Acute tryptophan depletion improves performance and modulates the BOLD response during a Stroop task in healthy females
Introduction
Acute tryptophan depletion (ATD) has been used as a model to study the effects of reduced central 5-HT (Nishizawa et al., 1997, Williams et al., 1999, Young et al., 1999) on cognitive performance and brain activation. In a previous study, our group showed that ATD increased the response in the dorsomedial prefrontal cortex (dmPFC) to negative feedback preceding a switch in response strategy (Evers et al., 2005). Activation in the dmPFC associated with negative feedback has been related to performance monitoring and cognitive control (see Ridderinkhof et al., 2004 for a review). According to one theory (Holroyd and Coles, 2002), the dmPFC response to negative feedback is linked to phasic changes in the midbrain dopamine system related to outcomes that are worse than expected. According to a second theory, dmPFC activation is related to conflict monitoring which becomes necessary when two competing response tendencies become active at the same time (Botvinick et al., 2004). In our previous study (Evers et al., 2005), it was unclear whether the dmPFC response to negative feedback was associated with an outcome that was worse than expected or conflict monitoring. Therefore, the present study examined brain responses to conflict in the absence of negative outcome. This was done in a combined cognitive and emotional Stroop task.
The studies examining the effects of ATD on performance in a Stroop task have been inconclusive thus far. Some studies reported improved performance (Coull et al., 1995, Rosse et al., 1992, Rowley et al., 1997, Schmitt et al., 2000), whereas other studies did not show an effect of ATD on Stroop performance (Gallagher et al., 2003, Horacek et al., 2005, Sobczak et al., 2002). Horacek et al. (2005) showed that ATD increased the blood oxygen level dependent (BOLD) response in the left bilateral mediofrontal, anterior cingulate and dorsolateral prefrontal cortex during Stroop performance in healthy volunteers. A problem with this study is that performance was measured outside the MRI scanner. It is therefore not possible to judge whether the participants carried out the task correctly during scanning. The current study used an event-related design to study the effect of ATD on the BOLD response during Stroop interference. Performance and the BOLD signal were recorded simultaneously.
5-HT has also been related to emotional processing. Previous studies revealed that ATD impaired the processing of positive information in healthy volunteers (Murphy et al., 2002) and remitted depressed patients (Booij et al., 2005). However, Booij et al. (2005) showed no effect of ATD on reaction times (RTs) and interference scores during an emotional Stroop task in currently depressed patients. In general, participants experience more interference from negative than from positive or neutral words (e.g. McKenna and Sharma, 1995) which can be explained by the higher threat caused by negative events in daily live (Mandler, 1975, Oatley and Johnson-Laird, 1987). Compton et al. (2003) showed that ignoring negative versus neutral words activated the bilateral occipito-temporal cortex and decreased the activation in the amygdala. Whalen et al. (1998) reported greater activation in the anterior cingulate cortex (ACC) for negative versus neutral words. However, Compton et al. (2003) did not find brain activation differences between positive and neutral words. To our knowledge, the present study is the first study examining the effect of ATD on the BOLD response during an emotional Stroop task.
To study the effect of ATD on cognitive Stroop interference and interference by emotional words, we designed a combined Stroop task in which congruent color (CC), incongruent color (IC), neutral (e.g. house, bean, coin), positive (e.g. proud, friend, smart) and negative (e.g. murder, bitch, death) words were presented in a semi-randomized order. Based on the studies discussed above, we hypothesized that (i) IC words cause more interference than CC words, (ii) IC words are associated with an increased BOLD response in the ACC, inferior parietal cortex, inferior frontal junction and middle frontal cortex (Laird et al., 2005) and (iii) ATD does not change or decreases Stroop interference and modulates the related BOLD response. With regard to the emotional Stroop part, we hypothesized that (i) RTs are higher for negative than for positive or neutral words because of the higher threat caused by negative events, (ii) the medial PFC, ACC and insula are activated during emotional words (for a review, see Phan et al., 2002) and (iii) ATD impairs performance on emotional words and affects the related brain activation.
Section snippets
Participants
Nineteen healthy female volunteers (aged between 19 and 33; mean age 22.3; SE of mean 0.7), mostly pregraduate students, were included in this study, which was approved by the Medical Ethics Committee of Maastricht University Hospital. Participants were recruited by local advertisements. The health status of the participants was checked by a medical questionnaire, which was evaluated by a medical doctor. The participants received no medication at the moment of inclusion, had never used
Results
Fifteen volunteers were successfully tested. Of the original 19 included volunteers, two dropped out after the first session because of nausea and vomiting, one volunteer panicked in the scanner, and imaging data for one participant were lost due to technical problems. Nine participants started in the BAL condition and six started in the TRP− condition.
Discussion
The present study investigated the effect of ATD on performance and the BOLD response during a combined cognitive and emotional Stroop task. In the cognitive part of the Stroop task, we showed first that ATD increased the BOLD signal in the ACC when IC words were compared with CC words in the first block of the Stroop task. Secondly, ATD increased the BOLD response in the left precuneus (BA 31) and cuneus (BA 18) during CC words. At the behavioral level, ATD decreased the interference score for
Acknowledgments
We thank Olga Reneerkens for test and analysis assistance and Jeroen van Deursen for radiographic assistance. This work was supported by a TOP grant (No. 912-02-050) from ZonMW-NWO and a grant from the Dutch Brain Foundation (Hersenstichting Nederland, 11F03(2).41).
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