One of the neural oddities of "declarative" memory--the recall of pastthings and events--is that some experiments have shown that recognizinga familiar object is accompanied by a reduction in activity of thebrain's memory centers in the medial temporal lobe. Such a reductionseems counterintuitive, since remembering seems to be a positive event.
Now, researchers led by Anthony D. Wagner, Brian D. Gonsalves, andItamar Kahn of Stanford University have documented this reducedactivity in humans and have demonstrated that the magnitude of the dipcorresponds with the familiarity of the objects.
In their experiments reported in the September 1, 2005, issueof Neuron, the researchers asked volunteers to look at series of facesas the subjects' brains were scanned using either of two techniques.One was functional magnetic resonance imaging (fMRI), which givesinformation on the location and amount of activation of brain regions,and the other was magnetoencephalography (MEG), which reports theprecise timing of brain responses. The technique of fMRI uses harmlessmagnetic fields and radio waves to map blood flow in the brain, whichreflects brain activity, and MEG detects the infinitesimal magneticfields generated by brain electrical activity.
The researchers asked the subjects to rate their familiaritywith each face as "remembering" if they strongly recalled the face,"knowing" if they had a feeling of recognizing the face, or "new" ifthey didn't recall seeing the face before.
The fMRI scans revealed that the decrease in medial temporallobe activity tracked the level of perceived memory strength for thefaces. What's more, the MEG studies revealed that this reduction beganvery rapidly during the recognition process.
Previous studies of the phenomenon have been done in monkeysand have measured only the electrical changes in individual neuronsduring memory tasks. Thus, wrote reviewers Chantel Stern and MichaelHasselmo in the same issue of Neuron, "Overall, the innovative use ofmultimodal techniques in this study reflects an important move toward atighter integration of neuroimaging data with the vast wealth of dataat the cellular and circuit level."
Gonsalves and his colleagues concluded that "medial temporalstructures, in the service of declarative memory, support recognitionof stimuli that were previously encountered, allowing organisms todiscriminate between novel and familiar items. The marked convergencebetween the present fMRI and MEG correlates of perceived memorystrength suggest that graded reductions in medial temporal corticalresponses support graded perceptions of item familiarity, providing abasis for such discriminations. As such, medial temporal mechanismsappear to rapidly signal knowledge about an item's relation to one'spast."
The researchers include Brian D. Gonsalves of Stanford University;Itamar Kahn of Stanford University and Massachusetts Institute ofTechnology; Tim Curran of University of Colorado, Boulder; Kenneth A.Norman of Princeton University; and Anthony D. Wagner of StanfordUniversity and the Athinoula A. Martinos Center for Biomedical Imaging,MGH/MIT/HMS. This study was supported by the National ScienceFoundation, NIMH, McKnight Endowment Fund for Neuroscience, EllisonMedical Foundation, and Alfred P. Sloan Foundation.
Gonsalves et al.: "Memory Strength and Repetition Suppression:Multimodal Imaging of Medial Temporal Cortical Contributions toRecognition" Neuron, Vol. 47, 751-761, September 1, 2005, DOI10.1016/j.neuron.2005.07.013 www.neuron.org
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