A standard practice in optometry is to measure the resolution acuity of the visual system at near distances. Commonly, near vision is assessed using standardized cards containing printed text/words,1 as opposed to single-letter targets, in which a threshold measure can be ascertained by determining the minimum print size required to correctly identify words presented at arm’s length from the body. Word acuity thresholds provide a good assessment of the functional performance of the visual system on common tasks performed close to the eye such as reading which involves the recognition of printed text that typically span central and the parafoveal areas of the retina. Word acuity thresholds are most important in the assessment of low vision patients2 in whom measures of visual function and performance are more important than those (e.g., single-letter charts) that largely reflect changes to the optics of the eye. For such patients, word acuity thresholds form the basis for treatment such as the power of the near addition or the prescription of magnification as an aid for near work.
Changes in the optics and physiology of the eye frequently lead to refractive error, which reduces the quality/sharpness of the image. Thus, previous studies have sought to examine how image defocus (blur) systematically affects the visual system’s ability to resolve spatial detail.3–12 Smith et al11 investigated how the degree of dioptric (D) blur affected single-letter acuity judgments and reported that the change in minimum angle of resolution (MAR) per diopter of blur was approximately 3.75 over a broad range of blur levels. Other investigations using single-letter acuity have reported comparable MAR/D values ranging from approximately 1 to 4, though the discrepancy in reported values is likely due to differences in the stimuli and testing conditions (e.g., lighting and pupil size) employed in these studies. Single-letter acuity measured at distance, however, is a poor indicator of the functional performance of the visual system.13 More recent studies also characterized how defocus influences near-visual acuity judgments.14 Chung et al4 demonstrated that dioptric blur greater than 2 D increases word acuity thresholds (for words at the threshold print size leading to 80% of the maximum reading speed performance, which in their study was approximately logMAR 0.9) and consequently reduces reading speed by approximately 23%. Also note that while the effect of blur on letter acuity (and grating acuity) bears a linear relationship, its effect on word acuity might be nonlinear. Collectively, these studies show that the resolution acuity of the visual system systematically changes with defocus, but importantly, changes in word acuity thresholds are well correlated with reading behavior and visual performance at near distances.
Previous studies that have characterized the effect of blur on near-visual acuity have typically done so using centrally presented high-contrast stimuli. Accordingly, they can only describe how word acuity is affected by defocus under ideal conditions in which the stimulus is highly detectable and the visual system is most optimal in resolving spatial detail. However, word acuity thresholds have been shown to be highly dependent on eccentricity and luminance contrast. Abdelnour and Kalloniatis15 noted that word acuity thresholds were the lowest for the central 2 degrees of the retina, but quickly increased at peripheral locations of 5 and 10 degrees. Luminance contrast also affected performance with higher thresholds observed for low-contrast targets. However, whether the effect of defocus on word acuity is additionally dependent on retinal eccentricity and luminance contrast is still unknown.
Previous studies using single-letter targets indicate that the visual system is much more tolerant to blur (requiring a greater change in blur to produce a just noticeable change in perception) at peripheral locations of the retina.16,17 This suggests that the effect of blur on word acuity might also change with retinal eccentricity, though whether and how additionally changing luminance contrast might influence performance remains unclear. The present study measures the effect of blur on word acuity as a function of eccentricity and contrast. In particular, we measured word acuity thresholds and determined the rate of change in word resolution per diopter of blur, but include target retinal eccentricity and luminance contrast as independent variables.
In addition to theoretical considerations, the present study has direct clinical implications for the assessment of low vision patients,10 in particular those with reduced contrast sensitivity in which targets are less visible, though note that a reduction in contrast sensitivity might increase perceived contrast consistent with the phenomenon of contrast constancy18,19 and or central visual field dysfunction (e.g., from age-related macular degeneration, AMD), which restricts vision to the periphery.20,21 While providing appropriate visual aid to low vision patients is a complex issue, characterizing the empirical differences between the central and peripheral vision will be useful in deciding what type of optical aids/correction might be best for low vision patients characterized by loss in contrast sensitivity and central vision. It has been shown that visual systems with low resolution tolerate defocus better than high-resolution systems. This difference in perception has been previously noted in patients with low vision in clinical references22,23 and empirically examined by Legge et al.24 The implication here is that because patients with low visual acuity are relatively insensitive to defocus, larger lens changes are necessary when conducting refraction. The results of the present study importantly contribute to the formulation of empirically derived clinical guidelines describing the required amount of effective blur for a just noticeable change in the resolution of targets located at different eccentricities and luminance contrast.
Six observers (three females and three males) volunteered to participate in this study. Anderson and Vingrys25 show that a sample size of five that show an effect implies that at a confidence of p = 0.05, it is possible to propose that at least 50% of the population will display the phenomenon (see also Siegel26). Observers were between 21 and 24 years of age, without history of ocular pathology or visual complaints. Best-corrected vision was 6/6 or better with all observers having a cylindrical correction of less than 0.75 D. All participants were required to have read and understood a participant information sheet and signed a consent form before the commencement of data collection; ethics approval was given by the University of Auckland Human Ethics Committee.
An artificial pupil of 3 mm was used in a trial frame to eliminate the effects of variable depth of focus and make the eye approximately diffraction limited.27 The measured natural pupil size was always larger than the 3-mm artificial pupil. The observer sat in front of a visual display terminal (driven at a frame rate of 80 Hz) with his or her left eye occluded. To minimize the effect of spectacle magnification, soft contact lenses were used to induce optical blur. The observers’ refractive error was corrected with the contact lenses (using spherical equivalent power) and adding positive power to the correction to create optical blur. Compensation was also made when the working distance altered the effective power of the blur (for power greater than 0.25 D, i.e., at working distances of 2 m or less). The observers were instructed to look at a large black fixation cross (6 cd/m2) and words were flashed as black text of various luminances on a white background (luminance of 30 cd/m2). The fixation target was removed after a variable foreperiod and a four-letter word was flashed on the screen for 90 ms using Microsoft PowerPoint fast flash presentation.15,28 Due to saccadic latency being 200 ms,29 the test duration we used eliminated the possibility that the observers were able to make saccadic eye movements towards the eccentrically placed word. The room was fully illuminated throughout the experiment keeping retinal illumination constant through the fixed pupil. The screen position was adjusted to eliminate reflections or glare.
Words were presented to the fovea, and below the point of fixation at 5 and 10 degrees. Common four-letter words were semi-randomly selected from the Oxford dictionary. We chose words that had a fairly even distribution of the first letter of the word and of words with ascenders and descenders to ensure an even distribution of word shape. All words were presented in lower case Times Roman.30 Three different word lists were used for each of the different contrast levels and retinal eccentricities. Each word list consisted of 85 words (thus 255 words in total), and each trial commenced at a different starting position. Using three different lists and different starting points and a total of 255 words minimized the possibility that observers memorized the words. As there were 255 words, there were 255 potential starting positions, and the probability that the same starting location is presented across the four different contrast levels and three different eccentricities was approximately 5%. Also, on average across the different stimulus eccentricities, word contrast levels, and trial repeats conducted in the present study (see below), the same word was presented approximately 30 times.
An initial test distance of 4 m was used and observers moved closer (2, 1, 0.5, and 0.25 m) when the largest size print could not be read. Dioptric blur calculations took into account the working distance to ensure that blur levels were appropriately adjusted. Each presentation was initiated when the examiner clicked the keyboard. The observers verbally reported the word or stated that it was illegible. No feedback was provided to the observer after the response. Each measurement was continued until the words become too small to read. This was the threshold value determined by Method of Descending Limits. The words were presented at various sizes with the resolution acuity calculated using the logMAR principle as specified by Bailey and Lovie.31,32 End point was reached when more than two errors were made on two consecutive sets of equal logMAR value.
The experiment was conducted at different eccentricities of 0, 5, and 10 degrees, using up to eight different blur levels of +0.5, +1.0, +1.5, +2.0, +2.5, +3.0, +3.5, and +4.0 D. There were four different Weber contrast levels of 90, 45, 10, and 4%, which required separate measurements (the 0.5-D steps were not conducted for the 45% contrast condition). Each observer repeated each combination of word contrast, blur, and eccentricity in a randomized order eight times and the mean was taken as the word acuity threshold for that particular condition.
As the pattern of data for the six observers was similar, the mean word acuity threshold (converted to MAR) is shown in Fig. 1 on a log axis as a function of the dioptric blur level. The different panels in Fig. 1 depict data for the four contrast levels used in the study. For each contrast level, word acuity thresholds were plotted separately for 0 (circles), 5 (squares), and 10 degrees (triangles) of eccentricity. Note that at 10 degrees, the majority of the observers were not able to read the 4% contrast words, and consequently this condition was omitted from the figure and the following analysis.
A number of findings present themselves. First, word acuity thresholds were in general affected by word contrast such that decreasing word contrast increased word acuity thresholds. A one-way ANOVA (with data collapsed across different eccentricities and blur levels) confirmed that this data trend was significant (F[3,489] = 73.90, p < 0.0001). Tukey post hoc comparisons of the four levels of word contrast indicate that mean word acuity (M: 13.13, 95% CI [11.99 14.27]) at 90% contrast was not significantly different from when the contrast level was 45% (M: 14.58, 95% CI [12.81 16.36]), p = 0.4892, but was significantly different from contrast levels of 10% (M: 32.71, 95% CI [29.80 35.62]) and 4% (M: 39.65, 95% CI [36.38 42.93]), p < 0.0001. Second, for each of the four contrast levels (i.e., each panel of Fig. 1), changing the level of optical defocus and retinal eccentricity increased word acuity thresholds.
To examine the statistical significance of the effect of both blur and eccentricity on word acuity thresholds, a repeated measures two-way ANOVA was conducted for each of the four contrast levels. Across all levels of word contrasts, these analyses reported main effects for both dioptric blur (90% contrast: F[8,45] = 102.6, p < 0.0001; 45% contrast: F[4,25] = 41.07, p < 0.0001; 10% contrast: F[8,45] = 18.55, p < 0.0001; 4% contrast: F[7,40] = 10.51, p < 0.0001) and retinal eccentricity (90% contrast: F[2,90] = 1755, p < 0.0001; 45% contrast: F[2,50] = 222, p < 0.0001; 10% contrast: F[2,90] = 881.3, p < 0.0001; 4% contrast: F[1,40] = 113.9, p < 0.0001). However, significant interaction effects were also observed at all contrast levels (90% contrast: F[16,90] = 74.19, p < 0.0001; 45% contrast: F[8,50] = 19.17, p < 0.0001; 10% contrast: F[16,90] = 47.90, p < 0.0001; 4% contrast: F[7,40] = 7.87, p < 0.0001). Importantly, an interaction effect suggests that, while increasing blur and eccentricity led to poorer word acuity thresholds, the effect of optical blur on word acuity thresholds was highly dependent on retinal location. As evident in Fig. 1, the effect of blur on word acuity change is greatest in central vision, but is considerably less at peripheral locations of 5 and 10 degrees (see Fig. 2 and below for further discussion).Third, under all contrast conditions, word acuity thresholds converged at the high blur levels.
In summary, these data show that word acuity thresholds are affected by blur, and overall performance changes with luminance contrast and retinal eccentricity. These results agree and replicate the results of Abdelnour and Kalloniatis15 who demonstrated a dependency of word acuity on stimulus contrast and eccentricity, but the present study extends these observations to include the effect of dioptric blur on word acuity.
In the introduction, we questioned whether the effect of blur on word acuity thresholds was dependent on the luminance contrast and retinal eccentricity of the target. To address this question, least squares regression analysis (using a semi–log-linear function of the form y = 10^(αx + β) where α is the slope and β is the y intercept) was conducted to establish the relationship between blur and word acuity thresholds for the different contrast levels and retinal eccentricities used. Estimated lines of best fit are shown in Fig. 1 (solid gray scale lines, average R2: 0.92). The derived slopes of these lines, which indicate the exponential rate of change in word acuity per dioptric blur, are plotted separately in Fig. 2. In Fig. 2, values for the different contrast conditions are grouped according to the retinal position of the target (error bars estimated from the variance of the best-fit lines signify 95% confidence intervals). A one-way ANOVA (collapsed across different word contrasts) confirmed that retinal eccentricity significantly affected word acuity change (F[2,63] = 84.59, p < 0.0001); as evident in Fig. 2, the effect of blur on word acuity is greatest for centrally presented targets (slope values > 0.15), but is much smaller at locations of 5 and 10 degrees (slope values < 0.12). Indeed, post hoc Tukey multiple comparison tests reported a significant difference in the mean slope between central vision and at 5 (mean difference: 0.124, p < 0.001) and 10 degrees (mean difference: 0.142, p < 0.0001), but not between the two eccentric locations (i.e., 5 vs. 10 degrees, mean difference: 0.024, p = 0.2578). When the slopes for different contrast levels were compared at each retinal location, a significant decline in the mean slope value (with decreasing contrast) was observed only at 0 degrees (one-way ANOVA, F[3,20] = 3.877, p < 0.024), but not at 5 degrees (one-way ANOVA, F[3,20] = 0.3831, p = 0.766) or 10 degrees (one-way ANOVA, F[2,15] = 0.308, p = 0.738) of eccentricity.
In summary, our data confirm that both retinal eccentricity and target contrast modulate the rate at which blur affects word acuity thresholds. Word acuity was most affected by blur when the target was presented centrally, and at this location performance was additionally dependent on the contrast of the stimulus. At peripheral locations, word acuity was less affected by blur, and contrast had minimal influence on the effect of blur on word acuity change.
How do our data compare with previous studies that have sought to determine the affect of blur on visual acuity? Note that previous studies examining the effect of dioptric blur on measures of acuity, in particular, letter acuity, have reported a linear change in MAR as a function of blur.11,33,34 Our data did not conform to this trend, but was best fitted with an exponential function which suggests that the effect of blur on word acuity is nonlinear, with much greater change at high levels of blur. While, as noted by Smith et al,11 it is difficult to make exact comparisons as pupil size fluctuations, contrast levels, light levels, and psychophysical procedures will affect threshold values, the difference between the present study and the abovementioned previous studies that report linear change might reflect an inherent difference in the resolution judgment of single letters and words. For the detection and resolution of words, observers will have to resolve a number of letters that are closely spaced and accordingly involve the interactions of adjacent contours (synonymous with visual crowding) which is known to impair acuity.35,36 Contour interaction effects might nonlinearly change with blur levels, but to our knowledge, no study has sought to thoroughly clarify this relationship. However, supporting evidence for this notion can be drawn from Pelli et al,37 who measured the maximum reading rate of words at different eccentricities. They noted that word reading rates decreased nonlinearly (with a log-linear slope) with retinal eccentricity consistent with words being progressively blurred and greater visual crowding in the periphery. Our findings are additionally consistent with Chung et al4 who examined word reading performance (in terms of maximum reading speed) and noted that the effect of dioptric blur on word acuity was also not linear, but noticeably increased at blur levels greater than 2 D.
We investigated the effect of retinal location, contrast, and optical blur on word acuity thresholds. We found that word acuity thresholds were affected by blur and this change was largest for central vision, but was much less in the periphery. This outcome suggests that in the case of word acuity, much like single-letter acuity, peripheral vision is much more tolerant to optical blur. Additionally, the effect of blur on word acuity thresholds was found to be dependent on contrast for central vision only. The change in threshold as a function of blur was not found to depend significantly on contrast levels at other locations. Therefore, the central retina is more sensitive to blur than the peripheral retina, and blur has a greater impact on word acuity when the stimulus was high in contrast. A possible reason for this difference may stem from a difference in sensitivity to spatial frequency information in central and peripheral vision. Note that optical blur is most effective in attenuating/removing the high spatial frequency components of the words used in the present study. Accordingly, this will have a greater effect on word acuity in central vision, as the fovea is attuned to fine spatial detail, but much less of an effect in the periphery, which is primarily sensitive to coarse spatial detail. This account of our data largely agrees with the findings of Chung and Tjan38 who demonstrated that the peak spatial frequency tuning for reading filtered words was scaled from high to low spatial frequencies when moving from central to peripheral (10 degrees of eccentricity) vision. The convergence of word acuity thresholds at 4 D blur for different contrast levels, as shown in Fig. 1, is also consistent with a spatial frequency explanation. At 4 D blur, words presented in central vision are sufficiently removed of high spatial frequencies, forcing vision to be based on low frequencies much like in peripheral vision.
In the present study, we found that luminance contrast modulated the effect of blur on word acuity thresholds in central but not at peripheral locations (see Fig. 2). This finding is consistent with Rovamo et al39 who demonstrated that spatial contrast sensitivity (for gratings) decreased with increasing eccentricity (cf., Legge and Kersten40), but this decrease was greater for higher than for lower spatial frequencies. Thus, peripheral vision which is tuned to coarse detail is relatively insensitive to contrast changes which accounts for the shallower slopes reported in Fig. 2 for off-axis locations. This effect might reflect differences in the extent of spatial summation between central and peripheral vision (see Strasburger, Rentschler, and Juttner41 for a review). It has been well established that spatial summation occurs over a much larger area in peripheral vision than in central vision, which reflects an inherent increase in receptive field size with increasing eccentricity (e.g., Oehler42). Note also that word acuity thresholds in the periphery are much higher than in central vision (see Fig. 1), which means that at threshold the print size is much larger in the periphery than in central vision. Accordingly, greater spatial summation in the periphery (and over a much larger target) might result in greater tolerance to contrast reductions, compared to central vision in which summation occurs over much smaller areas.
These findings can account for the difficulty reported in patients where monovision is used to assist in early presbyopia43 and the minimal impact on peripheral word acuity of optical blur induced by bifocal contact lenses.44 In addition, the results allow the prediction that it is possible to discriminate optical blur when a patient has normal contrast sensitivity and functional central retina versus one with normal contrast sensitivity but is using the peripheral retina. Given that the word acuity thresholds for different eccentricities converge for higher blur conditions (see Fig. 1), pinhole acuity should improve more for a patient with a functioning central retina. Further work is required to confirm this prediction.
The differences in the rate of word acuity threshold change as a function of optical blur is illustrated in Fig. 3 for two conditions: a 90% contrast condition in the fovea and a similar contrast level in the periphery. Note here that for clinical reference and comparison, word acuity is expressed in logMAR. If the criterion to discriminate a change in word acuity is 0.1 logMAR units (1.26 × MAR), then the required change in dioptric blur would be a total of 0.25 D (±0.125 D) for the central condition and 2.0 D (±1.0 D) for the peripheral condition (Fig. 3).45 By using the same change in word acuity of 0.1 logMAR units, we constructed Table 1 as a guide for the minimum dioptric change when refracting patients for central vision with contrast loss, or at different eccentricities at different contrast conditions. The refraction guideline does not depend on patient’s entrance visual acuity. For example, in patients with a healthy macula and good contrast sensitivity, 0.25 D will induce 0.1 logMAR change in word acuity, irrespective of the patients entering acuity as long as a chart which has a construction that provides an equal resolution demand irrespective of working distance or letter/word size (such as that used by the Bailey-Lovie charts). An important consideration for interpreting visual function loss in AMD is the extent of the lesion. Studies of patients with AMD indicate that anatomical and functional changes occur well outside the immediate lesion, which would imply worse visual performance than predicted when patients with healthy retinas are used to assess peripheral visual function.20,46–48
Optical, photoreceptor density, retinal circuitry, and higher neural factors combine to determine visual performance.49–52 Several features of the visual system vary with eccentricity. In peripheral vision (particularly in regions greater than 10 degrees, see Williams et al53), optical quality decreases,54 and the density of cones and ganglion cells diminishes rapidly.21,55,56 The regular cone mosaic in the fovea is disrupted by rods away from the fovea.21,57 The convergence of cone onto retinal neurons also increases.53 Moreover, the inverse cortical magnification factors rise precipitously with eccentricity.58–63
The word acuity threshold increase in the central retina was dependent on contrast level. The greatest change was shown by high-contrast words. These results are comparable to those obtained by Abdelnour and Kalloniatis15 who examined word acuity thresholds of targets (without blur) set to different contrast levels of 85, 45, and 10%. As noted above, the peripheral contrast sensitivity function (CSF) shows both a loss of contrast sensitivity and a restriction in the spatial frequency domain (high spatial frequencies loss) compared to the CSF measured with central fixation.39 The peak sensitivity of the CSF does not greatly change when the size of the stimulus is scaled with retinal eccentricity.51,64 Patients with central vision loss will consequently suffer contrast sensitivity reduction at mid-to-high spatial frequencies, but not necessarily to low spatial frequencies. As noted above, visual tasks requiring only lower spatial frequencies are more tolerant to defocus than tasks requiring higher spatial frequencies.16 Because individuals with central vision loss have both reduced contrast sensitivity in the periphery and poorer sensitivity to spatial detail, word acuity might not be greatly affected by further contrast loss and/or additionally image blur due to changes in the health of the eye. These considerations and the empirical data from this study allow us to formulate useful refraction guidelines outlined in Table 1.
Centre for Eye Health
University of New South Wales
Sydney 2052, NSW
This work was supported in part by a professorship to M.K. funded by the Robert G. Leitl estate, grants from the Australian Research Council Discovery project grant (DP11010471) to S.K., and the National Health and Medical Research Council (1033224) to M.K.
Received: November 27, 2012; accepted June 26, 2013.
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Keywords:© 2013 American Academy of Optometry
resolution; peripheral vision; reading; contrast; blur; word acuity; refraction; central vision