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The latest fitness fad has an unusual goal: bulking up the three pounds of mush between your ears. Sales of “brain fitness” software increased from a few million dollars in 2005 to $80 million in 2007, according to an estimate from the consulting firm Sharpbrains. Plenty more products are available on the web. Computerized cognitive training includes games that test memory, attention, and problem-solving skills, plus good old arithmetic and linguistic ability. Brain-training websites promise improved mental stamina, the ability to concentrate for longer periods, and a better memory for directions and names—even heightened mood.

The science behind these supposed brain boosters remains promising but inconclusive. Multiple studies in older adults indicate that on-screen exercises can slow age-related cognitive decline, but improving the cognition of other groups is another question. Recent findings do suggest that healthy younger adults can improve their smarts as well: The ability of twentysomethings to solve patterning problems improved after practicing on unrelated memory tests, one University of Michigan study found. The finding suggests that skills in one game can apply to different mental challenges, similar to the way running is good for parts of the body other than just the legs.

The following six websites all offer cerebral calisthenics, mostly built around quick, free mental stimulation that’s good for a break at work. Those who are more serious about online brain training may want to subscribe to track their progress. While the science sorts itself out, the websites already have one undeniable benefit: They’re fun

Brainist

A collection of free games and brain-related materials testing pattern-matching skills, memory, language and math skills, strategy, and more.

What’s unique: Sheer scope. Any and all things brainy and game-y can be found here, from number mnemonics to the most commonly used words in three languages to a speculative list of the smartest people ever.

Downsides: The site is a bit disorganized. You can’t track the gains you make in individual games.

Bonus: A classic IQ test like the one you might have taken as a teenager. Remember what you got then?

Summed up: All kinds of stuff to keep you entertained, though a bit scatterbrained. Check out the “Strategy Games” puzzle featuring six frogs sitting on seven rocks that must be hopped, one stone at a time, over each other’s heads. It’s marvelously frustrating.

Rating: 3 lobes

Fit Brains

Comprehensive brain fitness site featuring 10 games plus a word challenge for team play. Tests fall into five cognitive categories: memory, language, concentration, logic/reasoning, and visuospatial skills. Sports a blog and a list of scientific advisers. Boasts “Guilt Free Fun!”

What’s unique: Complex, richly illustrated and thought-out games with different levels and basic story lines. For example, in “Hidden Masterpiece” you are a painting-repair specialist who sells reconstructed works of art at auction, testing visuospatial ability and concentration.

Downsides: After a seven-day free trial expires, the site is $9.95 a month or $79.95 a year. Some games may actually be too complex and time-consuming. In “Busy Bistro” you scan ingredients and cooking instructions, then try to remember the items by filling out a virtual grocery list. That’s just for starters: One round takes almost five minutes, and there are five more courses to follow.

Bonus: Get real recipes from “Busy Bistro,” like Crab and Swiss Melts.

Summed up: Very dynamic. Feels as if you are playing a console-based game at times. You can subscribe to track your progress and meet other Fit Brainers. Suitable for the committed brain athlete.

Rating: 4.5 lobes

Enjoyable time killers, including standbys like checkers, sudoku, and chess, as well as original exercises like guessing (and then recalling) a country’s flag. Probes memory, pattern-matching abilities, spatial skills, and more.

What’s unique: Less hype—just lets you play. Simple scoring mechanism means you spend less time worrying about the clock and more solving the problem.

Downsides: Some of the games feel more, well, game-y than brainy.

Bonus: After entering a ticket number earned during your workout, you can see a prize image.

Summed up: A quick one-stop site for a dose of mental gymnastics. Lots of free games to choose from without any commitments, financial or otherwise. But you may not stick around for very long.

Rating: 3.5 lobes

Mostly a puzzle-book shop and source for brain-gain advice; the site also features games testing language, mathematics, and memory skills.

What’s unique: New types of crosswords involving letters or numbers, timed for speed. Example: An “alphabetic” has only 26 spaces, one for each letter of the alphabet, which comes up just once per game.

Downsides: Games can be played only once before it’s all reruns. Superbasic visuals are mostly black-and-white.

Bonus: Cool crosswords!

Summed up: Although the games are neat, getting to play them only once is a bummer.

Rating: 1 lobe

Brain Arena

Billed as “massive multiplayer online brain-training,” the Web site hosts contests challenging players’ visuospatial skills, arithmetic ability, and reaction time.

What’s unique: Competition among thousands of mind-gamers trying to beat one another’s scores. Winners crowned every 24 hours.

Downsides: Repetitive games that reward your quickness to click a mouse as much as your acumen. Almost 80,000 people have joined the site, but it still feels junior varsity: Some tasks, such as a tiered addition problem, are either poorly explained or not explained at all.

Bonus: Cheeky names for players’ rankings: You start off as a puny Habilis, work your way up to Sapiens—and if you keep at it, achieve Cibernetis status.

Summed up: The site feels a bit like it’s under development, but the league-play aspect is intriguing—especially for those with a competitive streak.

Rating: 2 lobes

A cleanly constructed site that is serious about representing the science of brain games, referencing studies and neuroscientists who support this approach to brain fitness. Tests memory, processing speed, attention, and cognitive control, which is basically impulse restraint.

What’s unique: Lets you set up a training program with regimented sessions and claims to offer a full workout in only 10 minutes.

Downsides: Just as expensive as Fit Brains but less visually appealing. Some games feel more clinical than creative.

Bonus: Waddling, animated penguinlike characters are cute.

Summed up: Solid, absorbing exercises that strike a good balance between basic playability and complexity, though a bit blah at times. After signing up, the Web site tracks your progress and tells you what to do. After all, if you’re doing neuro-workouts, you might as well get a personal trainer.

Rating: 4.5 lobes