Anyone who has tried to find an urgent e-mail amid masses of advertisements for dubious stock opportunities and sexual-enhancement drugs understands the critical importance of being able to filter out distracting information. That e-mail you seek may be in there, but it is lost among irrelevant clutter.

Although the capacity of our computer’s e-mail in-box is limited only by disk space, our mental “in-box” of working memory—the brain regions and processes that create temporary storage—is much more constrained. In fact, several decades of research have indicated that our capacity to hold information “in mind” for immediate use is limited to a mere three or four items.

Moreover, just as people vary in height and eye color, they also vary in the capacity of this memory in-box. Interestingly, these differences in working-memory capacity are strongly predictive of a person’s ability to perform abstract reasoning, mathematics and other forms of complex problem solving. This relation between memory capacity and fluid intelligence has motivated many scientists to try to understand why and how people differ in this important cognitive ability. A new study adds insights into that line of inquiry.

Hard Drive or Spam Filter?
There are two primary explanations for this severe limitation in working-memory capacity. First, it could be that storage space essentially determines working memory’s limits and that some people have larger “hard drives” than others do. The alternative theory is that capacity depends not on the amount of storage but on how efficiently that space is used. Thus, high-capacity individuals (who can remember more information at once and who tend to do better on aptitude tests) might simply be better at keeping irrelevant information “out of mind,” whereas low-capacity individuals may allow more irrelevant information to clutter up the mental in-box. The difference may just be a matter of having better spam filters.

Some of our own recent work on differences in controlling access to working memory has provided evidence favoring this mental spam-filtering idea. In one experiment, measuring electrical signals emitted by the brain enabled us to show that high-capacity people were excellent at controlling what information was represented in working memory: they let in information about relevant objects but completely filtered out that about irrelevant objects. Low-capacity individuals, in contrast, had much weaker control over what information entered the mental in-box; they let in information about both relevant and irrelevant objects roughly equally. Surprisingly, these results mean that we found that low-capacity people were actually holding more total information in mind than high-capacity individuals were—but much of the information they held was irrelevant to the task.

Where Is the Filter?
So the evidence is amassing that your mental spam filter largely establishes your working-memory capacity. Yet a critical question remains unanswered: Where in the brain does this spam filter reside?

According to a study published this past January in Nature Neuroscience, neuroscientists Fiona McNab and Torkel Klingberg of the Stockholm Brain Institute appear to have found its location. To do so, they had participants perform a working-memory task in which they had to recall the positions of red and yellow squares on a computer screen. Sometimes they were asked to remember all the items on the screen (both red and yellow), and other times they were asked to keep track of just the red items and to forget the yellow items—an act akin to filtering spam. A symbol presented at the start of each trial told them whether they had to focus on just red squares or let all the information from the display flow into memory. The researchers recorded the subjects’ brain activation using functional magnetic resonance imaging during this instruction period as a way of determining what parts of the brain became active as a person started up the mental spam filter.

McNab and Klingberg found that when participants were told they would need to filter the upcoming trial, parts of the basal ganglia (an area known to be important in movement, among other tasks) and the prefrontal cortex (considered to be the brain’s rationalizing, “thinking” part) became much more active than in the nonfiltering trials. And the researchers found that the jump in activity levels in these areas was largest for high-capacity individuals and smallest for low-capacity individuals. That is, when told they needed to filter, the high-capacity individuals ramped up activity in these brain regions to keep out irrelevant items. In contrast, the low-capacity individuals showed little additional activity in these areas when they were instructed to ignore the extraneous items. Thus, a leading candidate for the mental spam filter appears to be a cooperative effort between the basal ganglia and the prefrontal cortex.

In this filtering mechanism, the prefrontal cortex most likely provides details about the current task goals, and the basal ganglia lend the mental muscle to block out information that does not match these goals.

This role for the basal ganglia in helping to control the flow of information into working memory is quite similar to one of the basal ganglia’s other major functions, which is selecting which motor movements to use in a given context and suppressing the movements we do not want. Particularly intriguing is that the basal ganglia are evolutionarily ancient brain structures that have been highly conserved across species; even lizards have them. Consequently, what is thought to be our uniquely human ability to engage in abstract reasoning and problem solving appears to be dependent on brain structures that have been around far longer than humans have. The ability to filter out irrelevant spam, it seems, is critical for lizards as well as humans.