A change needs an explicit motion cue or marker to be noticed

What it is

Once a change has been identified as something that must be seen, only two routes reliably surface it: give the change itself a perceivable continuous process (a motion effect) that recreates a signal the visual system can catch automatically, or give up on automatic detection entirely and add an explicit marker (a badge, a highlight, a text callout) that the user finds through active search. These two routes solve different problems and are not interchangeable.

Why it happens

The motion route directly exploits the visual system's automatic orienting response to motion transients: as long as a shift in position, opacity, or size is continuous enough and lasts long enough, it recreates a local motion signal that pulls attention automatically, with no active search required. But this route is sensitive to exactly how continuous and how long: an animation completed in one or two frames collapses back into a hard cut, and an animation competing with other simultaneous motion elsewhere on the screen can get swamped by them.

The marker route does not depend on motion-transient detection at all — it gives the user a target to actively search for, effectively shifting the burden of discovering the change from automatic to controlled processing. The cost is that the user must first look toward that region and notice the marker's presence at all — if the user never looks that way, the marker fails just as completely as the animation, only for a different reason: the animation fails because no transient was generated, the marker fails because it was never searched for.

Where it stops holding

• The motion route requires that the user's gaze actually covers that region when the change occurs. If attention has already shifted elsewhere (switched tasks, looking at another window, changed tabs) when the change happens, even a perfectly continuous animation plays to no one — only a persistent marker, still visible when the user returns, works in that case.
• The marker route costs ongoing visual real estate. A marker that never disappears gradually becomes background noise and can itself become subject to habituation, weakening its effect over time — when a marker should disappear, and whether it should clear on the user's acknowledgment, is a separate design question that needs its own answer.
• Neither route works well when changes themselves are extremely frequent: frequent animations interrupt each other's visible windows, and frequent markers leave the interface permanently covered in indicators — which is functionally the same as having no markers at all.

Applying it

• If the change happens within the user's current field of view and needs immediate handling, favor motion: a positional animation should be long enough for the eye to actually catch a continuous process — do not pass off a one- or two-frame jump as an "animation."
• If the change might happen while the user is away (switched tabs, scrolled elsewhere), motion will necessarily fail — use a persistent marker instead, one that is still clearly visible when the user returns, rather than one that fades out automatically over time.
• Do not stack both an animation and a marker on the same change unless its importance genuinely warrants the redundancy — otherwise the interface looks overdone, which dilutes the credibility of "this is an important change" as a signal in its own right.
• Verification: test an animation-only version and a marker-only version separately, measuring detection rate after the user briefly leaves the interface and returns. Use that to judge which typical scenario the current case resembles, and pick one route accordingly rather than deploying both.

Related

• Same group: A5.04.1 Changes during a visual interruption go unnoticed · A5.04.2 Discrete UI refresh without a transition causes changes to be missed
• Nearby: A5.16 Habituation and prompt fatigue (a persistent marker becomes habituated and loses effect over time)
• Search terms: motion transient · explicit marker · change highlighting