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Science 23 December 2005:
Vol. 310. no. 5756, pp. 1963 - 1966
DOI: 10.1126/science.1117645
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Reports

Category-Specific Cortical Activity Precedes Retrieval During Memory Search

Sean M. Polyn1*, Vaidehi S. Natu2, Jonathan D. Cohen2,3 and Kenneth A. Norman2,3

1 Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104, USA.
2 Department of Psychology, Princeton University, Princeton, NJ 08544, USA.
3 Center for the Study of Brain, Mind, and Behavior, Princeton University, Princeton, NJ 08544, USA.


Fig. 1. Correspondence between the classifier's estimates of contextual reinstatement and verbal recalls for a single representative subject. Time is represented on the x axis; each time point represents one complete brain scan (lasting 1.8 s). For each brain scan, the classifier produced an estimate of the match between the current testing pattern and each of the three study contexts (strength of estimate appears on the y axis). The blue, red, and green lines correspond to the face, location, and object classifier estimates, respectively. The blue, red and green dots correspond to the face, location, and object recalls made by the subject (larger dots correspond to multiple items recalled during a single scan). The recall events are shifted forward by three time points to account for lag to the peak hemodynamic response (27). For illustrative purposes, the classifier estimate lines were temporally smoothed by replacing each point with the mean of that point and the immediately neighboring estimates (however, the correlations reported here were computed based on unsmoothed classifier estimates, as was the event-related average shown in Fig. 2). [View Larger Version of this Image (36K GIF file)]
 

Fig. 2. Event-related average of the classifier's estimates of contextual reinstatement for the time intervals surrounding recall events. Recall events were excluded from the event-related average if a same-category recall was made in the preceding 8 scans (equivalent to 14.4 s). The dotted line at t = 0 represents the scan on which the verbal recall was made. The "currently recalled" plot (black line) was constructed by averaging together classifier estimates for categories that were recalled at t = 0, but not in 14.4 s preceding t = 0. The "baseline" plot (purple line) was constructed by averaging together classifier estimates for categories that were not recalled at t = 0 or in the 14.4 s preceding t = 0. The "recently recalled" plot (red line) was constructed by averaging together classifier estimates for categories that were not recalled at t = 0, but were recalled (at some point) during the 14.4 s preceding t = 0 (28). The three plots have not been shifted to account for hemodynamic lag effects. Statistical comparisons focused on the difference between the currently recalled and baseline plots, because these two plots were matched for lack of recalls in the 14.4 s preceding t = 0. The currently recalled and baseline plots differ significantly starting at t = –3 (i.e., 3 scans or 5.4 s before the verbal recall). Significance was calculated using a one-tailed, paired-samples t test on the within-subject difference between the two plots (points marked with stars and circles differ at P < 0.01 and P < 0.05, respectively). [View Larger Version of this Image (27K GIF file)]
 

Fig. 3. The classifier-derived importance maps. Each voxel was assigned an "importance value" for each study context based on its influence on the classifier's estimate of reinstatement for that context (21). These maps average over all 9 subjects (whereas all classification was done on a within-subject basis). Each row depicts the map for a different study context, and each column depicts a different axial slice, spaced 12 mm apart. Voxels with positive importance values are colored red; voxels with negative importance values are colored blue. The colors fade into transparency as importance values approach zero, as shown on the color bar. FG, fusiform gyrus; PG, parahippocampal gyrus; MFG, medial frontal gyrus; PC, posterior cingulate; MTG, middle temporal gyrus (24). [View Larger Version of this Image (136K GIF file)]
 

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