Topographic analysis

For further analysis, ANOVAs for the selected time segments were performed throughout for different regions of interest, being prefrontal/ lateral frontal (FP1, FP2, F7, F8, FT9, FT10, F9,

F10, F9’, F10’), frontal (F3, F4, Fz), central-parietal (C3, C4, P3, P4, Cz, Pz), temporal (T7, T8, P7, P8,

P9, P10, TP9, TP10) and occipital (O1, O2, Iz, PO1, PO2) with repeated measures on type of composite (LL- vs. RR-composite), type of expression (anger vs. fear vs. happiness vs.

sadness), level of intensity (100% vs. 50%) and the additional factor hemisphere (left vs.

right) for regions of interest that did not include a midline electrode (prefrontal/lateral frontal and temporal region of interest). Regions of interest were defined by pooling electrodes that roughly showed a similar pattern of ERP response, irrespective of experimental condition being investigated (see Fig.8).

Individual electrodes within the above described regions of interest were not included as a separate factor for the ANOVAs, since regions of interest were considered to be homogenous.

Only effects that turned out to be significant in the global analysis were further analysed by ANOVAs for these defined regions of interest that were kept consistent across all different experimental conditions. Where appropriate, epsilon corrections for heterogeneity of covariance with the Huynh- Feldt method (Huynh & Feldt, 1976) were performed throughout.

Interactions with factor type of expression that turned out significant within the defined regions of interest for a specific time segments were further analysed. Post-hoc comparisons using Tukey hsd were made to determine the significance of differences.

Figure 8. Regions of interest having been defined.

3 Results 32

110-140ms (P1)

Post-hoc analysis for the first time segment revealed a significant interaction between type of expression and type of composite at temporal electrode sites, F(3,33)=4.65, p<.05, as well as a trend towards this interaction at frontal electrode sites, F(3,33)=2.95, p=.05. Fearful expressions as compared to any other expression, elicited a qualitatively different pattern of responses towards left vs. right composites, both at frontal (see Fig. 9) and at temporal electrode sites (see Fig. 10). At temporal electrode sites, a significantly larger amplitude resulted from left composites of fearful faces than from right composites of fearful faces (p<.05 (fear 50% vs. fear 100%), Tukey hsd).

Figure 9. Mean amplitude in µV for the time segment from 110- 140 ms at frontal electrode sites showing the interaction between type of expression and type of composite.

Figure 10. Mean amplitude in µV for the time segment from 110- 140 ms at temporal electrode sites showing the interaction between type of expression and type of composite.

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170-200ms (N170)

Post-hoc analysis of the second time segment revealed a significant effect of type of composite at temporal electrode sites, F(1,11)=5.23, p<.05, with right composites having a stronger influence on the N170 amplitude than left composites.

There was also a significant effect of level of intensity at occipital electrode sites, F(1,11)=7.07, p<.05, with faces presented in 50% intensity having stronger effects on the N170 amplitude compared to faces being presented in 100% intensity. There was also a trend towards this effect of level of intensity at central-parietal electrode sites, F(1,11)=4.64, p=.05., with higher amplitude of the N170 resulting from presentation of the more intense (100% version) faces.

The interaction between type of expression and type of composite turned out to be significant at occipital (F(3,33)=4.18, p<.05), central-parietal (F(3,33)=3.31, p<.05), temporal (F(3,33)=5.56, p<.01) and frontal (F(3,33)=4.26, p<.05) electrode sites. Fearful expressions modulated the general pattern of N170 amplitude towards left vs. right composites at occipital, temporal and frontal electrode sites. At occipital electrode sites (see Fig. 11), left composites of fearful faces had a significantly stronger effect on the N170 amplitude than right composites of fearful faces (p<.05, Tukey hsd). At temporal electrode sites (p<.01 (fear LL vs.

fear RR), Tukey hsd) as well as at frontal electrode sites (p<.01 (fear LL vs. fear RR), Tukey hsd), the N170

amplitude was significantly larger for right composites of fearful faces than for left composites of fearful faces (see Fig. 12 and Fig. 13).

Figure 11. Mean amplitude in µV for the time segment from 170- 200 ms at occipital electrode sites showing the interaction between type of expression and type of composite.

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Figure 12. Mean amplitude in µV for the time segment from 170- 200 ms at frontal electrode sites showing the interaction between type of expression and type of composite.

Figure 13. Mean amplitude in µV for the time segment from 170- 200 ms at temporal electrode sites showing the interaction between type of expression and type of composite.

Furthermore, there was a significant interaction between type of expression and level of intensity at prefrontal/ lateral frontal electrode sites, F(3,33)=3.63, p<.05, with sad expressions modulating the general pattern of N170 amplitudes towards faces presented in 100% vs. faces presented in 50% intensity that was present for all other expressions, as well as a trend towards this interaction at occipital (F(3,33)=2.79, p=.06) and central-parietal (F(3,33)=2.76, p=.07) electrode sites. At prefrontal/ lateral frontal electrode sites, N170 amplitude towards sad faces presented in 50% was larger than towards sad faces presented in

3 Results 35

100%, whereas every other expression yielded the opposite pattern of results, i.e. larger N170 amplitude towards more intense faces (100% intensity). Although sad expressions qualitatively popped out of the general pattern of results, post-hoc testing with Tukey hsd did not reveal any significant difference between any of the dependent variables (see Fig. 14).

Figure 14. Mean amplitude in µV for the time segment from 170- 200 ms at prefrontal/ lateral frontal electrode sites showing the interaction between type of expression and level of intensity.

200-300ms

Post-hoc analysis of the time segment from 200 to 300ms revealed a significant effect of level of intensity at occipital (F(1,11)=11.07, p<.01), central-parietal (F(1,11)=5.31, p<.05), temporal (F(1,11)=7.05, p<.05) and frontal (F(1,11)=5.32, p<.05) electrode sites. At occipital electrode sites, faces presented in 50% intensity elicited a larger amplitude than faces presented in 100% intensity. At central-parietal, temporal and frontal electrode sites, faces presented in 100% intensity elicited a higher amplitude than faces presented in 50% intensity.

The interaction between type of expression and type of composite turned out to be significant at temporal (F(3,33)=3.56, p<.05) and prefrontal/ lateral frontal (F(3,33)=3.46, p<.05) electrode sites. At temporal electrode sites, amplitude towards fearful expressions were strongly effected by type of composite - right composites of fearful faces elicited a significantly larger amplitude than left composites of fearful faces (p<.05 (fear LL vs. fear RR), Tukey hsd), whereas for every other expression, amplitude towards left vs. right composites did not significantly differ. Similar to the temporal region of interest (see Fig. 15), at prefrontal/

3 Results 36

lateral frontal electrode sites (see Fig. 16), amplitude towards fearful expressions were strongly effected by type of composite, with significantly larger amplitudes towards left composites of fearful faces than towards right composites of fearful faces (p<.01 (fear LL vs. fear RR), Tukey hsd).

Figure 15. Mean amplitude in µV for the time segment from 200- 300 ms at temporal electrode sites showing the interaction between type of expression and type of composite.

Figure 16. Mean amplitude in µV for the time segment from 200- 300 ms at prefrontal/ lateral frontal electrode sites showing the interaction between type of expression and type of composite.

3 Results 37

The interaction between type of expression and level of intensity turned out to be significant at occipital (F(3,33)=3.5, p<.05) and prefrontal/ lateral frontal (F(3,33)=3.44, p<.05) electrode sites. At occipital electrode sites, amplitude towards every type of expression but sadness was modulated by level of intensity, with faces presented in 50% intensity eliciting a larger amplitude than faces presented in 100% intensity (p<.01 (happiness 50% vs. happiness 100%), p<.01 (anger 50% vs. anger 100%), p<.01 (fear 50% vs. fear 100%) Tukey hsd) At prefrontal/ lateral frontal electrode sites, sad expressions as compared to every other expression elicited a qualitatively different pattern of amplitude, depending on the level of intensity. At this region of interest, amplitude towards faces presented in 100% intensity were larger compared to faces presented in 50%

intensity, except for sad faces that showed the opposite pattern of response (see Fig. 17).

Fig. XY

Figure 17. Difference waves for level of intensity (100% - 50%) for different time segments, separately for each individual expressions.

3 Results 38

300-500ms (P3)

Post-hoc analysis of the time segment from 300 to 500ms (P3) revealed a trend towards an effect of type of composite at prefrontal/ lateral frontal (F(1,11)=4.35, p=.05) and temporal (F(1,11)=4.19, p=.07) electrode sites, with right composites eliciting a larger amplitude at temporal electrode sites and left composites eliciting a larger amplitude at prefrontal/ lateral frontal electrode sites.

Level of intensity yielded a significant effect at every region of interest (occipital:

F(1,11)=35.46, p<.001; central-parietal: F(1,11)=13.1, p<.01; temporal: F(1,11)=16.53, p<.01; prefrontal/ lateral frontal: F(1,11)=4.92, p<.05; frontal: F(1,11)=14, p<.01). At occipital and prefrontal/ lateral frontal electrode sites, faces presented in 50% intensity elicited a larger amplitude than faces presented in 100% intensity. At central-parietal, temporal and frontal electrode sites, faces presented in 100% intensity elicited a larger amplitude than faces presented in 50% intensity (see fig.6 global analysis).

Furthermore, there was a trend towards an interaction between type of expression and level of intensity at central-parietal electrode sites, F(3,33)=3.5, p=.07.

500-800ms

For the time segment from 500 to 800ms, there was a significant effect of type of expression at temporal electrode sites, F(3,33)=10.87, p<.001, with happy faces eliciting the smallest amplitude compared to every other expression, with no difference in amplitude towards angry, fearful and sad expressions (p<.001 (anger vs. happiness), p<.001 (fear vs. happiness), p<.01 (sadness vs. happiness) , Tukey hsd) (see Fig. 18).

Figure 18. Mean amplitude in µV towards different expressions for the time segment from 500- 800 ms at temporal electrode sites.

3 Results 39

There was also a significant effect of level of intensity at central-parietal electrode sites, F(1,11)=16.08, p<.01, with faces presented in 100% intensity eliciting a larger amplitude than faces presented in 50% intensity.

The interaction between type of composite and level of intensity turned out to be significant at temporal (F(1,11)=7.82, p<.05) and frontal (F(1,11)=6.42, p<.05) electrode sites, as well as being present as a trend towards an interaction at occipital electrode sites, F(1,11)=4.48, p=.06. At occipital electrode sites, right composites presented in 50% intensity elicited a significantly larger amplitude than right composites presented in 100% intensity (p<.05 (RR 100% vs. RR 50%), Tukey hsd), whereas amplitude towards left composites did not differ regardless of the level of intensity. At temporal electrode sites, even though the pattern of amplitude for left and right composites seemed to be qualitatively different depending on the level of intensity, post-hoc testing with Tukey hsd did not reveal any significant difference between the experimental conditions. At frontal electrode sites, right composites presented in 100% elicited a significantly larger amplitude than right composites presented in 50%

intensity (p<.05 (RR 100% vs. RR 50%), Tukey hsd), whereas amplitude towards left composites did not differ regardless of the level of intensity.

4 Discussion 40

4 Discussion

4.1 Intensity task

In the first part of the experiment, the intensity task, subjects were simultaneously presented with two chimeric versions of a given original face stimulus, one being the left composite (LL) and the other one being the corresponding right composite (RR). Applying a two alternative forced- choice intensity judgement, subjects were asked to pick the stimulus that looked more intense to them. Subjects’ choice revealed a significant main effect for type of composite, with RR-composites judged as being more intense compared to corresponding LL- composites. Thus, composites consisting of right hemifaces were perceived as being more expressive. This very robust effect which is significant regardless of the type of expression, is quite contrary to the classical finding by Sackeim et al. (1978), describing a strong advantage for the left hemiface in terms of intensity of emotional expression (Sackeim et al., 1978).

It seems to be rather counterintuitive that the right hemiface, which is innervated by the left hemisphere, is perceived as being more intense, since the left hemisphere is the one that has in general been thought to be inferior for the processing of emotions. However, from a different point of view, namely the earlier described poser- perceiver- paradox, the present results in fact do make sense: Assuming that the right hemiface of a poser is actually more emotionally expressive, this information will fall into the left visual field of a perceiver, and consequently ends up in the perceivers’ right hemisphere, which is exactly the one that is thought to be superior in terms of emotion processing. Thus, from this perspective, information transfer between the transmitter and the receiver seems to be optimised, by differential contribution of the two cerebral hemispheres to the generating versus the perception of facial expressions.

However, concerning the innervation of facial muscles, the present results remain difficult to interpret. But even though facial muscles are mainly contrallaterally innervated, there is also some ipsilateral innervation, especially for the upper half of the face, where muscles receive bilateral neuronal input (Crosby, Humphrey, & Lauer, 1962). Moreover, with only static facial stimuli being used, there might also be the possibility that differences in emotional expression between the left and right hemiface rather lie in different time courses with right hemifaces being more expressive in the later or final stage of the display of a facial

4 Discussion 41

expression at which stimulus pictures of the posers had been taken. Although in general right hemispheric activation seems to be stronger for emotional processing, there is in fact also activation of the left hemisphere, but slightly later (Streit et al., 1999). Thus in order to interpret this unexpected right hemiface advantage for emotional expression found in the present study, it could be useful to focus on hemispheric differences not only in terms of localization, but also in terms of timing. However, even when hemispheric differences in timing are taken into consideration, it still does not sufficiently explain the very different outcome of the three chimeric studies that have been compared above (the present study, the one by Sackeim et al. (1978), and the one by Indersmitten et al. (2003)) since all of them were using static pictures, most likely all having been taken at a similar stage of displaying facial expressions, namely the final stage of expression.

In spite of these puzzling results, it is however indisputable that the reason for this right hemiface advantage must stemm from the poser and not from the perceiver, since stimuli were always presented centrally, and thus simultaneously supplying information to both hemispheres of the perceiver.

Generally, the results of the present intensity task are in itself quite consistent, but opposite to earlier studies (Indersmitten & Gur, 2003; Sackeim et al., 1978), indicating that not only choice of stimuli, but also the task itself might make a crucial difference.

While the studies cited above used a paradigm, where subjects were presented with only one stimulus at a time that had to be rated on a scale of perceived intensity, the present study applied a forced- choice paradigm, enabling direct comparisons between LL- and RR-composites to be done. Although the forced- choice task implemented here might be qualitatively different from a judgement that has to be made on a scale, it does not necessarily mean that the chosen approach is inappropriate for investigating differences between the two hemifaces in terms of intensity of emotional expression.

Regarding the choice of stimuli being used, it is important to mention that the stimuli taken for the study run by Sackeim et al. (1978) and those being used for the present study, were not identical, although they have been taken from the same stimulus set, namely the Ekman pictures of facial affect (Ekman, 1976). This stimulus set is highly validated and has been implemented in numerous studies investigating different aspects of facial expression.

For the present study, the choice of stimuli has been based on quality in terms of symmetry, brightness and contrast. Furthermore, an analysis of variance (ANOVA) did not reveal any systematic difference in size between left and right composites having been created from the original pictures. Sackeim et al. did not report having controlled for possible systematic

4 Discussion 42

differences between LL- and RR-composites, which might however be a crucial factor. It has repeatedly been reported that the right hemiface is larger than the left (Koff, Borod, &

Strauss, 1985; Nelson & Horowitz, 1980) and following the assumption of symmetrical movements respectively muscle activity being distributed on facial areas of different size, facial expressions would necessarily appear to be asymmetrical. However, direct testing of this size hypothesis did not reveal any statistically predictive value (Sackeim, Greenberg, Weimann, & Forman, 1984).

Indersmitten et al. (2003), proposing a more differentiated approach, replicated the Sackeim et al. (1978) findings, also claiming a dominant role for the left hemiface in terms of emotional intensity. However, Indersmitten et al. also used an intensity judgement on a scale, which might explain the similarity of their outcomes with those of Sackeim et al.. But, compared to the present study, apart from having implemented a different task, they also used a different stimulus set consisting of three dimensional pictures of posers (Gur et al., 2002) that were displaying either posed or evoked expressions. This might offer an explanation for differences in the outcome of the present study and the one by Indersmitten et al.. Moreover, it has unfortunately not been reported whether handedness of the posers used by Indersmitten et al. had been assessed, which is a very crucial factor indicating brain laterality, neither has it been reported what was actually meant by the term ‘moderate level’ of intensity that had been displayed by their posers.

4.2. Efficiency task

In the second part of the experiment, the efficiency task, subjects were presented with only one stimulus at a time that was either a left composite (LL) or a right composite (RR).

They were asked to classify these stimuli according to the corresponding type of expression (anger, fear, happiness, sadness), yielding measurements of speed and accuracy in this four alternative forced choice paradigm. The assumed dissociation between intensity and efficiency of emotional expression reported by Indersmitten et al. (2003) that was originally aimed to be replicated in the present study, however turned out to be different than expected:

Whereas Indersmitten et al. found a left hemiface advantage for emotional intensity and a right hemiface advantage for efficiency of emotional expression, the present study failed to show the assumed dissociation. First, contrary to earlier results, there has been a consistent right hemiface advantage for emotional intensity, regardless of type of expression (see

4 Discussion 43

intensity task). Secondly, even though the reported right hemiface advantage in terms of efficiency of emotional expression could partially be replicated, it turned out to be quantitatively as well as qualitatively different and even more consistent than reported by Indersmitten et al..

The right hemiface advantage for efficiency of emotional expression that could overall be demonstrated in the present study showed a similar pattern of results, be it for correct classifications or reaction times. However this relatively consistent effect for type of composite was modulated by type of expression, with only angry and fearful expressions showing a right hemiface advantage, whereas sad expressions showed a left hemiface advantage and happy expressions did not reveal any significant difference between LL- and RR-composites at all.

Even though the overall effect for type of composite did not reach significance for reaction times, there was a trend towards a right hemiface advantage (p= .06), which is congruent to the significant right hemiface advantage for correct classifications. Therefore, with correct classifications reflecting the pattern of results for reaction times, the present data should not be discussed in terms of a speed-accuracy trade-off.

Looking at each individual expression, performances of classification were as follows:

performance of classification for angry expressions was significantly better for composites, be it for reaction times or be it for percentage of correctly classified faces. RR-composites of fearful expressions were only classified more accurately but not significantly faster. Happy expressions were classified extremely fast and accurately without any difference in performance between LL- and RR-composites. As recognising happy faces seemed to be so much easier than recognizing any other expression, the very high performance level, likely engendering a ceiling effect, might possibly have covered underlying differences between LL- and RR-composites. Apart from the possibility of a ceiling effect for happiness, it is however well known that recognizing happy faces always seems to be a very easy task. (Hager & Ekman, 1979). In order to reveal any potential differences between LL-and RR-composites of happy faces, one would artificially have to make the task more difficult, e.g. by adding white noise to the stimuli. For sad expressions,

performance of classification for angry expressions was significantly better for composites, be it for reaction times or be it for percentage of correctly classified faces. RR-composites of fearful expressions were only classified more accurately but not significantly faster. Happy expressions were classified extremely fast and accurately without any difference in performance between LL- and RR-composites. As recognising happy faces seemed to be so much easier than recognizing any other expression, the very high performance level, likely engendering a ceiling effect, might possibly have covered underlying differences between LL- and RR-composites. Apart from the possibility of a ceiling effect for happiness, it is however well known that recognizing happy faces always seems to be a very easy task. (Hager & Ekman, 1979). In order to reveal any potential differences between LL-and RR-composites of happy faces, one would artificially have to make the task more difficult, e.g. by adding white noise to the stimuli. For sad expressions,

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