Closed loop degrades to open loop when the feedback channel is lost

What it is

If a movement that was relying on continuous feedback to correct itself step by step suddenly loses that feedback partway through — the screen goes black, the cursor freezes, the tracking signal drops — the correction process doesn't simply hold at the level it had reached before the interruption. It immediately loses the ability to keep correcting, and the rest of the movement has no choice but to run out on the direction and force already set at the instant of interruption, functionally identical to an open-loop movement that never had any feedback to begin with. This is called closed loop degrading to open loop: it isn't a deliberate switch in control mode — once the information source correction depends on disappears, open-loop is simply the only mechanism still capable of running.

Why it happens

Closed loop can keep correcting only because each round receives new feedback that genuinely reflects the current state; once that information pathway breaks mid-movement, the comparator has no new data to work with, and naturally can't generate a new correction command. The limb doesn't stop and wait for feedback to return — it keeps following the movement trend it was already executing at the moment of interruption, and everything in the trajectory after that point no longer contains any adjustment based on the current state — it has effectively become an open-loop execution that starts partway through. The earlier the interruption happens (leaving more distance and time for the movement to run), the larger the share of the movement that degrades to open-loop, and the closer the final error gets to what a purely open-loop movement over the same distance would produce.

Studying it

A common way to study this artificially cuts feedback at some predetermined fraction of the way through a movement (freezing cursor or visual display suddenly at the halfway point, for instance) and compares how final landing error changes depending on when the interruption occurs: the earlier the interruption, the closer the error gets to a fully open-loop condition; the later it occurs (closer to when the movement was already about to finish), the closer the error stays to a normal closed-loop condition. This curve of error against interruption timing can be used to estimate at which part of the movement closed-loop correction is mainly contributing to final accuracy for a given task.

Where it stops holding

This mechanism assumes the movement's total duration is long enough that closed-loop correction genuinely had room to operate in the first place; if a movement is already short and open-loop-dominated to begin with, a feedback interruption has little effect, since there's not much difference before and after degradation. Also, "degrading to open loop" describes a loss of correction ability — it doesn't mean the limb will immediately produce a violent or abnormal movement. If the error is already small at the moment of interruption, the degraded trajectory may differ only slightly from one that kept closing the loop; the effect of degradation only becomes pronounced when the interruption occurs early, while error is still fairly large.

Applying it

• Any operation that depends on real-time feedback to support closed-loop correction (dragging, continuous gesture aiming, fine slider adjustment) needs a fallback plan for brief interruptions to the feedback channel itself (network jitter, momentary tracking loss, dropped rendering frames) — for instance, pausing further execution of the action or freezing the current state once an interruption is detected, rather than letting the movement keep running on stale pre-interruption parameters.
• If feedback interruptions can't be avoided entirely, try to schedule the segments where interruption is likely to occur toward the point where the movement is nearly complete and error is already small, rather than early on when substantial correction is still needed — the same length of interruption causes far less accuracy loss late in a movement than early.
• How to check: in a test environment, artificially trigger a feedback interruption at different fractions of movement progress and log the final landing error for each interruption point. Comparing this curve against the error levels for the two extreme conditions — no interruption at all, and no feedback at all — reveals how tolerant the current system is to feedback interruption, and which phase most urgently needs fault-tolerant handling.

Related

• Same group: A8.07.1 Closed loop relies on continuous feedback to close in on the target step by step · A8.07.2 Each correction needs at least one full feedback-loop cycle · A8.07.3 Feedback delay beyond the loop cycle causes overshoot and oscillation · A8.07.4 Final landing accuracy is set by the closed-loop phase
• Nearby: A8.06 Open-Loop Control · A4.05 Proprioception and Kinesthesia
• Search terms: feedback dropout · closed-loop degradation · loss of feedback