Let me introduce you to the two poles in pop and rock. One is marked by authenticity, musicianship, a certain traditionalism. This is the pole that in critics’ discourse is called ‘rockism’ – the assumption that rock (or, at least, real people playing real instruments) is the normative state of music. The other is artificiality, brashness, a disdain for heritage – a celebration of everything that is inauthentic, where a good idea is worth 100 guitar lessons. And that pole is known as ‘poptimism’. Poptimism is why you end up with learned essays in the New Yorkeranalysing the singer Ariana Grande nicking a doughnut from a shop with reference to the work of John Ruskin. (Yes, that really happened.)

The US duo 100 Gecs are firmly at the poptimist pole. Their music fizzes, almost literally. It’s like a packet of sherbert dropped into a bottle of Tizer, possibly with a bomb of MDMA thrown in, too. It’s all E numbers, artificial flavours and colours and more added sugar than you could weigh. At times, at the Forum – so hot the air seemed to hang with sweat – it was like watching an avant-garde reimagining of the Australian children’s entertainers the Wiggles. From the moment Dylan Brady (in a wizard’s hat and cloak) and Laura Les (also in a multicoloured cloak, but no hat) walked on stage, it was much more like being at the circus than at a rock show. It was both confounding and fantastic.

100 Gecs have become just about the hippest thing in pop, in a way that would (and should) utterly mystify anyone old enough to have teenage kids. They make, say, Duran Duran look like the Velvet Underground in terms of seriousness of purpose, even if their music is the result of magpie minds and a keen vision. For all that it sounded like a children’s party, there was more going on in each song than was possible to comprehend.

But how to describe it? Beware, for here will follow a list of terms that will likely mean nothing to you. Broadly, they are what is known as hyperpop, which is electronic pop with all the characteristics exaggerated. And within that are what is apparently known as ‘nightcore’ (music artificially speeded up in the production process) and ‘chiptune’ (music based on electronic game tracks). But there’s also a whole lot of 1990s alt-rock in there – pop-punk and ska-punk especially. Even live, the vocals went through a variety of effects, and ‘live’ itself is something of a misnomer, for while the tracks were largely different from their recorded iterations, almost all of them involved Brady pressing play on a computer, then the two of them jumping around the stage shouting. The only time one knew everything was entirely live was when they played ‘gecgecgec’. They had stools brought out, and played it semi-competently on acoustic guitars, presumably in a comment on the pointlessness of authenticity.

They were onstage barely an hour, of which a good 15 minutes was messing around making stupid noises, including a couple of minutes of tuneless banging of what I think were some instruments from a gamelan ensemble, but couldn’t see for the dry ice. Yet none of it ever came across as cynical – no matter how short-changed one might feel rationally. That this was their belated UK debut perhaps accounted for some of the febrile excitement around the place, but it was a whizzbang explosion of entertainment, and beneath the seeming meaninglessness of tracks such as ‘Doritos and Fritos’ or ‘Hollywood Baby,’ there was a very old-fashioned pop songcraft at work. I went in already weary at the prospect, but left delighted.

The other pole was represented by Fleet Foxes, appearing at the lovely Islington Assembly Hall ahead of UK festival shows. You want authenticity, Fleet Foxes have got it by the bongful: beards, acoustic guitars, close harmonies, one member who spent much of the show playing tambourine, because a rattled tambourine can make all the difference to your sonic collage, man. Now, I’ve adored Fleet Foxes since first hearing them on MySpace way back when, and there’s no doubting the craft. But how authentic is it really, when Robin Pecknold – who has sold a ton of records, and who plays big rooms around the world – still comes on stage dressed like an engineering student who got out of bed late? It’s no more real than coming on dressed as a wizard. Just less fun.

The music was lovely – songs from their last album, Shore, were delivered with precision and grace, and early numbers ‘White Winter Hymnal’ and ‘Mykonos’ have lost none of their beauty. But, truthfully, it did not feel – to me, though it clearly did to the vast majority of the reverent crowd – like a living, breathing event. It was a recital. A terrifically done recital, but no more than that. Whereas 100 Gecs, no matter how little of it was actually live, felt wholly and completely alive.