Instant recall

Scientists have long held that memory retrieval is a constructive process in which bits of information supplied by different regions of the brain create one, unified memory. There has been less certainty about how fast memories are retrieved and how they may change over time.

Scott Slotnick, an assistant psychology professor at Boston College, specializes in visual memory, specifically “retinotopic” aspects of memory whereby recollections of an object you saw to your left stimulate activity (the electrical firing of neurons) on the right side of the brain and vice versa. In a recent study, Slotnick had two sets of undergraduate research subjects view abstract shapes reminiscent of a computer screen saver—Bezier curves filled with colored lines—on either the left or right side of a video screen. Then Slotnick replayed these shapes along with new curves, all shown in the center of the screen, and asked the students to identify old versus new as well as the original location of the old shapes. While the students answered, their brains were monitored with either electrical receptors (ERPs), which instantly register spikes in neural activity, or MRI scans, which take rapid images of the brain that are then spliced together like a movie reel.

The ERPs, which heretofore had not been much used in tracking memory development, revealed retinotopic effects occurring in the brain’s occipital and temporal regions (toward the back and sides of the head, respectively) within a quarter of a second, a speed far faster than typically attributed to memory retrieval. Indeed, Slotnick’s results suggest memories may take shape as much as four times faster than earlier studies have shown.

The brain’s right hemisphere showed the more prominent spikes in activity, perhaps due to its proclivity for processing coordinate data (the distance, for example, between a coffee mug and a table’s edge). By pinpointing the retrieval of spatial information, Slotnick’s results show memory recall with a rare level of detail.

In addition to the speed of the initial memory retrieval, Slotnick observed that the retinotopic activity in the subjects’ brains
continued intermittently for another second or so. This offers yet another indication, he says, that memory is an action “refined over time, as details are added.” Slotnick’s results appeared in the May 2009 issue of Brain Research.

Altered states

Plasma, the fourth state of matter (along with solids, liquids, and gases) constitutes some 99 percent of the universe’s visible matter—the stuff of coronas, solar cores, and comet tails. Plasma’s appearance on earth, however, is fleeting and limited—in lightning and flame, or in fluorescent light bulbs—and still little understood.

For the past three decades, the study of plasma, which is most simply described as electrically charged gas, has focused on “dusty” varieties, plasmas laced with solid particles ranging in width from nanometers to tens of microns. These particulates acquire an electrical charge from the plasma, causing all sorts of interesting results, such as, on a cosmic scale, the rings of Saturn.

Dusty plasmas have been much studied partly because of their importance to the semiconductor industry—plasmas are used in the manufacture of microchips, and the particles, produced during etching the chips, can cause contamination. But a subset of dusty plasmas that mimic the superdense, supercharged systems found at the center of stars is only beginning to reveal its secrets. Within these dense plasmas, dust can organize itself into states similar to liquid or solid, structures known as Coulomb crystals, and these may help reveal much about all four states of matter.

Recently, a team of physicists including Gabor Kalman, distinguished research professor at Boston College, and Stamatios Kyrkos, Ph.D.’03, a former graduate student of Kalman’s and current assistant professor at Le Moyne College, developed a theoretical description of dense dusty plasmas formed when a beam of charged particles penetrates plasma. Their theory, co-authored with Marlene Rosenberg, a researcher at the University of California, San Diego, appeared last June in the journal Physical Review Letters.

The team’s mathematical model supposes a two-dimensional crystalline lattice of charged particles penetrated by a charged beam, with both elements immersed in a plasma. The researchers calculated, among other effects, that the beam would cause electrostatic instabilities in the lattice—energetic ripples, so to speak—both along the beam’s progress and, most interestingly, perpendicular to it, generating the type of transverse waves produced by sound when it travels through solids.

The model, Kyrkos says, has broad application for basic scientific research. The lattices in these dense plasmas behave mathematically much like everyday solids and liquids, but, unlike solids, their atomic behaviors are easily observable to, at times, even the unaided eye. The similiarities in behavior suggest the team’s theory could provide a “good model for many condensed matter systems,” Kyrkos says.

The paper remains theoretical, but does provide a “solvable model” that, if proven correct, could advance study of phase transitions—the shift from one state of matter to another.

Paul Voosen ’03 is a writer based in Washington, D.C.

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