Memory at Work
Two MU researchers are closing in on the mysterious operations of short term, ‘working’ memory. Got that? By Mike Martin
Philosophers and scientists have for centuries puzzled over the workings of human memory, an aspect of human consciousness that, at least since the time of Aristotle, has been considered one of nature's most complicated creations.
But is it possible the world's great minds have spent the last two millennia over-thinking memory's complexities? A recent study by two University of Missouri researchers suggests this may in fact be the case -- at least for one important aspect of memory.
Jeff Rouder, an associate professor of psychology, and Curator's Professor of Psychology Nelson Cowan have found our "working memory," that faculty of the brain that allows us to retain information of immediate importance to us, may be very simple indeed.
"Most people believe the human mind is incredibly complex," Rouder says. "We were able to use a relatively simple experiment and look at how many objects can be in maintained in the human conscious mind at any one time. We found that every person has the capacity to hold a certain number of objects in his or her mind."
Their study, published in the April 22 edition of the Proceedings of the National Academy of Sciences, determined that the average person remembers a mere three or four things at any one time.
These recalled things, items such as numbers, letters, colors, and names, are then stored in the mind as discrete, well-defined bits of information. It's similar to the way a computer works, Rouder and Cowan say.
The researchers arrived at this conclusion after devising a unique version of a simple memory experiment.
First, they used a computer to quickly show study participants an array of one, two, three, or four differently colored squares. When quizzed by the researchers, subjects could typically remember the colors and, later, determine whether colors changed or appeared out of sequence.
Add a fifth square, however, and working memory falters.Study participants usually provided answers anyway but, thanks to a model Cowan devised that Rouder calls "exceptionally simple and elegant," the two researchers soon figured out subjects really didn't have a clue.
"When the square was not in working memory, the participant just randomly guessed," Rouder says."Each participant had a fixed capacity that could not be exceeded." That working memory had this "all or nothing" limitation came as something of a revelation to many experts in the field.
"I was surprised by this result, but find the study persuasive," says EEG Institute chief scientist
Siegfried Othmer, a Woodland Hills, Calif.-based author of several books on therapies for attention deficit disorders (ADD and ADHD).
"If there is indeed such a hard limit on visual working memory capacity, then it becomes easier to discern the influence of attention and other factors on working memory performance."
University of Notre Dame associate professor of psychology Bradley Gibson says he's been "thinking a lot about these issues lately in my own research, and I really liked this paper."
Gibson, who studies visual cognition, perception and attention, found the Rouder-Cowan study "eloquent." His own work, he adds, reinforces their conclusions.
"The strength of this study lies in its analytic treatment of the findings," Gibson says. "Because of its analytic rigor, it may well provide one of the most precise estimates to date of the capacity of working memory."
One way to think of working memory, explains Barbara Ingersoll, an associate professor of behavioral medicine and clinical associate professor of psychiatry at West Virginia University, is the ability to retain a phone number long enough to accomplish a given task -- such as dialing a phone.Figuring out how working memory functions is important, she adds, because it could improve therapies for various attention deficit disorders.
Gibson, who has performed significant research on working memory in children with ADHD, concurs. "Having a precise measure of working memory is important," he says, "because weaknesses in working memory have been shown to underlie academic difficulties in reading comprehension and mathematical competence, as well as clinical symptoms of inattentiveness."
Nelson Cowan has long been a leader in this search for such measures. Long before his most recent paper, he was well known among psychology researchers for developing "Cowan's Formula," a measure that estimates memory capacity by linking the art of "guesstimating" to visual memory. He's also been a leading debunker of outdated working-memory hypotheses.
"Before my 2001 paper, many researchers and laymen believed that working memory's capacity was about seven items," Cowan says."Other researchers believed the smaller figure of about three or four items.Still other researchers were perplexed by the discrepancy and concluded that the capacity ‘just depends' on many factors."
The 7-item memory model originated with psychologist George Miller in 1956, says Rouder. "Miller noted that in some simple tasks, people's performance would decline if more than seven items were presented." But that model began to falter when researchers embraced "chunking," the idea that the memory of seven items is actually a memory of three chunks of two items plus one more."We remember more than seven items when we can ‘chunk' two items into one new item," says Rouder.
People who memorize pi to hundreds of places are nothing more than good chunkers, Cowan adds.Most forms of mnemonics, that common memory aid beloved of elementary school teachers everywhere, is nothing more than good chunking.
As Gibson sees it,Cowan and his colleagues "have been arguing for some time that the estimate of 7-item retention was too high and was probably achieved in previous studies with the help of various strategies like chunking. One could argue that when strategies like chunking or rehearsing an answer that might inflate the capacity of working memory are prevented, the limit is lower—more like three or four objects."
Confirmation of the four-item memory model came about 10 years ago, when psychologists Steve Luck and Ed Vogel "published a very influential paper in which they asked if there was a basic-capacity limit in immediate consciousness."
Their approach, which Rouder calls "the gold standard for assessing the number of basic items in consciousness," was to flash five differently colored squares for about 100 milliseconds to study participants in a series of changing displays -- plenty of time for working memory to kick in, but not enough time to formulate an educated guess.
"People had to indicate whether the display was the same or had changed," Rouder explains. "Performance was perfect if the number of squares was small." But without the ability to guess, he adds, performance declined dramatically.
Using a variant of the same approach, Rouder and Cowan confirmed not only the four-item limit, but also its all-or-nothing nature.
All scientific discoveries are based on predictions, Rouder says. "I had the insight that the all-or-none model makes strong predictions about the relationship between correct and error responses."
The all-or-nothing model "made precise predictions that were borne out in the group as a whole and for the bulk of subjects individually, despite some individual differences," says Cowan. "People seemed to remember objects, not partial information from the objects."
By showing that nature loves the simplicity of an all-or-nothing approach—even with the complicated task of thinking and remembering—Rouder and Cowan believe they've simplified the study of attention-deficit disorder and learning disabilities that compromise working memory.
The study wasenlightening on several other levels as well.
With fellow research team members Richard Morey, Christopher Zwilling, Candice Morey and Michael Pratte, Rouder experienced what he calls "the power of collaboration between different viewpoints." A skilled mathematical analyst, "but not an expert in memory," Rouder found in Cowan "a memory expert not trained in mathematical analysis."
At first, Cowan's model seemed "too simple to be true," Rouder says. "But I was wrong. Nelson Cowan's simple model does an admirable job of passing a rigorous test."
For Rouder and Cowan, the logical next step is to extend their findings into other areas of cognition. An obvious choice, they say, involves investigations into the nature of long-term memory.
If short-term memory stores information in bytes, memory of the long-term variety stores it in megabytes.In fact, says Rouder, long-term memory storage -- remembering, say, that "most police cars have red and blue lights" -- is fairly limitless."We know seemingly countless pieces of information, such as faces or images," he says.
From an evolutionary perspective, it may seem counterintuitive that less memory is actually more. But natural selection may have restricted working memory's short-term cache to increase efficiency and reduce clutter around what Rouder says is the "gateway to long-term memory."
"It is biologically expensive to use information, and the brain's solution is to use a little bit of it at a time," Cowan says. "A few theorists have suggested that groups of three to seven items are logically optimal. If much more information were in short-term memory at the same time, the pieces might interfere with one another."