Episodic encoding is more than the sum of its parts: an fMRI investigation of multifeatural contextual encoding

Abstract

Episodic memories are characterized by their contextual richness, yet little is known about how the various features comprising an episode are brought together in memory. Here we employed fMRI and a multidimensional source memory procedure to investigate processes supporting the mnemonic binding of item and contextual information. Volunteers were scanned while encoding items for which the contextual features (color and location) varied independently, allowing activity elicited at the time of study to be segregated according to whether both, one, or neither feature was successfully retrieved on a later memory test. Activity uniquely associated with successful encoding of both features was identified in the intra-parietal sulcus, a region strongly implicated in the support of attentionally mediated perceptual binding. The findings suggest that the encoding of disparate features of an episode into a common memory representation requires that the features be conjoined in a common perceptual representation when the episode is initially experienced.

Figure 1

Schematic representation of experimental design. (A) Encoding phase. During each scanned encoding phase, volunteers made animacy judgments (living or non-living?) to a series of words presented one at a time, in one of four colors (red, green, blue and pink) and quadrants of the screen. Color and location of the stimuli were orthogonally varied. To direct processing emphasis towards the color of the stimuli (see Experimental Procedures), an additional set of words (accounting for one fifth of the total) were pseudorandomly interspersed among the critical study items. These words were presented in black and volunteers were instructed to perform size judgments (bigger or smaller than a shoebox?). Subsequent memory for these words was not tested. (B) Retrieval phase. Immediately following each encoding phase, volunteers made recognition memory judgments (old or new?) to studied and unstudied words. If a word was judged old, source judgments for color and location were required.

Figure 2

Feature-specific subsequent memory effects. (A) Location-specific subsequent memory effects. Anatomical overlay (top) and mean parameter estimates (and standard errors; bottom) of the cluster in retrosplenial/posterior cingulate cortex exhibiting significant (p < .001) subsequent memory effects associated with items for which location (but not color, p > .05) was later retrieved. (B) Color-specific subsequent memory effects. Surface rendering (top) and mean parameter estimates (and standard errors; bottom) of the cluster in posterior inferior temporal cortex exhibiting significant (p < .001) subsequent memory effects associated with items for which color (but not location) was later retrieved. Effects are displayed at p < .001 on a standardized brain. Both = subsequent memory effects for Both Correct; Color = subsequent memory effects for Color Only; Location = subsequent memory effects for Location Only. ** p < .01; *** p < .001

Figure 3

Multifeatural subsequent memory effects. Anatomical overlays and mean parameter estimates (and standard errors) illustrate clusters in right intra-parietal sulcus (top) and hippocampus (bottom) that exhibit subsequent memory effects associated with items for which both source features (but neither feature alone, p > .05) are later remembered. Intra-parietal and hippocampal effects are displayed at p < .001, and p <.005 respectively on a standardized brain. ** p < .005; *** p < .001

Figure 4

Feature-insensitive subsequent memory effects. Left anterior medial temporal subsequent memory effects invariant with respect to number of source features later retrieved (illustrated at p<.001) . Mean parameter estimates (and standard errors) are also shown. Both = subsequent memory effects for Both Correct; Color = subsequent memory effects for Color Only; Location = subsequent memory effects for Location Only; Item = subsequent memory effects for Item Only. * p < .05; ** p < .01; *** p < .001