Altered Eye Movements During Reading With Simulated Central and Peripheral Visual Field Defects

Abstract

Purpose: Although foveal vision provides fine spatial information, parafoveal and peripheral vision are also known to be important for efficient reading behaviors. Here we systematically investigate how different types and sizes of visual field defects affect the way visual information is acquired via eye movements during reading.

Methods: Using gaze-contingent displays, simulated scotomas were induced in 24 adults with normal or corrected-to-normal vision during a reading task. The study design included peripheral and central scotomas of varying sizes (aperture or scotoma size of 2°, 4°, 6°, 8°, and 10°) and no-scotoma conditions. Eye movements (e.g., forward/backward saccades, fixations, microsaccades) were plotted as a function of either the aperture or scotoma size, and their relationships were characterized by the best fitting model.

Results: When the aperture size of the peripheral scotoma decreased below 6° (11 visible letters), there were significant decreases in saccade amplitude and velocity, as well as substantial increases in fixation duration and the number of fixations. Its dependency on the aperture size is best characterized by an exponential decay or growth function in log-linear coordinates. However, saccade amplitude and velocity, fixation duration, and forward/regressive saccades increased more or less linearly with increasing central scotoma size in log-linear coordinates.

Conclusions: Our results showed differential impacts of central and peripheral vision loss on reading behaviors while lending further support for the importance of foveal and parafoveal vision in reading. These apparently deviated oculomotor behaviors may in part reflect optimal reading strategies to compensate for the loss of visual information.

Figure 1.

(A) Schematic diagram of the visual field. The approximate size of each sub-region of the visual field (fovea, parafovea, and perifovea) in degree units was given. (B) Task procedure. The sequence of one trial under a central scotoma condition was shown as an example. (C) Examples of text stimuli. (i) Five sentences within one text page under intact viewing (no scotoma) condition. (ii) Text page with simulated peripheral scotoma with a 6° aperture. (iii) Text page with simulated central scotoma with a 6° diameter. (D) Peripheral scotoma with five aperture sizes in degree units and the corresponding number of letters visible. (E) Central scotoma with five scotoma sizes in degree units and the corresponding number of letters masked. The orange bars in each example indicate the diameter of the central scotoma or the aperture of the peripheral scotoma (in degree units). For ease of visibility in the figure, the luminance of the scotoma is rendered two times darker than the original luminance.

Figure 2.

The relationship between reading speed and simulated visual field defects. The left and right panels plot reading speed (wpm) as a function of the size of the peripheral scotoma (i.e., aperture size of 2°, 4°, 6°, 8° and 10°), and the size of the central scotoma (i.e., diameter size of 10°, 8°, 6°, 4° and 2°) in log-linear coordinates, respectively. Each data point is the average value across all subjects in each condition (n = 24 for the peripheral scotoma condition, and n = 21 for the central scotoma condition). The critical aperture (or scotoma) size was shown with orange dashed arrows. Note that the sixth level (i.e., the rightmost datapoint on the x-axis) in both plots represents no scotoma (intact viewing) condition. For the peripheral scotoma condition, a visual angle of 33° sufficiently covers the text passage displayed on the screen, and was substituted for the aperture size of no scotoma condition for the sake of model fitting.

Figure 3.

Examples of eye movements during reading under (A) Intact viewing (no scotoma), (B) severe peripheral scotoma viewing (2° aperture), and (C) severe central scotoma viewing (10° diameter). Green circles represent a person's fixational eye movements. The radius of the circles indicates fixation duration with bigger circles corresponding to longer fixations. Note that each fixation position of the central scotoma condition represents the center point of the central scotoma as there is no method to accurately define which part of the visual field (or the retina) our subjects used for reading under this condition. Note that for ease of the visibility, the luminance of the text background was made darker than the original one.

Figure 4.

The relationships between the pattern of eye movements and different types of simulated visual field defects. The left and right panels plot each eye movement parameter as a function of the peripheral scotoma size (i.e., aperture size of 2°, 4°, 6°, 8° and 10°), and the central scotoma size (i.e., diameter size of 10°, 8°, 6°, 4° and 2°) in log-linear coordinates, respectively. Each data point is the average value across all subjects in each condition (n = 24 for the peripheral scotoma condition and n = 21 for the central scotoma condition). Note that the sixth level (i.e., the rightmost datapoint on the x-axis) in both plots represents no scotoma (intact viewing) condition. For the peripheral scotoma condition, a visual angle of 33° that sufficiently covers the text passage displayed on the screen, was substituted for the aperture size of no scotoma condition for the sake of model fitting. (A) Saccade amplitude (°). (B) Saccade velocity (°/sec). (C) Fixation duration (ms). (D) Number of fixations per line. (E) Proportion of regressive saccades (%). (F) Microsaccade rate (no./sec). (G) Microsaccade amplitude (°). The orange panels on the bottom represent a schematic diagram of changes in aperture or scotoma size. The critical aperture (or scotoma) size was shown with orange dashed arrows.

Figure 5.

Distribution of microsaccades and saccades. Probability density maps of (A) microsaccades and (B) saccades are shown in two-dimensional polar maps representing the visual field. Each density map shows the data from all subjects in each condition. Note that saccade maps are based on the data from both regular saccades and microsaccades. Density maps are compared for severe peripheral scotomas (2° and 4° aperture), severe central scotomas (10° and 8 ° diameter), and intact viewing (no scotoma). The color bar indicates the colors corresponding to different probability density values. The radii of the polar plots and the numbers in red color indicate retinal eccentricity in degree units.