Your brain may only be able to hold three or four things in your conscious mind at one time.

I forget how I wanted to begin this story. That's probably because my mind, just like everyone else's, can only remember a few things at a time. Researchers have often debated the maximum amount of items we can store in our conscious mind, in what's called our working memory, and a new study puts the limit at three or four.

Working memory is a more active version of short-term memory, which refers to the temporary storage of information. Working memory relates to the information we can pay attention to and manipulate.

Early research found the working memory cut-off to be about seven items, which is perhaps why telephone numbers are seven digits long (although some early telephone dialing started with a two- or three-letter "exchange," often the first letters of a community name, followed by four or five figures, e.g. PEnnsylvania 6-5000). Now scientists think the true capacity is lower when people are not allowed to use tricks like repeating items over and over or grouping items together.

"For example, when we present phone numbers, we present them in groups of three and four, which helps us to remember the list," said University of Missouri-Columbia psychologist Nelson Cowan, who co-led the study with colleagues Jeff Rouder and Richard Morey. "That inflates the estimate. We believe we're approaching the estimate that you get when you cannot group. There is some controversy over what the real limit is, but more and more I've found people are accepting this kind of limit."

The study was published April 14 in the journal Proceedings of the National Academy of Sciences.

Masters of memory

To prevent subjects from grouping or using other memory-aids, the researchers presented people with arrays of different-colored squares. The subjects were then shown an array of the same squares without the colors. Afterward, they were shown a single colored square in one location, and asked if the color matched that of the square in the same position at the beginning.

"What's nice about this visual task that they used is that it really makes it difficult to use some of those common strategies that are helpful with verbal lists," said Michael Kane, a psychologist at the University of North Carolina at Greensboro, who was not involved in the new study. "I think Cowan's work has really been convincing in this."

While the average person may only be able to hold three or four things in mind at once, some people have achieved amazing feats of working memory. Contestants at the World Memory Championships (most recently held in Bahrain in September 2007) often recall hundreds of digits in order after only five minutes. But even these masters of memory seem to start with the same basic capacities as everyone else, and improve their abilities with strategies and tricks.

"A very famous study was a test done of a long-distance runner who learned to associate digits together in ways that were meaningful to him with respect to running times," Kane said. "He could repeat back lists of up to 80 digits in the right order, but if you gave him a list of words, he was at seven plus-or-minus two like everyone else."

The new working memory study builds on previous research, but provides the most rigorous mathematical test of the three- to four-item estimate, Cowan said. The team used a mathematical model that assumed people have a fixed number of slots in their working memory, each one of which can only hold one item. When those slots are filled, the model predicted, people would make random guesses. Based on this assumption, the model was able to forecast the various results of the trials with impressive accuracy.

"It is a pretty simple mathematical model but it predicted a very exquisite pattern of data," Cowan told LiveScience. "The results really were simple. With a single value of working memory capacity we could really account for all those different scenarios."

Working memory and intelligence

Although there seems to be a cap on the average number of things a person can remember at once, basic working memory capacity does vary among individuals. Interestingly, those that test well on working memory tasks also seem to do well at learning, reading comprehension and problem solving.

"People accept that intelligence seems to be related to working memory," Cowan said. "The information you can hold in your mind at one time is the information you can interrelate. If you have a better working memory we believe that your problem-solving abilities are better."

Researchers don't know what causes these variations in working-memory abilities — perhaps they are genetic, perhaps they arise from differences in early childhood environments or education.

The good news is people can improve their performance on certain working-memory tasks with training. When children practice these tasks, over time they get better. And not only do their scores on the memory tasks improve, but their scores on tests of attention and reasoning can also rise.

"The jury is still out on how useful this will be, but it's at least suggestive that you can train skills at these tasks, and that this improvement can affect other things," Kane said. "We don't know quite how they work together, but attention and working memory seem to be very close cousins."

It's all in there

Researchers debate the relationship between working memory and long-term memory. While some hold that the two are independent storage facilities, others say working memory is simply the part of long-term memory that we can currently access.

Many scientists believe that almost all of our experiences are encoded into long-term memory, and that forgetting is simply a matter of not being able to access that memory.

"It's in there somewhere, the problem is just getting to it," Cowan said. "Everything gets encoded into long-term memory almost immediately, but it gets encoded in a way that may not be distinct enough to be retrieved."