And now for something completely different.

We talked about pocket universes a couple of weeks ago, when we discussed Joy Electric. It’s my way of describing the sealed-off idiosyncrasy of someone like Ronnie Martin, who follows his own set of rules and makes music that sounds like nothing else out there. Martin’s universe is cohesive – you can enter it at any point, and the rules remain pretty much the same. You’ll hear blippy techno-punk with soaring melodies performed on analog synthesizers.

But how about those artists who change the rules every time out? Those are rare, and always welcome. Take Terry Taylor as an oft-repeated-by-me example – Daniel Amos sounds different album to album, and bears little resemblance to Taylor’s work in the Swirling Eddies, the Lost Dogs and as a solo artist. The rules change, but the indefinable signature remains.

And yet Taylor will readily admit his influences – in fact, he wears them like a badge. Early DA took from classic country, and then from slashing new-wave, and the Lost Dogs crib from decades of American music. Taylor’s solo work has swung from Brian Wilson-esque pop to Johnny Cash-esque country and back again, and while all the songs sound like Taylor, they do draw from several traditions.

But what about the constantly-shifting artists who produce album after album that not only differs staggeringly from that artist’s other work, but from anything else out there? How many artists can consistently uproot the foundations of their own pocket universes without letting influences creep in? By my reckoning, very few – even the great Frank Zappa was a mélange of borrowed styles, from doo-wop to the orchestral work of Edgard Varese.

It’s true, of course, that no one’s work is completely original, but if there’s anyone making music that sounds otherworldly, it’s that Icelandic pixie Bjork. Starting with her tenure in the Sugarcubes, one of the strangest dancehall-reggae-pop bands ever, Bjork has cut a swath through the accepted ideas of pop, taking on bits and pieces of other idioms and wrapping them in her own sensibilities. No matter what she’s doing, she is always strange and fascinating to watch, and her work is always rewarding.

Her first two albums, Debut and Post, veered as far from the Sugarcubes as possible, spinning on vortexes of beats and bass, with the occasional brassy big band. With her third, the astonishing Homogenic, Bjork flipped the rules again, practically creating the technorchestral sound – synthesizer beats providing the crunchy foundation for an 80-piece orchestra. She moved into a colder and more ambient direction on Vespertine, with a slight detour into Broadway-from-Mars on Selmasongs, the soundtrack to her acting debut, Dancer in the Dark.

And now she’s done it again – she’s inverted the rules of her pocket universe. Bjork has never been one to do anything by halves, so when she proclaimed some months ago that “instruments are so over,” you had to expect that she would stick to that and deliver something like Medulla, her just-released fifth album. In simplest terms, Medulla is an a cappella project – there is almost nothing on here but the human voice. (The occasional piano and synthesizer only enhances the effect, and demonstrates that unlike her Dancer director Lars Von Trier, Bjork is not obsessive about her self-imposed limitations.)

This is not an album of voices, but rather one constructed from voices, and that’s an important distinction. If you’re expecting Take 6 or Bobby McFerrin here, you won’t get it. Most of the vocal tracks have been sampled, processed and mutilated into fascinating new forms, from the keyboard choral arrangements of “Where Is the Line” to the explosive beats of “Triumph of a Heart.” This is an icy, alien pop record at its core, but it’s infinitely more intimate than Vespertine, its closest cousin, simply because of the fragility of its production.

Contrary to some opinions, Bjork has not stumbled upon a completely original idea here. In fact, in 1985, Todd Rundgren made a record called (natch) A Cappella, consisting of nothing but his own sampled voice. Bjork’s album is conceptually similar, but it’s her own artistic thumbprint that sets Medulla apart. The record is eerily paced, and many songs contain no beats at all. Several of them are sung in Icelandic, and are revelatory for those who remain mystified by Bjork’s phrasing in English. In her native tongue, her trills and accents make perfect, even poetic sense. And of course, there are the multitude of other voices that contribute here, spicing up the tone.

And what voices they are. Former Faith No More singer Mike Patton lends his snarl to opener “Pleasure Is All Mine,” and the section in which his painful growls come out of the right speaker and Bjork’s pleasure-filled moans come out of the left is riveting. Robert Wyatt sings on “Submarine,” delivering the only non-Bjork lead vocal, and his shaky tenor sounds stunning when layered atop itself a dozen or more times. Bjork has also enlisted beatboxers Rahzel and Dokaka, and their work is amazing, especially on “Mouth’s Cradle” and closer “Triumph of a Heart.”

Elsewhere, Bjork sings with a full Icelandic choir, crafting minute-long interludes between the more experimental tracks. This is an uneven record – it flits about too radically and ends too quickly – but these sections work hard to tie it all together. For instance, it would have been even more jarring to slam from the majestic Olympics anthem “Oceania” into the crazed, dissonant “Ancestors” without the placid “Sonnets/Unrealities XI” in between. Still, there are no concessions to easy listenability here – Medulla is a defiantly odd and bizarrely beautiful record that sounds out of place next to… well, anything.

It’s also one of the best records of the year, and even I am getting tired of hearing myself say that. This has been an extraordinary year so far for dramatic, inspired works – so much so that I could easily fill all ten slots in my year-end list right now and come up with a respectable, even superb collection of albums. Even so, Medulla stands out as one of the most creative and rewarding of the bunch. This album, and in fact Bjork’s whole career, is the very definition of following one’s muse wherever it may lead. Damn the rules.

Next week, Tears for Fears. Rejoice, Liz!

See you in line Tuesday morning.