Behavioral Data
The difference in the accuracy for stimuli presented subliminally and supraliminally was used to see whether subjects were unable to detect target stimuli in the subliminal condition. Non-detection is defined as the case where the stimulus is perceived by the subject no more than in 50% of the times (Liddell et al, 2004; Merikle et al., 2001). Our results show an accuracy of 45.17 % for facial expression discrimination in the subliminal trials. These results don’t deviate significantly from chance. Supraliminal elaboration produced a corrected recognition for each emotional category (87.27 %), clearly above chance. These results bring us to the conclusion that the presentation paradigms were effectively good. Reaction times support this thesis: stimuli presented subliminally have longer RT than supraliminally presented stimuli, which suggest a difference in the two conditions with a faster recognition in the supraliminal condition.
A closer look at emotional faces, confirms the hypothesis that happy faces are perceived with more accuracy than negative ones. In line with Hess, Blairy and Kleck (1997), results show that in the supraliminal condition there is bigger accuracy for happy faces than negatives ones (fearful and angry).
Consistent with accuracy results, the RT data shows the same tendency. Indeed, in the supraliminal level, RTs to happy faces are faster than those of angry and fearful faces.
A possible explanation to the phenomena comes from Leppänen and Hietanen (2004). They suggest that the advantage may simply result from low-level physical differences in the stimuli that make happy expressions visually more distinctive and, thus, more recognizable than other facial expressions. Compared to neutral faces, happy expressions may contain greater physical changes in the configuration of facial features than expressions of negative emotions. These considerations concerning the greater visibility of happy facial expression are supported by our data: there are non significant statistical differences between sad and neutral faces. The same is true with regard to angry and neutral faces. Less
confusion is found between happy and neutral faces (Johnston, Katsikitis & Carr, 2001 cited in Leppänen and Hietanen, 2004). Moreover, expressions of negative emotions may be more difficult to produce on command, which makes them more heterogeneous than happy faces. In our paradigm it is possible that the diversity between positive emotional face and others emotional faces made them more recognizable.
EEG results
The ERPs analyses show an effect of the emotion arousal still on the N170 component, coherently with Miyoshi and colleagues (2004) who suggest that the emotional processing of emotional expression starts early in the occipito-temporal brain regions.
More in details, shown by post-hoc comparison of the emotional effect on N170 component neutral expression and happy expressions are statistically different from angry and fearful ones. This could suggest that the emotional expression analysis could begin from a valence evaluation (positive versus negative).
Worthy of notice, is the difference between the subliminal and supraliminal condition (fig. 12, page 38 and fig. 13, page 39) for the N170 component. In fact, in the subliminal condition there is more distinction between different emotions than in the supraliminal condition (results statistically confirmed by the post-hoc comparison which shows a difference between neutral and emotional expressions). This suggests a distinct non-overlapping neural substrate brain processing for emotional expressions at an unconscious level still at the level of the n170 component.
This interesting effect is more evident in the right occipito-temporal ROI (as clearly shown in the fig. 11). The statistically significant difference in the three ROIs used for the analysis of this component show a right lateralization of this effect in both presentation conditions.
On N170, an effect of selective attention was also presented. The N170 is enhanced in the target condition, where the subject had to pay attention at the emotional expression recorded. This suggests that emotional faces tend to attract attention (as e.g. still suggested by Pourtois and colleagues, 2004, with fearful faces facilitating spatial attention).
A presentation effect is also observed at this latency, where stimuli, presented supraliminally increase the amplitude of this the component. This result is in contrast with the results of Pegna and colleagues (2008) who find an enhancement of the N170 for the subliminal condition. This could be because in our paradigm we only retained correct responses which should have influenced the attentional brain processing. Notwithstanding these different results, we arrived at the same conclusion as Pegna and colleagues (2008) that emotional expressions modulate this component also in absence of awareness.
In the selective attention paradigm and in the presentation paradigm both targetness and supraliminal presentation represent the condition where there is less automatism, which assumes a bigger implication of cognitive resources.
These two conditions suppose more attentional efforts in emotional expression recognition.
At the level of the P2 component, there is no effect of emotion on the component at the occipito-temporal brain region.
On the contrary, the two awaited effects (selective attention and the presentation) are present with an enhancement of the component for respectively, the non target stimuli and the subliminal presentation.
What is interesting and observable watching the wave difference between subliminal and supraliminal on the P2 component, is that in the supraliminal condition there is hardly any effect on P2. This means that the P2 component in response to emotional face expressions appears only in the subliminal condition.
Other components (e.g. N2) may overlap with the P2 to some extent, making it difficult to distinguish clearly between them, depending on the location of measurement. This could be an explanation of the absence of the P2 in the supraliminal condition. Moreover, topographic maps of the P2 components show a strong influence of frontal areas in the supraliminal condition compared to the
subliminal condition. This larger distribution could explain the weaker effect on occipito-temporal areas.
The interaction effect between the presentation and the lateralization show that the effect is centrally distributed and that this effect is enhanced in subliminal condition compared to the supraliminal condition. Contrary to the N170 component, the P2 component is enhanced if there are less attentional resources implicated in emotional expression recognition.
In the same way of the P2 component, in the P300 component we observed a difference over the parietal region for the presentation type and the selective attention condition but in this case are the supraliminal condition and the target condition enhance the component. These results are in line with Liddell et al.
(2004) findings which show an increasing of this component in supraliminal condition. In facts, this component is sensitive to create a conscious emotional experience. The P300 is enhanced if the subject has to guess the meaning of the stimulus and if there's a bigger cognitive workload, which is the case of the supraliminal condition and target condition where the participant has to guess the valence of the emotional face perceived.
Anyway, it seem evident that the information presented to a subject under subliminal conditions may be perceived and processed at a higher level, even if the subject is not aware of this information. The difference between the peaks in the condition could be evidence of the conscious discrimination in emotional detection. Results from this study show that emotional stimuli are effective both in capturing attention and in eliciting autonomic responses even in higher levels.
Topographic map
Regarding the different topographic maps activation between N170 and P2 component we can make the hypotheses that unconscious information is at first elaborated within the posterior sites, and that this preliminary unconscious comprehension attends for a successive (conscious) process, that requires more anterior activation of the scalp for attention (represented from the P2 component).
Adolphs (2003) brought evidence of prefrontal activation in conscious
representation of emotional facial expression.
This result could also be in line with the selective attention effect, which requires frontal brain activation. Liddell et al. (2006) in a fMRI experiment studying emotional expression show activation of the medial prefrontal cortex only for the supraliminal condition.
Source localization
The results presented by the source localization in response to emotional faces show coherence with the Haxby model of face recognition (2000). In fact, we observe an activation of the extended system in the temporal lobe (limbic lobe) and amygdala, which are supposed to be involved in the emotional processing of faces. As explained in the introduction, Morris (1996), Sprengelmeyer and colleagues (1998), Adolphs and colleagues (1994) and Ledoux (2000) brought evidences for amygdala activation in reposes to fearful expressions as an immediate response to danger. These results are confirmed by our data.
Liddell et al. (2006) show a greater activation of the right amygdala in responses relating to fear expressions for the subliminal condition and a greater activation of left amygdala in response to the same stimuli for the supraliminal condition. It could be more interesting to observe whether this effect is present also in our study with distinct source localization between subliminal and supraliminal condition.
In general, these findings provide direct evidence that the human amygdala is engaged in processing the emotional salience of faces, with a specificity of response to fearful facial expressions.
General Discussion
The conclusion that emotional facial expressions can be perceived without consciousness is not surprising given the importance of emotional information for human survival. Emotional faces are treated even without consciousness and outside the focus of attention (as found out by Vuilleumier et al., 2002).
The paradigms used in this study allow us to observe the difference between the emotional face processing in subliminal versus supraliminal condition. There is a tendency of the enhancement of N170 due to negative emotions. The awareness effect of emotional expression on N170 in both subliminal and supraliminal conditions provide further evidence that emotional face processing begins within 170 ms of visual presentation, consistent with the hypothesis of rapid non-conscious processing for these stimuli.
The source localization and N170 component are coherent in showing stronger enhancement for negative emotions, in line with literature and in contrast with behavioral results. This dissociation shows that behavioral planning and execution are not directly linked to the brain activation and to the pathways of emotional processing.
This study has show that awareness and selective attention strongly influence all the components analyzed. A point worth observing is the direction of this effect.
Supraliminal and target conditions point to an implication of attentional resources in emotional expression recognition. On the contrary in the subliminal and non target condition, attentional resources do not affect performance. The results of the three components analyzed show that components are enhanced from conditions the degree of cognitive resources involved in the task was higher (supraliminal and target condition) or lower (subliminal and non target conditions). This confirms the influence of attentional resources on emotional face brain processing, thus supporting the thesis presented in many studies that emotional expression tend to attract attention.
For the future development of this research, it would be interesting to observe the difference between sexes in emotional expression recognition. Many studies (e.g.
Thayer, J., 2000) show a female advantage in judging facial features.
Another issue worth considering is the incorrect behavioral responses. Moreover, it is worthwhile to examine the difference in ERPs correlates obtained from erroneous judgments of facial expressions.
Other components should also be better analyzed. For instance, one could examine the C1 to see whether there are still some difference in emotional
responses at this earlier level (as suggest by Proverbio, Del Zotto and Zani, 2010).
A frontal analysis of N2 (orienting and automatic aspects of face processing) could be interesting for the analysis of attentional effects.
For better spatial resolution it could be furthermore interesting to examine EEG and fMRI together.
The weakness of this study became apparent whilst the experiment was progressing. For example, having 16 variables could enable a better understanding of the interaction effects. However, this would result in greater difficulty observing every single variable in detail. Moreover, analyzing every single effect of every variable would lead to a loss in statistical power.
Another problem appeared when comparing correct responses only in subliminal and supraliminal condition. In fact, for the subliminal condition we had less retained trials compared to the supraliminal condition, which in terms of statistical power could have had an influence.
Moreover, considering neutral emotional expressions in the behavioral tasks as a positive one could have influenced results because ERP data showed that neutral emotions are processed differentially from emotional expressions. This could have confused the subject's recognition abilities and consequently affected the brain processing of this emotion.
Conclusions
Our study confirmed emotional faces as special kinds of stimuli. They influence our cognitive resources in both conscious and unconscious presentation in different ways and even at a behavioral level.
Moreover, evidences of a large and specific neural substrate for emotional expressions treatment have been found with a particularity for fearful faces in the amygdala activation as danger reaction (as supported by Ledoux, 2000).
Emotional face processing would reside in a particular combination of discrete processes, including physical, affective and cognitive analysis such as attention and awareness. On the other hand, the influence of facial expression on early variables (N170), suggest that there is some specific brain pathways for the treatment of emotional facial expressions, and that this is true even at in unconscious condition.
The influence of arousal effect on emotional expressions is complex, and more research is needed in order to understand these phenomena in greater detail.
Nevertheless, this study remains a topic with a high projection in psychophysiology.
Acknowledges
I thank Alan Pegna, who accepted me as student. Not only did he give me all the resources I needed, but also his open-minded personality which proved useful both during the discussions and the moments of hardship.
I thank Marzia Del Zotto, who shared with me her knowledge. She devoted her time giving me all the knowledge and tools I will need to think and behave like the researcher I want to become.
I thank Lore Legrand and all the Lenpsy Laboratory team for having always been so nice to answer my questions.
I thank my friends; their encouragements and their faith in my skills have been of great support. In particular: Alicia, Nora, Valentina, Masha, Margherita, Natalina, Vera, Giaime, Véronique and Julien.
I thank all the participants who gave their time to take part in my study.
Finally, I would like to thank my parents for the unconditional support they have given me throughout university and life. Their attitude has been the driving force behind my passion and motivation in pursuing my career.
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