Visual Crowding in Glaucoma

Abstract

Purpose: Crowding refers to the phenomenon in which objects that can be recognized when viewed in isolation are unrecognizable in clutter. Crowding sets a fundamental limit to the capabilities of the peripheral vision and is essential in explaining performance in a broad array of daily tasks. Due to the effects of glaucoma on peripheral vision, we hypothesized that neural loss in the disease would lead to stronger effects of visual crowding.

Methods: Subjects were asked to discriminate the orientation of a target letter when presented with surrounding flankers. The critical spacing value (scritical), which was required for correct discrimination of letter orientation, was obtained for each quadrant of the visual field. scritical values were correlated with standard automated perimetry (SAP) mean sensitivity (MS) and optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness measurements.

Results: The study involved 13 subjects with mild glaucomatous visual field loss and 13 healthy controls. Glaucomatous eyes had significantly greater (worse) scritical than controls (170.4 ± 27.1 vs. 145.8 ± 28.0 minimum of visual angle, respectively; P = 0.007). scritical measurements were significantly associated with RNFL thickness measurements (R2 = 26%; P < 0.001) but not with SAP MS (P = 0.947).

Conclusions: In glaucoma patients, a pronounced visual crowding effect is observed, even in the presence of mild visual field loss on standard perimetry. scritical was associated with the amount of neural loss quantified by OCT. These results may have implications for understanding how glaucoma patients are affected in daily tasks where crowding effects may be significant.

Figure 1

A real-world example of visual crowding. While fixating the middle of the keyboard, note the relative ease of detecting and identifying the key to the left (presented in isolation) versus the key on the right (presented with clutter).

Figure 2

Screenshots of the visual crowding test. While fixating the center dot, subjects were required to identify the visual target stimulus (either upright or downright letter “T”) surrounded by radial flanking distractors (letter “H”). Target stimuli were presented at 10° of eccentricity, one quadrant at a time, in random order: (A) upper left quadrant, (B) lower left quadrant, (C) upper right quadrant, and (D) lower right quadrant. Note the variable spacing between the target and the flankers. The distance at which flankers degraded the performance in recognizing the target was considered the critical spacing.

Figure 3

Scatterplot illustrating the relationship between critical spacing and retinal nerve fiber layer (RNFL) thickness.

Figure 4

Scatterplot illustrating the relationship between critical spacing and standard automated perimetry (SAP) 24-2 mean sensitivity (MS).

Figure 5

Examples of psychometric functions (A) obtained in the inferior nasal quadrant of the visual field from a normal patient (B) and a glaucoma patient (C) with their respective SAP and OCT results.

Figure 6

Illustration of the smallest/best (A), the typical (B), and the largest/worst (C) critical spacing results from the normal group, and the respective curves for the glaucoma group (D–F).