Because we are developing field expansion devices based on HMDs, we measured the dispersion of eye movements relative to the head. We believe that this measure could help us define the necessary field of view to be used in such displays. A wide field of view is one of the most difficult parameters to achieve in HMD design and is usually associated with loss of resolution.
18 We also hoped to gain a better understanding of the variables that affect eye movements in the face of the challenges imposed by severe PFL (tunnel vision) when walking.
Our findings in normally sighted subjects (dispersion of 9.7° vertically and 14.2° horizontally) are comparable with the findings in a previous study conducted under similar conditions.
19 Bahill et al.
19 used electro-oculography (EOG) and reported only the distribution of saccade amplitude during extended walking on a college campus. They found that most saccades measured less than 15°. In the patients with PFL, we found dispersions of 9.5° in the vertical direction and 9.4° for the horizontal (approximately two thirds that of the normally sighted). Despite the small sample size, the difference in horizontal dispersion between patients with PFL and normally sighted subjects was found to be statistically significant.
Other studies of the effects of restricted visual fields on eye movements have been performed in different contexts. The recording methodology used by Turano et al.
6 was similar to ours, but they measured only a few seconds of indoor walking and analyzed mainly gaze position, identifying objects that were fixated. Other studies focused on gaze movements (meaning compound eye+head position) during search tasks on displays, either by normally sighted people with simulated PFL,
20 or by patients with PFL using an augmented-viewing device.
16
The implication from our results is that head-mounted mobility visual aids for severe PFL may be effective, even with a relatively narrow field of view. A field of view covering four times the dispersion found would keep approximately 96% of fixations inside the active area of the visual aid, if eye fixations are assumed to be normally distributed. Consequently, desirable fields would be approximately 40° × 40°. On the one hand, immersive HMDs prevent any vision outside the active display area and may indeed require a field of view that wide. The required field may be even wider if the full extent of the residual field is to be within the display all the time. On the other hand, an open display design permits natural viewing outside the active area of the display. Therefore, to provide augmented visual information, the field of view of an HMD, to be implemented in a mobility visual aid for PFL, need not be more than double the eye position dispersion found in the PFL group. Hence, displays that subtend approximately 20° both horizontally and vertically may be sufficient. Such a display assures that, most of the time (approximately 64%), the line of sight of users with PFL will be within the active display field, providing useful information. Manufacturers of HMD-based magnifying low-vision aids generally seek wider fields of view. However, displays with a narrower field of view available at a lower price would make potential aids for PFL more affordable. Smaller displays should help patients with PFL locate their targets of interest within the display itself, since, in most cases, they would not have to scan the active area. With a smaller display, the patients would have less trouble identifying their own gaze location within the display. This self-localization helps them to achieve a quicker correspondence with the target location in the real environment. Luo and Peli
16 proposed the inclusion of guide grids in the active area of an HMD to help patients locate the center of the display. A similar difficulty in self-locating within a display field was noted for simulated PFL in a visual search experiment with targets presented on computer monitors.
20
We found a relative change in the vertical scan dispersion between indoor and outdoor environments for all patients with PFL. This behavior is probably associated with differences in the navigation and obstacle-avoidance demands of the two environments. On the one hand, a patient walking indoors might expect to find an even floor, with little concern for low-lying obstacles or abrupt changes in elevation. On the other hand, walking outdoors entails increased concern for ground-level obstacles (e.g., uneven pavement and curbs) and head-level obstacles (e.g., low tree branches). This would require less attention to the ground and a consequent reduction in the vertical dispersion indoors. Most of our patients with PFL used long canes to monitor the ground for obstacles and uneven pavement. One would expect that a patient who does not use a long cane might exhibit an even wider dispersion of vertical eye movements in outdoor mobility than we have found. Indoor mobility comes with an increased need to locate orientation features such as doors and hallways, as shown by Turano et al.
6 However, the horizontal angular span of such features is limited by the structure of the corridors. While outdoors, objects located at more distant lateral locations are used mostly for navigation rather than safe mobility (obstacle avoidance) and they can usually be spotted from a farther distance where their angular span is limited. Yet, the normally sighted subjects exhibited horizontal dispersions that were wider than their vertical dispersions. Possible reasons for this are discussed in the following text. These differences suggest that while a display with wider horizontal than vertical field (landscape mode) may be better for normally sighted users, a square or even a portrait mode display with wider vertical span may be more useful for patients with PFL, especially outdoors. For two of the three PFL subjects, who completed indoor and outdoor walks with a long cane, we found a trend toward an expansion of the vertical dispersion downward when outdoors (although the main effect was a symmetrical expansion). Patients who do not use long canes may tend to scan even more downward vertically. Thus, we suggest that an asymmetric setting of the display relative to the camera, using more of the display to cover the lower than upper field, may also be beneficial.
6
The limited size of the display field of view does not imply that patients with PFL never use larger eye movements. In fact, we have found that they do perform occasional large scanning eye movements. Therefore, such visual aids should neither restrict nor block normal eye movements, nor restrict the dynamic visual field.
5 Moreover, since the severely restricted visual field of patients with PFL can be fully blocked by a small obstruction, the design of such displays and their field of fixation
21 22 should avoid even small peripheral obstructions (5–10°); therefore, immersive HMDs should be avoided for mobility applications. The term “clearance” of a visual aid, defined as the overall unblocked visual field, was adopted to indicate the field of view that allows unrestricted eye scanning by the user, both inside and outside the active display area of the visual aid.
15 As an example, some studies reported that patients with PFL found the field of the multivision night-vision goggles restrictive, even though they had a relatively wide display field of 32° × 24°.
23 24 25 The concept of clearance suggests that a narrow display is more likely to be useful if it is embedded within a clear carrier lens, which is one of the implementations we have proposed
15 and implemented.
16 Clearance of an HMD is not dissimilar to the peripheral field that is available to spectacle wearers outside of the rim of the frame, providing a non–optically corrected, yet important, area of visual field.
A plausible explanation for our main finding of reduced horizontal eye position dispersions in patients with severe PFL is that head movements play an important role in scanning. This must be confirmed in future studies recording eye and head movements under similar conditions. Patients with PFL may have a better sense of direction when they use their body heading as their main reference. Some of the subjects reported such a strategy when debriefed. They reported selecting a reference or landmark directly ahead in the direction in which they were walking and trying to keep that landmark in view, only briefly shifting their gaze to scan the path between them and the landmark or to check for likely sources of hazards. They indicated that it was quite difficult to recover their landmark if their gaze had been shifted away by scanning eye movements. By comparison, scanning with head movements seemed to facilitate recovery of the primary position of gaze and facilitate regaining of the landmark.
Patients with PFL may use a wider scanning strategy in some situations, although on average they show a reduced dispersion of eye position while walking. A wide scanning strategy may be used at a critical street crossing or when searching for a misplaced object. Such situations may be observed more readily by rehabilitation personnel, which may explain the clinical impression of increased scanning by these patients.
For normally sighted observers, saccades are usually aimed at some peripheral visual target. It is possible that the patient with PFL would not make eye movements aimed anywhere outside of the field, because of the lack of peripheral stimulation and therefore would have a restricted range of eye movements—a possibility that may be the main reason for our finding. This concept of saccade inhibition is implicit in the results of Luo and Peli.
16 They showed that patients with PFL have a reduced search time and an increased directness of the gaze path to a target placed outside the visual field, when using either auditory clues or an augmented-viewing aid. The augmented-viewing aid and auditory cues provided the missing peripheral information necessary to induce larger gaze movements toward the target stimulus. Cornelissen et al.
20 also speculated about the effect of PFL in limiting the ability to program efficient eye movements. Furthermore, there is evidence from reading eye movements in patients with hemianopia, that the lack of peripheral stimulation reduces saccade length; right hemianopes generally make smaller-amplitude saccades to the right along a line than do the normally sighted, and left hemianopes make many small-amplitude leftward saccades when returning to the beginning of the next line rather than the single large leftward return sweep saccade made by normally sighted readers.
26 27 A possibly related effect was described by Hassan et al.,
28 who showed that a moderate reduction in the visual field due to glaucoma had an impact on head-movement patterns while crossing streets; the patients with glaucoma did not exhibit head movements consistent with maximizing safety. The authors hypothesized that this result could be due to the dynamic nature and complexity of the street-crossing task. However, it is possible instead to advance an explanation based on the lack of visual stimulation from the peripheral field. Important objects such as oncoming cars were not noted and therefore did not induce shifting of the gaze to the blind field.
Thus, we believe that the absence of visual stimulation is probably the main cause of our primary finding of reduced horizontal ocular scanning in people with severe PFL. If this were the case, we might expect to find that horizontal dispersion would decrease as field size decreased; however, we did not find such a correlation, presumably because the sample size and range of visual field sizes examined were too limited. On the other hand, the strategy reported by patients of using proprioceptively cued head movements to restore context would also account for the narrower eye movement dispersion found and are not necessarily related to field size.
The authors thank Miguel Angel García-Pérez and members of the Peli Laboratory for suggestions and Alexander K. Nugent for helping in data collection and analysis.