<preform>Head/Case, Soho, London</br>Tejas Verdes, Gate, London</br>Tim Fountain - Sex Addict, Royal Court Upstairs, London</preform>

Talk, talk, talk. It comes with the terrain: many dramatists are obsessed with language, its power and its failings. What's fascinating about Ron Hutchinson's new play, Head/Case, is that his characters, Tracy and Julia, are suffering from contrasting kinds of speech disorders. In Caroline Hunt's excellent production, we are in a hospice with one fading sofa and one narrow bed marooned in a chilly, white chamber (design by Tom Piper). Claire Cogan's Tracey stands there jabbering. She looks like an ogress but is oddly childish and funny.

Talk, talk, talk. It comes with the terrain: many dramatists are obsessed with language, its power and its failings. What's fascinating about Ron Hutchinson's new play, Head/Case, is that his characters, Tracy and Julia, are suffering from contrasting kinds of speech disorders. In Caroline Hunt's excellent production, we are in a hospice with one fading sofa and one narrow bed marooned in a chilly, white chamber (design by Tom Piper). Claire Cogan's Tracey stands there jabbering. She looks like an ogress but is oddly childish and funny.

We glean she is Irish and was brain-damaged years ago when hit by a brick during a sectarian riot. Though trying to make a new start and behave "normally", she hears voices and talks compulsively, often going round in small verbal circles. By comparison, Sarah Cattle's reed-thin Julia - middle-class, English, and hurt in a car crash - is almost mute. She moves in weird, marionette-like slow-motion, as if struggling to overcome complete inertia and leaning backwards to avoid social contact. When she opens her mouth, it is as if she is dragging every word up from some unfathomable well. She says that she has only partially come out of her coma, has "got stuck somewhere" and is emotionally numb. While starting to look like a co-dependent unit, each of them speaks of being a split personality. Tracy is also visited by Jimmy (Jonjo O'Neill), who insists he still loves her and who sings tender, haunting Irish ballads, yet harbours destructive urges.

Being no neuropsychologist, I can't say how closely Tracy and Julia resemble actual patients with frontal lobe injuries. Head/Case is, pretty evidently, a blend of medical research and poetic licence, but what's clever is that Hutchinson is using his fictional medium to question how the mind may confuse imaginary and real people. The play becomes a brainteaser as you try to work out who genuinely exists outside of Tracey's head. At the same time, the protagonists become allegorical representatives of national identities, somehow knocked into being stereotypically Irish and English (loquacious and dangerous vs taciturn and cold) by having, mentally, been in the wars. These racial differences are dubious, but Hutchinson is alert to that. You could also see the play as a portrait of a single person with conflicting sides to their character.

Though a couple of heavy-handed passages could be pared, Head/Case is rewardingly multilayered and deserves to become a small-scale modern classic. Co-presented by Coventry's Belgrade Theatre, the RSC and Soho, Hunt's production is beautifully acted and startlingly eerie, especially when (thanks to ingenious lighting tricks) further ghostly doppelgängers start seeping out of the walls.

Tracy and Julia's almost liturgical phrases have some striking affinities with the five women in Tejas Verdes - a daisy-chain of monologues by Fermín Cabal (translated from the Spanish by Robert Shaw). Here, however, we are in Chile and Cabal's characters are compelled to speak of their memories of the repression, torture and killings that followed Pinochet's coup in 1973. The first unquiet spirit we encounter is one of the "disappeared", who runs over and over her happy youthful memories and the graphic horrors of her incarceration. The subsequent witnesses include her friend, who turned informer, a cemetery caretaker, and a military doctor who would prefer to sweep the whole episode under the carpet.

Thea Sharrock's production, her first as the Gate's artistic director, creates an ominous promenade setting. Armed with a torch, you wander through a maze of filing cabinets and tombstones into a dark forest, with leaf-mould sinking under your feet and tree trunks snagging your clothes. Her cast give fine, subtle performances, too. Shereen Martineau is outstanding as the dead girl, materialising out of the shadows like a delicate, lost fawn, followed by Diana Hardcastle as her guilt-ridden friend and Gemma Jones as the salt-of-the-earth caretaker. Still, I'm not wholly convinced by Cabal's writing. Tejas Verdes is certainly topical, the cruelty described is horrendous and the play is, apparently, based on real people. Nevertheless, the end result is discernibly artificial. I'd rather watch a straight documentary than have Cabal pushing my emotional buttons.

Finally, bearing witness becomes a curiously lightweight affair inTim Fountain - Sex Addict. This writer-performer has, allegedly, had erotic relations with 5,000 men and two women so far, and he is intending to up his total via this show. Essentially, TF-SA is a one-man comedy confessional (for over-18s). En route, Fountain tosses in some very frank chat about being a scat queen and flashes up pornographic slides of tumescent bits and pieces, orifices and bodily fluids. Then he gets chatting online, and on his mobile, with a bunch of eager, consenting strangers - heading off at the end to shag whichever one got the audience's vote. He takes a video camera with him, so the start of each show includes some footage of yesterday's conquest.

There are, perhaps, two remarkable things about this show. On the one hand, it's dull. You feel you've seen or heard about it all before, even when you haven't. Maybe this is because Fountain himself confesses he's jaded, and is sluggish with his comic timing. On the other hand, TF-SA proved morally thought-provoking after the event. I couldn't help laughing, then felt seriously worried about the masochist who rang in, panting, on press night. Other wannabe dates seemed peculiarly innocent, while the home-movie from the night before was unexpectedly endearing: a chubby bloke sweetly chuckling about how Fountain had failed to get it up. It was like Sex in the City with Dame Thora Hird.

k.bassett@independent.co.uk

'Head/Case': Soho, London W1 (020 7478 0100), to 29 Jan; 'Tejas Verdes': Gate, London W11 (020 7229 0706), to 5 Feb; 'Tim Fountain - Sex Addict': Royal Court Upstairs, London SW1 (020 7565 5000), to 29 Jan