Ask a model that reads text and pictures together which of several images a question refers to, and accuracy drops as more images are added. Large are networks whose layers contain attention heads, small units that each pull information from parts of the input. This work ranks heads two ways: how much attention each puts on the target image, and whether that image can be recovered from what each head passes onward.
The second test trains a small image matcher on each head's output; the two rankings disagree sharply, with fewer than half the top heads on both lists.
Switching heads off shows multi-image answering depends most on heads that both point at and pass on the target image. Pointing is not writing.
