Nosce te ipsum – Socrates revisited? Controlling momentary ruminative self-referent thoughts by neuromodulation of emotional working memory
Introduction
We all spend time thinking about our own feelings, thoughts and behavior. Self-reflection enables us to create and clarify the meaning of past and present experiences (Boyd & Fales, 1983). This issue of self-referent thinking has ever inspired many scholars. Ancient philosophers have pondered on the nature and fundamental conditions of human self-focus as part of a greater understanding of personal identity. The classical aphorism “Nosce te ipsum” (Know thyself), attributed to Socrates, reflects the ultimate goal of using systematic self-questioning to increase one's self-knowledge and, as a result, to attain wisdom. However, although this process facilitates coping under a variety of circumstances (Austenfeld and Stanton, 2004, Pennebaker and Seagal, 1999, Trapnell and Campbell, 1999), it might turn out to be unconstructive when self-referent thoughts become rather abstract, evaluative and self-critical (e.g., “Why do I always react this way?”, Watkins, 2008). Moreover, self-referent thoughts might evolve in rumination – “a class of conscious thoughts that revolve around a common instrumental theme and that recur in the absence of immediate environmental demands requiring the thoughts” (Martin & Tesser, 1996, p. 7). Although these repetitive thoughts do not necessarily have unconstructive consequences (Watkins, 2008), depression vulnerable individuals have the tendency to focus their thoughts on negative information and personal concerns. It is therefore crucial to understand how self-evaluative ruminative thoughts can be regulated in order to prevent them from becoming unintentional and unconstructive, particularly in individuals who demonstrate a tendency to ruminate in everyday life.
Here, a fundamental question is whether the occurrence of ruminative thoughts could be attributed to underlying cognitive phenomena, such as information processing. In studies investigating this question, participants are usually required to make retrospective reports on the content of their usual thoughts, whether or not referring to a moment when they feel down, sad or depressed (e.g., Joormann et al., 2006, Joormann and Gotfib, 2008). These studies tap into stable dispositions of rumination, and have shown that these trait scores are associated with cognitive information processes such as working memory for negative information (i.e., angry faces) (De Lissnyder, Koster, Derakshan, & De Raedt, 2010). During everyday functioning, however, ruminative thoughts of healthy individuals fluctuate continuously from one moment to another (Moberly & Watkins, 2008). Therefore, it might be important to assess these thoughts as how they occur naturally. In a prior study, we looked into brain activation over a period of 8 min, and afterwards asked participants to report their momentary thoughts during this period. Results revealed that the more unwanted self-referent ruminative thoughts were present during the 8 min rest period, the less dorsolateral prefrontal cortex (DLPFC, Brodmann 9/46) activation was observed (Kühn, Vanderhasselt, De Raedt, & Gallinat, in press). Interestingly, the DLPFC seems not only involved in momentary ruminative thinking (e.g., Cooney et al., 2010, Kühn et al., 2013), but also in the active maintenance of attentional demands and contextual information during cognitive tasks (MacDonald et al., 2000, Ochsner and Gross, 2005) and cognitive control for mental representations in working memory (Barch et al., 1997, Braver et al., 2001, Braver et al., 1997, Curtis and D'Esposito, 2003, Goldmanrakic, 1994).
Working memory (WM) refers to a limited-capacity system to simultaneously maintain and select information over a short period of time in the service of current cognitive processes (Baddeley, 1992, Miyake et al., 2000, Smith and Jonides, 1999). The representations maintained in WM can be emotional or not, and the selection of information depends on its importance in the attainment of goals. In fact, it has been hypothesized that the relation between DLPFC and rumination would be mediated by WM operations (Joormann, Yoon, & Zetsche, 2007). In other words, the cognitive capacity to adequately update negative mental representations in WM, which is associated with DLPFC activation, might influence the occurrence of momentary self-referent thoughts, preventing these thoughts to become uncontrollable and unconstructive (e.g., rumination). This is in line with cognitive theories proposing that a reduced capacity to shift away from internal representations of negative information could be the functional process underlying the inability to control self-referent ruminative thoughts, which can result in a tendency to continuously ruminate over negative thoughts and feelings (e.g., Koster et al., 2011, Mor and Winquist, 2002, Watkins, 2008). Based on these models, we specifically hypothesized that momentary ruminative self-referent thoughts would be associated with WM processes to update and shift away from angry faces, an ability which is associated with the DLPFC.
To experimentally address this hypothesis, we used Transcranial Direct Current Stimulation (tDCS) which is a non-invasive neuromodulatory technique. This neuromodulatory technique consists of the application of a weak, direct electric current through electrodes positioned over one's scalp, which are able to reach the neuronal tissue and induce polarization-shifts on the resting membrane potential (Brunoni et al., 2011). Anodal stimulation generally facilitates cortical activity, whereas cathodal tDCS has opposite effects. In many previous studies it could already be demonstrated that tDCS of the left DLPFC enhances cognitive processes, both for non-emotional (e.g., Fregni et al., 2005, Leite et al., 2011, Mulquiney et al., 2011) as emotional processes (Boggio et al., 2007, Wolkenstein and Plewnia, 2013). In the current experiment, we used anodal left tDCS to causally modulate DLPFC neural activation in order to enhance WM operations, and subsequently explored the occurrence of self-referent ruminative thoughts. Based on the literature on cognitive deficits inrumination, we used the Internal Shift Task (IST, De Lissnyder, Koster, & De Raedt, 2012), an experimental paradigm to evaluate the ability to update and shift between specific emotional and non-emotional representations in WM (Chambers et al., 2008, Garavan, 1998). Twenty minutes after the end of the stimulation, we explored the occurrence of momentary ruminative self-referent thoughts during an unguided rest period. We asked individuals to rest without any specific task – which is known to result in a stream of undirected free thoughts – and self-referent ruminative thoughts – were assessed during this period using a short self-report questionnaire (subjects were not aware of the purpose of our study). In this paper, momentary self-referential ruminative thinking refers to a temporary cognitive thought pattern that is highly dependent on situational cues but that is independent of mood. Trait rumination, on the other hand, is defined as “behaviors and thoughts that focus one's attention on one's depressive symptoms and on the implications of those symptoms”, and is considered a habitual thinking response to sadness (Nolen-Hoeksema, 1991, p. 569).
Our study hypothesis is based on cognitive theories which propose that a reduced capacity to shift away from internal representations of angry faces (e.g., negative information) could be the functional process underlying the inability to control self-referent ruminative thoughts. Therefore, we hypothesized that the effect of anodal left DLPFC stimulation (versus sham) on momentary ruminative self-referent thoughts would be mediated by the influence of tDCS on WM operations to update and shift away from negative representations. Because the level of momentary self-focus is related to the habitual tendency to ruminate in everyday life (Moberly & Watkins, 2008), we predicted that trait rumination scores would moderate the relationship between neuromodulation, working memory operations for negative material and momentary ruminative self-referent thoughts.
Section snippets
Participants
Thirty-two right handed volunteers (20F/12M) ranging from 18 to 36 years in age (M=22.28, SD=3.74) participated in this study. All participants were university students or had a university degree and reported no history of depression or anxiety. The study was approved by the institutional ethics committee of the Mackenzie Presbyterian University, Brazil and by the National Ethics Committee (SISNEP, Brazil).
Trancranial direct current stimulation
Direct electrical current was applied by a saline-soaked pair of surface sponge rubber
Effects of tDCS on mood: analysis of variance
The two Stimulation (tDCS, sham)×Time (T0, T1,T2) repeated measures ANOVAs with positive and negative affects as dependent variable revealed a significant main effect of Time for PA, [F(2, 30)=17.86, p<.0001, ηp2=.54], and for NA, [F(2, 30) =6.82, p=.004, ηp2=.31]. Paired t-tests revealed that both in the sham and tDCS condition, participants reported less PA and NA towards the end of the experiment: T1 to T0 (p’s<.02) and T2 to T1 (p’s<.05). For both PA and NA, the main effect of Stimulation
Discussion
We investigated the influence of DLPFC neuromodulation on working memory (WM) operations and momentary ruminative self-referent thinking. Anodal tDCS of the left DLPFC (compared to sham) did not directly influence the occurrence of momentary ruminative self-referent thoughts. However, we observed that the influence of anodal tDCS (and not of sham stimulation) on momentary ruminative self-referent thinking was mediated by the enhancement of WM operations for angry faces. In other words, the
Acknowledgments
M.A.V. (FWO08/PDO/168) is a postdoctoral research fellow of the Research Foundation Flanders (FWO). Preparation of this paper was also supported by Grant BOF10/GOA/014 for a Concerted Research Action of Ghent University (awarded to R.D.R.). P.S.B. is supported by a CNPq researcher Grant (305718/2009-6). T.L. would like to thank the Research Foundation Flanders (FWO) for financial support (Grant G.0111.12).
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