Abstract
BackgroundThe prefrontal cortex has been implicated in episodic memory and the awareness of memory. Few studies have probed the nature and necessity of its role via brain stimulation. There are uncertainties regarding whether the hemisphere of stimulation predicts effects on memory and whether effects of stimulation are format‐specific, with most previous studies utilizing verbal/semantic stimuli.ObjectiveOur primary objective was to determine if theta‐burst transcranial magnetic stimulation (TBS) to prefrontal cortex modulates visual memory accuracy, visual memory awareness, or both, and whether these effects depend on brain hemisphere.MethodsWe administered TBS to 12 individuals in either left prefrontal, right prefrontal, or a sham location on three separate days. We then administered a visual associative‐memory task incorporating global‐level awareness judgments and feeling‐of‐knowing (FOK) judgments on test trials for which retrieval failed.ResultsOverall memory accuracy significantly improved after right hemisphere TBS compared to sham. Simultaneously, subjects were relatively underconfident after right TBS, suggesting minimal awareness of memory accuracy improvements. The correspondence between FOKs and later recognition accuracy suggested a pattern of disruption in prospective memory monitoring accuracy after left TBS.ConclusionsOur findings provide unique evidence for improved visual memory accuracy after right prefrontal TBS. These results also suggest right prefrontal lateralization for visual memory and left‐hemisphere specialization for item‐level prospective memory awareness judgments. Taken together, these results provided continued support for noninvasive stimulation to prefrontal cortex as a means of potentially improving memory and causally influencing prospective memory awareness.
Highlights
The prefrontal cortex has been associated with a variety of roles in episodic memory (e.g., Blumenfeld & Ranganath, 2007; Fletcher, Shallice, Frith, Frackowiac, & Dolan, 1998; Henson, Shallice, & Dolan, 1999; Simons & Spiers, 2003; Ranganath, Johnson, & D'Esposito, 2000; Ranganath & Gregor, 2003; Tulving, Kapur, Craik, Moscovitch, & Houle, 1994), including encoding and retrieval operations crucial for memory accuracy as well as introspective functions related to the conscious experience of memory content (e.g., Baird, Smallwood, Gorgolewski, & Margulies, 2013; Gagnon, Schneider, Grondin, & Blanchet, 2011; Fleming & Dolan, 2012; Fleming & Frith, 2014; Metcalfe & Schwartz, 2016; Modirrousta & Fellows, 2008; Wheeler, Stuss, & Tulving, 1997)
These previous studies suggest it is possible that prefrontal TBS can improve recognition accuracy, the location and hemisphere of stimulation are important variables that seem to predict the effects of TBS on memory which have not been thoroughly explored
Global awareness judgments indicated that individuals were largely unaware before taking the test of the fact that memory accuracy was going to be higher following right prefrontal TBS
Summary
The prefrontal cortex has been associated with a variety of roles in episodic memory (e.g., Blumenfeld & Ranganath, 2007; Fletcher, Shallice, Frith, Frackowiac, & Dolan, 1998; Henson, Shallice, & Dolan, 1999; Simons & Spiers, 2003; Ranganath, Johnson, & D'Esposito, 2000; Ranganath & Gregor, 2003; Tulving, Kapur, Craik, Moscovitch, & Houle, 1994), including encoding and retrieval operations crucial for memory accuracy as well as introspective functions related to the conscious experience of memory content (e.g., Baird, Smallwood, Gorgolewski, & Margulies, 2013; Gagnon, Schneider, Grondin, & Blanchet, 2011; Fleming & Dolan, 2012; Fleming & Frith, 2014; Metcalfe & Schwartz, 2016; Modirrousta & Fellows, 2008; Wheeler, Stuss, & Tulving, 1997). Despite an enormous relevant fMRI literature, very few studies have used noninvasive brain stimulation methods to test the nature of prefrontal contributions to episodic memory These studies have generally applied transcranial magnetic or direct current stimulation unilaterally to either left or right prefrontal cortex and identified changes in memory accuracy and/or awareness (Blumenfeld, Lee, & D'Esposito, 2014; Chua & Ahmed, 2016; Demeter, Mirdamadi, Meehan, & Taylor, 2016; Javadi & Walsh, 2012; Köhler, Buckner, & Milner, 2004). We previously found no effects of bilaterally administered TBS on recognition accuracy (Ryals, Rogers, Gross, Polnaszek, & Voss, 2015), there were effects of stimulation on memory awareness (described ) Together, these previous studies suggest it is possible that prefrontal TBS can improve recognition accuracy, the location and hemisphere of stimulation are important variables that seem to predict the effects of TBS on memory which have not been thoroughly explored. These results provided continued support for noninvasive stimulation to prefrontal cortex as a means of potentially improving memory and causally influencing prospective memory awareness
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