Mind’s Limit Found: 4 Things at Once
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."