Been there, done that!

One of the neural oddities of "declarative" memory--the recall of past things and events--is that some experiments have shown that recognizing a familiar object is accompanied by a reduction in activity of the brain's memory centers in the medial temporal lobe. Such a reduction seems counterintuitive, since remembering seems to be a positive event.

Now, researchers led by Anthony D. Wagner, Brian D. Gonsalves, and Itamar Kahn of Stanford University have documented this reduced activity in humans and have demonstrated that the magnitude of the dip corresponds with the familiarity of the objects.

In their experiments reported in the September 1, 2005, issue of Neuron, the researchers asked volunteers to look at series of faces as the subjects' brains were scanned using either of two techniques. One was functional magnetic resonance imaging (fMRI), which gives information on the location and amount of activation of brain regions, and the other was magnetoencephalography (MEG), which reports the precise timing of brain responses. The technique of fMRI uses harmless magnetic fields and radio waves to map blood flow in the brain, which reflects brain activity, and MEG detects the infinitesimal magnetic fields generated by brain electrical activity.

The researchers asked the subjects to rate their familiarity with each face as "remembering" if they strongly recalled the face, "knowing" if they had a feeling of recognizing the face, or "new" if they didn't recall seeing the face before.

The fMRI scans revealed that the decrease in medial temporal lobe activity tracked the level of perceived memory strength for the faces. What's more, the MEG studies revealed that this reduction began very rapidly during the recognition process.

Previous studies of the phenomenon have been done in monkeys and have measured only the electrical changes in individual neurons during memory tasks. Thus, wrote reviewers Chantel Stern and Michael Hasselmo in the same issue of Neuron, "Overall, the innovative use of multimodal techniques in this study reflects an important move toward a tighter integration of neuroimaging data with the vast wealth of data at the cellular and circuit level."

Gonsalves and his colleagues concluded that "medial temporal structures, in the service of declarative memory, support recognition of stimuli that were previously encountered, allowing organisms to discriminate between novel and familiar items. The marked convergence between the present fMRI and MEG correlates of perceived memory strength suggest that graded reductions in medial temporal cortical responses support graded perceptions of item familiarity, providing a basis for such discriminations. As such, medial temporal mechanisms appear to rapidly signal knowledge about an item's relation to one's past."