1.6.1 Behavioural data
The present study aimed to replicate the classical findings by Sackeim et al. (1978), suggesting that the left hemiface is more emotional expressive than the right hemiface.
Additionally, it was aimed to check whether intensity and efficiency of emotional expression can actually be dissociated, as stated by Indersmitten et al. (2003). To investigate these two aspects of emotion processing, two different tasks were implemented, one dealing with the intensity of emotional expression (intensity task) and the other one dealing with the efficiency of emotional expression (efficiency task).
Although thematically related to the above cited studies, the present study used a stimulus set and an experimental paradigm that was different from these previous studies, trying to overcome some their inherent methodological weaknesses.
Stimuli used for the present study consisted of a set of chimerics (either being composed of left hemifaces (LL) or right hemifaces (RR)) that were created from original pictures of faces. The original face stimuli were all displaying one of the basic emotions (anger, disgust, fear, happiness, sadness or surprise) in two different levels of intensity. There were faces in 100% intensity level (standard emotional expressive faces) and faces in 50% intensity level,
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which have been created by morphing a neutral face and an expressive face. Both tasks were basically using the same stimuli, with stimuli for the efficiency task just being a subset of the stimulus set for the intensity task. This reduction in the number of basic emotions being investigated in the efficiency task had to be made due to experimental constraints.
1.6.1.1 Intensity task
Regarding the intensity task, it was expected that the type of chimeric (either LL-composite or RR-LL-composite) would influence subjects’ choice when asked to pick the more expressive one from a set of corresponding LL- and RR-composites (stimulus pairs of a given trial differed only in terms of type of chimeric). Furthermore, there was also the hypothesis that the type of expression might as well have an influence on subjects’ choice referring to the question which one of a corresponding chimeric looked more intense. Finally, two different levels of stimulus intensity have been applied in order to check the possibility that perception of intensity of a given chimeric pair and choice respectively does also depend on the level of stimulus intensity.
Thus the intensity task focused on three variables, namely on the type of chimeric (left (LL) vs. right (RR) composites), on the type of expression (anger, disgust, fear, happiness, sadness, surprise) and on the level of intensity (100% vs. 50%). These variables might influence the perceived intensity of expressive faces and consequently serve as a hint to support any of the above outlined models of hemispheric contributions to the processing of facial expression.
1.6.1.2 Efficiency task
Regarding the efficiency task, it was aimed to test the hypothesis stated by Indersmitten et al. (2003) claiming that the efficiency of facial expression is better in the right than the left hemiface. Thus, it was assumed that performance of classification might be different for RR- vs. LL- composites. Apart from this expected difference in performance depending on the type of chimeric, there was also the possibility that the type of expression as well as the level of intensity might affect or modulate the efficiency with which stimuli are classified.
Investigating the assumption of a dissociation between intensity and efficiency of emotional expression, this task had been designed to complement the intensity task and hopefully offering further insight to the processing of facial expression.
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1.6.2 Electrophysiological data
Apart from studying behavioural effects of the perception of facial expressions, the present study also tried to identify neuronal correlates and timing characteristics of different expressions by recording event-related potentials (ERPs) for the classification task (efficiency task).
ERPs, which are time-locked changes in the electrical activity of the brain, vary with stimulus- and response- characteristics, and they are defined by typical components, such as a specific pattern of amplitude/polarity, latency and topography (see Rugg, 1995). Since the temporal resolution of ERPs is extremely high and therefore matching the time scale of emotional phenomena under investigation, it seems to be an optimal non-invasive electrophysiological method to make inferences about cortical processes underlying the perception of facial expressions. Even though, spatial resolution of the EEG is rather poor compared to alternative functional brain imaging methods, conclusions about local activation and topographic characteristics of ERP components can still be drawn, based on a limited level of spatial accuracy. Using a 32- channel set-up, the methodological drawback of EEG, namely limited spatial resolution, is still at an acceptable level, with the primary focus being on the temporal characteristics of emotion processing anyway.
By recording ERPs in addition to the collection of behavioural data, the present study aimed to identify neuronal correlates and timing characteristics towards different facial expressions, to compare the two different levels of intensity that have been used, and finally to compare brain responses towards different types of composites, consisting either of left or right hemifaces. Eventually, ERP data might also be a hint to solve the earlier described poser-perceiver paradox, since the additional electrophysiological data obtained from the efficiency task directly relates to the perceiver.
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