It is generally accepted that some visually impaired subjects have more difficulty performing activities of daily living than is suggested by their visual acuity (see, for example, Rubin et al.
50 ). In the particular case of real-world tasks that require the judgment of depth and space, no study has been attempted to explain why certain visually impaired subjects have difficulties with these tasks and what visual factors might be predictive of these difficulties. In the present study, we developed a questionnaire that enabled us to discriminate between different levels of self-reported difficulty experienced in such tasks by visually impaired individuals.
The results of the Rasch analysis indicate that the SLQ is a valid questionnaire, as the infit and outfit statistics seem favorable, with very little noise and dependency in the data, with the exception of items 8 (quantity of food to put on plate) and 23 (walking in a straight line). These items may be misfitting, because subjects may have misunderstood the question or may have confounded the question with other factors. For example, walking in a straight line may have been confounded with physical ability. The easiest item on the questionnaire was fastening a button. This is not a surprising finding, as subjects may be able to do this task by touch, irrespective of the level of acuity and precision. In contrast, threading a needle requires good acuity and precision; and, unsurprisingly, this was the most difficult item on the questionnaire. As discussed in the results section, the SLQ may not distinguish between subjects at the higher end of the ability scale. It can be argued that questions are not required that would distinguish between people with high ability, because such persons do not have any functional difficulty and therefore do not need any help. Questions are required that distinguish people with low and medium levels of ability and the items in the questionnaire seem to cover this range. Although the SLQ was found to be a valid measure, it was validated on a small sample in subjects mainly affected by ARM (32 subjects who took part in this study had ARM), who were specifically chosen for the study. In the future, the SLQ should be validated on a larger, more random visually impaired population and test–retest values need to be established.
The main hypothesis in the present study was that laboratory measures of spatial localization would predict self-reported difficulty with spatial tasks much better than traditional clinical measures of visual function, such as visual acuity or contrast sensitivity. The laboratory measures investigated were vernier acuity and bias and bisection acuity and bias. We thought that measures of vernier and bisection bias would predict self-reported difficulties with spatial tasks better than vernier and bisection acuity; because, theoretically, bias or constant error should correlate with spatial localization, whereas precision or variable error (vernier and bisection acuity) should correlate with resolution. In the present study, the only bias measure that correlated significantly with self-reported spatial localization difficulties was vernier bias. The other measures of bias did not correlate significantly. Measures of precision, however, seemed to be better correlated with the spatial localization score, with both vernier acuity and bisection acuity for the larger target size correlating significantly. When a stepwise regression analysis was performed, it was found that vernier acuity and contrast sensitivity were the two variables that best predicted the spatial localization score. It appears that visually impaired subjects, particularly those subjects with ARM, have greater difficulty with precision judgments and may simply be more variable at performing tasks, and this variability may explain why they have difficulty with tasks that require judgments of space and depth. Turano and Schuchard
6 and Turano
7 found localization defects in individuals who had from retinitis pigmentosa, and these defects seemed to be independent of visual acuity and visual field loss. The finding of the present study that changes in contrast sensitivity correlate significantly with the variability in the questionnaire score is similar to results in several previous studies. It has been shown that contrast sensitivity correlates significantly with various tasks of daily living, such as reading performance,
51 52 53 mobility,
54 and perception of faces
55 and with difficulties in tasks requiring distance judgments.
3
Although there was a small but significant correlation between the questionnaire score and visual acuity, other measures of visual function were better predictors of self-reported difficulty with real-world spatial tasks. Other investigators have found a similar lack of correlation between acuity and practical tasks, such as mobility,
52 in individuals who are visually impaired. The low association between the questionnaire score and visual acuity in this study, however, may be due to the low-vision population that was selected. For this study, individuals were selected who had fairly good visual acuity, but they had complaints that were not thought to be explained by visual acuity alone. The sample that was selected for this study was therefore not a typical low-vision sample and, had a more typical sample been selected, a better association may have been found. Rubin et al.
3 studied a random sample of 220 community-dwelling adults in the United States and found that visual acuity was associated with difficulty in tasks requiring good resolution and changing light conditions. Other investigators
40 56 57 58 59 60 have also found good correlation between visual acuity scores and ability to perform activities of daily living. For example, Gothwal et al.
59 administered the L. V. Prasad-Functional Vision Questionnaire (LVP-FVQ) to 78 visually impaired Indian school children and reported a Pearson correlation coefficient of −0.57 between the person ability scores and logMAR visual acuity. The LVP-FVQ was also administered to 128 visually impaired adults, and a similar correlation was found between the person ability scores and LogMAR visual acuity.
60 Massof and Fletcher
40 also showed a good correlation (
r = −0.53) between the NEI-VFQ person–ability scores and visual acuity. These investigators used a larger and more random sample of visually impaired subjects and that may explain in part the difference in results.
Only approximately half the variability observed in the questionnaire score was attributable to the visual factors chosen for this study. This may be because this study selected only a range of visual factors that could be measured in a clinic or a laboratory. The activities of daily living on which the questionnaire is based are complex tasks and are also dependent on many nonvisual factors, such as levels of motivation, personality, and social support. Many investigators have speculated that these factors play an important role in the performance of day-to-day activities (see for example Szlyk et al.,
61 Rosenbloom,
62 Davis et al.,
63 and Jang et al.
64 ). Physical and mental health,
65 age, and manual dexterity may also have accounted for some of the variability in the questionnaire scores. Some subjects had arthritis, although none thought that it was severe enough to limit them physically. It is also possible that visual field loss correlates significantly with self-reported localization difficulties; however, visual fields were not measured in this study, because we thought that the available means of testing visual fields, such as automated perimetry, Amsler grid or the tangent screen, would not yield reliable results, particularly since it has been shown that patients with a central scotoma often fixate with a preferred retinal locus instead of the fovea, despite verbal instructions being given.
66
Possible mechanisms for localization defects in individuals who are visually impaired have not been investigated. A vernier acuity loss in persons with amblyopia is well reported and suggested to be due to undersampling
67 or neural disarray.
68 According to the undersampling theory, because of neuronal loss, subjects with amblyopia do not have enough neurons to be able to determine object position with certainty. Although the origin of loss is presumed to be cortical, it is conceivable that the cellular loss at the level of the retina could help explain the presence of localization defects in visually impaired subjects. Most of the visually impaired subjects who took part in our research had ARM, and it is associated with a loss of visual cells due to the degeneration of RPE cells.
69 This loss could quite easily result in a sparse number of cells, which might explain poor positional ability in visually impaired individuals, especially individuals who have macular degeneration. According to the neural-disarray theory, spatial position “tags” or neural signals may be altered or scrambled so that the cellular response is mislabeled, coming from the wrong position in space. As with the undersampling theory, the origin is presumed to be cortical. However, it is conceivable that in a disease such as ARM, photoreceptors could be moved out of position by hemorrhage and or drusen resulting in an alteration or scrambling of signals. An alternative third explanation could be that poor localization may simply be due to visual field defects, particularly if these defects are in the central visual field. Eccentric viewing caused by a central scotoma would also lead to the use of peripheral retina, which has poorer localization abilities.
70
It seems plausible that the duration of the visual impairment would play an important role in determining level of difficulty; however, it was not possible to determine accurately the onset of vision loss in most of the subjects who participated in the study. The onset of vision loss reported in this study was based on the subject’s memory, which may not have been accurate. Many subjects reported that they could not remember which eye had been affected first and were reporting approximate values for onset. One would expect that the longer the duration, the more likely it would be that the individual would have adapted to the visual loss. Several of the visually impaired subjects in the study had one eye that was more severely affected, and some of these subjects reported that they had great difficulty in judging distances and depth at the onset of their vision loss; however, they were able to adapt to their loss within a few months. Most of the subjects reported stable vision for several months before this study. It may be that if vision is constantly changing, individuals experience greater difficulty, since they have to readapt constantly to their new visual status.
An interesting aspect of this study was that only 5 of the 42 visually impaired subjects had measurable stereo acuity on the Frisby test. Rubin et al.
71 tested individuals using the Randot Circles test (Stereo Optical, Chicago, IL) and found that elderly individuals who have four or more lines of difference (0.4 logMAR or greater) in visual acuity or 0.60 log units or more of difference in contrast sensitivity between the two eyes, lacked stereo acuity. All the five individuals who had measurable stereo acuity in the present study had acuity differences of less than 0.4 and contrast differences of less than 0.6 log units between the two eyes. Nine other subjects also had similar acuity and contrast differences, yet did not demonstrate measurable stereo acuity. Most of these nine subjects had only slight to moderate difficulty in performing localization tasks, and it was surprising to note that they lacked stereo acuity. It is hard to establish what could be the cause of this, even after accounting for the fact that all the subjects who took part in the present study were elderly and that stereo acuity declines with age.
72 73 One possibility is that subjects were not able to see the pattern on the Frisby test plates, but that seems unlikely, because the Frisby test plates are of high contrast and all the subjects who participated in the study reported that the patterns on the Frisby plates were visible. Likely causes for this decline should be investigated further.
The SLQ can be used to measure self reported difficulty with real world localization tasks. The questionnaire takes less than 15 minutes to administer and does not involve complicated equipment. Information can also be gained by measuring contrast sensitivity, which would probably be the preferred method of choice for clinicians. Vernier acuity testing requires more sophisticated instrumentation that limits its use to a laboratory; however, it may be possible to design a simple vernier test that can be used in a clinical setting. Further research should be undertaken to explore this possibility.
Various studies have shown that visual tasks such as vernier acuity improve with practice.
74 75 However, most of these studies have found that training an individual on one task does not necessarily lead to improvement on another task, and many practice sessions are required before thresholds improve (see, for example, Fendick and Westheimer
76 ). Therefore training for visually impaired subjects would probably be best directed to the task itself rather than the underlying visual function. Another practical recommendation for individuals with spatial localization difficulties would be to improve the contrast of the task.