The present results demonstrate that persons with real PFL show distortions in spatial representations similar to those observed when PFL was simulated in participants with normal vision.
12 In particular, mean placement errors for the real PFL participants increased with decreasing FOV size. Further analyses showed that this effect is driven in large part by a tendency to underestimate the distances to the statues. Signed angular offsets were also comparable between the two groups. When comparing the performances of participants with real PFL to the simulated PFL participants in the real-world task, the same trends across FOV size were observed for placement and distance errors. Across all performance measures, no effect of environment or FOV × environment interaction was observed. This suggests that the distortions observed in the spatial representations of the real PFL and the simulated PFL participants in the virtual environment task are related to the loss of the peripheral visual field and not to an experimental artifact. Collectively, then, the behavioral data indicate a relationship between the ability to create veridical spatial representations and FOV size, with distortions increasing as FOV size decreases, regardless of the nature of the field loss (simulated or natural) or the type of environment (virtual or real world).
There were, however, some unexpected results in the behavioral data from the participants with 20° FOV sizes. Two of the RP participants with 20° FOV showed an overestimation for signed distances, indicating a bias in the opposite direction compared with that predicted from the simulated PFL participants’ data. Although overestimating distances may carry more functional disadvantages than underestimating distances, such as bumping into objects that are not continually monitored, when coupled with the absolute distance errors, the results show that the magnitude of these participants’ biases still falls within the range predicted from the simulated PFL participants’ data. In addition, though the signed angular offsets of the RP participants were similar to those of the simulated PFL participants, the absolute angular offsets of six of the nine RP participants were smaller in magnitude than expected, indicating that these RP participants were either slightly better at representing the orientations of the statues or at maintaining their heading while walking to the statues.
In contrast to the behavioral results, the eye movement and gaze strategies of the RP participants in the present study showed marked differences from those observed with the simulated PFL participants. First, fixation durations for both the testing and the learning phases were shorter for all but one of the RP participants than for the simulated PFL participants. Previous research
16 has shown that persons with PFL caused by ocular disorder exhibit shorter fixation durations than do normal-vision controls when completing a dot-counting task but not a visual search task. It has also been found that simulating PFL in normal-vision observers can lead to increases in fixation duration with decreasing FOV size.
17 Given that there is no consensus on how fixation durations typically change with reductions in FOV size, it is not clear whether the deviation in fixation durations between the RP and the simulated PFL participants here is the result of adaptive strategies on the part of the RP participants, a response to the FOV restrictions by the simulated PFL participants, or both.
Analyses of gaze strategies also showed that the RP participants fixated on the statues to a greater extent than the simulated PFL participants in both the learning and the testing phases. The small decrease in the proportion of statue fixations in the testing phase is not surprising given that the statues were only visible in this part of the experiment after placement by the participants. Although this result is consistent with previous studies indicating that persons with PFL tend to focus their gaze on the target when walking,
18 19 the present study is limited in determining whether this result reflects a conscious strategy used by the RP participants.
The results of the present study also suggest that the effect of PFL on spatial representations may depend not only on the absolute size of the remaining visual field but also on the parts of the visual field that are spared. Specifically, the RP participant with large annuli and a 30° central visual field showed larger signed distance errors and angular offsets than the other RP participants, with and without annuli, and the simulated PFL participants. When asked whether she perceived the annuli in her peripheral visual field, the participant responded that she did not. Instead, she reported “seeing” a larger, continuous FOV. Although both participants with annuli had large areas of spared vision, the annuli of the participant with a 30° central FOV spanned both hemifields, creating a unified ring around the remaining central region in her binocular FOV, whereas the annuli of the other participant did not. For both participants, however, annuli would not have provided more direct visual input during testing because the regions spanned by their annuli fell outside the FOV provided by the screens of the headset, with a maximum eccentricity of 25.5° horizontally and 20.5° vertically.
In conclusion, the present results extend previous findings demonstrating distortions in perceived eccentricity
9 and online perception of distances
10 in persons with PFL, and they have important implications for the development of orientation and mobility rehabilitation protocols. The results indicate that the strategies used by the RP participants did not effectively compensate for their FOV loss in a task that involved remembering spatial layouts. As such, the present findings may help to explain some of the navigation difficulties reported by persons with PFL when completing similar tasks in their daily lives.
1 Furthermore, because previous studies
18 19 have found a tendency for persons with PFL to use certain types of strategies during navigation that do not involve actively learning spatial layouts, it may be helpful for persons with PFL to engage in training that focuses on deficits in encoding and retrieving spatial configurations and object locations in memory. However, further work is needed to understand the mechanisms driving these distortions and what compensatory strategies, if any, are effective in reducing them.
The authors thank Judy Hao for help with data collection and analyses.