As even the most ambitious rockers have learned, pushing the limits of one's art is laudable, but so is realizing when one should apply the brakes. Pop history is dotted with cautionary tales of musical explorers who reached that crucial point of no return, sacrificing much of their audience and spending the rest of their careers searching for a path back. The art-rock crowd has often succumbed to this temptation, exemplified by Yes' 1974 ''Tales From Topographic Oceans,'' four 20-minute meanderings that tested even the patience of the drooler nation. Yes were never the same again -- nor were, to cite other examples, Joni Mitchell after her jazz project ''Mingus,'' Styx after the schlock opera ''Kilroy Was Here,'' or Pearl Jam after the drony ''No Code.'' Every so often, a band that ventures afield manages to find its way home: U2's recent return to form, after their halfhearted techno phase, is a rare example of a graceful artistic U-turn.
Progheads at heart, Radiohead have now arrived at that dangerous crossroads. Actually, they first set foot there last fall with the self-conscious sonic wanderings of ''Kid A.'' Even its artwork and tour paraphernalia -- spooky barren glaciers! -- seemed like rough drafts of the landscapes Roger Dean contributed to old Yes albums. Now, an unexpectedly quick eight months later, they give us Amnesiac, recorded at the same time as ''Kid A.'' In contrast to that album, which worked very, very hard not to sound like the Radiohead many of us had come to know and love, ''Amnesiac'' was rumored to be a more grounded effort, more accessible and song-oriented than the hums, burrs, and sonic bric-a-brac of its predecessor.
''Amnesiac'' is certainly less post-rock than ''Kid A,'' yet ''Kid B'' would not have been an inappropriate name for it. The band exhibits the same fascination with lo-fi rhythms and keyboards that chime like doorbells, and the same disinterest in building tracks around guitars. The key differences lie in Thom Yorke's voice, which is more prominent and less altered than it was on ''Kid A,'' and in the presence of fewer instrumental or novelty-effect tracks. In general, ''Amnesiac'' is easier to digest and takes far less time adjusting to than did ''Kid A.''
Then why is ''Amnesiac'' a more frustrating, even infuriating, work? The band proclaimed ''Kid A'' as something unusual from the git-go, so the shape-changing songs on its first half weren't a complete surprise. It was easy to forgive Radiohead their indulgences, especially since the sonic rapture they convey so well was in full effect. It's not Radiohead's fault that we were expecting a more traditional follow-up to ''Kid A.'' But it is their fault that what they serve up on ''Amnesiac'' is neither a grand song cycle a la ''OK Computer'' or ''The Bends,'' nor wholly satisfying experiments like the most successful parts of ''Kid A.''
''I Might Be Wrong'' is mostly a cyclical guitar riff with indecipherable lyrics. (Once again, the band has refused to supply lyric sheets, so we're left to guess.) ''You and Whose Army?'' has a creepy beauty, with Yorke's drowsy voice resting on a piano bed, but it soon devolves into subpar Moody Blues. With its rippling-water guitars and wan Yorke delivery (of lines like ''Look into my eyes/I'm not coming back''), ''Knives Out'' feels more like a leftover from Radiohead disciples Coldplay than the real thing. ''Pyramid Song'' has potential: ''I jumped in the river and what did I see?/ Black-eyed angel swam with me'' is a striking opening line, but like many of the other tracks, it evaporates before your ears. These may be ''songs,'' but they feel unfinished, lacking power.
Other songs merely feel like gauzy outtakes from ''Kid A.'' ''Pull Pulk Revolving Doors'' reprises the distorted vocals and staticky percussion of the earlier album. One of the high points of ''Kid A,'' ''Morning Bell,'' is reprised here in a droopier, soupier version dubbed ''The Morning Bell Amnesiac.'' Judging by that song, it's hard to tell what's more coy: Yorke's voice filtered through a machine, or Yorke's normal voice at its sleepiest.
Yorke and the band may not care that they are aggressively, increasingly impenetrable, and maybe they shouldn't. But their inward turn is beginning to feel like an act of hostility and contempt. By the sound of it, Radiohead have strayed off into the same territory Yes did over a quarter century ago -- and two pieces of marginalia in a row don't bode well for the outcome. In Amnesiac's first song, ''Packt Like Sardines in a Crushd Tin Box,'' Yorke sings, tersely and defensively, ''I'm a reasonable man/Get off my case.'' If Radiohead continue at this pace, his audience's demands and expectations will be the least of his and his bandmates' concerns.