4.4. Controlling for Physical Properties of Stimuli

Although the findings we reported above clearly support the hypothesis that emotional picture content results in more predominant perception, it cannot be completely ruled out that physical properties of pictures, as well as self-report of perception, could have influenced the results in these studies. To account for this and to validate our previous findings, we conducted four more experiments with the objective to largely eliminate the influence of physical properties of stimuli and minimize response biases in self-report of perception.

In the first experiment, we presented two physically identical geometric gratings which only differed in spatial orientation. In order to introduce differences in emotional valence we used a differential fear conditioning paradigm. Thus, the two stimuli which had neutral valence at the outset were given different emotional valence by our experimental manipulation (Alpers, Ruhleder, Walz, Mühlberger, and Pauli, 2005). Interestingly, we were able to interleave conditioning and binocular rivalry trials. This helped us to document that more aversive experience with a given grating with a given orientation changed its influence on predominance across trials. As a result, the aversively conditioned pattern was perceived as the dominant percept for a longer period of time during the trials when compared to the perception before the fear conditioning, and it was reported more and more frequently as the first percept of a trial. However, these effects were rather small compared with the effects in studies using emotional IAPS pictures and emotional facial expressions. This can probably be best explained by the fact that geometric patterns are not evolutionary relevant, even if they aquire emotional relevance after fear conditioning. Thus, underlying neuronal processes of emotional processing are probably less pronounced here than in experiments with stimuli that are evolutionary prepared or are naturally relevant.

In a second study we presented schematic emotional faces which are more biologically relevant but differ in physical characteristics. We designed neutral, positive, and negative facial expressions by arranging nearly identical picture elements (also see Lundqvist, Esteves, and Öhman, 2004). Although those faces are rather simple, several studies have demonstrate that schematic faces can elicit similar emotional reactions as photographs of faces (Bentin and Gollanda, 2002; Eastwood, Smilek, and Merikle, 2003; Lundqvist and Öhman, 2005).

In this experiment schematic emotional faces clearly predominanted compared with neutral faces (Experiment 2, Alpers and Gerdes, 2007). The pattern of results was very similar to the pattern reported above for photographic emotional faces. Taken together, both control-experiments demonstrate that

These results are consistent with other findings showing that meaningful pictures dominate over meaningless ones in binocular rivalry (Yu and Blake, 1992). However, as to the mechanisms, we have not yet shown that dominance was mediated by activation of emotional neuronal circuits (the amygdala, for example). Nonetheless, predominance of emotional stimuli in binocular rivalry is another piece of evidence for preferential processing in the visual stream. Whether this is based on automatic processes (Öhman, 2005), higher order (cortical) attentional processes (Pessoa, et al., 2003) or an interaction of both is a challenging problem for future research.

5. Conclusion: "Blind" in One Eye - But not When It Comes to Emotion

The preferential perception of emotional pictures in binocular rivalry is clearly consistent with results from other experimental paradigms, such as the faster detection of emotional stimuli in search tasks (Hansen and Hansen, 1988; Öhman, et al., 2001). Furthermore, the results are consistent with findings form psychophysiological studies which show stronger activation of the visual cortex when looking at emotional pictures (Alpers, et al., 2005; Herrmann, et al., 2008; Schupp, et al., 2003).

An evolutionary advantage of faster detection of potentially meaningful stimuli accounts for an easier processing of emotional material (Öhman and Mineka, 2001). However, with the paradigm we described here we were not able to verify whether activation of emotional neuronal circuits is in fact responsible for the competitive strength of emotional pictures in binocular rivalry.

With regard to neuronal substrates in which binocular rivalry is processed, it can be hypothesized that feedback from subcortical circuity such as the amygdala (Amaral, Price, Pitkanen, and Carmichael, 1992) and the anterior cingulate cortex (Posner and Raichle, 1995) may be involved in processes leading to conscious perception. As we explained in the introduction, emotional material activates the amygdala in binocular rivalry even when it is suppressed. Because different stages of processing in primary and extrastriatal areas are involved in binocular rivalry (Kovacs, et al., 1996; Logothetis, et al., 1996; Sheinberg and Logothetis, 1997), it is apparent that influences from emotional processing centers could take effect.

While some experiments with different paradigms found emotion specific effects which suggest that preferential processing of negative pictures is specific (Hansen and Hansen, 1988; Öhman, Lundqvist, et al., 2001), our experiments with binocular rivalry point to a dominance of both positive and negative pictures.

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