1 SEM.

Figure 5

First saccade-landing site after each target move. (a) Probability density maps of the retinal position of a target at the completion of the first saccade following target movement (the same format as in Figure 4a). (b) The mean variance (BCEA) of the first saccade-landing site as a function of block number. The solid orange lines indicate the average variance value across subjects. Error bounds are ±1 SEM.

Figure 6

Improvements in recognition and visual search performance. (a) Recognition accuracy (% correct) in the target following and recognition phase (Phase 1) as a function of block number. (b) Visual search accuracy (% correct) in the visual search phase (Phase 3) as a function of block number. (c) Time taken to complete a search trial (only a trial with correct response) as a function of block number. The solid orange lines indicate the average value across subjects. Error bounds are ±1 SEM.

Figure 7

Changes in visual function between pre- and posttests. Improvements in posttests with respect to pretests are shown on the y axis. A value greater than 0 (positive sign) means improvements in performance. On the x axis, “TL” means trained locations (region corresponding to a clear window) while “UL” indicates untrained locations (the opposite from the trained locations). Error bounds are ±1 SEM.