Factors affecting crowded acuity: eccentricity and contrast

Abstract

Purpose: Acuity measurement is a fundamental method to assess visual performance in the clinic. Little is known about how acuity measured in the presence of neighboring letters, as in the case of letter charts, changes with contrast and with nonfoveal viewing. This information is crucial for acuity measurement using low-contrast charts and when patients cannot use their fovea. In this study, we evaluated how optotype acuity, with and without flankers, is affected by contrast and eccentricity.

Methods: Five young adults with normal vision identified the orientation of a Tumbling-E presented alone or in the presence of four flanking Tumbling-Es. Edge-to-edge letter spacing ranged from 1 to 20 bar widths. Stimuli were presented on a white background for 150 ms with Weber contrast ranging from -2.5% to -99%. Flankers had the same size and contrast as the target. Testings were performed at the fovea, 3°, 5°, and 10° in the inferior visual field.

Results: When plotted as a function of letter spacing, acuity remains unaffected by the presence of flankers until the flankers are within the critical spacing, which averages an edge-to-edge spacing of 4.4 bar widths at the fovea and approximately 16 bar widths at all three eccentricities. Critical spacing decreases with a reduction in contrast. When plotted as a function of contrast, acuity only worsens when the contrast falls below approximately 24% at the fovea and 17% in the periphery, for flanked and unflanked conditions alike.

Conclusions: The letter spacing on conventional letter charts exceeds the critical spacing for acuity measurement at the fovea, at all contrast levels. Thus, these charts are appropriate for assessing foveal acuity. In the periphery, the critical spacing is larger than the letter spacing on conventional charts. Consequently, these charts may underestimate the acuity measured in the periphery because of the effects of crowding.

Figure 1

Subject data with high contrast stimuli at 3 degrees in the lower vision field demonstrating the two line fit of acuity vs. letter spacing. The datum plotted at a letter spacing marked with “∞ represents unflanked acuity. The critical spacing is where the two lines intersect. Error bars indicate the standard deviation between the thresholds from the subject’s two separate staircase runs. To the right of the critical spacing acuity is flat, implying that it is unaffected by crowding. To the left of the critical spacing, adjacent characters are within the “crowding zone”, and thus crowding is evident. The slope in this portion is constrained to −1.

Figure 2

Individual subject data showing acuity versus letter spacing at the four eccentricities tested (different shaded curves in each panel), at all stimulus contrasts. Each column is a given contrast and each row is a particular subject. Error bars indicate the standard deviation between the thresholds from the subject’s two separate staircase runs, and the colored lines show two-line fits. In each plot, the lowest curve comprises the foveal condition, with each successive eccentricity (3°, 5°, and 10°, respectively), stacked above.

Figure 3

Critical contrasts for each eccentricity at the various letter spacings, averaged over all subjects. Error bars represent the standard deviation between the five subjects on the given condition.

Figure 4

Critical spacing plotted as a function of eccentricity for contrasts of −99% (black dots), −12.5% (gray dots), and −2.5% (white dots). Each point represents the average of the five subjects, and error bars indicate the standard deviation. The dotted line shows the spacing of standard chart designs following Bailey-Lovie guidelines, which have 1 letter width (5 bar widths) between each character. Values that fall below the dotted line indicate acuity measurements not limited by crowding based on the letter spacing of a standard letter chart; acuity measurements that fall above the line will be limited by crowding with the letter spacing recommended by Bailey-Lovie chart design. We chose to show the critical spacing for −12.5% contrast to illustrate that for the commercially available low contrast versions of the Bailey-Lovie or ETDRS charts, which have a contrast close to −12.5%, the letter spacing is smaller than the critical spacing in the periphery. Hence, acuity measured using these low contrast charts for patients who cannot view foveally may underestimate the peripheral acuity.

Figure 5

Schematic demonstration of a modified Bailey-Lovie/ETDRS chart with 3 letter widths (15 bar widths) critical spacing. Every other line was removed, and every other character of the remaining lines was removed.