Maar uses the medusa metaphor in two ways, the latter of which bears some resemblance to Freud's conception, as I understand it. The first kind of medusa experience is as you've described it, a feeling of oneness with the universe, a sense of perfect purpose and order (the universe filtered through the individual, as light through a jellyfish). But Maar's point is that in VN's books, this "shimmer" doesn't last and is replaced by the Medusa experience, "the connection between the self and the world has been severed" (8). Maar doesn't mention it, but I think of John Shade's fits where he feels distributed through time and space, but then reawakens with a sense of shame and disgust. He gives several other examples.
As an aside, I have noticed a similar effect used repeatedly in Anna Karenina, which I've been reading this summer--the moment of clear perception, quickly followed by the scales falling from the disillusioned person's eyes. Is this a particular feature of Russian lit in general, or just in Tolstoy and VN?