Brian Ferneyhough Day, Barbican Centre

Earlier this month something happened to me that's never happened before. Brian Ferneyhough's Sixth String Quartet roughed-up my critical faculties and left them for dead. I couldn't tell you what had happened, why, in what order, when. As it finished, small birds circled my head. So I entered Brian Ferneyhough Day yesterday at the Barbican as one would an egg-beater, knees a-knocking.

I needn't have. The day was a revelation. As is usual with these BBC Symphony Orchestra composer portraits, many different ways into the composer's oeuvre were proferred. The first reductivist hook (to scare you away or reel you in, who knows?) had been bandied about all week (all his life): that Brian Ferneyhough (pictured below right) composed the most complex music known to man, that his notation was denser than a sandy beach, that his scores were bigger then the Great Wall of China (see picture below left). All of which Guinness Book of Records-type hyperventilating got a bit trying. Tom Service's morning talk, however, mostly honed in on the whys not the wows.

One way to absorb his music, Service suggested, was to get off on the performer's struggles. A musician's attempts to realise the myriad instructions - each note often having a dozen different signposts delineating dynamic, tonal or timbral definition (see picture above) - was where enjoyment was to be had. Time and Motion Study II (1973-6), a physical and violent work for cello and electronics, excelled in this way. But wasn't it veering suspiciously close to Marina Abramovich's performance art? Pain and experience as entertainment? This early-ish work, however, also had a clear programmatic element to it - an original subtitle was to be "Electric Chair" - by which one could quite easily follow things. Both these guides suggested a remarkable continuity with a Modernist-Romantic ethos. Neither did much for me.

Another way to unlock Ferneyhough's music would have been to appreciate its realisation of his philosophical and theoretical positions. "What Brian Ferneyhough doesn't know about Hegel, Adorno and critical theory", Tom Service informed us, "is not worth knowing." Is there anything worth knowing about Adorno or Hegel? Service's sentence sent a shiver down my spine. Mistrust was calmed by the benign and only intermittently obscurantist presence of Ferneyhough himself, who bumbled around the Barbican with his straggly hair like a member of Time Team trying to find his way to the next trench.

But critical theory is critical theory, notation notation and music music. That is, none of the verbal and mental arabesques (some of which made no sense at all) that had fed Ferneyhough's works mattered. Talk of notational excess, emotive programmes, critical theory, even the discussions of formal matters of placing and structure, were distracting. We were being led on a very well-meaning but wrong-headed goose chase. For, in its sweep, energy, expression and effect, the music on its own was as clear as day. That is, it was quite brilliant.

La terre est un homme was like moving through a throbbing city: a completely intoxicating experience

Some revealed a linear clarity. The Second String Quartet (1980), gracefully played by the Quatuor Diotima, works itself up into a synchrony, breaks into a polyphonous mess, and winds up in a lazy mode blowing pitched bubbles. Carceri d'invenzione III (1986) is busier and more intense but no less readable. We are very clearly in the imaginary prisons of the Piranesi's famous prints, each woodwind instrument squirming away garrulously, desperately and diligently as the domineering beat of the percussion periodically flays them. It's a compelling work right to the mystical end, where it empties out into a smokey smear of bass flute and cymbal.

The earliest works, denuded of the later complexity of notational expression, were less emotional, more a piece with the cooler work of Webern and Boulez, and no less engaging. The Sonatas for String Quartet (1967), in which the four players (pretty uniquely for Ferneyhough) seem to work collegially to evoke their gently harmonised pictures, was dreamy and delectable. His strange little vocal exercise, Missa brevis (1969), whose meticulous rendition by the BBC Singers (with the help of several tuning forks that they bounced off their heads) shows just why they're one of the jewels in the BBC musical crown, was formally quite fascinating.

The episodic nature of the Sonatas for String Quartet ties it in with Plötzlichkeit (2006), the most recent. Both seem to be informed by the narcolepsy from which Ferneyhough suffers. A liminal stream of humming and singing stitches together bouts of activity that circles the orchestra. Scenes are snatched in the manner of a hyperactive Godard movie. And while every section gets a chance to be heard - the detuned harps acting as offstage commentators - the ever-present brass distribute a sunny glare to the activity, summoning up the light of mid-morning.

And then there are the deliberately, calculatedly chaotic works, such as the one that had stumped me the other week, which aren't as common in number as one is led to believe. Last night we received one, La terre est un homme (The World is Man) (1967-9) (score pictured right), and in the hands of the great Martyn Brabbins and an unflappable and impassioned BBC Symphony Orchestra it swept all before it. Its dramatic intensity and visceral kick (similar in scope and power to Sir Peter Maxwell Davies's Worldes blis) are sourced from the vast multi-part string writing, whereby virtually each and every violin was wriggling his or her own individual way through the life of this piece. This aviary of noise is undergirded with - and finally crushed by - the sound of sustained woodwind and percussion. It was like moving through a throbbing city: a completely intoxicating experience.

Rarely does one come out of these monolithic days with an unequivocally positive feeling for the composer concerned. Few have the variety to sustain a whole day's worth of music. But with Brian Ferneyhough, the more one heard, the better it got. More, please.

• Listen to Brian Ferneyhough Day on BBC iplayer
• Total Immersion continues with Unsuk Chin on 9 April and Peter Eötvös on 14 May
• See what's on at the Barbican Centre

Find Brian Ferneyhough on Amazon