Occluding one eye does not switch off its input, and rivalry can still occur

What it is

A common assumption behind monocular near-eye display devices (monocular smart glasses, a heads-up display that only overlays digital content on one side) is: since only one eye is looking at the digital content, the other eye is "not being used," so its input can be ignored. That assumption is wrong — as long as that eye remains open and is normally viewing the real environment, what it receives is a genuine, active stream of visual input, and it does not automatically "switch off" just because the device isn't deliberately sending it content. What actually happens is this: one eye sees a view overlaid with digital content (or partially obstructed by the display optics), while the other eye sees the completely unmodified real environment. These two streams disagree with each other wherever the fields of view overlap, which still satisfies the condition for binocular conflict and can still trigger binocular rivalry — rather than being calmly ignored by the user as the design intended.

Why it happens

The trigger condition for binocular rivalry only cares about whether the content the two eyes receive at the same location in the visual field is reconcilable — it doesn't care whether one of the two streams is "specially designed digital imagery" or "unmodified real world." As long as the two streams differ, in the overlap region, in a way normal fusion cannot handle — for example, if one side of the field of view has a ring of abrupt brightness or contrast change caused by the display bezel, optical path structure, or the boundary of the overlay content, while the other side has no such boundary at all — the area around that boundary satisfies the condition for triggering competitive suppression. On a monocular near-eye display this typically shows up as luning: the edge of the display's imaging area usually has a ring of dimming or contrast falloff due to optical design constraints. This transition band objectively exists in the field of view of the eye wearing the display, but the corresponding location in the other, fully bare eye has no such dimming transition. When both eyes are open and trying to normally fuse the overlapping field of view, this localized boundary difference triggers localized competitive suppression, which users subjectively report as that region flickering, edges looking blurry, or a hard-to-pin-down visual discomfort — not simply "the display isn't bright enough." This explains why truly monocular viewing (closing or fully covering the other eye) and "both eyes open but only one has digital content" are two completely different visual states, and the latter is far more prone to problems than the former.

Studying it

• Subjective darkening/flicker report collection: having participants wearing a monocular near-eye display report, under different content, ambient brightness, and display bezel designs, whether they perceive edge-of-field darkening, localized flickering, or visual discomfort, noting the specific location where it occurs, and comparing this against the display's optical parameters (field-of-view angle, edge vignetting curve).
• Adapting binocular rivalry quantification paradigms: borrowing the methods used in rivalry research to measure dominance-period duration and switching frequency, treating the edge region of a monocular display as a "localized binocular conflict zone" to quantify how much stronger the competitive suppression is there compared to the center of the field of view.
• Typical independent variables: the display's physical field-of-view angle and bezel position, the difference in ambient lighting between the eye wearing the device and the bare eye, and the content's contrast at the edge of the field of view.
• Typical dependent variables: subjective discomfort ratings for the edge region, the proportion of reports of darkening/flickering, and the difference in subjective experience between both eyes open versus only the device-wearing eye open.

Where it stops holding

• This entry concerns scenarios where the device itself allows — or even assumes — the user keeps both eyes open. If actual use genuinely requires or habitually involves covering the non-display eye (with a truly opaque patch, say), that eye really has no effective input and the rivalry condition doesn't hold — that case falls outside what's discussed here.
• How noticeable the effect is depends on the display's field-of-view size, how steep the edge vignetting is, and the ambient lighting where it's worn; a larger field of view and a gentler edge transition generally make the problem less noticeable, so it can't be assumed that every monocular display suffers this to the same degree.
• What's described here is a localized rivalry phenomenon triggered by a content boundary while both eyes are open, which is a different problem from generally poor visibility caused by an overall display that's simply too dim or too low-contrast — the two should be triaged separately, not conflated.

Applying it

• When designing a monocular near-eye display, don't assume the non-display eye "isn't involved" — use both eyes open, attempting to fuse both fields of view, as the default test condition, rather than only testing the display's appearance with one eye covered.
• For the vignetting/darkening transition at the edge of the field of view, prefer a gentle gradient over a hard boundary (avoid a sharply localizable brightness discontinuity) — a softened transition generally lowers the odds of triggering localized competitive suppression.
• When triaging a user report of "the edges look blurry" or "my eyes feel uncomfortable," first check whether there's a visible contrast discontinuity at the boundary between the display's field of view and the real-world view, rather than defaulting to assuming it's an overall brightness or resolution issue.
• Verification: have a participant wear the device normally with both eyes open for a while, then specifically ask whether the edge region of the field of view shows flickering, intermittent darkening, or discomfort, and compare that against the same region's subjective experience with the non-display eye closed — if closing that eye clearly improves things, that confirms the problem is rivalry from binocular conflict rather than the display's own brightness or clarity being insufficient.

Related

• Same group: A1.33.1 Conflicting binocular input can produce rivalry instead of fusion · A1.33.2 Most people have a dominant eye, and aiming/pointing defer to it · A1.33.4 Failure of binocular fusion produces diplopia or monocular suppression
• Nearby: A1.15 Visual Fatigue and Accommodative Load
• Search terms: luning · monocular near-eye display · binocular rivalry · field-of-view edge artifact