Surrounding environments in conjunction with the situations we encounter everyday require us to apply our cognition, plans, and goals to structure our actions through the top-down organization of our attention (Allport, 1993; Desimone and Duncan, 1995; Milner and Goodale, 2006; Fuster, 2009). In determining what is important during any given goal directed activity, we need to suppress erroneous actions that may arise due to the ambiguity of our surroundings (Schall, et al., 2002). When faced with competing demands contrary to habit with more than one option for behavior, further complex decision-making strategies and coordination of actions are required (Miller and Cohen, 2001; Behrens et al., 2007; Cisek et al., 2009). Previous literature on the prefrontal cortex (PFC) has found this region to be implicated in higher cognitive functions, including long term planning, response suppression, and response selection (Miller, 2000; Duncan, 2001; Miller and Cohen, 2001). This role is exerted when learned and expected stimulus associations that guide behavior are violated and inhibition of the prepared response is necessary to redirect attention (Nobre et al., 1999). A key function of the PFC is evident in situations when we need to draw on our internal representations of goals and use the most appropriate rules to achieve them (White and Wise, 1999; Asaad et al., 2000; Wallis et al., 2001; Everling and DeSouza, 2005). A classic task that demonstrates such situations in a laboratory setting is the Stroop (1935) task.

In the Stroop task, participants must name the color of ink in which a color word is written (Stroop, 1935; MacLeod, 1991). When the ink and the word refer to different colors, a conflict between what must be said and what is automatically read occurs. The participant must resolve the conflict between two competing processes: word reading and color naming. Our urge to read out a word leads to strong “stimulus–response (SR)” associations; hence inhibiting these strong SRs is difficult and prone to more errors. Such interference is due to the effortless nature of reading, and the less habitual behavior of color naming (MacLeod, 1991). When task irrelevant information such as the color word wins priority in processing by grabbing our attention, the PFC must exert top-down control in order to solve such biased processing of task contents (Milham et al., 2003). The medial frontal region has been implicated in control of voluntary actions, especially during tasks requiring a choice between competing responses (Schlag-Rey et al., 1997; Carter et al., 1999; Ito et al., 2003; Sumner et al., 2007). The anterior cingulate cortex (ACC) particularly, is involved in a large range of behavioral adjustments and cognitive controls namely detecting conflicts caused by competition of responses/behaviors, more broadly in processing cognitively demanding tasks (Carter et al., 1998; Kerns et al., 2004; Fellows and Farah, 2005). Imaging studies have confirmed increased activation within the PFC, particularly the ACC, which is proposed to be caused by the cognitive interference that results from simultaneous processing of two stimulus features with contrasting SR associations (Pardo et al., 1990; Bench et al., 1993; Vendrell et al., 1995; Bush et al., 1998; Barch et al., 2001; Zysset et al., 2001; Melcher and Gruber, 2009). Greater conflict-related activity within the ACC associated with the high-adjustment (i.e., incongruent) trials (MacDonald et al., 2000; Kerns et al., 2004) supports a role of this region in conflict monitoring and its engagement in cognitive control as stated by the conflict hypothesis (Carter et al., 1998; Botvinick et al., 2004). A host of imaging studies have reinforced the role of the orbitofrontal and inferior regions of the PFC in the regulation of emotion (Lévesque et al., 2004; Ohira et al., 2006). These two areas have been particularly implicated in suppression and reappraisal of negative emotional stimuli (Ochsner et al., 2004; Phan et al., 2005). The inferior frontal cortex has also been implicated in response conflict and inhibition as well as in selective attention processes (Kemmotsu et al., 2005). Studies on the go/no-go task (Menon et al., 2001; Rubia et al., 2001), the Eriksen flanker task (Bunge, 2002), and the Stroop task (Bush et al., 1998; Matthews et al., 2004; Kemmotsu et al., 2005), have found a role of the inferior frontal cortex on the no-go and incongruent conditions of these tasks. The activation of the inferior frontal region during the no-go trials and the incongruent conditions suggests the involvement of this area in restraining a more habitual response in favor of a more effortful one.

Based on our previous results on the anti-saccade and the Stroop task (DeSouza et al., 2003; Ovaysikia et al., 2008), we set out to further investigate the role of the medial PFC (mPFC) in an extended face processing network (Haxby et al., 1996; Ó Scalaidhe et al., 1997; Marinkovic et al., 2000). Using affective faces in an interference task (emotional Stroop) similar to the original Stroop-color/word, now introduces response conflict in conjunction with emotion of faces. This emotional Stroop task involves a picture–word interference (Rosinski et al., 1975; Beall and Herbert, 2008), in which descriptive words are superimposed over the pictures of individual faces. These words indicate the expression (happy, neutral, sad) that is either congruent or incongruent with the emotion depicted by the face in the picture (Figure 2A). By pairing word reading with a behavior such as recognition of face expressions, we will be able to see whether socially relevant stimuli (i.e., faces) alter the automatic processing of reading, as shown by the classic Stroop effect. We hypothesize that a conflict will occur because people's reading abilities interfere with their attempt to correctly identify face expressions, hence recruiting similar frontal regions as the classic Stroop (Pardo et al., 1990; Bench et al., 1993; Vendrell et al., 1995; Bush et al., 1998; Zysset et al., 2001; Botvinick et al., 2004; Melcher and Gruber, 2009).

We wanted to shift the participants’ attention to either the word or to the face by switching instructions to report the written word or the face expression across blocks or individual trials, in order to examine which conflicting paradigm is modulated by the instruction both in behavior and across the mPFC network. In a previous experiment (Ovaysikia et al., 2008), we examined the overlapping neural systems recruited during the classical Stroop-color/word task and the anti-saccade task (Hallett, 1978; DeSouza et al., 2003; Figure 1). At first glance, the Stroop task and the anti-saccade task seem to involve very diverse neural mechanisms, since one appears to be more of a cognitive task (i.e., the Stroop) whereas the other seems to require less cognition but more visual attention (i.e., the anti-saccade). We were interested in their common association in the inhibition of a highly automatic process. Our novel emotional Stroop paradigm would allow us to further examine this suppression network and its activity pattern in the mPFC.

When comparing the two behaviors of recognition of face expressions and reading, one which is assumed to be an innate ability (Meltzoff and Moore, 1983; Valenza et al., 1996; Farah et al., 2000; Sugita, 2009) and the other which is a learned behavior, it would be expected that the innate ability be processed more automatically in our brain. However, contrary to popular assumptions (Öhman, 2002; Dolan and Vuileumier, 2003), our results demonstrated the opposite. We found that attending to words is processed in a more automatic manner than attending to faces. The automatic nature of word reading which in our task integrated affective processing through emotional words, is concluded from the shorter response latencies and smaller number of errors yielded during the word instruction condition (Figures 3 and 4). This finding is in contrast to a study by Beall and Herbert (2008) that used affective faces and words in a similar modified version of the Stroop task. Beall and Herbert (2008) found that words produced larger interference effects than face expressions, suggesting that affective faces were processed more automatically. Discrepancies between our findings could be due to differences in stimuli presentation time and saliency, and also the instructions given to the participants. In the Beall and Herbert (2008) study, the face stimuli was presented for 1 s compared to the 250 ms in our study; the longer presentation time would allow for the face expressions to be processed more completely and hence reported at a faster rate. Their superimposed words were placed right on the nose of the face stimuli (very small font size) in a dark gray color, making the expressions easier to identify than ours that was placed diagonally in an attempt to cover as much of the face as possible. The color of the words were also not as salient as the red we used in our experiment; the more salient color could have caused reporting the word to be an easier task and hence leading to the faster RT on our word instruction. Additionally, in the Beall and Herbert (2008) study, participants were asked to respond by reporting whether the emotion is positive or negative whereas in ours they were simply reading the written words or reporting the particular face expression. The longer time during the word instruction in their study could also be due to the extra step that the participant has to undergo to identify each word or expression as either a positive or negative one.

Our brain selectively processes not only the more salient stimuli in the environment but also those that are most relevant to behavior. Whether a behavior is an effortless one such as word reading because of over-learning or is an instinctive one based on its importance for survival (i.e., recognition of emotions), our brain needs to prioritize these in the most efficient way possible. One way the brain achieves such efficient processing is through hierarchical organization (Felleman and van Essen, 1991; Badre and D'Esposito, 2009) of the processing sequence. By placing all the information competing for neural processing on a continuum of rankings, behaviors with greater importance (i.e., stronger SR associations) are processed in a faster manner. Another possible explanation could be that word reading is computed faster because more neural resources have been dedicated to its processing over the years, making our brain more efficient and hence faster at such coding. One could also argue that equal neural resources have been dedicated to expression processing and that a key difference may be one's awareness of doing so (Lamme, 2003). We are much more aware of what we are doing when word reading, whereas often, interpretation of expression is done “unconsciously” or without awareness. The complex nature of a face compared to a word could also be a contributing factor to the longer behavioral latencies observed during the processing of face expressions in our data.

We observed that the mPFC (in Figures 5B,E) was more activated during trials that required the suppression of a more automatic action in favor of a controlled, less automatic one, which was also observed in our previous findings for the anti-saccade task and the Stroop-color/word task (Ovaysikia et al., 2008). Our present results show that different regions of the mPFC are activated in each of the two instruction conditions (attend to the word or attend to the expression). Though both the expression instruction and the word instruction conditions entailed inhibitory responses, the distinct pattern of activation within the PFC areas suggests the involvement of a suppression mechanism specific to the type of inhibition at hand. Recognition of face expressions may be more difficult to suppress because of the socially relevant nature of faces to us humans. In our task, the familiarity of some of the faces could have also had an effect in distracting the participants as their attention would have been captured by the face due to the self-referential significance attached to it. The more difficult nature of expression recognition is behaviorally evident through the longer RTs and increased number of errors made on this condition compared to the word instruction condition (Figure 3). During the expression instruction condition, a region within the ACC was activated (Figure 5B), which could also be indicative of the greater complexity level of reporting face expressions in presence of incongruent emotional words. Overall, we make the assertion that the more conflicting nature of the suppression, the more anterior the mPFC activation will be.

The observed bilateral activation of the IFG during the attend to word and expression instructions is consistent with previous studies showing inferior frontal activity in response inhibition tasks (Konishi et al., 1998; Menon et al., 2001; Kemmotsu et al., 2005). This activation was a result of the contrast of incongruent trials compared to congruent trials, as has also been demonstrated in previous Stroop tasks (Matthews et al., 2004). Recognition of others’ face expressions is assumed to be an adaptive behavior with substantial social importance attached to the understanding of these expressions. The involvement of the IFG has been established in recognition and interpretation of others’ actions (Chong et al., 2008). Our present study showed the left IFG's sensitivity to face expressions as we observed significant BOLD signal differences between all three expressions of happy, neutral, and sad (Figure 6). Hence not only is there a suppression mechanism accounting for the higher signal intensity on the incongruent trials but the particular face expressions presented within those trials also have an effect on this region's activation. The emotion-specific activation of the left IFG has previously been demonstrated using disgust, fearful, and angry face expressions (Sprengelmeyer et al., 1998), which can now be extended to include happy, neutral, and sad expressions from our results. The linguistic processing of emotional information has also been shown to activate the right IFG (Rota et al., 2009), which in our task is evidently carried out in an automatic manner under both instruction conditions, but the BOLD intensities in this region were not significantly different between happy, neutral, and sad expressions. It is possible that by attending to the face expressions of the presented faces in the experiment, participants experienced those affective states by making inferences from the emotional faces. The presence of such experience is to be expected especially since half of the faces viewed by the participants in this study were of familiar individuals with whom they would have had personal acquaintances. The involvement of IFG in emotional empathy is evident through the lack of such ability in patients with lesions in this region (Shamay-Tsoory et al., 2009).

An interesting pattern of FEF activation was observed with lateralization to the right hemisphere during attend to the word instruction and to the left hemisphere during attend to the expression instruction. The FEF region has well-established connections with the ventral stream areas as well as area V4 which are involved in attentional processing (Schall et al., 1995; Moore and Fallah, 2001; Gregoriou et al., 2009). During the word instruction of the task, the participants are required to ignore the face in order to successfully read the word. Perhaps, it may be that the attentional suppression of affective faces is recruiting the right FEF, given that the right hemisphere is known to be dominant in the processing of face expressions (Natale et al., 1983; Hauser, 1993; Borod et al., 1998; Killgore and Yurgelun-Todd, 2007). The right hemisphere of the prefrontal regions is also implicated when conflict arises at the response level, whereas the left hemisphere is more involved in attentional control at non-response levels (Milham et al., 2001). In contrast, during the expression instruction of the task, participants need to successfully suppress their urge to attend to the superimposed word in order to report the face expression. Since reading is dominated by the left hemisphere (Perret, 1974) in our 10 right-handed participants, we propose that the left FEF is now only laterally recruited during this subset of the task as it requires the suppression of word reading. Our results cannot be explained using the hemispheric dominance in attentional control view (Milham et al., 2003), since our task design does not allow us to disambiguate between conflict at response levels and non-response levels; we are assuming the conflict is occurring just prior to the response level in both instruction sets.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.