Impact of simulated central scotomas on visual search in natural scenes

Abstract

Purpose: In performing search tasks, the visual system encodes information across the visual field at a resolution inversely related to eccentricity and deploys saccades to place visually interesting targets upon the fovea, where resolution is highest. The serial process of fixation, punctuated by saccadic eye movements, continues until the desired target has been located. Loss of central vision restricts the ability to resolve the high spatial information of a target, interfering with this visual search process. We investigate oculomotor adaptations to central visual field loss with gaze-contingent artificial scotomas.

Methods: Spatial distortions were placed at random locations in 25° square natural scenes. Gaze-contingent artificial central scotomas were updated at the screen rate (75 Hz) based on a 250 Hz eye tracker. Eight subjects searched the natural scene for the spatial distortion and indicated its location using a mouse-controlled cursor.

Results: As the central scotoma size increased, the mean search time increased [F(3,28) = 5.27, p = 0.05], and the spatial distribution of gaze points during fixation increased significantly along the x [F(3,28) = 6.33, p = 0.002] and y [F(3,28) = 3.32, p = 0.034] axes. Oculomotor patterns of fixation duration, saccade size, and saccade duration did not change significantly, regardless of scotoma size.

Conclusions: There is limited automatic adaptation of the oculomotor system after simulated central vision loss.

Figure 1

Illustration of spatial distortion in natural image. Distortions were smoothly blended into a random location with a Gaussian (σ_x,y = 1°). Even though these images are probably unfamiliar and the first order statistics of the distorted region do not differ from the rest of the image, the distortion (ringed with a red dashed circle that was not present in the experiments) can be detected with minimum effort.

Figure 2

Mean normalized search time as a function of scotoma size. Search time was defined as the time elapsed between the start of a trial and the mouse button press ending the trial. Error bars show ±95% confidence intervals.

Figure 3

Frequency distribution of saccade amplitude (degrees) for the control condition.

Figure 4

Distribution of log₁₀ mean saccade amplitudes for eight subjects, indicated by the caption. Cumulative proportion of saccade amplitudes for (a) control no scotoma condition, (b) scotoma σ_x,y = 1°, (c) scotoma σ_x,y = 2° and (d) scotoma σ_x,y = 4°. It is important to note that the cumulative proportions for each subject, represented by a different color, sum to unity.

Figure 5

(a) Mean normalized micro-saccade standard deviation as a function of scotoma size for x-axis (dashed line) and y-axis (solid line). (b) Mean bivariate contour ellipse area (BCEA) as a function of scotoma size.

Figure 6

Absolute distance, in degrees, of final fixation locations relative to the geometric center of the spatial distortion. Each plot is a representative sample for (a) control condition for subject LM, (b) σx,y=1° for subject CV, (c) σx,y= 2° for subject SM, (d) σx,y=4° for subject MS.