Roughly speaking, what the neurologists have discovered is that Ocean, locked inside a scanner with a sketch pad and pencil, uses different parts of his brain to draw different things: faces from photographs, abstract geometric figures and, for some unspecified reason, letters from the Korean alphabet. His brain also worked differently from that of some poor sap known to posterity only as "the non-artist", who is given to doing mere "slavish copying" when presented with his sketch pad. The exhibition shows this by contrasting MRI images of Ocean's brain at work, various bits of it lit in orange according to the task it is performing, with those of the non-artist aforementioned. And then, of course, there is the plastic head, the frontal lobes of which throb an angry red, bearing as they do the neurological brunt of Ocean's creative genius.
Surprised? Well, no. If there are extraordinary scientific revelations contained in any of this, they are either beyond the comprehension of the average Portrait Gallery punter (this one included) or inadequately explained. Who could be surprised to find that different brains work differently when presented with different tasks? This revelation is particularly underwhelming in the last part of the show, which invites us to look with new eyes at four portraits chosen by Ocean: Rembrandt's picture of Margaretha de Geer, Gwen John's Portrait of a Nun, Warhol's Marilyn Diptych and Frank Auerbach's Head of Catherine Lampert. Rather than demystifying the creative process, the sloppy curatorship of "The Painter's Eye" only makes it more mysterious still. Here, apparently, is a discrete and homogeneous subset of homo sapiens called "artists" - or, even more specifically, "portrait painters" - whose brains glow a uniform orange when presented with a sketch pad in a magnetic scanner. In spite of this, they produce works as extraordinarily different as Warhol's obsessive-compulsive Marilyns and Auerbach's impastoed vision of Ms Lampert. As with Jonathan Miller's patronising exhibition on the neurological implications of reflection in art at the National Gallery last year, the NPG's show assumes that visitors will be drawn from another subset of humanity called "people who frequent art galleries", clearly a synonym in the Wellcome Foundation's mind for "people who failed their physics O-level". In any case, trying to excite the public with extruded plastic heads in a room that also contains Rembrandt's sublime portrait of Jacob Trip's widow is an uphill struggle. The National Portrait Gallery is enough of a Wunderkammer without flashing frontal lobes.
Still, you can't help wondering just which areas of Glenn Brown's brain must have lit up when he was painting pictures like Bertrand Russell at the BBC (1999) and Death of a Disco Dancer (1998), currently on show at the Jerwood Gallery. Shortlisted last year for the Jerwood Art Prize, Brown has spent the last eight years repainting Auerbach portraits in his own style. Since this style is the diametric opposite of Auerbach's - Brown paints with a photorealist exactness and a fine squirrel-hair brush; Auerbach in vast, gloopy strokes that slop somewhere between expressionism and abstraction - the whole thing seems more than faintly perverse.
And perversity is precisely where the considerable power of Brown's work lies. He does not simply paint Auerbach's subjects: he paints Auerbach's brush-strokes, copying them from photographs onto a canvas balanced on his lap, flattening the wild impasto of the originals into a wafer-thin, meticulously brittle surface so flawless that it, too, looks photographic. Every muddy trench of Auerbach's technique, every globule, each viscous string, is copied exactly. Like six-foot models of Salisbury Cathedral built in matchsticks, two things confound the viewer of Brown's work: the sheer scale of the undertaking and the bizarre unsuitability of the medium for the task to which it has been applied.
As with matchstick cathedrals, the viewer is left scratching his head as to why Brown should have gone to all the trouble. The Jerwood's exhibition notes suggest things to do with "critical discourses surrounding originality and authorship", and they are doubtless all true. (Brown did go to Goldsmiths, after all.) But if there is something conceptual going on here, there is also something more straightforwardly painterly. For all its meticulousness, it is certainly not the slavish copying portion of Brown's brain that must flash orange when he is at work. These are not copies of portraits but portraits of portraits. It seems obvious to say that you do not set out to do what Brown does unless you feel strongly about your subject: the question that remains unanswered, having looked at his work, is whether his strong feelings towards Auerbach are ones of homage or its opposite. It is easy to read the extraordinary perversity of Brown's painting as a love-letter to Auerbach's gloopiness, but it is equally easy to read it as a corrective to it. In flattening itself to the canvas, Brown's style plays its cards close to its chest. It is this sphinx-like quality that makes his work both oddly troubling and unpleasantly powerful.
'The Painter's Eye': National Portrait Gallery, WC2 (0171 306 0055) to 13 June. 'Glenn Brown': Jerwood Gallery, SE1 (0171 654 0171) to 23 May.