<figure><img src="image_124.png" /></figure>

<figure><img src="image_125.png" /></figure>

<figure><img src="image_126.png" /><figcaption>**Fig. 4.** EEG group results (n = 16) according to the target modality and the congruency. (a) ERPs for the auditory target (C-A: congruent auditory target; I-A: incongruent auditory target) and the visual target (C-V: congruent visual target; I-V: incongruent visual target). The black arrow designates the N270 peak. (b) Scalp maps and difference maps of the N270 peaks (t = 269 ms post-stimulus onset) for the 2 modality targets and congruency conditions. (c) Mean N270 amplitudes (time window [230–320] ms) for the central electrodes (10–20 system) from anterior (left) to posterior (right) sites. A: auditory target, V: visual target; C: congruent condition, I: incongruent condition.</figcaption></figure>

activation in a large region of the right prefrontal cortex (PFC) for the incongruent condition. This region extended from the inferior to the superior frontal gyrus (IFG, SFG), including the DLPFC. (see Table 2). Similarly, we found a greater activation of a large part of the right PFC for the modality-switch session compared to the no-switch session. Although this area was more superior then the conflict-related one, the right DLPFC was also included. Finally, the conjunction map between conflict and modality-switch effects showed that only the rDLPFC was activated during both processes (Fig. 6).

**4. Discussion**

The goal of this study was to evaluate the responsiveness of the N270 amplitude to (a) the conflict strength in a spatial audiovisual conflict and (b) an additional working memory effort induced in a switching target-modality task. To do so, we designed a simultaneous audiovisual target and distractor task and created two types of conflict (visual target with auditory distractor and auditory target

with visual distractor) that needed an easy target-modality maintenance in a first place. In a second part of the experiment – using the same set of stimuli – we added distinct auditory stimuli that indicated the participants to switch from one target modality to the other. Hence, an increase in the task difficulty was provoked by the frequent switch which led to an additional effort to maintain the target modality in working memory among two alternating ones. The findings of this study provide a first evidence that the N270 amplitude may by modulated by the strength of the conflict (modality-dependent in this case) and that the supramodal conflict processing and the active maintenance in working memory can be both evaluated through the N270 amplitude modulations within the right DLPFC, but do not interact with each other.

**4.1. Spatial conflict**

Behavioral conflict costs have been observed for both visual and auditory target conditions. Moreover, the visual dominance in a spatial context led to a larger behavioral conflict cost when
<figure><img src="image_127.png" /><figcaption>**Fig. 5.** ERP group average (n = 16) results according to the modality-switch and the congruency for the auditory and the visual targets. C-A: congruent auditory target; I-A: incongruent auditory target; C-V: congruent visual target; I-V: incongruent visual target. No switch: no modality-switch blocs of the first session. Switch: modality-switch of the second session. The black arrow designates the conflict-related N270 peak.</figcaption></figure>

**Table 2**

Brain regions that showed significant differential activation between conditions as a function of congruency and modality-switch at the individual N270 peak latency.

<table><thead><tr><th>Anatomical region</th><th>BA</th><th>MNI Coordinates</th><th>Log-F-ratio</th></tr></thead><tbody><tr><td colspan="4"><em>Congruency : I > C</em></td></tr><tr><td rowspan="4">R Middle Frontal Gyrus</td><td>10</td><td>(45, 50, 15)</td><td>0.97</td></tr><tr><td>10</td><td>(45, 45, 15)</td><td>0.96</td></tr><tr><td>10</td><td>(40, 45, 15)</td><td>0.95</td></tr><tr><td>46</td><td>(45, 45, 20)</td><td>0.92</td></tr><tr><td>R Superior Frontal Gyrus</td><td>8</td><td>(30, 40, 45)</td><td>0.92</td></tr><tr><td>R Middle Frontal Gyrus</td><td>9/46</td><td>(45, 40, 25)</td><td>0.91</td></tr><tr><td>R Inferior Frontal Gyrus</td><td>45/46</td><td>(35, 35, 15)</td><td>0.91</td></tr><tr><td colspan="4"><em>Modality - switch : Sw > NoSw</em></td></tr><tr><td>R Superior Frontal Gyrus</td><td>9</td><td>(45, 35, 35)</td><td>0.98</td></tr><tr><td>R Anterior cingulate</td><td>32</td><td>(10, 45, 10)</td><td>0.97</td></tr><tr><td rowspan="2">R Medial Frontal Gyrus</td><td>10</td><td>(10, 50, 10)</td><td>0.96</td></tr><tr><td>10</td><td>(10, 50, 15)</td><td>0.93</td></tr><tr><td>L Inferior Frontal Gyrus</td><td>45</td><td>(-55, 30, 5)</td><td>0.91</td></tr><tr><td>L Superior Frontal Gyrus</td><td>9</td><td>(-20, 40, 40)</td><td>0.91</td></tr></tbody></table>

R: right hemisphere; L: left hemisphere; I: Incongruent; C: Congruent; Sw: Switch; NoSw: No-Switch. BA: Brodmann Area; MNI: Montreal Neurological Institute coordinates. Corrected *p*<0.05.

participants had to inhibit an incongruent visual distractor while detecting the auditory target position. Similarly, we found that both auditory and visual target tasks induced significant modulations of the N270 amplitude over a large fronto-central area of the scalp. Whatever was the target modality, the spatial incongruity created by the crossmodal distractor led to significant larger N270 amplitude compared to the congruent conditions. These results are in accordance with all SDJ task results that have shown an increase of the N270 amplitude within the mismatch condition compared to the match condition. They add, furthermore, interesting complementary information to the N270 literature. Yang and Wang [36] found that the spatial discrepancy of two stimuli presented in a sequential order elicited a larger N270 than the spatial match. They hence concluded that the N270 should correspond to the brain activity for processing spatial discrepancy. Zhang et al. [37], however, pointed out the fact that sequential stimuli presentation in SDJ tasks involves comparison processes between a cued stimulus attribute and a subsequent one. Thus, N270 modulations in SDJ tasks could result from a “template mismatch” [13] between the second stimulus and the mental template created from the first
<figure><img src="image_128.png" /><figcaption>**Fig. 6.** Graphical representation of the N270 current density for the congruency effect (a) and the modality-switch effect (b) from sLORETA. Coordinates in MNI space in mm. Corrected *p* < 0.05. (c) Conjunction map of the congruency effect (red) and the modality-switch effect (blue) plotted with micron; intersection MNI coordinates: 45, 40, 30 (purple); overlay threshold > 0.907; corrected *p* < 0.05.</figcaption></figure>

stimulus. In our task the conflict arose from audiovisual spatial incongruity – left hand side *versus* right hand side – of simultaneous target and distractor presentations. Thereby, both stimuli were processed and compared at the same time inducing a competition between the target and the distractor. We hence confirm Yang and Wang [36] and Zhang et al. [37] results by giving additional evidence that the N270 could be the cortical electrophysiological signal of the supramodal conflict processing in absence of template representation.

## 4.2. Target modality and conflict strength

More interestingly, the reaction times confirmed that it was more difficult to inhibit a visual distractor when focusing on spatial attributes of an auditory target than the converse in our paradigm. This difference in behavioral effects supports previous findings of a visual dominance in spatial target detection [7,14]. This mandatory result allowed us to dispose of two conflicting tasks that solely differed in terms of conflict strength. At the electrophysiological

level, we found that the crossmodal spatial incongruity not simply evoked a N270 but led to a greater – and more largely distributed over the scalp – N270 effect for the auditory target than for the visual one. This was due to a larger N270 amplitude for the auditory incongruent condition compared to the visual incongruent one and no difference when comparing the congruent ones. According to the present results, we now argue that the behavioral cost is quantifiable at the electrophysiological level through the conflict-related N270 modulations: the higher the conflict, the larger the conflict-related N270 amplitude. We thus bring clues that the N270 could be a qualitative (conflict occurrence) and quantitative (sensitive to the conflict strength) marker of the conflict processing.

## 4.3. Target-modality maintenance and working memory

The second motivation of this study was to evaluate the impact over the conflict-related N270 amplitude of the active maintenance in working memory. In all the aforementioned N270 studies participants had to give their answer according to a relevant stimulus
dimension that remained the same within a session. In the second session of the present study, we added a distinct auditory switch signal indicating the participants that they had to change the target modality for their response (randomly every 4–10 trials). While waiting for this signal they had to retain in working memory the current target modality among two possible for correct responses – compared to solely one modality in the first session – and to update it from the switch signal. We found that increasing the working memory effort in the task led to a general increase of reaction times in the detection of the visual and the auditory targets, confirming the need of an additional cognitive effort for correct responses. This working memory effort also led to a significant global effect over the N270 component corresponding to a general increase of its amplitude. Based on these observations, we hence argue that the active maintenance of the target modality in the modality-switch session compared to the no-switch session has led to a supplementary cognitive effort observable at both the behavioral (i.e., global increase of reaction times) and the electrophysiological levels – in terms of an increase of the N270 amplitude in all conditions. This suggests that conflict processing and working memory share common resources. An important nuance however, is that, contrary to our initial hypotheses, the increase of difficulty in active maintenance of the target modality affected equivalently interference (incongruent trials) and facilitation (congruent trials) processes at both behavioral and electrophysiological levels. Thereby, conflict processing and active working memory in our task did not interact with each other although they may share common neural subtract. This result may be interpreted in terms of a cumulative effect over the N270 amplitude, comparable to those observed for different ERP components in the literature [5,15,37].

## 4.4. N270 and DLPFC

As suggested by several studies [6,20,29] the right DLPFC is a key area for the inhibition of non-pertinent motor plans. A significant part of the literature however, showed that other parts of the prefrontal cortex (PFC) are also involved in motor output inhibition [9,12]. This apparent divergence may find a uniformity in the paper of Munakata et al. [21] where authors have shown that both the ACC/Pre-SMA, the rIFG and the rDLPFC may be involved in the inhibition of non-pertinent motor plans. They suggest that two distinct PFC neural mechanisms act for response “inhibition”. On the one hand, the rDLPFC would be in charge of an active maintenance of the target attributes in corresponding areas via excitatory neurons, hence indirectly inhibiting other attributes by subsequent lateral inhibition within these specific areas. On the other hand, the ACC/Pre-SMA and rIFG would be involved in a more direct inhibition mechanism via excitation of subthalamic nucleus (STN) which then provides global inhibition over the output of the basal ganglia. In accordance with Zhang et al. [38] hypotheses and the work of Munakata et al. [21], our source localization results showed that while the conflict-related N270 was generated by both the right IFG (BA 45/46) and the right DLPFC (middle frontal gyrus, BA9/46), the conjunction source localization for the conflict processing and working memory engagement showed only the rDLPFC as a common area for these processes. One plausible explanation is that selecting the relevant modality among audiovisual information by an active maintenance in working memory and selecting the relevant target modality by inhibiting the distractor in crossmodal presentation are both processes that belong to the executive attention function of the PFC [10,19].

## 4.5. Limits

In the present study we were not able to isolate a N200 component, which is however known to be a marker of the conflict

monitoring. This point perfectly illustrates the difficulty in some cases to identify overlapping components, particularly for temporarily close ones and when one of them has a larger amplitude. Thus it is likely that a N200 has been evoked but it could have been overlapped by the large amplitude of the N270. Further technical investigation to isolate the N200 component is needed and should help in understanding the link between the N200 and the N270. In addition, one could argue that the N270 component, when it has a fronto-central scalp distribution, actually corresponds to an anterior N200. For example, in their extensive N200 review, Folstein and Van Petten [13] cited a work achieved by Zhang et al. [39] to illustrate the impact of the perceptual matching and response conflict over the N200. Looking at the cited paper however, revealed that the conflicting situation induced a N270 whereas a N2b was present solely in the congruent processing condition. In addition, other studies [16,33] have made distinctions between the N200 and N270 showing that both components can be elicited successively within the same task. The temporal proximity of the N200 and the N270, along with previous fMRI studies showing ACC and DLPFC activations in mismatching tasks [8,28,38], is a supplementary argument in favor of a plausible cingulo-prefrontal network that would be involved in mismatching information processing [38]. Finally, the participants' mean age was 50 years old (i.e., middle age) for the purpose of future comparisons with brain damaged patients. Therefore, it is legitimate to put our results into the perspective of the group's age. Some studies have shown differences in conflict effects between young and middle-aged persons. For example Mager et al. [17] have found age effects over the level of performance that corresponded to a larger conflict effect for middle-aged than for young participants albeit present in both groups. At the electrophysiological level however, they did not find any difference between these two groups which is in favor of the results presented in this paper.

## 5. Conclusion

In the present study we analyzed ERPs and cortical current density to investigate the sensitivity of the N270 component to the conflict strength in an audiovisual conflict task and to the load in working memory involved in active maintenance of the target modality. Our findings suggest for the first time that the N270 amplitude is sensitive to the conflict strength of relevant attributes (i.e., the stronger the conflict, the larger the conflict-related N270 amplitude). In addition, increasing the load in active maintenance did not impact the conflict cost but led to systematic larger N270 amplitudes suggesting common neural substrate for motor inhibition and working memory as revealed by an increase in the right DLPFC current density at the time point of individual N270 peaks for both processes.

## Acknowledgements

The authors want to thank Blanchard, C for her help with data acquisition. This study was supported by the Midi-Pyrénées Regional Council grants 05006110, R07054BB, the AXA research fund and DGA-MRIS.

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<figure><img src="image_129.png" /></figure>