Distinct patterns of functional and effective connectivity between perirhinal cortex and other cortical regions in recognition memory and perceptual discrimination.

Traditionally, the medial temporal lobe (MTL) is thought to be dedicated to declarative memory. Recent evidence challenges this view, suggesting that perirhinal cortex (PrC), which interfaces the MTL with the ventral visual pathway, supports highly integrated object representations in recognition memory and perceptual discrimination. Even with comparable representational demands, perceptual and memory tasks differ in numerous task demands and the subjective experience they evoke. Here, we tested whether such differences are reflected in distinct patterns of connectivity between PrC and other cortical regions, including differential involvement of prefrontal control processes. We examined functional magnetic resonance imaging data for closely matched perceptual and recognition memory tasks for faces that engaged right PrC equivalently. Multivariate seed analyses revealed distinct patterns of interactions: Right ventrolateral prefrontal and posterior cingulate cortices exhibited stronger functional connectivity with PrC in recognition memory; fusiform regions were part of the pattern that displayed stronger functional connectivity with PrC in perceptual discrimination. Structural equation modeling revealed distinct patterns of effective connectivity that allowed us to constrain interpretation of these findings. Overall, they demonstrate that, even when MTL structures show similar involvement in recognition memory and perceptual discrimination, differential neural mechanisms are reflected in the interplay between the MTL and other cortical regions

Revisiting Cognitive and Neuropsychological Novelty Effects

Recent proposals have attributed a key role to novelty in the formation of new episodic memories. These proposals are based on evidence of enhanced memory and greater metabolic activity in the hippocampus in response to novel relative to familiar materials. However, such novelty effects are incongruous with long-standing observations that familiar items and lists are associated with better memory than novel ones. In four experiments, I explored possible reasons for this apparent discrepancy. In Experiment 1, I directly tested whether previously observed novelty effects were the result of novelty, discrimination demands, or both. I used linguistic materials (proverbs) to replicate the novelty effect but found it occurred only when familiar items were subject to source confusion. In Experiment 2, to examine better how novelty influences episodic memory, I used experimentally familiar, pre-experimentally familiar, and novel proverbs in a paradigm designed to overcome discrimination demand confounds. Memory was better for both types of familiar proverbs. These cognitive results indicate that familiarity, not novelty, leads to better episodic memory for studied items, regardless of whether familiarity is experimentally induced or based on prior knowledge. I also conducted two fMRI experiments to evaluate the neural correlates of the encoding of novel and familiar forms of information. In Experiment 3, I compared the neural encoding correlates of source memory for novel and familiar visual scenes using fMRI. Replicating previous neuroimaging studies, I observed an anterior novelty-sensitive region of the hippocampus specialized in novelty encoding. Unlike past studies, I also probed for familiarity-encoding regions and identified such regions in the posterior hippocampus. I replicated this pattern in Experiment 4 using proverbs as stimuli. As in Experiment 2, I found the effect held whether familiarity was based on prior knowledge or experimental induction. In both fMRI experiments, anterior and posterior hippocampal regions were functionally connected with different large-scale networks, helping to explain local variation in hippocampal functional specialization in terms of different neural contexts. Together, these experiments show that stimulus familiarity enhances episodic memory for materials, and that novelty is processed differently, not preferentially, in the hippocampus. A new model of hippocampal novelty processing is proposed.Ph

Brain meta-state transitions demarcate thoughts across task contexts exposing the mental noise of trait neuroticism

Explicit self-reflection is unreliable for measuring thoughts. Here, the authors use brain data to implicitly pinpoint transitions between thoughts and find thought turnover to be reliably predicted by narrative events during movie-viewing, as well as by greater trait neuroticism at rest

Briefly Cuing Memories Leads to Suppression of Their Neural Representations

Previous studies have linked partial memory activation with impaired subsequent memory retrieval (e.g., Detre et al., 2013) but have not provided an account of this phenomenon at the level of memory representations: How does partial activation change the neural pattern subsequently elicited when the memory is cued? To address this question, we conducted a functional magnetic resonance imaging (fMRI) experiment in which participants studied word-scene paired associates. Later, we weakly reactivated some memories by briefly presenting the cue word during a rapid serial visual presentation (RSVP) task; other memories were more strongly reactivated or not reactivated at all. We tested participants' memory for the paired associates before and after RSVP. Cues that were briefly presented during RSVP triggered reduced levels of scene activity on the post-RSVP memory test, relative to the other conditions. We used pattern similarity analysis to assess how representations changed as a function of the RSVP manipulation. For briefly cued pairs, we found that neural patterns elicited by the same cue on the pre- and post-RSVP tests (preA–postA; preB–postB) were less similar than neural patterns elicited by different cues (preA–postB; preB–postA). These similarity reductions were predicted by neural measures of memory activation during RSVP. Through simulation, we show that our pattern similarity results are consistent with a model in which partial memory activation triggers selective weakening of the strongest parts of the memory

Multiple-object Tracking as a Tool for Parametrically Modulating Memory Reactivation

Converging evidence supports the "nonmonotonic plasticity" hypothesis, which states that although complete retrieval may strengthen memories, partial retrieval weakens them. Yet, the classic experimental paradigms used to study effects of partial retrieval are not ideally suited to doing so, because they lack the parametric control needed to ensure that the memory is activated to the appropriate degree (i.e., that there is some retrieval but not enough to cause memory strengthening). Here, we present a novel procedure designed to accommodate this need. After participants learned a list of word-scene associates, they completed a cued mental visualization task that was combined with a multiple-object tracking (MOT) procedure, which we selected for its ability to interfere with mental visualization in a parametrically adjustable way (by varying the number of MOT targets). We also used fMRI data to successfully train an "associative recall" classifier for use in this task: This classifier revealed greater memory reactivation during trials in which associative memories were cued while participants tracked one, rather than five, MOT targets. However, the classifier was insensitive to task difficulty when recall was not taking place, suggesting that it had indeed tracked memory reactivation rather than task difficulty per se. Consistent with the classifier findings, participants' introspective ratings of visualization vividness were modulated by MOT task difficulty. In addition, we observed reduced classifier output and slowing of responses in a postreactivation memory test, consistent with the hypothesis that partial reactivation, induced by MOT, weakened memory. These results serve as a "proof of concept" that MOT can be used to parametrically modulate memory retrieval-a property that may prove useful in future investigation of partial retrieval effects, for example, in closed-loop experiments

Multiple-object Tracking as a Tool for Parametrically Modulating Memory Reactivation

Converging evidence supports the "nonmonotonic plasticity" hypothesis, which states that although complete retrieval may strengthen memories, partial retrieval weakens them. Yet, the classic experimental paradigms used to study effects of partial retrieval are not ideally suited to doing so, because they lack the parametric control needed to ensure that the memory is activated to the appropriate degree (i.e., that there is some retrieval but not enough to cause memory strengthening). Here, we present a novel procedure designed to accommodate this need. After participants learned a list of word-scene associates, they completed a cued mental visualization task that was combined with a multiple-object tracking (MOT) procedure, which we selected for its ability to interfere with mental visualization in a parametrically adjustable way (by varying the number of MOT targets). We also used fMRI data to successfully train an "associative recall" classifier for use in this task: This classifier revealed greater memory reactivation during trials in which associative memories were cued while participants tracked one, rather than five, MOT targets. However, the classifier was insensitive to task difficulty when recall was not taking place, suggesting that it had indeed tracked memory reactivation rather than task difficulty per se. Consistent with the classifier findings, participants' introspective ratings of visualization vividness were modulated by MOT task difficulty. In addition, we observed reduced classifier output and slowing of responses in a postreactivation memory test, consistent with the hypothesis that partial reactivation, induced by MOT, weakened memory. These results serve as a "proof of concept" that MOT can be used to parametrically modulate memory retrieval-a property that may prove useful in future investigation of partial retrieval effects, for example, in closed-loop experiments

Mechanisms supporting superior source memory for familiar items: A multi-voxel pattern analysis study

Recent cognitive research has revealed better source memory performance for familiar relative to novel stimuli. Here we consider two possible explanations for this finding. The source memory advantage for familiar stimuli could arise because stimulus novelty induces attention to stimulus features at the expense of contextual processing, resulting in diminished overall levels of contextual processing at study for novel (vs. familiar) stimuli. Another possibility is that stimulus information retrieved from long-term memory (LTM) provides scaffolding that facilitates the formation of item-context associations. If contextual features are indeed more effectively bound to familiar (vs. novel) items, the relationship between contextual processing at study and subsequent source memory should be stronger for familiar items. We tested these possibilities by applying multi-voxel pattern analysis (MVPA) to a recently collected functional magnetic resonance imaging (fMRI) dataset, with the goal of measuring contextual processing at study and relating it to subsequent source memory performance. Participants were scanned with fMRI while viewing novel proverbs, repeated proverbs (previously novel proverbs that were shown in a pre-study phase), and previously known proverbs in the context of one of two experimental tasks. After scanning was complete, we evaluated participants’ source memory for the task associated with each proverb. Drawing upon fMRI data from the study phase, we trained a classifier to detect on-task processing (i.e., how strongly was the correct task set activated). On-task processing was greater for previously known than novel proverbs and similar for repeated and novel proverbs. However, both within and across participants, the relationship between on-task processing and subsequent source memory was stronger for repeated than novel proverbs and similar for previously known and novel proverbs. Finally, focusing on the repeated condition, we found that higher levels of hippocampal activity during the pre-study phase, which we used as an index of episodic encoding, led to a stronger relationship between on-task processing at study and subsequent memory. Together, these findings suggest different mechanisms may be primarily responsible for superior source memory for repeated and previously known stimuli. Specifically, they suggest that prior stimulus knowledge enhances memory by boosting the overall level of contextual processing, whereas stimulus repetition enhances the probability that contextual features will be successfully bound to item features. Several possible theoretical explanations for this pattern are discussed

Briefly Cuing Memories Leads to Suppression of Their Neural Representations

Previous studies have linked partial memory activation with impaired subsequent memory retrieval (e.g., Detre et al., 2013) but have not provided an account of this phenomenon at the level of memory representations: How does partial activation change the neural pattern subsequently elicited when the memory is cued? To address this question, we conducted a functional magnetic resonance imaging (fMRI) experiment in which participants studied word-scene paired associates. Later, we weakly reactivated some memories by briefly presenting the cue word during a rapid serial visual presentation (RSVP) task; other memories were more strongly reactivated or not reactivated at all. We tested participants' memory for the paired associates before and after RSVP. Cues that were briefly presented during RSVP triggered reduced levels of scene activity on the post-RSVP memory test, relative to the other conditions. We used pattern similarity analysis to assess how representations changed as a function of the RSVP manipulation. For briefly cued pairs, we found that neural patterns elicited by the same cue on the pre- and post-RSVP tests (preA–postA; preB–postB) were less similar than neural patterns elicited by different cues (preA–postB; preB–postA). These similarity reductions were predicted by neural measures of memory activation during RSVP. Through simulation, we show that our pattern similarity results are consistent with a model in which partial memory activation triggers selective weakening of the strongest parts of the memory

Hippocampal correlates of reading comprehension, word recognition, and typed free recall

An investigation into reading comprehension and its behavioural and neuroanatomical predictors