The stuff of memories

Can animals recall specific events from their lives? Aristotle and many scientists after him argued that only humans have the ability to remember what, where and when a particular event occurred, said Howard Eichenbaum, PhD, a psychology professor at Boston University. Experiments conducted over the last decade have shown otherwise.

Eichenbaum and his colleagues have devised clever tests that show that rats, indeed, have specific memories, and they've pinpointed where that occurs in the brain.

"Now we have an animal model, we can see how memory really works," he said.

The trick to testing a rat's ability to recall an event lies in creating an experiment in which rats can't simply rely on familiarity to solve a puzzle, Eichenbaum said. Familiarity, he explained, is a vague sense that you've seen, done or been somewhere before, while recall is the sharp memory of the time and place of a particular event.

In one study published in 2008 in Nature (Vol. 11, No. 1), Eichenbaum and his postdoctoral fellow Magdalena Sauvage, PhD, gave rats pairs of cups filled with a particular scent mixed into a particular digging material (such as oregano mixed into plastic beads or coffee mixed into wood chips). Later, the researchers tested the rats on both the same odor-mix pairs (for example oregano in plastic beads) and pairs that involved a mismatching of the odor and the mix (for example, oregano in wood chips), and the rats had to respond differently to the pairs they had experienced before and the new pairings. That procedure, said Eichenbaum, helped ensure that the animals couldn't rely on a vague impression of having smelled oregano or dug in beads before. Rather, they had to remember the particular association between each odor and the material in which it was mixed. The rats quickly learned only to dig for treats in the scent and mix combination they'd sniffed before, but rats that had their hippocampi removed responded nearly at random.

"This all suggests that the hippocampus plays a crucial role in combining what-where-when information," said Eichenbaum.

After Eichenbaum and his colleagues had shown that rats indeed have episodic memories, and that these memories involve associations of what and where, they explored how the brain integrates information to create them. In one experiment, Eichenbaum and student Robert Komorowski recorded the activation of cells in rats' hippocampi as they sniffed scented sand that was placed at different locations on the floor. They also recorded the cells' activation when testing the animals' memory of which smell belonged where.

During the learning phase, the animals' hippocampal neurons tended to fire when the rat sniffed at a particular location, but the cells did not discriminate between the scents. Over time they pared down their firing to times when a smell appeared in a place they had encountered it before. Misfiring tended to lead to false responses, according to the study, which was published in July in the Journal of Neuroscience (Vol. 29, No. 31).

"Hippocampal cells start as place cells and eventually acquire position information," Eichenbaum said.

Past research finds that two different streams of visual information feed into the hippocampus: The dorsal stream provides information about the kind of object one has seen and the ventral stream records contextual information, Eichenbaum said. Taken together, this research suggests that the hippocampus integrates perceptual information and, when called upon, retrieves it in the form of episodic memory.