a growing body of literature reports that computerized attention training programs are successful in reducing symptoms of anxiety and depression . these attentional bias modification ( abm ) procedures intend to reduce or even reverse patients ' habitual tendency to direct their attention towards negative information . most of these studies use variants of the dot probe task . in abm variants of this task , participants are implicitly taught to direct their attention away from negative information ( a picture or word ) . this is accomplished by administering several hundreds of trials that involve brief simultaneous presentations of negative and neutral information , followed by a target . this target is systematically presented at the location of the neutral information . in a recent meta - analysis , an effect size of .61 the authors concluded that these training programs show promise as a new treatment but also qualify the research on abm as immature . nine of the twelve studies were conducted in nonclinical samples ( anxious students or adults ) , while only three studies concerned patients with an anxiety disorder . the effect sizes across trials also varied substantially , and an asymmetrical funnel plot was found , indicating a possible publication bias . furthermore , it remains unclear whether the treatment actually exerts an effect on attentional bias to threat , the presumed mechanism of action . mediation analyses , if performed , give equivocal results , and in one study participants who were trained to direct attention towards and away from threat showed comparable reduction of emotional reactivity . studies are needed that systematically vary variables such as stimulus durations , stimulus content , and sessions durations to establish the optimal training parameters . currently and remitted depressed patients show attentional bias for sad words and faces ( reviews : [ 6 , 7 ] ) and , compared to healthy controls , lack a bias towards positive information [ 8 , 9 ] . such biased information processing is theorized to play a role in the aetiology and maintenance of major depressive disorder . four sessions of abm over the course of two weeks had no immediate effect on depression scores of 34 dysphoric students ; however , a significant difference was observed between the abm - treated and control groups at two weeks of followup . a mediation analysis that supported the proposed working mechanism , the reduction of attentional bias for negative information over the course of treatment , attentional bias at followup , however , was not reported . moreover , attrition was very high at 47% , rendering the conclusion that the treatment had the intended effect premature . in another paper two studies were presented that tested the effects of an abm procedure based on the emotional spatial cueing task , which is closely related to the dot probe task . the treatment did not affect attentional bias in depressed patients nor did it affect their symptom severity . in a dysphoric student sample , treatment also had no effect . in a post hoc analysis , however , symptom severity was decreased in a mildly dysphoric subgroup , whereas increased symptoms were observed in those with moderate to severe symptoms at baseline .   in summary ; abm treatment for anxiety disorders has been called promising , but a close look at the individual clinical trials reveals many open questions , while abm for depression is even less studied . two studies have claimed small effects in nonclinical samples including an adverse effect in some participants . the abm parameters used in both studies differ substantially from each other and from those typically used in anxiety disorder studies . instead of carrying out another randomized controlled trial with one of these treatments , we propose that intensive case studies are needed to select the most promising approaches for evaluation in clinical trials . the current study is such a case study in which the effect of two parameters of the dot probe abm paradigm , stimulus duration and aiming at inducing towards positive information versus reducing bias towards negative information , will be evaluated in a dysphoric student sample . the depression abm procedure designed by wells and beevers   differs from abm procedures for anxiety disorders . sad ( depression relevant ) pictures were used rather than threatening stimuli , as well as exceptionally long stimulus durations ( 3000  ms for faces and 4500  ms for scenic pictures ) . the spatial cueing abm procedure introduced by baert and the rationale is that , compared to anxiety , attentional biases in depression may be more pronounced at longer stimulus durations , after more elaborated processing and greater activation of relevant schemata . a recent meta - analysis , however , found similar effect sizes for studies assessing attentional bias in depression , dysphoria , or induced dysphoria at 500  ms stimulus duration ( d = 0.54 , 7 studies ) and at 1000  ms or higher ( d = 0.59 , 9 studies ) . in addition to bias towards sad information , remitted and currently depressed patients also show a reduction in bias towards happy faces compared to healthy controls [ 8 , 9 ] . this combination of biases was also found in dysphoric students   and girls at risk for depression . the spatial cueing abm procedure tested by baert and colleagues was designed to induce bias towards positive information as well as reduce bias towards negative information . one other study , that did not explicitly target nor measure depression symptomatology , successfully modified attentional bias using a dot probe abm procedure to induce bias towards positive word stimuli in healthy students at a stimulus duration of 500  ms . eye - tracking revealed that abm reduced the time participants spent gazing at negative portions of images .   in the present study , 30 students with mild to moderate symptoms of depression engaged in one of six variants of the abm paradigm for four sessions within a week . each condition was defined by a combination of one of two training directions , away from neutral towards happy information or away from sad towards neutral information , and either a long , a short , or a variable stimulus duration . to our knowledge the rationale is that , instead of training participants to direct their attention at a specific point in time , a variable stimulus duration will train participants at several stages of attention orienting simultaneously . we used an 85% congruency contingency , that is , 85% of the trials were congruent or incongruent trials , dependent on the training direction . this allowed monitoring attentional bias per session and also obscured the rationale of the study from the participants . attentional bias was assessed throughout the four training sessions and at pre- and posttraining using a separate stimulus set . upon completion of the training procedure , participants were interviewed regarding their awareness of the task mechanics , as unpublished data suggest that training may only have an effect on symptoms if participants remain unaware of the congruency contingency .   in summary , we tested the effects of six variants of an abm procedure in dysphoric students . the main outcome measure was the change of attentional bias , assessed during the training as well as posttraining on a separate set of stimuli . we hypothesized that attentional bias would change in the intended direction during the course of treatment and that this change would be generalized to untrained stimuli . potential participants filled out the depression subscale of the hospital anxiety and depression scale ( hads ) . participants who scored between 4 and 8 and had not previously participated in abm studies were eligible . a dutch general population sample had a mean hads - d score of 3.4 , whereas psychiatric outpatients had a mean score of 9.3 . depressive symptoms were assessed with the beck depression inventory - ii ( bdi - ii ) . four separate stimulus sets were created , two containing pictures of happy and neutral expressions and two containing pictures of neutral and sad expressions . the facial expressions were correctly identified at least 60% of the time in ratings obtained from a female student population . the two sets for each emotion ( happy , sad ) contained different actors and were matched on hit rates , valence , and arousal ratings . mean hit rates , valence , and arousal ratings for each set are presented in table 1 . the probe was a small black square ( 15  15 pixels ) that was shown either upright ( square ) or rotated 45 degrees ( diamond ) . participants were seated circa 60  cm from the display resulting in a horizontal distance of 16.0 of visual angle between the midlines of the two stimulus pictures and the two possible probe locations . the horizontal width of the stimulus pictures and the probe was 8 and 0.4 , respectively . throughout the procedure a single trial started with a 750  ms fixation cross - followed by the stimulus images in a horizontal arrangement . upon offset of stimulus pictures the probe appeared at the location previously taken by either stimulus . the probe remained on the screen until the participant responded by pressing one of two mouse buttons , corresponding to the two possible probe identities ( square / diamond ) . a 750  ms intertrial interval preceded the next fixation cross . during the pre- and posttraining bias assessment tasks the probe appeared on the location previously occupied by the neutral ( incongruent trial ) or the emotional stimulus ( congruent trial ) with equal probability ( congruency contingency : 50% ) . emotional stimulus location , probe identity , and stimulus identities were randomized and counterbalanced within the congruent and incongruent trials . within each training session of 200 trials , 170 trials ( 85% ) were either happy congruent ( neutral towards happy conditions ) or sad incongruent ( sad towards neutral conditions ) . these 60 trials were used for the  on - line  assessment of attentional bias within each training session . six versions of the dot probe task all differed on two dimensions : direction of training ( sad towards neutral or neutral towards happy ) and stimulus duration ( 500  ms , 3000  ms , or variable ) . within each block of 40 trials in the variable stimulus duration conditions , the stimulus durations 500 , 1125 , 1750 , 2375 , and 3000  ms were randomized and counterbalanced within each combination of trial congruency ( for the 50% congruency trials ) , location of the emotional stimulus , and probe identity . participants were scheduled to visit the laboratory on a monday , tuesday , thursday , and friday within one week . the appointments were scheduled at the same time each day with a maximum deviation of 1.5 hours . upon completion of the procedure participants the ethics committee of the institute of psychology at leiden university approved the study , and informed consent was obtained . the first session started with filling out the questionnaires , followed by 100 trials of a dot probe bias assessment task ( congruency contingency of 50% ) and 200 trials of the training ( congruency contingency of 85% ) . the fourth session started with 200 training trials , followed by 100 assessment trials , filling out questionnaires and the debriefing procedure . at the first session participants were required to take as many practice trials as needed for six consecutive correct answers with a minimum of 10 trials . for sessions 2 , 3 , and 4 only six practice trials with four consecutive correct trials short breaks were given every 40 trials ; participants could take a few seconds rest and continue the task with a mouse click . after 200 trials in the first and the fourth sessions participants were required to take a 5-minute break . in sessions 1 and 4 the program switched between the pre- and posttraining assessment trials and the training trials without notice . following wells and beevers , we did not mention the word  training  to the participants , nor any other phrase that implied that their mood might change . participants were informed that we were interested in the effects of repeated engagement in a task . the debriefing at the end of the last session consisted of a funnelled interview in which participants were first asked to guess the purpose of the study followed by a series of open questions informing whether they noticed anything specific regarding the faces , the location of the faces , the probes , the location of the probes , and whether anything else had changed when the images changed ( the switch from the training to the postassessment in the fourth session ) . afterwards , the experimenter explained the concept of attentional bias , the dot probe task , and the rationale of the dot probe training . participants were then told that they could have been in either a training or a control condition ( congruency contingency 85% or 50% ) . all participants had to guess which condition they had been in and rate how certain they were of their answer on a scale ranging from 50 to 100% . this percentage , reversed for those who believed to have been in the control condition , was used as an index of awareness . fifty - two of these scored between 4 and 8 points and were subsequently invited . these three participants were replaced to obtain a total of 30 participants .   the pre- and posttraining assessment trials as well as the 60 trials with a 50% congruency contingency within each of the four training sessions were included in the analyses . inspection of the error rates per participant and session showed that none of the participants made more than 10% errors within a single session . inspection of the remaining data showed that their distribution was skewed , and we therefore used median instead of mean response times to calculate the bias indices . bias indices were calculated separately for each participant and each of the six phases ( pretraining , training sessions 1 to 4 , and posttraining ) by subtracting the median response time for congruent trials from the median response time for incongruent trials . nonparametric tests showed no differences between the groups at baseline ( kruskal - wallis tests , all p > .5 ) . the bias indices obtained during each of the four sessions for each individual participant are shown in figure 1 . note that for the upper three rows ( neutral - to - happy conditions ) an increase in bias index was the intended effect of training , while for the lower three rows ( sad - to - neutral conditions ) bias index was intended to decrease . visual inspection of each row of panels in figure 1 learns that none of the training conditions had a consistent effect on the participant 's bias indices . the neutral - towards - happy 500  ms condition ( top row ) had the most consistent effect : relatively large increases in bias indices in three out of five participants , while the other two participant 's bias indices remained relatively stable . during the posttraining interview , participant 3 in this condition reported to have noticed at some point that the location of the probe and the happy face may have been associated . the participant then tested and rejected this idea , probably due to the 85% contingency . both 3000  ms conditions ( rows 2 and 5 ) as well as the sad - towards - neutral variable condition ( bottom row ) showed a change in the intended direction in two participants and a change in the opposing direction in another . the neutral - towards - happy variable condition ( row 3 ) had only one participant who showed the expected change in bias index , while two participants showed a change in the nonintended direction . the neutral - towards - sad 500  ms condition ( row 4 ) had one participant whose bias changed slightly in the nonintended direction , while the other four participant 's bias indices remained largely stable .   in order to assess whether a change in bias during the training transferred to a changed bias for the untrained stimulus pictures used in the pre- and posttraining assessment , we calculated two new variables : 
 ( 1)sct(successful  change  trained  stimuli )  = bias  index  s4bias  index  s1,scu(successful  change  untrained  stimuli )  = bias  index  postbias  index  pre . sct and scu are multiplied by 1 for the sad - towards - neutral conditions so that a positive value of sct and scu represents a change in the intended direction , while a negative value represents a change in the nonintended direction . a larger value represents a larger change in bias index at session 4 ( sct ) or posttraining ( scu ) , as compared to session 1 and pre - training , respectively . figure 2 illustrates that changes in bias during the training did not systematically transfer to the untrained stimuli used in the pre- and postassessment . bias indices of 16 participants changed in opposing directions , while five participants had negative values for both sct and scu . bias index changed in the intended direction for both trained and untrained stimulus pictures in nine participants , as indicated by positive values of sct and scu . for one of these participants ( p2 in the sad - towards - neutral 500  ms condition ) the values of sct and scu were 7 and 9  ms , respectively . these values are so small that they likely fall within the range of measurement error . the remaining eight participants whose bias changed in the intended direction for both trained and untrained stimulus pictures were scattered over the six conditions . three conditions ( sad - towards - neutral 500  ms and the two variable ms conditions ) each contained two such participants , the neutral - towards - happy 500 and 3000  ms conditions each had one such participant , whereas the sad - towards - neutral 3000  ms condition had no such participants . wilcoxon signed ranks tests comparing the bdi and bai scores pre- and posttraining for the entire sample indicated no effects of training ( z = 1.54 , p = .12 and z = 1.44 , p = .15 , resp . ) , as was expected . however , mean bai scores decreased significantly within the sad - towards - neutral conditions ( z = 2.35 , p = .02 ) but not within the neutral - towards - happy conditions ( z = .52 , p = .61 ) . mean bdi scores did not change significantly within either the sad - towards - neutral or the neutral - towards - happy conditions ( z = 1.16 , p = .24 and z = 1.03 , p = .30 , resp . ) .   at the start of the posttraining funnelled interview , sixteen participants believed to have been in a ( nonexistent ) control condition , whereas 14 participants thought to have received training . the confidence with which participants believed to have received training correlated significantly with the change in bias for untrained stimuli ( scu ; rs = .57 , p = .001 ) but not with change in bias index for the trained stimuli ( sct ; rs = .02 , p = .93 ) nor with the absolute bias indices obtained during session 4 and the posttraining assessment ( p = .47 and p = .27 , resp . ) . from the scatterplot showing the relation between scu and the confidence rating ( figure 3 ) it was observed that this effect was restricted to participants who believed to have received training ( confidence ratings > 50% ) . indeed , within the group that indicated to have received training , confidence ratings correlated significantly with scu ( rs = .82 , p < .001 ) , while the correlation was nonsignificant within the group that believed not to have received training ( rs = .04 , p = .89 ) . these correlations differ significantly ( test of fisher z transformed correlation coefficients : z = 2.92 , p = .004 ) . we studied six variants of an abm procedure in a multiple case series design , in order to select the most promising variant for further testing in future controlled clinical trials . this makes it very unlikely that any of these abm versions will have a specific effect on symptoms in controlled studies .   during the training , the bias index changed in the intended direction in 17 out of 30 participants . for nine of these 17 participants the bias index for untrained stimulus pictures also changed in the intended direction . this lack of generalization is problematic as generalization of a modified bias is theoretically the key mechanism of abm . in the study by wells and beevers   ( testing an abm variant comparable to the current studies ' sad - to - neutral 3000  ms condition ) for instance , depressive symptoms were unaffected during the training but this is consistent with a model in which cognitive changes only lead to symptomatic changes after interaction with the environment , as has also been proposed for pharmacologically induced cognitive changes [ 24 , 25 ] . we did not measure longer term effects , but since the effects of training on attentional bias , if any , did not generalize , a delayed effect on symptoms is quite unlikely . furthermore , 14 of our 30 participants showed a change of bias in the wrong direction . self - reported anxiety symptoms did decrease significantly within the sad - towards - neutral conditions , however . although the lack of a control treatment makes this finding hard to interpret , we argue that this is a chance finding : an effect on symptoms was not expected , the training was designed to target depression rather than anxiety , and our data do not support the working mechanism that would theoretically mediate such an effect . abm studies reporting effects on self - reported anxiety measures often do not relate effects to changes in attentional bias , however .   in an unpublished study comparing implicit and explicit instructions conditions , it was found that explicit instructions increased the effect of abm on attentional bias but abolished the effect on emotional reactivity . in the current study participants were kept unaware of the study 's objectives , but awareness of training contingency was assessed in a funnelled interview following the procedures . one participant reported to have considered the possibility that the probe systematically appeared on the location of one type of stimulus , while 16 out of 30 participants believed to have been in a nonexistent control treatment . the more certain participants were that they had received treatment , the more their bias index for untrained stimuli pictures had changed in the intended direction . no correlation existed between certainty of receiving training and the direction and magnitude of change in bias index during the training . this could be a reflection of demand characteristics in participants who were implicitly aware of the training contingency . it can not be ruled out that such an effect could also influence self - reported measures of symptoms . further research into the role of awareness in abm seems warranted . in summary , this single case series approach enabled us to scrutinize the effects of six variants of abm for depression at the individual 's and at training session levels . since we did not observe any consistent effects on attentional bias , we advice not to proceed yet to rcts to test dpt - based abm for depression . given the large placebo effect in clinical trials of affective disorders , the risk of nonreplicable findings would be large .
background . attention bias modification ( abm ) is a new treatment for affective disorders . 
 a meta - analysis of abm for anxiety disorders showed that the effect size may be large but the number of studies is low . 
 the working mechanism is still unclear , and little is known about the optimal treatment parameters . 
 abm for depression is much less studied . 
 a few studies claimed positive effects but the sample sizes are low . 
 furthermore , the treatment parameters varied widely and differed from the anxiety literature . aim . to select the most promising version of abm for depression for further evaluation in clinical trials . 
 methods 
 . multiple case series design . 
 we tested six versions of abm that varied on stimulus duration and training direction . 
 thirty students with mild to moderate symptoms of depression underwent four sessions of abm . 
 change of attentional bias was measured during each session . 
 generalization of treatment effects and the role of awareness of receiving training were also investigated . 
 results . 
 none of the investigated versions of abm had a consistent effect on attentional bias . 
 changes of attentional bias in individual participants the effects did not generalize to untrained stimuli . 
 conclusion . 
 it is unlikely that any of these abm versions will have a specific effect on symptoms in controlled studies .