The body supplies a location reference that does not need vision

What it is

A hand goes to a belt buckle while the eyes stay on the road. Pocket, iliac crest, mid-chest — these locations are known without being seen. The body supplies a location reference that does not need vision: if an input target is stored in body coordinates, the hand can arrive without looking, because proprioception is already reporting the limbs relative to the torso.

This is not “the interface travels with the person” — travel is the next leaf. This one says only: the body is a map that can be read without being lit. The eyes can do something else.

Why it happens

Proprioception, from muscle spindles, joint receptors and skin stretch, continuously encodes limb angles relative to the torso. The encoding still runs in the dark and with the head turned away. People use it every day to find a nose, a pocket, an earphone cable, without first opening a visual search. Park a clickable or grabbable on that map — at the hip, a palm in front of the chest — and arrival becomes a rehearsed movement, not a visual servo into empty air.

A visual referent cannot do this. A button stored in the room, once out of view, can only be guessed from a remembered spatial model; a wrong guess means looking back. A body referent unloads “where” from visuospatial working memory onto a sense that is always on. A headset already hides much of the room, so the channel’s value rises: visual landmarks of the room thin out; the buckle is still there.

Studying it

Have people reach a target without seeing hand or target, comparing one fixed on the body with one fixed in the room that was seen a moment ago.

Independent variables: target stored on the torso versus in the room, whether a glance is allowed mid-reach, a concurrent visual task as load. Dependent variables: spatial error on first arrival, number of look-backs, arrival time, verbal “how I knew where it was.”

Classic proprioceptive pointing (touch your nose eyes-closed, return to a just-touched spot on the forearm) is a relative. XR adds a layer: the headset already limits the view, visual landmarks are scarce, so a body referent should outperform more than it would in an open room. If the on-body target shows reliably smaller eyes-closed error, the vision-free reference has been measured.

Where it stops holding

Proprioception drifts. A long-held pose, or an arm passively moved and replaced, biases “where my hand is.” When tracking draws the virtual hand apart from the real one, vision overwrites proprioception, the body map is rewritten, and an eyes-closed reach misses. Vestibular disease, some strokes, and users who have long leaned on visual localisation have a weak channel. Targets far from the torso, outside the everyday “I could find that with my eyes shut” ring (overstretched fingertips, the back of the head), degrade the body referent into ordinary motor memory. Lab eyes-closed tasks overstate daily life: people still glance. Vision-free is a capability, not a habit.

Applying it

• Put entries that must be reachable while looking elsewhere (system menu, emergency exit, mic mute) at hip, belt, or mid-chest — places findable eyes-closed — not only where the head has to look down to search.
• Do not require people to see their own hand before using these entries; if they must see to click, the body reference is unused.
• Alignment of virtual and real hands matters especially at these sites; otherwise the body map and the picture fight.
• How to check: have someone stare at an object unrelated to the menu and open that entry. One-shot arrival means the reference holds. Looking down first, or fishing in the air, means the location was never written into the body.

Related

• Same group: N2.07.2 Body-anchored interfaces travel with the person · N2.07.3 Choice of reference frame decides what happens after a turn · N2.07.4 The head is the worst body site for an anchor · N2.07.5 A waist anchor is more stable than a shoulder anchor · N2.07.6 A wrist anchor brings the interface into view at the cost of raising the arm · N2.07.7 Anchor distance must scale with body size, not a fixed length
• Nearby: N3.01 Locking Modes · N3.08 World-locked, Body-locked, and Head-locked
• Search terms: proprioceptive spatial reference · egocentric coding · body-relative target