Neural substrates of mnemonic discrimination: A whole-brain fMRI investigation

Abstract

Introduction: A fundamental component of episodic memory is the ability to differentiate new and highly similar events from previously encountered events. Numerous functional magnetic resonance imaging (fMRI) studies have identified hippocampal involvement in this type of mnemonic discrimination (MD), but few studies have assessed MD-related activity in regions beyond the hippocampus. Therefore, the current fMRI study examined whole-brain activity in healthy young adults during successful discrimination of the test phase of the Mnemonic Similarity Task.

Method: In the study phase, participants made "indoor"/"outdoor" judgments to a series of objects. In the test phase, they made "old"/"new" judgments to a series of probe objects that were either repetitions from the memory set (targets), similar to objects in the memory set (lures), or novel. We assessed hippocampal and whole-brain activity consistent with MD using a step function to identify where activity to targets differed from activity to lures with varying degrees of similarity to targets (high, low), responding to them as if they were novel.

Results: Results revealed that the hippocampus and occipital cortex exhibited differential activity to repeated stimuli relative to even highly similar stimuli, but only hippocampal activity predicted discrimination performance.

Conclusions: These findings are consistent with the notion that successful MD is supported by the hippocampus, with auxiliary processes supported by cortex (e.g., perceptual discrimination).

Keywords: cortex; episodic memory; fMRI; hippocampus; mnemonic discrimination.

Conflict of interest statement

None declared.

© 2020 The Authors. Brain and Behavior published by Wiley Periodicals, Inc.

Figures

Figure 1

The Mnemonic Similarity Task (MST) used here consisted of an incidental study phase followed by a test phase. During the study phase, participants made “indoor”/“outdoor” judgments to a series of objects. During the test phase, participants made “old”/“new” judgments to novel foils, high‐ or low‐similarity lures, and repeated targets. For both phases, intermittent left‐ or right‐facing arrows were presented, and participants were asked to judge the respective direction

Figure 2

Four hippocampal clusters identified in the voxel‐wise step function analysis are displayed in red‐yellow on coronal slices (top right of each bar graph). Displayed results were thresholded uncorrected (p < .05, ≥20 contiguous voxels), presented in Montreal Neurological Institute (MNI) 152 space, and in radiological orientation (right = left). For reference, a hippocampal subfield template identified using a multi‐atlas model (see Stark & Stark, 2017 for details) that was aligned to Montreal Neurological Institute (MNI) 152 space displays DG/CA3 (red), CA1 (blue), and subiculum (green) subfields on coronal slices (bottom right of each bar graph). For illustrative purposes, bar graphs display the parameter estimates from each hippocampal cluster identified from the voxel‐wise analysis. Of note, the directionality of the bars in this Figure is inconsequential because the direction of fMRI activity is dependent on multiple elements of the design, including the specific trials being contrasted, the frequency of those conditions, and which conditions are in the model. Thus, we assess step function patterns independent of the direction of the difference. Consistent with mnemonic discrimination, the average activity to correct “new” responses to foils and low‐ and high‐similarity lures differs from activity to correct “old” responses to targets. Error bars display standard error of the mean

Figure 3

Whole‐brain clusters exhibiting significant mean and mnemonic discrimination effects are displayed on axial slices. Displayed results were cluster extent corrected at Z > 2.7, p < .05, presented in Montreal Neurological Institute (MNI) 152 space and in radiological orientation (right = left)

Figure 4

Two occipital clusters identified in the voxel‐wise step function analysis are displayed in red‐yellow on axial slices (right of each bar graph). Displayed results were cluster extent corrected at Z > 2.7, p < .05, presented in Montreal Neurological Institute (MNI) 152 space and in radiological orientation (right = left). For illustrative purposes, bar graphs display the parameter estimates from each occipital cluster identified from the voxel‐wise analysis. Consistent with mnemonic discrimination, the average activity to correct “new” responses to foils and low‐ and high‐similarity lures differs from activity to correct “old” responses to targets. Error bars display standard error of the mean

Figure 5

Scatterplots display relationships between MST performance and parameter estimates from step function activity. For (a) and (b), clusters from the step function analysis and voxel‐wise correlations are shown in red‐yellow and in blue, respectively. Significant relationships were seen between mnemonic discrimination (lure discrimination index, LDI) and activity in right hippocampal body/head (a), and between recognition performance (d′[T,F]) and activity in right hippocampal head (b) and right occipital cortex (c). Dashed lines represent 95% confidence intervals