Skin Conductance Response

Electrodermal activity represented by maximum and mean skin conductance response to each slide was averaged across stimuli belonging to the same picture category (pleasant, neutral, unpleasant).

Resulting mean scores were analyzed by an ANOVA with the between factor group (patients and controls) and the repeated measurement factor picture category. Only the first 48 trials were included with the intention to exclude habituation to affective slide contents. Data from 7 control subjects and 2 patients had to be excluded from analyses as a result of instrument and processing problems during SC-recordings.

Fig. 4.1.11.

Connectivity between prefrontal cortex and other brain regions;

Both dorsolateral and ventromedial prefrontal areas are reciprocally connected with visual association areas.

(adapted from

Wood & Grafman, 2003)

Mean Skin Conductance Response

For mean skin conductance response a main effect of picture category was observed with emotional slides eliciting higher SCR-Amplitudes than neutral ones (see Fig. 4.2.1.). Post hoc tests revealed statistically significant differences between pleasant and neutral pictures (p< .005) and between unpleasant and neutral stimuli (p< .05). Although pleasant slides seemed to elicit a higher SC response than unpleasant ones, this discrepancy did not reach significance (p< .08).

MEAN SCR-AMPLITUDE Main Effect of Picture-Category F(2,70)=8.04, p<.001, Epsilon = .87

SCR Amplitude (µS)

0,0 0,2 0,4 0,6 0,8

Pleasant Neutral Unpleasant

Fig 4.2.1. Mean Amplitude of skin conductance response (SCR) for each picture category (mean +/- standard error)

( * indicate post-hoc significance p< .05; ** indicate significance p< .01)

Mean amplitude of skin conductance responses did not significantly differ between the two participant groups. Scatterplots in Figure 4.2.2. illustrate SCR amplitudes of individual patients and controls elicited by each stimulus condition. Except for single outliers, data of the two groups show a quite similar distribution, irrespective of picture category.

Fig. 4.2.2. Mean SCR-Amplitude for individual subjects in each group. Values are separately

presented for each picture category (pleasant, neutral, unpleasant)

Maximal Skin Conductance Amplitude

The peak of skin conductance amplitude during the 1 to 5 s post stimulus period significantly differed between picture categories (F(2.70)= 6.64; p< .005) with pleasant slides eliciting a higher peak amplitude than neutral (post-hoc: p< .005) and unpleasant pictures (p< .05). Although a difference can be observed between maximal skin conductance response to unpleasant pictures (mean: 1.44 µS) and neutral slides (mean: 1.16 µS), it does not reach statistical significance (p<

.09). No significant effects including the variable “group” could be found. Table 4.2.1. provides an overview on peak SCR amplitudes of patients and controls obtained for each of the three picture categories.

Table 4.2.1. Maximal SCR-Amplitudes (and standard errors) of participant groups for each condition

Peak SCR-Amplitude

Pleasant Pictures Neutral Pictures Unpleasant Pictures Patients (N = 21) 1.98 ( 0.39) 1.35 (0.27) 1.42 (0 .27)

Controls (N = 16) 1.67 ( 0.35) .91 ( 0.21) 1.48 ( 0.30)

Thus, SC responses did not reveal any differences between TBI patients and healthy subjects. In both groups, mean and maximal skin conductance responses (SCRs) were substantially larger for emotionally salient pictures (pleasant and unpleasant) as compared to neutral stimuli.

Discussion

The present data confirm a series of studies reporting that the magnitude of SCR, which indexes sympathetic nervous system activation, is significantly influenced by the arousal level of the presented pictures ( Bradley et al., 2001; Codispoti et al., 2001; Cuthbert et al., 1996). With respect to the importance of stimulus valence for electrodermal responses, empirical evidence is rather equivocal. The present findings suggest that pleasant stimuli evoke stronger SC responses than unpleasant picture, which is in agreement with a recent finding by Cuthbert and colleagues (2001).

Furthermore, in the present thesis, stronger processing of pleasant slides was also indicated by a larger late positive potential for this stimulus category. Other investigations have revealed an opposite pattern with unpleasant slides eliciting the largest skin conductance changes (e.g. Bradley et al., 2001). The latter study also investigated different content groups within the category of pleasant pictures and came to the conclusion that only very explicit erotic contents, such as erotic couples and opposite-sex erotica, led to noticeably enhanced SC responses. Thus, the fact that in the present investigation, 14 out of the 25 pleasant IAPS pictures involved erotic stimuli (see Appendix A), could be a possible reason for the large increment of electrodermal responses to pleasant slides.

In terms of Lang’s model on emotion, these findings on appetitive motivation suggest that a specific threshold of motivational activation is necessary before greater sympathetic activity is initiated and that personally highly relevant cues, such as erotic stimuli, may be needed to prompt strong sympathetic engagement. Consistent with this notion, Bradley et al. (2001) demonstrated that other pleasant picture contents rated as similarly arousing (e.g. adventure, extreme sports) failed to evoke equally large SC responses. Furthermore, given that the present sample contained nearly only males, an influence of gender can be assumed to account for the valence effect with men showing more aroused responses to pleasant, especially erotic content. This view is consistent with recent fMRI study that revealed more pronounced cortical activation during presentation of pleasant material in men compared to women (Lang et al., 1998; Wrase, Klein, Gruesser, Hermann et al., 2003).

Contrary to our predictions, TBI patients did not show an attenuated SCR during viewing of arousing slides. Though no similar experiment with a comparable clinical sample of traumatic brain injured patients has been published so far, evidence from human lesion studies argues in favor of an impairment of SC activation following damage to the frontal lobes. Tranel and Damasio (1994) and Zahn et al. (1999) investigated electrodermal responses to IAPS pictures in patients with lesions of various parts of the frontal cortices and revealed that patients’ SC responses to arousing stimuli were evidently attenuated or even completely absent. An analogous finding was described by

Angrilli and co-workers (1999) who reported a lower autonomic response to emotional pictures in a single TBI patient with a circumscribed damage to the right orbitofrontal and anterior cingulate cortex.

A possible explanation to the discrepancy between these results and the unimpaired electrodermal activation in TBI patients found in the present investigation could be the important differences regarding the exact lesion location in the respective clinical samples. Whereas patients in the Tranel and Damasio study (1994) and the Zahn et al. (1999) study had lesions comprising the anterior cingulate cortex, in no single patient was the anterior cingulate gyrus affected in the present investigation. Even more importantly, both earlier studies came to the conclusion that bilateral and unilateral lesions of the anterior cingulate cortex were particularly effective in producing deficits of SC responding. Findings demonstrated that neither ventromedial prefrontal nor orbitofrontal lesions alone were sufficient enough to cause electrodermal hyporesponsivity to emotional pictures. In light of these considerations, the unimpaired SC response of patients in the present study provides further evidence for the assumption that the anterior cingulate cortex plays a decisive role in SC modulation.