Without any feedback, control has to stay open-loop, and accuracy has a fixed ceiling

What it is

In some settings, feedback isn't just "too slow to be used" — it doesn't exist at all: a patient with severely impaired proprioception, a finger numbed by anesthesia, a blind-operation scenario where the object being manipulated can't be seen at all. In these settings, movement has to run entirely on open-loop execution from start to finish, because there's simply no available information to correct course with. Accuracy in these settings has a fixed ceiling set by the body's own output variability — a ceiling that can't be broken through by "giving the user a bit more time" or "asking them to focus harder," because what's limiting it isn't time or attention, but the sheer absence of feedback.

Why it happens

Where the accuracy ceiling comes from depends on whether a movement could, in principle, have used feedback to pull error back toward the target. If a feedback channel is objectively unavailable, a movement has no choice but to rely entirely on the force, direction, and timing parameters set before launch — and those parameters carry inherent output variability that repeated attempts don't make disappear. Under normal circumstances, that variability can be partly cancelled by in-flight feedback correction, but when feedback is absent, that cancellation path doesn't exist, so the error shows up unchanged in the final result. This is why the accuracy ceiling in these settings is "fixed": it's set by the variability of the body's own output, not by how long the movement took or how much the user wanted to get it right.

Studying it

Two paths are commonly used to study this. One studies patients with severely impaired proprioception, observing what level of accuracy their movements stabilize at without normal sensory feedback, and whether that level improves with practice or extended time. The other artificially induces feedback loss in healthy participants (fully occluding vision, locally anesthetizing a finger), compares accuracy between the feedback-absent and feedback-normal conditions, and tests whether extending movement time narrows that gap — if extending time fails to improve accuracy under the feedback-absent condition, the limitation genuinely comes from the absence of feedback itself, not from insufficient time.

Where it stops holding

"A fixed accuracy ceiling" describes performance during the period when feedback is absent — it doesn't mean the ceiling is entirely immovable. Extensive, specific training can, in principle, reduce the body's own output variability and nudge that ceiling upward a little, but that's a long-term motor-learning change, not something achievable within a single task by giving more time or more cues. Also, most so-called "no-feedback" scenarios are really missing just one feedback channel (vision, say) while proprioception and other channels may still be partly available, so the real accuracy ceiling tends to be somewhat more forgiving than the extreme case of no feedback whatsoever.

Applying it

• For interactions with a genuine feedback gap — a device occluded by the hand so the contact point can't be seen, a glove blocking tactile sensation, an operation that has to be performed blind — don't expect more time or more instructional text to raise accuracy; instead accommodate this fixed ceiling by enlarging targets and adding redundant confirmation channels (an audio cue, haptic feedback standing in for vision) rather than fighting the ceiling.
• When setting acceptance criteria for feedback-absent scenarios, calibrate the tolerance against the measured accuracy distribution for that action under a genuinely feedback-absent condition, rather than reusing the accuracy requirement meant for normal-feedback conditions — otherwise a large share of operations that are actually performing normally will get misjudged as failures.
• How to check: have users repeat the target operation with the relevant feedback channel occluded, and log the accuracy distribution. If extending the allowed time per attempt doesn't noticeably narrow that distribution, the current accuracy ceiling really is set by the absence of feedback, and the fix should come from redundant channels or target size — not from further relaxing the time limit.

Related

• Same group: A8.06.1 An open-loop action does not draw on feedback once it has launched · A8.06.2 Fast, short movements are dominated by an open-loop component · A8.06.3 Open-loop error comes from programming precision, not execution correction
• Nearby: A4.05 Proprioception and Kinesthesia · A8.26 Non-Manual Limb Input
• Search terms: deafferentation · open-loop accuracy ceiling · blind operation