Elsevier

NeuroImage

Volume 167, 15 February 2018, Pages 211-223
NeuroImage

The effect of prior knowledge on post-encoding brain connectivity and its relation to subsequent memory

https://doi.org/10.1016/j.neuroimage.2017.11.032Get rights and content

Abstract

It is known that prior knowledge can facilitate memory acquisition. It is unclear, however, whether prior knowledge can affect post-encoding brain activity to facilitate memory consolidation. In this fMRI study, we asked participants to associate novel houses with famous/nonfamous faces and investigated how associative-encoding tasks with/without prior knowledge differentially affected post-encoding brain connectivity during rest. Besides memory advantages in the famous condition, we found that post-encoding hippocampal connectivity with the fusiform face area (FFA) and ventral-medial-prefrontal cortex (vmPFC) was stronger following encoding of associations with famous than non-famous faces. Importantly, post-encoding functional connectivity between the hippocampus (HPC) and FFA, and between the anterior temporal pole region (aTPL) and posterior perceptual regions (i.e., FFA and the parahippocampal place area), together predicted a large proportion of the variance in subsequent memory performance. This prediction was specific for face-house associative memory, not face/house item memory, and only in the famous condition where prior knowledge was involved. These results support the idea that when prior knowledge is involved, the HPC, vmPFC, and aTPL, which support prior episodic, social-evaluative/schematic, and semantic memories, respectively, continue to interact with each other and posterior perceptual brain regions during the post-encoding rest to facilitate off-line processing of the newly formed memory, and enhance memory consolidation.

Introduction

Making associations between different kinds of information is an important way of building our knowledge system (Halford et al., 2010). Studies have shown that these associative processes can be facilitated by previous experiences or prior knowledge, namely the knowledge one has acquired and brings to bear in acquiring new information (Kan et al., 2009, Poppenk et al., 2010a, Reder et al., 2013, Sharon et al., 2011). At the neural level, it has also been shown that prior knowledge can modulate brain activity during encoding and retrieval to enhance new memory processing (Liu et al., 2017, Maguire et al., 1999, Poppenk et al., 2010b, Staresina et al., 2009, van Kesteren et al., 2010a, van Kesteren et al., 2010b). In addition, there is growing evidence that changes in resting functional connectivity after encoding reflect early memory consolidation (de Voogd et al., 2016, Gruber et al., 2016, Hermans et al., 2017, Staresina et al., 2013, Tambini et al., 2010, Tambini and Davachi, 2013), but there is little research on whether prior knowledge can also influence this process. In this paper we show that prior knowledge can promote post-encoding functional connectivity in regions implicated in memory and perception, and that this effect is related to subsequent associative memory.

A large body of animal literature has shown that post-encoding brain activity, e.g., in the hippocampus (HPC), may reflect memory replay or reactivation and contribute to memory stabilization (Foster and Wilson, 2006, Jadhav et al., 2012, Knauer et al., 2013, Ólafsdóttir et al., 2016, O'Neill et al., 2010, O'Neill et al., 2008, Oudiette and Paller, 2013, Silva et al., 2015, Singer and Frank, 2009, Sutherland and McNaughton, 2000, Wilson and McNaughton, 1994). Other brain structures such as the striatum, ventral medial prefrontal cortex (vmPFC), visual, and motor cortex are also involved in the replay of previous experiences during post-learning rest or sleep (Gomperts et al., 2015, Hoffman and McNaughton, 2002, Ji and Wilson, 2007, Pennartz et al., 2004, Ribeiro et al., 2004). In line with the animal literature, human functional neuroimaging studies have found that learning experiences can modulate resting-state brain connectivity (Albert et al., 2009, Deuker et al., 2013, Groen et al., 2011, Hasson et al., 2009, Urner et al., 2013, Wang et al., 2012, Zou et al., 2013), and that the HPC activity during post-encoding rest can be correlated with memory performance (Tambini et al., 2010, Staresina et al., 2013, Tambini and Davachi, 2013, de Voogd et al., 2016, Gruber et al., 2016, Hermans et al., 2017). These observations support the idea that post-encoding neural activity may reflect early memory consolidation processes by which newly-encoded memory becomes more stable. Intriguingly, evidence from recent animal and human neuroimaging studies has also shown that prior knowledge can facilitate this memory consolidation process (Hennies et al., 2016, Tse et al., 2007). This raises the important question of whether prior knowledge can directly affect brain activity during an immediate post-encoding time window to contribute to memory formation in humans.

Although a recent study found that higher sleep spindle density during post-encoding sleep predicted better schema-related memory (Hennies et al., 2016), to the best of our knowledge, only two human neuroimaging study (van Kesteren et al., 2010a; Schlichting and Preston, 2016) have directly examined prior knowledge effects on brain activity during post-encoding awake rest. However, both studies mainly focused on the connectivity between the HPC and vmPFC, based on the assumption that the vmPFC plays an important role in supporting existing structured mental representations or memory updating (van Kesteren et al., 2012, Zeithamova et al., 2012, Zeithamova and Preston, 2010). While this assumption is reasonable, other regions, such as the anterior temporal pole (aTPL), also have been shown to support prior knowledge (Kan et al., 2009, Liu et al., 2017, Sharon et al., 2011, Staresina et al., 2009) and should be considered in the study of prior knowledge effects on new learning.

To tackle these questions, we need to design experiments that can elicit different types or components of prior knowledge, such as semantics, social emotions, perceptions, or episodic memories, which can be supported by well-known distinct brain regions and the connections between them. Then, we can use fMRI to examine how functional connectivity among these different brain regions can be affected by prior knowledge manipulations in the preceding encoding tasks. Following this idea, in this study we designed an associative memory task that required forming associations between two items, a house and a face, for which there are well-delineated and differential localizations in the brain. The houses were always novel, but to manipulate prior experience, the faces were those of famous people or of people unfamiliar to the participants. Specifically, participants were asked to associate pictures of novel houses with pictures of either famous or nonfamous faces in different blocks. After each encoding block, participants rested in the scanner. Because famous faces can elicit rich stores of social/affective, semantic, and perceptual knowledge, as well as episodic memories, each supported by different brain systems (Fairhall and Ishai, 2007, Gobbini et al., 2004, Gobbini and Haxby, 2007, Ishai, 2008, Renoult et al., 2012, Ross and Olson, 2012, Simmons et al., 2010), this fame manipulation created two conditions, famous vs. nonfamous, in which prior knowledge effects at the brain level can be examined during post-encoding rest.

Similar to the previous study that focused on the encoding phase (Liu et al., 2017), we included the HPC, aTPL, vmPFC, parahippocampal place area (PPA), and fusiform face area (FFA) as our regions of interest (ROIs). We chose these ROIs because the literature has shown that the medial temporal lobe, especially the HPC, plays an important role in associative encoding (Davachi, 2006, Davachi and Wagner, 2002, Eichenbaum et al., 2007) and post-encoding memory reactivation (O'Neill et al., 2010). Previous studies have also shown that the vmPFC and aTPL support social evaluative and semantic processing (Etkin et al., 2011, Grabenhorst and Rolls, 2011, Luo et al., 2010, O'Reilly, 2010, Patterson et al., 2007, Roy et al., 2012) and contribute to prior knowledge facilitation effects (Kan et al., 2009, Liu et al., 2017, Ross et al., 2011, Sharon et al., 2011, Staresina et al., 2009, van Kesteren et al., 2012). The two posterior perceptual regions, i.e., the PPA and FFA, support the processing of house and face stimuli, respectively (Kanwisher, 2010), and their activation can be affected by prior knowledge related to familiar faces (Bar et al., 2008, Gobbini and Haxby, 2006, Liu et al., 2014). Importantly, all of these ROIs, which support different components of memory processing, showed increased activity during face-house associative encoding when prior knowledge was involved (Liu et al., 2017), raising the possibility that they also play a role in post-encoding memory processes. Therefore, by comparing the functional connectivity among these ROIs between the famous post-encoding and nonfamous post-encoding rest periods, the current study enables us to investigate how prior knowledge affects post-encoding brain activity and impacts subsequent memory.

The literature on memory consolidation has shown that the medial temporal lobe, especially the HPC, is crucial for supporting new memory processing, whereas neocortical regions may play a greater role in supporting already consolidated memories (Frankland and Bontempi, 2005, Moscovitch et al., 2005, Nadel and Moscovitch, 1997, Squire and Alvarez, 1995). This switch in neural substrates may also underlie changes in the nature of these memories following consolidation (Nadel and Moscovitch, 1997, Wiltgen and Silva, 2007, Winocur and Moscovitch, 2011). Because it has been found that prior knowledge can facilitate memory consolidation (Tse et al., 2011, Tse et al., 2007), we hypothesized that the anterior cortical regions, namely, the vmPFC and aTPL, should form stronger resting connectivity with the HPC, PPA, and FFA after the famous compared to the nonfamous encoding condition, reflecting stronger cortical involvement from anterior brain regions during early memory consolidation when prior social/evaluative or semantic knowledge was involved. These connectivity measures should also better predict associative memory performance in the famous than the nonfamous condition. Moreover, our earlier finding that the HPC and FFA/PPA activations were stronger in the famous than nonfamous encoding condition (Liu et al., 2017), would lead to the prediction that the connectivity between the HPC and PPA/FFA should also be stronger during the famous post-encoding than the nonfamous post-encoding rest, reflecting stronger episodic binding processes when prior knowledge is present.

Section snippets

Participants

Twenty young adults (12 females), between 18 and 24 years of age (Mean = 21.3, SD = 1.49), were recruited from the University of Toronto's St. George campus. They were all right-handed, native English speakers, and free of any psychiatric or neurological conditions. The participants were paid $76 and gave their informed consent. The study was approved by the Research Ethics Board at Baycrest Center for Geriatric Care (University of Toronto).

Overview

There were 3 resting scans in this experiment (Fig. 1

Behavioral results

First, we found that associative memory performance (hit rate - false alarm rate) was better for the famous (M = 0.36, SD = 0.21) than the nonfamous (M = 0.19, SD = 0.14) condition (t(19) = 4.60, p < 0.0002, Cohen's d = 1.03). The correlation between associative memory performance in the two conditions was also significant (r = 0.47, p = 0.035). Interestingly, participants' age predicted associative memory performance in the famous (r = 0.60, p = 0.005), but not the nonfamous condition (r

Discussion

Using an explicit encoding task in which participants associated novel houses with either famous or nonfamous faces, we investigated how associative encoding with or without prior knowledge involvement differentially affected post-encoding brain connectivity. We hypothesized stronger post-encoding connectivity in the famous than non-famous condition among regions whose activation at encoding was found to play a role in associative memory performance. For the most part, our results were

Conclusion

The current findings, consistent with our hypotheses, suggest that when prior knowledge is involved, the HPC, vmPFC, and aTPL, which support prior episodic, social-evaluative/schematic, and semantic memories, respectively, continue to interact with each other and the posterior perceptual brain regions (e.g., the PPA and FFA) during the post-encoding rest to facilitate off-line processing of the newly formed memory and lead to better memory for it. Our findings may also provide preliminary

Author contributions

Z.-X.L. and M.M. designed research, Z.-X.L. performed research, Z.-X.L. analyzed data with help from M.M., and C.G., Z.-X. L., M.M., and C.G. wrote the paper.

Conflicts of interest

No conflict of interest.

Acknowledgement

This study was supported by Natural Sciences and Engineering Research Council of Canada RGPIN 8347 to M. M. Z.-X. L. was also supported by NSERC PDS scholarship. We would like to thank Dr. Kang Lee at the Institute of Child Study, University of Toronto, for his helpful comments and suggestions. We also gratefully acknowledge Marilyne Ziegler for her help in programming the experimental task, and Yasha Khatamian and Annette Weekes-Holder for their help in designing the fMRI protocol.

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