Artists have toyed with abstraction and figuration for centuries - there are wonderfully free passages in Velázquez's and Rembrandt's paintings, for instance, that coalesce into a readable image only from afar.

But many artists today make this grey area itself the subject, and Parasol Unit has chosen four painters and a sculptor who do so with varying degrees of success.

Visible Invisible is built around the paintings of US-based British artist Cecily Brown and young London artist Shaun McDowell - to my mind the weakest in the show. Brown's paintings almost obliterate their forms in a blizzard of gestural marks, and while she delights in the sheer stuff of paint, the effect appears showy rather than profound.

McDowell displays similar exuberance as he reflects his passion for a lover whose presence is felt but never seen, even smudging and clawing at the paint with his hands in one series. But most of his paintings offer little beyond their initial energy and immediacy.

At the other extreme are Maaike Schoorel's white canvases, whose sparse, delicate dabs of pastel form images that gradually reveal themselves as plays on classical genres - a still life or a portrait, for instance. Slow-burning but immensely satisfying, Shoorel's paintings apparently effortlessly achieve a synthesis of intimacy and grandeur that remains out of McDowell's and Brown's grasp.

Best of all are Katy Moran's teasing small paintings whose scratches, stains and whirls are seemingly on the brink of forming a recognisable image and whose titles - Daniel, Lazy Wears Blue - force you to delve deep in search of a subject.

But they remain a mystery, leaving you absorbed in Moran's imaginative use of her medium, and in the strange atmospheres she evokes.

The quality of the artists is inconsistent (and Hans Josephsohn's brass sculptures of abstracted female forms feel out of place) but this is still a thoughtful exhibition on an enduringly fertile seam of artistic activity.

Until 7 February (020 7490 7373, www.parasol-unit.org).