They could be extremely obvious, like bright red dots, or more nuanced, like lone mountaineers climbing Icelandic glaciers.
In the Blink of a Mind — Attention
Jessica Brillhart

We may also call them as individuation markers: points of contrast between more than at least one element at a given time (in the example of lone mountaineers the list would be the co-existence of these contrasts: object vs. subject; foreground vs. background; action vs. inaction; color vs. monochrome; warm color vs. cool color, so on)

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