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Hi Chris,
I experience this phenomena regularly with NGC 3918 and with NGC 3242.
It is simply caused by the eye flitting around a little and the pulsating comes about
as you go from the cones to the rods and back again as you alluded to.
It is such a powerful illusion that it can seem to take on a life of its own even
when you make a conscious effort to try and not shift your gaze.
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