Abstract
Purpose:
Eccentric viewing is a common strategy used by people with central vision loss (CVL) to direct the eye such that the image falls onto functioning peripheral retina, known as the preferred retinal locus (PRL). It has been long acknowledged that we do not know whether the PRL used in a fixation test is also used when performing tasks. We present an innovative method to determine whether the same PRL observed during a fixation task was used to watch videos and whether poor resolution affects gaze location.
Methods:
The gaze of a group of 60 normal vision (NV) observers was used to define a democratic center of interest (COI) of video clips from movies and television. For each CVL participant (N = 20), we computed the gaze offsets from the COI across the video clips. The distribution of gaze offsets of the NV participants was used to define the limits of NV behavior. If the gaze offset was within this 95% degree confidence interval, we presumed that the same PRL was used for fixation and video watching. Another 15 NV participants watched the video clips with various levels of defocus blur.
Results:
CVL participants had wider gaze-offset distributions than NV participants (P < 0.001). Gaze offsets of 18/20 CVL participants were outside the NV confidence interval. Further, none of the 15 NV participants watching the same videos with spherical defocus blur had a gaze offset that was decentered (outside the NV confidence interval), suggesting that resolution was not the problem.
Conclusions:
This indicates that many CVL participants were using a PRL to view videos that differed from that found with a fixation task and that it was not caused by poor resolution alone. The relationship between these locations needs further investigation.
The visual system may adjust to a loss of foveal vision by using a functioning peripheral area of the retina to perform visual tasks that the nonfunctioning fovea would normally accomplish.
1 This pseudofovea is known as the preferred retinal locus, or PRL.
2–4 Previous studies have shown that most people with bilateral macular disease use a single retinal area during fixation that is normally located near the edge of the central scotoma.
1,2,5,6 Typically, fixation tests are used to determine the location of the PRL. The problem that has been long acknowledged is that we cannot determine whether the PRL measured in a fixation test is also used when performing other tasks. There is some evidence of the PRL varying with luminance
7 and multiple PRLs.
8–11 It is not clear whether these multiple PRL locations are truly separate or whether they are unrepeatable local increases in frequency caused by the stochastic nature of short measurement periods. Only a handful of people with truly separate PRLs, and PRLs that can be used at will, have been reported.
9,10,12 Sullivan and Walker
13 found that the area used to fixate while pointing may be larger than the fixational PRL, be offset from the measured fixational PRL, and vary between eyes. Fixation is typically measured monocularly, but most visual tasks are performed binocularly; yet often, monocular PRLs are not in corresponding retinal locations.
14,15 Thus, comparing monocular fixation measurements to activities performed binocularly may be misleading. A method of measuring the binocular PRL has been reported,
16 though the instrument is not widely available and its reported measurement error is smaller than might be expected (and is much less than that with a different method that we have not published).
Apart from the study of pointing,
13 to our knowledge, there are no systematic approaches to identify the location of the PRL used while performing activities of daily living. Here we present an innovative method to determine whether the same PRL measured during a fixation task was used to watch videos. This approach rests on two assumptions: (1) that calibration of a gaze-tracking system, in which the participant is asked to look at a fixation target in multiple locations, determines the fixational PRL (i.e., participants are expected to use the same fixational PRL when fixating at each target during the process, so, once calibrated, the system tracks the fixational PRL), and since we calibrated with both eyes viewing, we track the binocular fixational PRL; and (2) that participants using a PRL to watch a video will look in similar locations to people with normal vision (NV). This democratic center-of-interest (COI) approach uses the tendency of people with NV to look at the same things most of the time in directed videos (i.e., those in which the presentation of the content was planned).
17,18 For our analysis, we assume that people with central vision loss (CVL) will do the same but with less ability or due to impaired vision, including reduced spatial resolution, contrast sensitivity, and impacts of crowding (identifying objects of interest), poor eye movement control (being able to direct the gaze to the target location), and unstable fixation (holding the gaze at the visual target).
19 Also, people with CVL may look at slightly different aspects of objects of interest, for example, looking at external features of a face more than internal features.
20,21 Thus, the gaze of a person with CVL at an object might be offset from that of a person with NV. The direction of that offset is not likely to be consistent (systematic bias), as the external features are distributed around the face and are likely to have offsets that vary between objects. So, for our analysis, it may introduce a wider distribution of gaze locations, but not a bias direction.
In our first study, we measured the difference between the (binocular) gaze location and the (binocular) fixational PRL (as found by the gaze-tracking calibration process) in a group of people with CVL and a case-matched, NV, control group. We hypothesize that some participants with CVL will use the same PRL as that used in a fixation test while others would use a different PRL location. We anticipated high variability between participants due to individual differences that probably relate to the differences in the shape and location of the central scotomas.
Further, we set out to determine whether the reduced resolution experienced by people with CVL exclusively explains the fact that CVL participants often did not look in about the same place as the NV participants. To answer this question, in our second study, additional NV participants wore hyperopic defocus lenses of different powers to induce different optical blurs while they viewed the video clips. Blur induced by defocus and diffusive (translucent) lenses has been previously used to simulate impaired vision.
22–27 We examined whether they still located the COI despite the blur. We anticipated that the difference between the presumed video-watching PRL and the fixational PRL would not significantly change, if poor resolution was the cause of the gaze offset.
We compared the distributions of gaze-offset distances between the three groups. NV-match participant distributions varied in BCEA (spread), but were all close to the COI (
Supplementary Fig. S1). Their average BCEA was 4.61 (range, 2.7–8.6) deg
2 and the average gaze offset was 1.16° (range, 0.8°–1.7°). These gaze offsets may represent gaze calibration errors. The maximum error was consistent with our calibration criterion of less than 1.5° error (before data were collected).
As shown in
Figure 3, CVL participants had wide distributions, and many CVL participants had distributions that are clearly not centered, being shifted away from the COI, which corresponds to the fixational PRL. The distributions that were shifted away from the COI indicated that the participant was using a different PRL to view the video clips than when looking at a fixation target. The BCEA of the CVL group (average 15.2, range, 2.3–33 deg
2) was larger than for the NV-match group (mixed-effects regression,
z = 3.81,
P < 0.001). The average gaze offset varied between individuals and was usually larger than for the age-matched NV participant (
Fig. 4). The average gaze-offset distances of the CVL group were larger than the NV-match group (
z = 5.14,
P < 0.001;
Fig. 5B).
Four CVL participants were considered to be using their fovea in at least one eye (
Table 1: P2, P4, P5, and P17). Their BCEA was relatively smaller than that of the other CVL participants (
z = −1.71;
P = 0.09). This was particularly clear for P4 and P5, who had “central,” “small” gaze-offset distributions (
Figs. 3,
5). P2 had a gaze offset that was very close to the 95% confidence interval for the NV-match group (
Fig. 5). P17 had a gaze-offset distribution that was not centered (
Figs. 3,
5). P17 had macular scarring in both eyes that produced multiple, small, central scotomas, so a gaze offset might have been made to obtain a more open view of objects of interest. In this small sample, there was a trend for higher binocular letter CS scores to be related to smaller BCEA values (
z = 1.93;
P = 0.054) and smaller gaze offset (
z = 1.79;
P = 0.074). We found no relationship between the spread of the gaze offset and visual acuity, age, or sex.
Nine of the 20 subjects with CVL had an average gaze offset that was less than 3° (subjects P2, P3, P4, P5, P7, P8, P10, P11, and P14), and thus might have followed an alternative video scanpath rather than the democratic video scanpath (see analyses of unrelated video clips described above). Of those nine subjects, eight subjects had a NSS score that was above the 95% confidence limit of NSS scores (2.03) when using an unrelated scanpath, and thus were probably following the democratic scanpath. One subject, P2, had a low NSS score (0.9), and thus might have been using an alternative scanpath. P2 had geographic atrophy with central islands in each eye that included the fovea (visual acuity of 20/84 in the right eye and 20/34 in the left eye). While the foveas were used for fixation of small stimuli (e.g., letters during visual acuity measurement and fixation stimuli), it is likely that he used an alternative location for viewing video. Thus, we expected an offset gaze distribution. The lack of a gaze offset and a low NSS score could have resulted from the use of more than one video-watching PRL.
Presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Seattle, Washington, United States, May 2016.
The authors thank Dylan Rose and Sarah Sheldon for assistance with data collection. They also thank John Ackerman for his technical advice on the analysis of the COI.
Supported by National Eye Institute Awards R01EY019100 and P30EY003790.
Disclosure: F.M. Costela, None; S. Kajtezovic, None; R.L. Woods, None