You never quite know what to expect from Edmund McMillen. The neurotic dreamworlds of Aether. The cryptic poetry of Coil. The sheer tingling grossness of Meat Boy. McMillen consistently dances on the outside edge of predictable Flash gaming, with a unique and recognizable art style, and dark themes that bite close to the psychological bone. Here he is collaborating with Tommy Refenes, author of the highly anticipated Goo! (no relation to World of Goo) and a bit of an experimental gameplay maverick in his own right. The result is one of the best action games I've played in Flash, although its appeal may be limited for casual game fans.

Grey Matter is an anti-shooter, which means that you can't actually shoot. You are the bullet, and you attack by directly colliding with the exposed brain-meats of your enemies. Control your movement with the [Arrow Keys]. The [Space Bar] gives you a short burst of speed, and [Shift] slows you down, which is very useful for weaving through tightly-packed enemy fire. Various creepy-crawlies will attack from all sides of the screen, and although their bodies are vulnerable, their bullets, tails, teeth, and lasers are deadly. Basically anything flashing brightly can kill you. (By the way, if you're sensitive to flashing light patterns, skip this one.)

Press [P] or [Esc] to pause the game and enter the power-up screen, where you can spend points on shields, shields, and some other bonuses that are a lot less important than shields. Naturally the higher your score, the faster you can max out your bonuses, so it pays to master the combo system. Ram three sequential enemies of the same species to increase your score multiplier. This also triggers a Trinity Attack, which destroys anything caught inside the triangle formed by the last three baddies you killed. (Tip from the comments: "If you've purchased the "Options" power-up, holding down Shift will not only slow you down, it will extend the options away from you, so you have a larger kill range." Thanks, LSN!)

You'll need nimble fingers to play Grey Matter, because of the small moving targets you have to hit, and the sheer volume of bullets they'll send at you. Apparently, you can play using an XBox controller using a program called FlashJoystick (also by Refenes), but I don't have one, so I don't know if it makes the game any easier. Please let us know if you can get that feature to work.

Analysis: Grey Matter looks great and sounds even better, thanks to the music of Danny Baranowsky, who also composed for Meat Boy and Gravity Hook. The gameplay has all the depth of a modern professional shoot-'em-up, thanks to the combo system. Grey-Matter is perfectly playable without using the Trinity Attacks, but if you do employ them, it becomes almost like a hyperactive, twitchy puzzle game.

There are only a handful of levels, but they get real intense real fast. Beating the game repeatedly unlocks a few extra modes, including direct access to the battle with the incredibly dangerous and aggressive final boss. I can't say there's a lot of replay value beyond that, but like all the best action games, the pleasure here is in mastering the technique, rather than merely surviving.

Most of the neurological nightmares you'll fight can only fire when they are facing you, so try to attack from behind. You can travel on the decorative border on the edges of the screen, while they can not. So one possible strategy is to make calculated blitzing runs and return to relative safety. You'll probably die anyway, but at least you'll have a strategy.

The most interesting thing about Grey Matter for me, though, is the theme of mental illness threaded through it, in the form of ambiguous quotations at the loading screen and between levels. Initially these quotes may seem like window-dressing, but they left a footprint on my mind long after I shut down the game. McMillen has left the connection between the theme and the gameplay deliberately vague, but anyone who has had experience with depression or schizophrenia will find echoes of that darkness in Grey Matter's weird, abstract brain-scape. It's an intimate stamp on a memorable shoot-'em-up. If you can handle the difficulty, this comes highly recommended.

Play Grey Matter.

If the game runs slowly, you can also download it here.