twenty - three patients were administered a high - dose and low - dose atd mixture ( 100 vs. 25  g amino acids ) in a double - blind randomized crossover design with two sessions , separated by at least 4  days ( booij et al .
the 100  g and 25  g atd mixtures have previously been shown to lower plasma trp levels by approximately 90% and 50% , respectively , in this sample ( booij et al .
, 2005b ) as well as in other samples ( e.g. , booij , van der does , haffmans , & riedel , 2005a ) .
the study was approved by an independent medical ethics committee ( metigg , utrecht ) and was performed according to their guidelines and regulations .
all patients were informed about the study by their clinician and in detail by one of the investigators ( l.b . ) , and provided written informed consent .
the sample has been described in detail previously ( booij et al . , 2005b ) .
inclusion criteria were : age between 18 and 65  years ; ongoing treatment with an ssri or a serotonin noradrenaline reuptake inhibitor for at least 4  weeks , meeting dsm - iv criteria for depression in full or partial remission , and a hamilton depression rating scale ( hrsd , 17 items ) ( hamilton , 1960 ) lower than 15 ( frank et al . , 1991 ) .
exclusion criteria were : substance abuse within the past 3  months , psychosis ( lifetime ) , major physical illness , lactation , and pregnancy .
after excluding two drop - outs and two statistical outliers , 19 participants remained for statistical analyses ( cf .
booij et al . , 2005b ) . as described in the original article ( booij et al . ,
2005b ) , depressive symptoms were assessed with the 10-item montgomery asberg depression rating scale ( madrs ) ( montgomery & asberg , 1979 ) .
the sleep items were omitted , since this could not change within an atd session .
diagnoses , and demographic and clinical background variables were verified with the structured clinical interview for dsm - iv ( scid - i ) ( first , spitzer , gibbon , & williams , 2005 ) . the simon task consisted of 64 trials presented in two consecutive blocks in which the stimulus interval differed ( 2,250  ms fixed vs. 2,2505,500  ms variable ; originally included to investigate a possible effect of task complexity )
right  was presented in randomized order either at the left or the right side of the screen .
participants were instructed to respond to the meaning of the word ( target)that is , making a left - hand response to the word  left  and a right - hand response to the word 
the same number of compatible ( distracter location matches the target word ) and incompatible ( distracter location conflicts with the target word ) stimuli was used . 1 venous blood ( 10  ml ) was taken in the morning , 6  hr after atd and the next day ( t + 24 ) and was analyzed for both total plasma trp and the other large neutral amino acids ( fekkes , vandalen , edelman , & voskuilen , 1995 ) .
mood was assessed 1  hr before atd ( t  1 ) , 6.5  hr later ( t + 6.5 ) , and the next morning ( t + 24 ) .
the simon task (  left / right task  ) was administered ( booij et al . , 2005b ) approximately 5.5  hr after administration of the atd mixture .
the order of the low - dose session and the high - dose session was counterbalanced . cognitive performance and mood
repeated measures anovas were used for sequential analysis of simon performance , separately for correct reaction time ( rt ) and percent accuracy . in order to measure sequential adjustments in simon task performance ,
we not only included the factor compatibility of the current trial as is usually done , but we also added the factor  compatibility of the previous trial .  standard conflict adaptation  that is , the modulation of the compatibility effect as a function of previous - trial compatibility ( cf .
1a)should yield an interaction effect between current and previous trial compatibility ( egner , 2007 ; gratton et al .
, 1992 ) . to analyze direct effects of the atd manipulation on conflict adaptation , intervention ( baseline vs. low - dose vs. high - dose atd )
the effect of mood state on conflict adaptation , for the low - dose atd and high - dose atd sessions separately , was analyzed by using mood scores ( measured at + 6.5  hr ) as a covariate .
additionally , we also ran separate anovas in which we included the binary between - subjects factors ongoing treatment ( ssri vs. snri ) and mood response ( absence vs. presence of depressed mood increase from t-1 to t + 6.5 in the high - dose atd session).fig .
1a conflict adaptation in the simon task as evidenced by a reduced compatibility effect after conflict trials versus nonconflict trials .
b more depressive symptoms ( madrs score ) after the high - dose atd intervention are associated with increased conflict adaptation a conflict adaptation in the simon task as evidenced by a reduced compatibility effect after conflict trials versus nonconflict trials .
b more depressive symptoms ( madrs score ) after the high - dose atd intervention are associated with increased conflict adaptation to visualize the hypothesized association between mood and conflict adaptation , we calculated standard individual conflict - adaptation scores by subtracting the interference effect following a correct conflict ( incompatible ) trial from the interference effect following a correct nonconflict ( compatible ) trial ( cf .
1a ) . before averaging sequential conditions for each individual , the first trial of each block ,
trials following an error , and trials with rts not fitting the outlier criterion ( deviating more than 2.5 sds from the individual condition - specific mean ) were excluded from analyses .
the sample has been described in detail previously ( booij et al . , 2005b ) .
inclusion criteria were : age between 18 and 65  years ; ongoing treatment with an ssri or a serotonin noradrenaline reuptake inhibitor for at least 4  weeks , meeting dsm - iv criteria for depression in full or partial remission , and a hamilton depression rating scale ( hrsd , 17 items ) ( hamilton , 1960 ) lower than 15 ( frank et al . , 1991 ) .
exclusion criteria were : substance abuse within the past 3  months , psychosis ( lifetime ) , major physical illness , lactation , and pregnancy .
after excluding two drop - outs and two statistical outliers , 19 participants remained for statistical analyses ( cf .
as described in the original article ( booij et al . , 2005b ) , depressive symptoms were assessed with the 10-item montgomery asberg depression rating scale ( madrs ) ( montgomery & asberg , 1979 ) .
the sleep items were omitted , since this could not change within an atd session .
diagnoses , and demographic and clinical background variables were verified with the structured clinical interview for dsm - iv ( scid - i ) ( first , spitzer , gibbon , & williams , 2005 ) .
the simon task consisted of 64 trials presented in two consecutive blocks in which the stimulus interval differed ( 2,250  ms fixed vs. 2,2505,500  ms variable ; originally included to investigate a possible effect of task complexity ) . the word  left  or 
right  was presented in randomized order either at the left or the right side of the screen .
participants were instructed to respond to the meaning of the word ( target)that is , making a left - hand response to the word  left  and a right - hand response to the word 
right,and to ignore its location ( distracter ) , as fast as possible . the same number of compatible ( distracter location matches the target word ) and incompatible ( distracter location conflicts with the target word ) stimuli was used . 1
venous blood ( 10  ml ) was taken in the morning , 6  hr after atd and the next day ( t + 24 ) and was analyzed for both total plasma trp and the other large neutral amino acids ( fekkes , vandalen , edelman , & voskuilen , 1995 ) .
mood was assessed 1  hr before atd ( t  1 ) , 6.5  hr later ( t + 6.5 ) , and the next morning ( t + 24 ) .
the simon task (  left / right task  ) was administered ( booij et al . , 2005b ) approximately 5.5  hr after administration of the atd mixture .
the order of the low - dose session and the high - dose session was counterbalanced . cognitive performance and mood
repeated measures anovas were used for sequential analysis of simon performance , separately for correct reaction time ( rt ) and percent accuracy . in order to measure sequential adjustments in simon task performance ,
we not only included the factor compatibility of the current trial as is usually done , but we also added the factor  compatibility of the previous trial .  standard conflict adaptation  that is , the modulation of the compatibility effect as a function of previous - trial compatibility ( cf .
1a)should yield an interaction effect between current and previous trial compatibility ( egner , 2007 ; gratton et al .
, 1992 ) . to analyze direct effects of the atd manipulation on conflict adaptation , intervention ( baseline vs. low - dose vs. high - dose atd )
the effect of mood state on conflict adaptation , for the low - dose atd and high - dose atd sessions separately , was analyzed by using mood scores ( measured at + 6.5  hr ) as a covariate . additionally , we also ran separate anovas in which we included the binary between - subjects factors ongoing treatment ( ssri vs. snri ) and mood response ( absence vs. presence of depressed mood increase from t-1 to t + 6.5 in the high - dose atd session).fig .
1a conflict adaptation in the simon task as evidenced by a reduced compatibility effect after conflict trials versus nonconflict trials .
b more depressive symptoms ( madrs score ) after the high - dose atd intervention are associated with increased conflict adaptation a conflict adaptation in the simon task as evidenced by a reduced compatibility effect after conflict trials versus nonconflict trials .
b more depressive symptoms ( madrs score ) after the high - dose atd intervention are associated with increased conflict adaptation to visualize the hypothesized association between mood and conflict adaptation , we calculated standard individual conflict - adaptation scores by subtracting the interference effect following a correct conflict ( incompatible ) trial from the interference effect following a correct nonconflict ( compatible ) trial ( cf .
1a ) . before averaging sequential conditions for each individual , the first trial of each block ,
trials following an error , and trials with rts not fitting the outlier criterion ( deviating more than 2.5 sds from the individual condition - specific mean ) were excluded from analyses .
as described in detail elsewhere ( booij et al . , 2005b ) , high - dose atd but not low - dose atd led to a both statistically and clinically significant induction of depressive symptoms as measured by madrs scores 6.5  hr after depletion [ 7.9  7.8 vs. 3.7  3.8 , mean  standard deviation ; t(18 ) = 3.34 , p < .005 ] .
the simon task produced a standard interference effect : incompatible trials produced longer rts than compatible trials , f(1 , 18 ) = 23.47 , p < .001 , mse = 1933.65 .
the analysis also revealed a robust conflict - adaptation effect as indicated by an interaction between current- and previous - trial compatibility , f(1 , 18 ) = 38.27 , p < .001 , mse = 1143.45 .
1a shows , the interference effect was eliminated after conflict ( incompatible ) trials but not after nonconflict ( compatible ) trials , indicating standard conflict adaptation  that is , reduced interference after conflict trials .
this reduction in interference was driven by both postconflict speeding of incompatible trials [ illustrating that increased conflict - driven control reduces interference ; t(18 ) = -4.2 , p < .001 ] and by postconflict slowing of compatible trials [ illustrating that increased conflict - driven control reduces facilitation ; t(18 ) = 4.5 , p < .001 ] .
although participants made only a few errors in general , error rate analyses still showed standard interference , f(1 , 18 ) = 5.38 , p < .05 , mse = 0.001 , and conflict adaptation , f(1 , 18 ) = 6.30 , p < .05 , mse = 0.001 , effects .
in addition , they revealed a main effect of previous compatibility , f(1 , 18 ) = 4.46 , p < .05 , mse = .002 , indicating improved accuracy after conflict . notably , no main effects or interactions with congruency or conflict adaptation in rt or accuracy were observed for atd intervention .
thus , atd did not have an effect on interference ( as reported earlier by booij et al .
ongoing treatment did not have main or interactive effects.table 1behavioral data for all sessions and conditionsreaction time ( in milliseconds)error rate ( in percentages)msemsebaselinecompatible trial following a compatible trial65514.40.60.3incompatible trial following a compatible trial70512.33.61.2compatible trial following an incompatible trial67612.10.90.4incompatible trial following an incompatible trial67511.61.00.5low - dose sessioncompatible trial following a compatible trial65016.81.40.9incompatible trial following a compatible trial70618.22.91.0compatible trial following an incompatible trial68816.70.90.9incompatible trial following an incompatible trial68014.40.80.8high - dose sessioncompatible trial following a compatible trial64918.31.40.8incompatible trial following a compatible trial71119.92.30.9compatible trial following an incompatible trial66717.30.90.5incompatible trial following an incompatible trial67816.41.10.8 behavioral data for all sessions and conditions an ancova using mood score as a covariate confirmed our hypothesis : depressed - mood scores during the high - dose atd condition predicted increased conflict adaptation in rt as indicated by a significant three - way interaction between mood , current - trial compatibility , and previous - trial compatibility , f(1 , 17 ) = 5.30 , p < .05 , mse = 396.75 . as fig . 
1 shows , individuals with more depressive symptoms after the atd intervention showed more conflict adaptation in the simon task . as is typically observed ( chepenik et al . , 2007 ; van steenbergen et al . , 2010
2 moreover , no mood effects were found for accuracy , thus showing that the effect on conflict adaptation could not be attributed to a speed  accuracy tradeoff . because the low - dose atd session did not lead to any mood changes ( booij et al . , 2005b )
, data from this session were used for a control analysis : no association between mood and performance emerged.3
we report the first evidence for a link between low tryptophan concentrations , depressed mood , and conflict adaptation in remitted depressed patients : individuals with higher levels of depressive symptoms following high - dose atd showed increased conflict adaptation .
this finding is in line with predictions derived from mbm theory ( gendolla , 2000 ) , with earlier behavioral and physiological observations from mood - induction studies in healthy populations ( e.g. , gendolla , abele , & krusken , 2001 ; gendolla & krusken , 2002 ; van steenbergen et al . , 2010 ) , and with neural evidence ( e.g. , evers et al . , 2009 )
our present study has important implications for understanding how depressive affect regulates cognitive control . in line with mbm theory ( gendolla , 2000 )
, our data illustrate that depressed mood per se does not have motivational implications ( as would be indicated by a modulation in attentional interference effects ) , but may facilitate increased cognitive control after a behavioral challenge .
interestingly , this effect was observed in a relatively low - demanding simon task in which people were merely instructed to do their best ( see also brinkmann & gendolla , 2007 ) , and in the context of depression scores that were mainly below the cut - off value for a depression diagnosis , but were still clinically relevant and much larger than the effect of mood inductions in healthy participants .
note that our findings suggest that depressive symptoms as induced by atd did not simply mimic the effect of depressive symptoms in currently depressed patients .
that is , whereas mdd has been associated with reduced postconflict adaptation ( meiran et al . , 2011 ; pizzagalli , 2011 )
mbm theory ( gendolla , 2000 ) has proposed that the association between mood and effort mobilization is nonlinear , because it is mediated by task difficulty appraisal .
accordingly , negative mood will facilitate demand - driven adjustments in effort only up to a certain level : if demands become perceived as too high to actively cope with , negative mood may actually trigger demand - driven disengagement ( cf .
evidence for this effect has been reported in mood - induction studies and can also be shown in dysphoric participants when they perform tasks with extremely high fixed demands ( brinkmann & gendolla , 2008 ) .
taken collectively , these observations suggest an inverted - u relationship between depressive symptoms and conflict adaptation ( see brehm & self , 1989 ) .
it is an important aim for future studies to understand the generalizability of these findings and to disentangle the effects of increased negative affect and putative reduced availability of resources ( e.g. , due to rumination ) in mdd ( cf .
mbm theory assumes that the interaction between both factors determines the actual appraisal of the demand , which in turn modulates effort mobilization .
this hypothesis is now ripe for further testing in other studies using sequential analyses of conflict - task performance . at the neural level
, the joint impact of depressed mood and demand evaluation on subsequent effort mobilization and cognitive control may be associated with ( hyper)activation of the anterior cingulate cortex ( acc ) , a region important for signaling the need for more cognitive effort to the dorsolateral prefrontal cortex ( dlpfc ) ( botvinick et al . , 2001 ; davidson , pizzagalli , nitschke , & putnam , 2002 ; olvet & hajcak , 2008 ; pizzagalli et al . ,
dlpfc circuit in unipolar depression also produces catastrophic reactions to errors ( for a review , see pizzagalli , 2011 ) .
further study is needed to understand the exact neuromodulating role that 5-ht may play in this affective regulation ( cf .
future studies that combine neuroimaging methods with effort - related physiological and behavioral measures will advance our understanding of the functional role of the acc in the affective ( dys)regulation of adaptive control to fluctuating task demands .
 negative affect in healthy populations regulates the appraisal of demanding situations , which tunes subsequent effort mobilization and adjustments in cognitive control . in the present study , we hypothesized that dysphoria in depressed individuals similarly modulates this adaptation , possibly through a neural mechanism involving serotonergic regulation . 
 we tested the effect of dysphoria induced by acute tryptophan depletion ( atd ) in remitted depressed patients on conflict adaptation in a simon task . 
 atd temporarily lowers the availability of the serotonin precursor l - tryptophan and is known to increase depressive symptoms in approximately half of remitted depressed participants . 
 we found that depressive symptoms induced by atd were associated with increased conflict adaptation . 
 our finding extends recent observations implying an important role of affect in regulating conflict - driven cognitive control . 
Method
Participants
Diagnoses and symptoms
Conflict adaptation
Procedure
Data analysis
Results
Discussion