Head-lock is the most uncomfortable mode for prolonged use

What it is

Pinning a full page of instructions to the centre of the view for ten minutes is far more uncomfortable than pinning the same page to a board in the room. Head-lock is almost unnoticeable for a short toast; once it becomes a long reading surface or a resident layer, it is the most likely of the three lock modes to drive eye strain, pressure, and sickness. The discomfort comes from the content remaining on the retina, not from what the text says.

Why it happens

The vestibulo-ocular reflex stabilises the world on the retina: the head turns, the eyes counter-rotate, so the room does not smear. Head-locked content is already nailed to the display plane, so its retinal image does not move when the head does. In the same head movement the world is being stabilised by VOR (or smeared by display persistence) while the HUD travels with the camera — two optic-flow fields stacked. To read the HUD, the wearer has to suppress the eye movement that should have stabilised the world; to look at the world, the HUD occupies central vision and comes along. The conflict integrates over minutes.

The second layer is that there is nowhere to look away. A world-locked board leaves the fovea on a turn; a body-locked tray can be glanced off. Head-lock removes “turn the face away” as a rest path. Typical HUDs also sit at a near virtual depth, so vergence parks near while accommodation stays on the headset focal plane, and that near conflict becomes a resident load. Tracking micro-jitter is magnified in central vision into a swimming panel, which is harder to read than a still world.

Studying it

Present the same material head-locked and world-locked, stretch the session from two minutes to fifteen or more, and do not allow the headset to come off in between. Sample SSQ or VRSQ several times in-experience; a single score at the end is too late. Companion measures: blink rate, the moment people say they want the interface torn off, time until they dismiss the content themselves.

Independent variables: lock mode, session length, HUD eccentricity and virtual depth. Dependent variables: SSQ total and oculomotor subscale, the in-experience discomfort curve, early-termination rate.

A seated reading task that forbids head turns underestimates the cost, because the VOR conflict has not been triggered. People need to change heading naturally while they read.

Where it stops holding

A reticle in a fighter or in surgical navigation is built for a glance; brief head-lock barely registers. Thin, low-contrast lines in a dark field are more tolerable than a large opaque panel. People with very low sickness susceptibility can finish a chapter under head-lock — the individual range is wide, and “I was fine” is not product evidence. Video see-through makes a large head-locked panel worse than optical see-through: it covers an already digitised world, so the flow conflict is complete. A status lamp that lasts a few hundred milliseconds is out of scope — there is not enough time for the conflict to integrate.

Applying it

• Take reading, tables, and anything that must be stared at out of head-lock and hang them in the room or on the body. Leave head-lock for glance marks: a reticle, a recording dot, a tracking-lost glyph.
• If a prototype must use head-lock as the main surface for a while, shrink the area, move it off the view centre, and give an explicit dismiss or re-parent control. Resident by default is uncomfortable by default.
• Sample in-experience: a 0–10 ocular-fatigue item every five minutes, not a satisfaction form at the end.
• How to check: run the same task fifteen minutes head-locked and fifteen minutes world-locked. If the oculomotor SSQ subscale is higher under head-lock, or more people ask to kill the interface mid-session, do not leave the main reading surface in view coordinates.

Related

• Same group: N3.01.1 World-locked content stays fixed in the environment · N3.01.2 Body-locked content moves with the person but not with head turn · N3.01.3 Head-locked content stays in the same place in the view
• Nearby: N3.08 World-locked, Body-locked, and Head-locked · N2.07 The Body as Reference Frame
• Search terms: head-locked discomfort · HUD cybersickness · visor-locked fatigue