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initial attentional capture. Taken together, data from this paradigm demonstrates that emotional pictures are perceived more intensively.
Keywords: binocular rivalry, emotional pictures, visual perception, preferential processing
1. Preferential Processing of Emotional Stimuli
We are constantly exposed to a multitude of visual stimuli. Evaluating this information and responding with adequate behavior where necessary, helps us to survive. Identifying threatening stimuli seems to be especially important as we need to repel danger or protect ourselves from it as quickly as possible. Evidence for a privileged role of aversive stimuli in perception and attention processes can be found in a number of convincing research paradigms.
For example, visual search tasks show that angry faces can be detected very quickly and pop out among a variety of neutral faces (Hansen and Hansen, 1988; Öhman, Lundqvist, and Esteves, 2001). Similarly, fear relevant stimuli like spiders and snakes surrounded by neutral objects can be found more quickly (Öhman, Flykt, and Esteves, 2001). In the so called dot-probe paradigm, participants respond faster to test probes (letters, for example) that appear on the spot where a fear relevant, as opposed to a neutral, stimulus was presented beforehand (Bradley, Mogg, Millar, Bonham-Carter, Fergussoon, Jenkins et al., 1997; Mogg and Bradley, 2002). Many scientists assume that this allocation of attention is based on automatic processes which operate independently from conscious processing. Convincing evidence for this assumption comes from experiments which reveal psychophysiological reactions to emotional stimuli even when they were not consciously perceived, for example when they were presented subliminally and masked by another picture (Dimberg, Thunberg, and Elmehed, 2000; Öhman and Soares, 1994).
1.1. Two Pathways for the Processing of Emotional Stimuli
Fast and automatic perception of emotionally relevant visual stimuli calls for rapid neuronal processing. The amygdala is central for the processing of emotional cues and especially for fear relevant information (LeDoux, 1996; Morris, Friston, Buchel, Frith, Young, Calder et al., 1998). Based on a number of animal studies, LeDoux (1996) demonstrated that sensory information of external stimuli can reach the amygdala via two relatively independent pathways. On the
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one hand, information from the retina projects to the sensory cortex, via the sensory thalamus, for a higher-level analysis. This pathway is rather slow but encompasses detailed information and LeDoux therefore calls it the “high road” of emotional processing. Cortical areas process this input before it reaches the amygdala, from where emotional reactions can be elicited and modulated. Then, a more time consuming and more elaborate processing of emotional stimuli can be guaranteed (Pessoa, Kastner, and Ungerleider, 2002). Even more complex reciprocal influences of emotion and attention can further modulate processing of visual perception within a network of frontal and parietal brain regions (Pessoa, et al., 2002; Windmann, Wehrmann, Calabrese, and Güntürkün, 2006).
Figure 1. Two pathways of processing of visual emotional stimuli (after LeDoux, 1996).
In addition to this cortical pathway, a subcortical pathway for immediate processing of emotional information exists, where sensory information reaches the amygdala via direct thalamic projections. This is thought to be independent from the cortical analysis mentioned above. This direct projection from the thalamus to the amygdala only provides for a crude analysis but it is much faster than the one from the thalamus to the amygdala via the sensory cortex. Thus, this so called “low road” represents a shortcut which bypasses cortical areas. Along this pathway, information about emotional relevance can reach the amygdala much more rapidly (see Figure 1). This allows for a direct and fast response to potentially dangerous stimuli before the analysis is complete. Thus, emotional reactions that are initiated by the amygdala enable for effective fight or flight
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responses (e.g., defensive reflexes or an increased respiration rate and heart rate – see Alpers, Mühlberger, and Pauli, 2005).
Figure 2. Amygdala projections to the visual cortex of the macaque brain (Amaral, et al., 2003). L - lateral nucleus, Bmc - magnocellular division, BI - intermediate division, TEO – optic tectum , TE – telencephalon.
Support for the independence of the "low road" from cortical processing comes from animal studies and studies with cortically blind patients. They demonstrate that processing of visual emotional information is indeed possible without involvement of intact cortical circuitry. For example, cortically blind patients show physiological responses to emotional stimuli even if they are not able to consciously perceive them (Anders, Birbaumer, Sadowski, Erb, Mader, Grodd et al., 2004; Hamm, Weike, Schupp, Treig, Dressel, and Kessler, 2003).
The “low road” has an additional function: It can modulate cortical processing on the “high road”.
Once again, it was shown in animal studies that there are direct neuronal projections from the amygdala to cortical visual areas (V1 or V2, for example) (Amaral, Behniea, and Kelly, 2003; Amaral and Price, 1984) (see figure 2). Evidence supporting that such neuronal circuits also exist in humans has been provided since (Catani, Jones, Donato, and Ffytche, 2003). These projections make it possible that "quick and dirty" subcortical processing can influence more
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elaborate processing in cortical areas which would allow for enhanced conscious perception and processing (Pessoa, Kastner, and Ungerleider, 2003).
Electrophysiological (Schupp, Junghöfer, Weike, and Hamm, 2003; Stolarova, Keil, and Moratti, 2006), as well as hemodynamic evidence (Herrmann, Huter, Plichta, Ehlis, Alpers, Mühlberger et al., 2008; Lang, Bradley, Fitzsimmons, Cuthbert, Scott, Moulder et al., 1998) shows that emotionally relevant stimuli are accompanied by increased activation of visual areas in the occipital lobe. It is very plausible that this may be partly initiated by input from the “low road” in addition to top-down input from higher cortical areas such as directed attention.
1.2. Intensive Processing of Negative Valence or of Arousal?
In additon to multiple findings documenting a preferential processing of negative stimuli in the amygdala there is growing evidence for an equally intensive processing of arousing positive stimuli. According to the Emotionality Hypothesis, all emotional stimuli are selectively processed, independent of their specific valence. And indeed, processing stimuli which are associated with reward involves similar brain circuits as the processing of cues for danger (Berridge and Winkielman, 2003; Davis and Whalen, 2001). As it can be seen in functional magnetic resonance imaging (fMRI) and positron-emission-tomography (PET) studies, processing of positive as well as negative words is associated with higher amygdala activation (Hamann and Mao, 2002). Furthermore, higher amygdala activation in response to positive and negative as opposed to neutral pictures has been observed (Garavan, Pendergrass, Ross, Stein, and Risinger, 2001; Hamann, Ely, Hoffman, and Kilts, 2002). Electroencephalography (EEG) findings also support the notion that strongly activating affective pictures are processed faster and more intensely in the visual cortex (Cuthbert, Schupp, Bradley, Birbaumer, and Lang, 2000; Schupp, et al., 2003).
Thus, the intensity of emotional arousal seems to be more crucial than the specific affective valence of a stimulus. Also, these findings suggest that the amygdala may be involved in the processing of positive as well as negative emotional stimuli. In conclusion, it could be expected that positive as well as negative pictures boost visual perception.