Competition for priority in processing increases prefrontal cortex s involvement in top-down control: an event-related fmri study of the stroop task

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1 Cognitive Brain Researh 17 (2003) loate/ ogbrainres Researh report Competition for priority in proessing inreases prefrontal ortex s involvement in top-own ontrol: an event-relate fmri stuy of the stroop task a, b,,1 a Mihael P. Milham *, Marie T. Banih *, Vikram Bara a The Bekman Institute, University of Illinois at Urbana-Champaign, 405 N. Matthews, Urbana, IL 61801, USA b Department of Psyhology, University of Colorao at Bouler, E213-E Muenzinger Hall, 345 UCB, Bouler, CO 80309, USA Aepte 28 February 2003 Abstrat Prior work iniates that various aspets of task-irrelevant information (e.g. its saliene, task-relateness, emotionality) an inrease the involvement of prefrontal ortex (PFC) in top-own attentional ontrol. In light of these finings, we hypothesize that PFC s involvement inreases when task-irrelevant information ompetes for priority in proessing. In an event-relate fmri stuy using an oball variant of the Stroop task, we examine the generality of this hypothesis using three manipulations esigne to inrease the ability of task-irrelevant information to ompete for priority in proessing. First, we investigate how the frequeny of ourrene of task-irrelevant information affets PFC ativity. Seon, we examine whether onfliting olor information (i.e. inongruent trials) inreases ativity in regions of PFC that are similar to or istint from those sensitive to infrequently ourring task-irrelevant information. Finally, we examine the impat of the number of levels at whih onflit oul our (e.g. non-response only, non-response an response). Ativity in posterior orsolateral an posterior inferior PFC inrease for infrequently ourring task-irrelevant information, being largest when the task-irrelevant information ontaine onfliting olor-information. In ontrast, inreases in mi-orsolateral prefrontal ortex s ativity were only note when onfliting olor information was present, being largest when onflit ourre at multiple levels. The anterior ingulate was primarily sensitive to the ourrene of onflit at the response level with only a small sub-region exhibiting sensitivity to non-response onflit as well. From these finings we suggest that posterior DLPFC an PIPFC are involve in biasing proessing in posterior proessing systems, mi-dlpfc is involve in biasing the proessing of the ontents of working memory, an ACC is primarily involve in response-relate proesses Elsevier B.V. All rights reserve. Theme: Neural bases of behavior Topi: Cognition Keywors: Attentional ontrol; Stroop; Conflit; fmri; Oball; Prefrontal; Anterior ingulate; Frequeny 1. Introution provies a top-own bias that favors the seletion of task-relevant information (e.g. [2,22,29,33,38]). Here we The ability to selet between task-relevant an task- argue that suh a bias is espeially important for exerting irrelevant information is a basi aspet of attentional attentional ontrol when task-irrelevant information an funtion. Neuropsyhologial an neuroimaging stuies effetively ompete with task-relevant information for iniate a role for prefrontal ortex in this proess, as it priority in proessing. As isusse by Frith [17] an others (e.g. [15]), the pereptually-relate properties of a task-irrelevant stimulus *Corresponing authors. Tel.: ; fax: (e.g. brightness) an inrease its saliene relative to a aresses: mmilham@s.psyh.uiu.eu (M.P. Milham), task-relevant stimulus, thereby favoring its proessing an mbanih@psyh.olorao.eu (M.T. Banih). allowing assoiate representations to effetively ompete 1 Tel.: ; fax: for priority in proessing. Top-own ontrol by prefrontal / 03/ $ see front matter 2003 Elsevier B.V. All rights reserve. oi: / S (03)

2 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) ortex is require to overome suh a bias [25]. Another neutral trials. On response-eligible inongruent trials, the situation in whih top-own ontrol is require beause inongruent olor-wor names a possible response (e.g. the task-irrelevant information an effetively ompete for wor blue in yellow ink, when blue, yellow, an green priority in proessing ours when task-irrelevant infor- are the possible ink olors). For these trials, task-irrelevant mation is proesse relatively automatially. In suh ases, information is present an an ompete for priority in task-irrelevant representations an/ or assoiate responses proessing at both the response an non-response levels. are ativate to a high egree [12]. A lassi example is On response-ineligible trials, the inongruent olor-wor provie by the Stroop task, in whih an iniviual must oes not name a possible response (e.g. the wor purple ientify a wor s ink olor while ignoring its ientity. in yellow ink, when blue, yellow, an green are the Sine wor-reaing is a relatively automati proess, the possible ink olors), an hene an only ompete at nonwor ativates task-irrelevant information (i.e. semanti response levels. Consistent with our hypothesis, DLPFC an phonologial representations as well as their assoiate ativity was greater, relative to neutral trials, for responseresponses) to a high egree, espite attempts to ignore it. eligible trials, whih oul introue ompeting infor- Beause proessing of this task-irrelevant information mation at both response an non-response levels of proours to a high egree, it is extremely effetive at essing, than for response-ineligible trials, whih introue interfering with the proessing of task-relevant informa- ompeting information at just non-response levels. Intertion. As a result, there is an inrease nee for top-own estingly, we note inreases in ativity (relative to neutral ontrol by PFC to selet the orret information that trials) within anterior ingulate ortex only for responseshoul be use to guie performane, as emonstrate in eligible inongruent trials, not response-ineligible trials, numerous stuies of the Stroop task an its variants impliating this prefrontal struture in response-relate [2,31,32]. Finally, other attributes of task-irrelevant in- proesses, not top-own ontrol (see [31,37], an [36] for formation, suh as its emotional value, an sometimes aitional eviene of the ingulate s ativity being linke inrease its ability to ompete. For example, emotionally speifially to response-relate proesses). laen information, espeially when arousing or negative, is Here, we further explore the possibility that PFC s effetive at apturing attention [51,53], leaing to an involvement in ontrol is epenent upon the ability of inrease nee for top-own ontrol by PFC [51,53]. task-irrelevant information to ompete for priority by Two reent stuies in our laboratory support the iea examining another variable, frequeny of ourrene. that PFC s involvement in top-own ontrol inreases Although fmri stuies using priming an oball when task-irrelevant information an effetively ompete paraigms have onsistently emonstrate greater neural for priority in proessing. In one stuy [31], we foun ativity in prefrontal regions for the proessing of relagreater PFC ativity when task-irrelevant information is tively novel or infrequently ourring information when it relate to the task at han than when it is not. More is task-relevant, suh investigations have not fouse on speifially, in the olor-wor Stroop task, we foun information that is task-irrelevant. When task-irrelevant inreases in PFC s ativity when the task-irrelevant in- information ours infrequently it shoul be better able to formation introue by the wor was olor-relate (rela- ompete for priority in proessing, as it is relatively novel tive to neutral wors), regarless of whether or not it an hene more salient [7]. onflite with task-relevant olor information. Thus, we To test our hypothesis that PFC s involvement in ontrol observe greater PFC ativity for both ongruent (e.g. the inreases in response to the ability of task-irrelevant wor re in re ink) an inongruent (the wor blue in information to ompete for priority in proessing, we re ink) olor-wor trials relative to neutral-wor trials etermine if PFC s ativity is greater for infrequently (e.g. the wor bon in re ink). The task-irrelevant ourring task-irrelevant information than frequently oinformation introue by the wor is better able to urring task-irrelevant information. To aomplish this, we ompete for priority in proessing on both ongruent an employe an oball variant of the Stroop task that mae inongruent olor-wor trials than on neutral-wor trials, use of two sets of neutral wors the first was baseline as it is relate to the task onept (olor). We observe neutral trials, whih omprise the majority of trials aitional PFC ativity for inongruent trials relative to (86%), an the seon was oball neutral trials, whih ongruent an neutral-wor trials, probably ue to their ourre with a muh lower frequeny. If as we suggeste, ability to ompromise seletion of the orret response. PFC s involvement in top-own ontrol is epenent upon The finings of a seon stuy by our laboratory are the ability of task-irrelevant information to ompete for also onsistent with our hypothesis, as they suggest that priority in proessing, inreases in ativity shoul be note ativity within PFC inreases with the number of levels at for oball neutral trials relative to baseline neutral trials. whih a stimulus an ompete for priority in proessing The seon goal of the present stuy was to etermine if [32]. Using a manual version of the olor-wor Stroop the subregions of PFC sensitive to infrequently ourring task, we examine inreases in neural ativity assoiate task-irrelevant information overlap with those engage with two ifferent types of inongruent trials (response- when task-irrelevant information onflits with task-releligible, response-ineligible) relative to that assoiate with evant information, as on inongruent trials in the Stroop

3 214 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) task. To aomplish this, we inlue oball inongruent oball trials were inlue: inongruent-eligible, inontrials as well as oball neutral trials. Task-irrelevant gruent-ineligible, an neutral. information introue by inongruent olor-wors has a The stimuli onsiste of wors presente in one of three unique ability to ompete for priority in proessing, as it is olors: blue, green an yellow. The wor set for inonboth relate to the task at han an able to interfere with gruent-eligible trials onsiste of the wors BLUE, seleting the orret representation on whih a response GREEN an YELLOW printe in an inongruent olor shoul be base. Hene, we examine whether the ai- (e.g. the wor blue in green ink) while that for inontional ability of task-irrelevant information to ompete gruent-ineligible trials onsiste of the wors RED, with task-relevant information on inongruent trials leas ORANGE an BROWN (e.g. the wor re in blue to further inreases in ativity within the same regions of ink). For the oball neutral trials, we mae use of a wor PFC as ativate by neutral oball trials, or whether set onsisting of three semantially relate wors ( MILE, ativation is observe in aitional an istint subregions YARD, METER ) that have no assoiation with olor. of the PFC. For the intervening baseline neutral trials, the wor set The thir goal of our stuy was to re-examine the onsiste of six olor-neutral wors, unrelate to the impat of response eligibility on inreases in neural oball neutrals. It is important to note eah baseline ativity within prefrontal regions. A prior stuy in our neutral wor appeare with a frequeny of one in every laboratory [32] iniate that the ACC was impervious to seven trials, while eah oball wor appeare with a the ourrene of onflit at non-response levels. Not only frequeny of one in every 42 trials. Thus, baseline neutral i we fin that it exhibite greater ativity uring wors appeare six times as frequently as oball wors response-eligible than response-ineligible trials, but we (neutral, inongruent-ineligible, inongruent-eligible). foun no signifiant ifferene in ativity for response- Eah sanning session onsiste of four runs, onsisting ineligible an neutral trials. Thus, as long as the wor i of 257 trials per run presente at a rate of one trial every 2 not name a possible response, no ACC ativity was note. s (36 oball trials were presente per run in a ranom However, other stuies have iniate that the ACC is orer we ensure that eah oball trial type ourre 12 most responsive to novel or infrequent events [8,54]. Thus, times per run). Eah trial onsiste of a 300-ms fixation we reasone that if the ACC has any sensitivity to non- ross followe by a 1200-ms presentation of the wor an response relate onflit, it might be etetable with an 500-ms inter-trial interval. oball esign in whih the trial types of interest our with low frequeny Proeure To review, we mae use of an oball esign, in whih the majority of trials were neutral trials (referre to as Partiipants were instrute to ientify the ink olor in baseline neutrals), with oball trials ourring one every whih eah wor was presente, while ignoring the wor s six to eight trials. Three oball trial types were inlue: ientity. Responses were aquire using a three-button (1) neutral wor trials; (2) response-ineligible inongruent response pa. trials; an (3) response-eligible inongruent trials. While we expete that all three oball types woul proue 2.3. Data aquisition inreases in neural ativity ue to their low frequeny of ourrene, we expete ativations to be more extensive A GE Signa (1.5 T) magneti resonane imaging system for interferene trials, ue to their ability to ativate equippe for eho-planar imaging (EPI) was use for ata representations of onfliting olor information, whih aquisition. Sixteen right-hane native English-speaking woul provie them with a greater ability to ompete for partiipants were inlue in our stuy. For eah run, a priority in proessing. Finally, we expete response-elig- total of 261 EPI images was aquire (TR52000 ms, ible trials to proue greater inreases in neural ativity TE540 ms, flip angle5908), eah onsisting of 10 onthan response-ineligible trials, as they an ompete with tiguous slies (thikness58 mm, in-plane resolution53.75 task-relevant information at more levels of proessing. mm), parallel to the AC-PC line. A high-resolution 3D anatomial set (T1-weighte three-imensional spoile graient eho images) was ollete for eah partiipant, 2. Materials an methos as well as T1-weighte images of our funtional aquisition slies. The hea oil was fitte with a bite bar to 2.1. Stimuli/esign minimize hea motion uring the session. Stimuli were presente on a goggle system esigne by Magneti The Stroop task was programme using Mel V2.0 an Resonane Tehnologies. presente using an IBM-PC ompatible omputer. We mae use of an oball esign, in whih partiipants were 2.4. Image proessing presente a series of neutral wor trials, with an oball trial ourring every six to eight trials. Three types of Within-subjet statistial analyses were arrie out using

4 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) FEAT, the FMRIB Easy Analysis Tool ( types iniviually relative to baseline neutrals an onwww.fmrib.ox.a.uk/ fsl). The first five volumes of eah ute a onjuntion analysis. More speifially, for eah run s time series were isare to allow the MR signal to region (luster) ientifie as ative, we alulate the mean reah steay state. Prior to statistial tests, images were parameter estimate (aross voxels within the region) for motion orrete using MCFLIRT ([24]). The following eah oball trial type for eah partiipant. Then for eah pre-statistial proessing was applie: (1) spatial smooth- region, we teste for the presene of inreases in neural ing using a Gaussian kernel of FWHM 8 mm; (2) mean- ativity relative to neutral trials for the oball trial type base intensity normalization of all volumes by the same iniviually at a level of P,0.05. In orer to pass the fator; (3) non-linear high-pass temporal filtering (Gaus- onjuntion test, a region ha to pass for all three sian-weighte LSF straight line fitting, with sigma535.0s); onitions, proviing a onjuntion probability of P, an (4) Gaussian low-pass temporal filtering (HWHM For regions in whih multiple peaks were 2.8s.) ientifie, rather than testing the region as whole, we arrie out a onjuntion test at eah peak, using the mean 2.5. Statistial analyses signal for a sphere efine aroun eah peak (iameter5 five voxels). The same approah was employe to verify Statistial analysis was arrie out for eah partiipant that inreases in neural ativity assoiate with onfliting using FILM (FMRIB s Improve Linear Moel) with loal task-relevant olor information (inongruent-ineligible an autoorrelation orretion ([24]). Event-relate responses inongruent-eligible.neutral) were not riven by a single were moele using the SPM 99 hemoynami response. trial type. In our moel, we inlue three preitors: (1) FRE- QUENCY OF OCCURENCE, responses assoiate with all oball trials relative to the baseline neutral trials; (2) 3. Results STROOP INTERFERENCE TRIALS, responses assoiate only with inongruent trial types (greater for both 3.1. Regions sensitive to the frequeny of irrelevant inongruent trial types relative to oball neutral trials); information an (3) RESPONSE CONFLICT DURING STROOP INTERFERENCE TRIALS, responses speifi to inon- The first goal of our analysis was to ientify regions gruent-eligible trial types (inongruent-eligible. sensitive to the frequeny with whih task-irrelevant inongruent-ineligible). Temporal erivatives for eah materials were presente (see Fig. 1A an Table 1). preitor were inlue in the moel. Regression analyses reveale inreases in ativity through- Parameter estimate (PE) maps for eah partiipant for out left orsolateral prefrontal ortex (primarily posterior) eah preitor were transforme into a ommon stereotaxi an posterior inferior prefrontal ortex ommon to all three spae [47] using MeX 3.4 (lanmark-base image regis- oball trial types (neutral, inongruent-ineligible, inontration, polynomial transformation, trilinear interpolation) gruent-eligible) relative to baseline neutrals. Our onfir- ( mex.sensor.om). The parameter estimates for matory onjuntion analysis verifie that these inreases in eah preitor were then entere into a ranom effets ativity relative to baseline neutral trials are present for all group analysis, in whih we teste if the parameter three oball trial types when ompare to baseline estimates aross all partiipants were signifiantly greater neutrals, although as an be seen in Fig. 1B not neessarily than 0. We use a signifiane level of P,0.001 to to an equal egree. Of most importane is the fining that threshol our statistial maps. In orer to protet against the oball neutral trials yiele signifiantly greater false positives ue to multiple omparisons, a ontiguity ativity in these regions than i baseline neutral trials, threshol (minimum spatial extent) of 10 voxels was iniating that ativity in these regions reflets the greater employe [16]. In orer to ifferentiate between regions ability of infrequently ourring (i.e. novel) task-irrelevant within an ative luster, we use a peak etetion algo- information to ompete with task-relevant information for rithm to ientify peak ativations within a luster [34]. For priority in proessing an thereby inrease the nee for eah luster, we report the enter of intensity an peak top-own ontrol. ativation(s). The regions ientifie in this analysis are the same regions impliate in implementing attentional ontrol 2.6. Confirmatory analysis aross a variety of other manipulations that inrease the nee for top-own ontrol [2,22,29,31]. Thus, the present In orer to onfirm that the inreases in neural ativity finings support the iea that these regions are responing ientifie when ontrasting oball trial types (neutral, to the ability of task-irrelevant information to ompete inongruent-ineligible, inongruent-eligible) with baseline with task-relevant information, regarless of the speifi neutral trials reflete inreases in ativity for all three manipulations employe to vary this ability (e.g. infrequent oball trial types (as oppose to only inongruent trial events, onflit). types, for example), we examine eah of the oball trial Consistent with prior stuies emonstrating the exist-

5 216 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) Fig. 1. Regions sensitive to infrequently ourring task-irrelevant information. (A) All three oball trial types proue inreases in neural ativity within left prefrontal ortex (enter of intensity: x5240; y518; z524; see Table 1), inluing posterior orsolateral prefrontal ortex (BA 6/ 9) an posterior inferior prefrontal ortex (BA 44). (B) Results of the onfirmatory analysis iniating that the parameter estimates for all three types of oball trials (relative to baseline neutral trials) were signifiant. ene of a fronto-parietal iruit unerlying attentional within the left AIPS (r50.78, P,4310 ) an right AIPS ontrol, this ontrast also reveale inreases in parietal 23 (r50.66, P,6310 ). However, some aution nees to be ativity in response to infrequently ourring task-irrele- taken with respet to these finings onerning parietal vant information. More speifially, we note bilateral regions, as they faile to pass our onfirmatory onjuntion ativity within the anterior inferior parietal sulus (AIPS) analysis ue to variability in the response to oball an nearby regions of the inferior parietal lobule. These neutral trials aross iniviuals. regions are highly similar to those reporte in stuies of Finally, inreases in ativity were note in the superior attentional funtion employing other paraigms an ma- frontal gyrus (SFG), ajaent to anterior ingulate ortex. nipulations (e.g. [18,31,52]). Reent work has suggeste This ativation is similar to that note by Hopfinger et al. that these ativations in parietal ortex may reflet gene- [22] when using a ueing paraigm to ientify regions ration of a top-own signal that moulates proessing in involve in top-own ontrol. Thus, onsistent with prior extrastriate regions (see [14] for a review). Given its suggestions, the meial wall oes appear to be involve in onnetivity with posterior DLPFC [42], it is possible that attentional ontrol to some egree, but primarily the PFC may rely upon its interations with parietal ortex (at superior frontal gyrus, not the ingulate region as postleast to some egree) in orer to implement top-own ulate by some moels of attention [45]. influenes. Consistent with this notion, aross partiipants, In sum, the presentation of infrequently ourring taskleft PFC ativity was signifiantly orrelate with ativity irrelevant information proue inreases in ativity 24 Table 1 Regions sensitive to the frequeny of irrelevant information (oball trials) a b b Regions BA Cluster size x y z Max sig Mean sig L. mile oipital gyrus L. inferior oipital gyrus L. mile oipital gyrus 18/ L. orbital gyrus R. mile oipital gyrus R. mile oipital gyrus 18/ R. mile temporal gyrus L. superior frontal gyrus L. inferior frontal gyrus L. inferior frontal gyrus L. mile frontal gyrus 6/ L. mile frontal gyrus R. inferior frontal gyrus R. meial frontal gyrus L. inferior parietal lobule R. inferior parietal lobule a x, y, z oorinates speify the Talairah oorinates for eah luster s enter of intensity. b Signifiane levels reporte in terms of 2log(probability). Clusters in whih more than one peak ativation was etete [34]. Passe onfirmatory analysis.

6 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) Table 2 Regions sensitive to Stroop interferene regarless of the level at whih onflit ours (inongruent-eligible an inongruent-ineligible.neutral) a b b Region BA Cluster size x y z Max sig Mean sig L. inferior frontal gyrus R. inferior frontal gyrus L. mile frontal gyrus 46/ R. mile frontal gyrus 46/ R. mile frontal gyrus R. mile frontal gyrus 46/ R. pre-entral gyrus R. ingulate gyrus (ACC) R. inferior frontal gyrus L. preuneus ortex R. inferior parietal lobule L. superior parietal lobule R. superior parietal lobule R. preuneus ortex L. uneus gyrus L. lingual gyrus a x, y, z oorinates speify the talairah oorinates for eah luster s enter of intensity. b Signifiane levels reporte in terms of 2log(probability). Clusters in whih more than one peak ativation was etete [21]. Passe onfirmatory analysis. throughout a istribute network of regions, inluing leaing to a more bilateral pattern of ativation. Our DLPFC, posterior inferior frontal ortex, IFC, the anterior onfirmatory analyses verifie these inreases in neural interparietal sulus, superior parietal lobule an superior ativity. In one of our prior stuies of the Stroop task, we frontal ortex, regarless of whether or not this infor- foun that suh inreases in PIPFC ativity were also mation onflite with the task-relevant information. assoiate with ongruent trials [31]. In tanem, these finings suggest that PIPFC is responing to the fat that 3.2. Regions sensitive to Stroop interferene regarless the task-irrelevant information is relate to the task onof the levels at whih onflit an our (inongruent- ept (i.e. the task-irrelevant information is relate to olor eligible an inongruent-ineligible.oball neutral) when the task is olor ientifiation) [31]. Although the posterior DLPFC was ativate by all The introution of onfliting olor information by an oball trials (with inongruent-eligible trials prouing inongruent olor-wor proue further inreases in PFC the largest ativations), only inongruent oball trials ativity, regarless of whether or not the wor name a proue signifiant inreases in ativity within mipossible response (Table 2). Not only i both inongruent DLPFC (BA 46/ 9) (See Fig. 2A). Given moels sugtrial types proue more wiesprea ativity within L. gesting that mi-dlpfc is involve in monitoring an PIPFC (relative to oball neutral trials), but they also manipulating information in working memory [42], this proue inreases in ativity within R. PIPFC as well, ativity probably reflets the nee to selet between olor- Fig. 2. Regions sensitive to stroop interferene. (A) Both inongruent trial types proue inreases in neural ativity within mi-dlpfc (BA 46/ 9) bilaterally [enters of intensity: (L): x5246; y540; z524; (R): x548; y538; z524, see Table 2]. (B) Results of the onfirmatory analysis iniating that only the parameters estimates for inongruent trials (both eligible an ineligible) were signifiant relative to neutral baseline trials. Thus these regions only showe inreases in neural ativity for inongruent trial types (relative to baseline).

7 218 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) representations in working memory that are task-relevant tasks as well [4,31,32,52], often in tanem with the AIPS. (i.e. those relate to ink olor) an task-irrelevant (i.e. Finally, inreases in ativity were also note with the those relate to the olor wor). Supporting suh a lateral inferior parietal lobule, a region that has been onlusion, the inreases in mi-dlpfc ativity were impliate in non-spatial attention [52]. Overall, these ata aompanie by inreases in ativity within preuneus suggest that as the nee for attentional resoures inreases, ortex (aross subjets, L. DLPFC an preuneus were lateral inferior parietal an superior parietal are reruite in signifiantly orrelate: r50.59, P,0.02), a region whih aition to AIPS. has reently been emonstrate to be more sensitive to working memory than attentional emans [27]. Confir Regions sensitive to the ourrene of response matory analyses iniate that ativity was signifiantly onflit uring Stroop interferene trials (inongruentinrease in both response-eligible an response-ineligible eligible.inongruent-ineligible) trials in mi-dlfpc relative to oball neutral trials. Fig. 2B shows the parameter estimates for eah of the oball Consistent with prior stuies of response onflit an/ or trial types relative to baseline neutral trials. Notie that inhibition (e.g. [18,32]), the ourrene of onflit speativation in this region is speifi to inongruent trial ifially at the level of response (inongruent-eligible. types, as there was no inrease in neural ativity for inongruent-ineligible) proue inrease ativity oball neutral trials relative to baseline neutral trials (i.e. throughout a istribute network of strutures (see Table the parameter estimates for oball neutral trials were not 3). Relative to inongruent-ineligible trials, inongruentsignifiantly greater than zero). eligible trials proue aitional ativations bilaterally The introution of onfliting task-irrelevant olor throughout prefrontal ortex. Inreases were observe in information proue inreases in ativity within regions anterior inferior prefrontal ortex, onsistent with prior of parietal ortex as well. More speifially, inreases in stuies of onflit resolution at the level of response (e.g. ativity were note bilaterally within the superior parietal [18,48]). Inreases in ativity were also note throughout lobule. While early stuies of attentional ontrol note DLPFC (posterior, mile, anterior) an PIPFC bilaterally, ativity within the superior parietal lobule for tasks most likely refleting the inrease attentional an working involving spatial attention (e.g. [13]), several stuies have memory emans assoiate with response onflit. Furreporte ativity in superior parietal lobule for non-spatial ther reruitment of ativity was note within preuneus Table 3 Regions sensitive to the ourrene of response onflit uring Stroop interferene trials (inongruent-eligible.inongruent-ineligible) a b b Region BA Cluster size x y z Max sig Mean sig L. mile frontal gyrus L. inferior frontal gyrus 44/ L. mile frontal gyrus L. mile frontal gyrus L. inferior frontal gyrus R. inferior frontal gyrus 45/ R. mile frontal gyrus R. mile frontal gyrus R. inferior frontal gyrus R. inferior frontal gyrus R. ingulate gyrus 32/ R. ingulate gyrus L. ingulate gyrus 23/ R. ingulate gyrus 23/ L. ingulate gyrus R. preentral gyrus R. supramarginal gyrus 39/ R. supramarginal gyrus R. inferior parietal lobule R. inferior parietal lobule R. preuneus ortex L. superior parietal lobule 7/ Preuneus ortex L. inferior oipital gyrus a x, y, z oorinates speify the talairah oorinates for eah luster s enter of intensity. b Signifiane levels reporte in terms of 2log(probability). Clusters in whih more than one peak ativation was etete [34].

8 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) ortex as well, suggesting an even greater inrease in inreases of ativity in posterior-dlpfc an posterior working memory emans assoiate with the ativation IPFC. of ompeting response-relate representations. Although these regions of the fronto-parietal attentional The presene of onflit at the level of response network exhibite inreases in ativity anytime the ability proue extensive patterns of ativity throughout anterior of task-irrelevant information to ompete for priority in ingulate ortex (ACC), a fining onsistent with prior proessing was inrease (i.e. ue to frequeny of ourstuies of the Stroop task [2 4,9,10,20,28,29,31,32,35,49] rene or onflit at response an non-response levels), as well as various paraigms fousing on resolution of some subregions were only sensitive to onflit. More response onflit an/ or ompetition (e.g. [37,48]). While speifially, mi-dlpfc an anterior inferior prefrontal ativations within anterior ingulate ortex were most ortex only exhibite inreases in ativity when the wor prominent, inreases in ativity were also note within introue olor information that onflite with taskmi- an posterior ingulate ortex. relevant information (i.e. as on inongruent trials). These inreases were largest when onflit oul our at both 3.4. Behavioral results response an non-response levels. In aition, right-hemisphere involvement was only note uring inongruent Four partiipants were exlue from the analysis of trials. We posit that the ativity within these regions behavioral measures ue to tehnial iffiulties with the reflets the inrease attentional emans assoiate with response evie. Mean reation times for all oball trial inongruent trials, not neessarily the ourrene of ontypes were signifiantly longer than those for baseline flit. This onlusion is base on the results of the present 26 neutrals (P,6310 ), iniating that infrequently our- stuy in onjuntion with our prior finings [31]. The ring events le to more potent interferene. We also foun regions that showe inrease ativity in response to eviene of a Stroop interferene effet, as the mean inongruent trials in the urrent stuy were not iential to reation time to inongruent oball trial types were those that were speifi to onflit in our prior stuy in signifiantly longer than for neutral oball trials (P,23 whih we ompare ativation in inongruent as ompare ). Finally response-relate onflit inreases interfer- to ongruent an neutral trials. ene as mean reation time for inongruent-eligible trials Of note, the ifferential ativation of anterior an was longer than to inongruent-ineligible trials (P,23 posterior inferior prefrontal orties observe in the pres ). Overall, these behavioral finings are onsistent ent work is onsistent with a framework reently propose with those of prior stuies employing similar trial types by Wagner et al. [50] as part of effort to unerstan PFC s [30], an iniate that we were able to ifferentiate involvement in working memory an exeutive ontrol. inrease attentional emans on the basis of frequeny, More speifially, they propose that AIPFC is speifially Stroop interferene, an whether onflit was present at involve in onitions requiring ontrolle semanti retriboth response- an non-response relate levels or just at eval (e.g. retrieval of weakly assoiate information as non-response levels. oppose to strongly assoiate information), whereas PIPFC is involve in retrieval an seletion proesses for a variety of stimulus types (e.g. phonologial, non- 4. Disussion phonologial, early semanti). Consistent with their proposal, we foun that PIPFC was sensitive to ompeting Our finings iniate that neural ativity within DLPFC task-irrelevant information regarless of its semanti rean PIPFC, two regions previously impliate in top-own lateness to task relevant information (i.e. oball neutral, ontrol, is influene by three fators: the frequeny with inongruent-ineligible, inongruent eligible), while AIPFC whih task-irrelevant information is presente, the ability was uniquely responsive to trial types on whih semanti of task-irrelevant information to introue onfliting task- onflit oul our (i.e. inongruent-ineligible, inonirrelevant information, an the number of levels at whih gruent-eligible). suh onflit or interferene an our. As suh, our results Overall, the ata provie eviene that multiple regions are onsistent with the hypothesis that top-own ontrol by of PFC (i.e. posterior orsolateral an posterior inferior PFC inreases in response to the ability of task-irrelevant PFC) were sensitive to both low frequeny of ourrene information to ompete for priority in proessing. In- an the onflit/ interferene engenere by inongruent reases in ativity assoiate with infrequently presente trials. Of note, some PFC sub-regions (i.e. mi-dlpfc) task-irrelevant information were note within posterior- were primarily sensitive to the onflit/ interferene en- DLPFC an posterior-ipfc, two regions that we have genere by inongruent trials. We posit that this pattern of previously argue are involve in moulating ativity ativation provies important information about the nature within posterior proessing regions to ensure task-appro- of mehanisms of attentional ontrol in the PFC. In priate performane [2 4,32]. The introution of task- partiular, as isusse next, we posit that mi-dlpfc an irrelevant information that onflits with task-relevant posterior-dlpfc are likely to play ifferent roles in information uring inongruent trials proue further attentional ontrol.

9 220 M.P. Milham et al. / Cognitive Brain Researh 17 (2003) Differentiating mi- an posterior-dlpfc within the ventral (non-spatial) visual proessing stream. Given that both proessing streams have been note to be We along with others have posite that PFC works to involve in olor [6] an wor proessing [26], it is not maintain proper task performane through two mehanisms surprising to fin both regions ative for the olor-wor (e.g. [31,50]): (1) the moulation of proessing in posterior Stroop task. In fat, aross our stuies of the olor-wor proessing systems (i.e. amplifying neural ativity within Stroop task, these two prefrontal regions show highly task-relevant proessing systems); an (2) the biasing of similar patterns of ativity. seletion proesses within working memory. Though speulative, we posit that posterior DLPFC is primarily 4.2. ACC an response onflit responsible for the moulation of proessing in posterior pereptual proessing systems, as it is a region that is Consistent with the finings of our prior stuy [32] repeately ativate in stuies of attentional ontrol an is ativity within the ACC is greatly inrease for inonmore highly interonnete with posterior visual proess- gruent response-eligible trials ompare to response-ineliging regions than other portions of DLPFC ible trials. Suh finings one again impliate the ACC as ([5,43,44,23,1,46]; see [42] for a review). Consistent with playing a major role in response-relate proesses. this interpretation, all infrequent trial types, regarless of Although the finings of our prior stuy suggeste that whether or not they ontaine onfliting information, inreases in ACC ativity are speifi to inongruentproue inreases in ativity within posterior DLPFC. eligible trial types, our primary an onfirmatory analyses Furthermore, ativity within these regions inrease with etete a region of ACC (towars BA 9) that exhibite attentional emans (i.e. baseline neutral,oball inreases in ativity for both inongruent trial types neutral,inongruent-ineligible,inongruent-eligible). (inongruent-ineligible an inongruent-eligible), though In ontrast, we posit that mi-dlpfc is responsible for far greater for inongruent-eligible trial types (see Tables 2 biasing seletion of task-relevant representations within an 3). A stuy by Braver et al. [8] may provie an working memory. Suh a suggestion is onsistent with explanation for these latter finings. They foun that the animal an neuroimaging stuies alike, that have shown ACC s ativity is sensitive to the frequeny of an event, that mi-dlpfc is responsible for monitoring an man- exhibiting greater ativity to infrequent events. While the ipulating representations in working memory ([39 41]; see ratio of inongruent trials (eligible or ineligible) to neutral [42] for a review). Furthermore, this region laks the iret trials in our prior stuy was 50:50, the ratio of inongruent onnetivity with major portions of extrastriate an parietal trials (eligible or ineligible) to neutral trials in the present regions, making a iret role in the moulation of neural stuy was 2:21. This relatively low frequeny of ourativity within posterior pereptual proessing regions less rene appears to have amplifie ativity in the ACC. In likely. Our ata are onsistent with suh an iea, as only fat, even the ativity speifially assoiate with inoninongruent oballs, not neutral oballs, evoke signifi- gruent-eligible trials (inongruent-eligible.inongruentant inreases in ativity within mi-dlpfc. Inongruent ineligible) is far greater in the present stuy than our prior olor wors are more apable of taxing working memory s stuy (see Table 3 an [32]). As suh, the resiual ACC resoures beause they ativate representations of two ativity assoiate with an inongruent-ineligible trial may olors, the wor s ink olor an the olor name by the have been amplifie in the present stuy, making it more wor. In ontrast, neutral wors only ativate one repre- etetable. Hene, the present stuy s finings are ompatsentation of olor, namely that of the ink olor. Further- ible with our prior stuy s, as the ACC appears to be muh more, our prior work reveale inreases in ativation of more sensitive to ourrene of onflit at the response mi-dlpfc for ongruent olor wors relative to neutral level than at the non-response level. However, uner wors, as they also ontain two soures of olor in- speifi onitions, a sub-region of the ACC was apable formation, one in the ink olor an one in the wor [31]. of showing some egree of sensitivity to onflit at non- Wagner et al. [50] mae a proposal similar to the one response levels. suggeste here, though they attribute the role of mou- Our finings have impliations for the onflit monilating neural ativity within posterior proessing regions to toring theory [11,55], one of the more popular theories PIPFC (referre to as VLPFC in their work) rather than regaring the funtion of the ACC ativity. The fining of posterior DLPFC. While the two proposals may at first inrease ativity within one subregion of the ACC for seems isharmonious, it is important to note that Petries both response-eligible an response-ineligible trials sug- [42] argue that posterior DLPFC is highly interonnete gests that it is possible that ACC monitors for the with the orsal (spatial) visual proessing stream, while ourrene of onflit at both response an non-response PIPFC (an inferior portions of VLPFC) is highly inter- levels of proessing. However, this ability was restrite to onnete with the ventral (non-spatial) visual proessing a speifi portion of the ingulate, towars BA 9. Furtherstream. Thus, posterior DLPFC may work to moulate more, this effet was only observe when the task-irreleneural ativity within the orsal (spatial) visual proessing vant information ourre infrequently, thereby inreasing stream, while PIPFC may work to moulate neural ativity its saliene. Given the results of our prior stuy an the

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