In contrast to its better-known counterpart to the north, the 40th Corcoran Biennial didn’t pretend to provide an overview of current American art. Instead it presented New York as the latest in a series of “regional” art centers being considered by the Biennial. (The West and the Midwest were featured in the two previous editions; still to come are the Southeast, the Mid-Atlantic, and New England.) Curator Ned Rifkin (now at the Hirshhom Museum) redeemed this somewhat droll conceit by narrowing the focus of the exhibition to the work of a group of midcareer artists, all of them abstract painters,