Emotions are considered here as evolved modules organizing and modulating multifaceted functions such as sensory intake, information processing, and various responding levels (e.g., subjective experiences, physiology and behavior). The functionality and hardware of such processes has been shaped in order to appropriately cope the recurrent encounter with adaptive problems in phylogenesis and ontogenesis (Cacioppo et al., 1999; Öhman & Mineka, 2001; Tooby & Cosmides, 1992). Based on the universal principles of approach and avoidance, basic motivational systems (appetitive and aversive) are assumed to provide the fundament for the evolvement of multifaceted human emotion response repertoire (Buck, 1994; Lang et al., 1997; Öhman et al., 2000). For instance, procreation, sustaining organisms’ integrity and the adaptation to environmental conditions are vital objectives motivating specific feelings and behavior. Moreover, various functions developed to organize human being and behavior in relation to their social and cultural contexts (e.g., communication, emotion expression and perception;
Ekman, 1999; Izard, 1977; Scherer, 1984).
Human emotion and behavior is embedded in environmental settings, thus, any action strongly depends on perceptual information up-take, and therefore, is gated through attentional processes. Not only the detection of action cues but also the fine-tuning and re-adjustment of ongoing responses require iterative situational updates. Accordingly, a seemingly endless stream of sensory and interoceptive input is needed to be encoded and integrated by information processing systems. Apparently, sensoric information processing needs to be selective. According to this perspective, both emotion and attention deal with processing priorities organizing adequate responding in a highly complex environment (Oatley & Johnson-Laird, 1987).
Recent attention theories commonly refer to limited capacity of processing resources leading to the necessity of input selection (Broadbent, 1958; Desimone &
Duncan, 1995; Deutsch & Deutsch, 1963; Hopfinger, Luck & Hillyard, 2004; Kastner &
Ungerleider, 2000; Treisman, 1969). In detail, selective attention refers to the aspect that not all stimuli are equally processed, i.e., some stimuli, events or attributes are given longer, more effective and elaborate processing than others (Compton, 2003; Kastner &
Ungerleider, 2000, 2001; Pashler, 1998). Currently, two broader selection mechanisms have been proposed in visual attention research: Firstly, explicitly directed attention, which implies voluntarily and goal-directed attention towards task-relevant locations, visual features, objects or higher-order semantic categories (Beck & Kastner, 2005; Hillyard &
Anllo-Vento, 1998; Posner, 1980). That is, if one actively directs attention (top-down) to a particular location in a complex scene, processing of information at the attended location will be facilitated and processing of irrelevant objects will be suppressed (Beck & Kastner, 2005). Secondly, selective attention may be sensory-driven (bottom-up) by stimulus characteristics such as physical key features (e.g., velocity, location; Corbetta, Miezin, Dobmeyer, Shulman, & Petersen, 1990; Luck, Woodman, & Vogel, 2000; Luck & Ford, 1998; Posner, 1980). Similarly, stimuli depicting implicit emotional/motivational significant information (e.g., related to procreation or life-sustaining topics) have been suggested to guide selective attention and receive preferential processing (Compton, 2003;
Stockburger, Schmälzle, Flaisch, Bublatzky, & Schupp, 2009; Lang et al., 1997; Schupp et al., 2003b, 2006; Vuilleumier, 2005). Unlike voluntary attention, these attentional processes are assumed to operate rather spontaneously, that is, in passive viewing conditions without explicit instruction to attend.
In this context, natural pictures depicting emotional and motivational significant scenes (e.g., mutilation, threatening or erotic scences) are useful stimulus materials to
examine the interaction of both implicit and explicit attention processes under conditions of competition (Kastner & Ungerleider, 2000; Pessoa & Ungerleider, 2004; Schupp, Stockburger, Bublatzky et al., 2007, 2008; Vuilleumier, Armony, Driver, & Dolan, 2001) and cooperation (Compton, 2003; Compton et al., 2003; Schupp, Stockburger, Codispoti et al., 2007). For instance, research on visual search revealed an ‘attention capture’ by features such as movement or abrupt onsets (Yantis & Jonides, 1984, 1990) and emotional salient cues (Hansen & Hansen, 1988; Öhman, Flykt, & Esteves, 2001). Similarly, more complex emotional stimuli have been shown to attract attention and benefit from a processing advantage over neutral stimuli (Calvo & Lang, 2004; Öhman, Lundqvist, &
Esteves, 2001; Schupp et al., 2006). That is, environmental information depicting emotionally significant meaning guide selective attention and receive enhanced processing (e.g., Lang, et al., 1997; Cacioppo et al., 1999; Bradley, Codispoti, Cuthbert, & Lang, 2001; Öhman, Flykt, & Lundqvist, 2000; Hamm, Schupp, & Weike, 2003; Schupp et al., 2003b; Vuilleumier, Armony, & Dolan, 2003). Building upon this, explicitly directing attention to (or away from) emotional significant stimuli/features has been shown to modulate perceptual processing, physiological responding and behavior (Schupp, Stockburger, Codispoti et al., 2007).
Picture Viewing Paradigm
Humans perceive their environment predominantly visually. Thus, the presentation of pictures depicting real life contents provides a distinguished methodological approach to evoke content-related emotional states and to establish a contextual environment in an experimental laboratory setting. Since stimulus features (e.g., physical characteristics, presentation time) can be held controlled, responses measured during passive picture viewing strongly relate to processes elicited by the stimulus content. Furthermore,
concurrently performed active tasks (e.g., count emotional cues) allow to examine the interface of emotional and attentional processes.
To promote these benefits in emotion research, Lang and colleagues collected a set of naturalistic photographs (International Affective Picture System, IAPS; Lang, Bradley,
& Cuthbert, 2008) standardized with respect to subjective evaluations (e.g., Self-Assessment-Manikin, SAM; Bradley & Lang, 1994). As illustrated in Figure 1.2, a multitude of pictures depicting neutral and emotion-related scenes can be organized regarding their emotional value in an affective space spanned by basic dimensions of emotion: Valence and arousal (Bradley, 2000; Bradley & Lang, 2000). Thus, the IAPS set is designed to allow (1) for experimental control in stimulus selection, (2) comparisons of results from different studies, and (3) exact replications within and across research labs.
Fig. 1.2. The affective space as defined by means of pleasure and arousal for IAPS pictures (Lang et al., 1997). Vectors indicate hypothetic motivational systems organizing affective evaluations (from Schupp, Cuthbert, et al., 2004).
Until today, a large body of research utilized the IAPS with different methodological approaches, psychophysiological measurements and parameters. In the
following subsections, major results are reviewed focusing on affective modulations as revealed by peripheral measurements, neuroimaging and electrophysiological parameters.
Peripheral Psychophysiology of Emotion
To promote the survival of individuals and species, fast and accurate responses to environmental changes are essential (Öhman & Mineka, 2001). In this context, emotions are discussed as action dispositions (Frijda, 1986, Lang et al., 1997), providing the readiness to act and respond. Although not all emotional experiences yield an overt behavior, physiological activity in behavior relevant organ systems are reflected in psychophysiological parameters.
When viewing pictures with emotional and neutral contents, several response systems covary with affective valence and/or arousal, as defined by evaluative judgements (see Figure 1.3; Bradley et al., 2001). Accomplishing expressive and signal functions, valence sensitive responses are shown in facial muscle movements (Ekman & Friesen, 1975). For instance, zygomatic “smile” EMG activity increases linearly with rated pleasantness, and conversely, judged unpleasantness with the corrugator “frown” EMG activity (Bradley et al., 2001; Dimberg, 1990). As a cardio-vascular parameter, heart rate generally prompts marked deceleration during passive picture viewing suggested to indicate orienting, whereas pleasant in comparison to unpleasant pictures show greater accelerative responses related to motor preparation (Bradley et al., 2001; Lacey, 1967;
Palomba et al., 1997; Vila et al., 2003). Other response parameters covary with rated arousal rather than affective valence. For instance, regardless of picture valence, electrodermal activity and viewing time increase linearly with rated arousal of picture contents (Calvo & Lang, 2004; Greenwald, Cook, & Lang, 1989; Lang, Bradley, &
Cuthbert, 1990).
Fig. 1.3. Mean skin conductance changes and startle blink responses when viewing specific pleasant, neutral and unpleasant picture contents (from Bradley et al., 2001).
Of special interest, the defensive startle reflex (measured by the orbicularis eyeblink EMG activity) evoked by acoustic startle probes covaries with both rated picture valence and arousal in an interactive way (Berg & Balaban, 1999; Hamm, Greenwald, Bradley & Lang, 1993; Lang et al., 1997; Vrana, Spence & Lang, 1988). In reference to viewing neutral pictures or baseline startle reflex, the eyeblink activity decreases with rated
pleasantness and increases with unpleasantness. This dichotomy is even more pronounced for highly arousing than for stimuli or contextual settings rated lower in arousal (see Figure 1.3; Bradley et al., 2001; Schupp, Cuthbert, et al., 2004). Moreover, valence modulated blink reflex has been shown to be sensitive to individual differences and vary with clinical symptoms (Grillon & Baas, 2003; Hamm et al., 1997). Therefore, the affective eyeblink modulation supports the motivational priming hypothesis assuming that fundamental motivational approach and avoidance systems are linked to action programs, which are more responsive in the presence of congruently valenced environmental stimuli and contexts (Lang et al., 1997). Since this pattern is observable in humans and other animals, the startle reflex serves as an excellent variable to outline and test fundamental models of emotion (Lang & Davis, 2006).
Neuroimaging of Emotion
Research in mapping neural structures underlying emotional processes and experiences forged ahead dramatically with the advent of neuroimaging techniques.
Distinct neuronal activation patterns are observed consistently in viewing pictures of emotional significant in comparison to neutral pictures. Specifically, enhanced activity for affective materials (pleasant and unpleasant) is demonstrated in brain areas involved in visual information processing, for instance the occipital and fusiform gyri (see Figure 1.4;
e.g., Bradley et al., 2003; Junghöfer, Schupp, Stark, & Vaitl, 2005, Junghöfer, Peyk, Flaisch, & Schupp, 2006; Lang, Bradley, Fitzsimmons, et al., 1998; Sabatinelli, Flaisch, Bradley, Fitzsimmons, & Lang, 2004, Sabatinelli, Bradley, Fitzsimmons, & Lang, 2005).
Furthermore, signal changes in visual cortex vary with rated stimulus arousal (high arousing > low arousing > neutral stimuli; Bradley et al., 2003; Junghöfer et al., 2005).
Apart from modality specific processing areas, emotional modulation has been reported in
several cortical and sub-cortical structures suggested to mediate emotional processing and expression, such as the amygdala (e.g., Hadjikhani & de Gelder, 2003; Hamann, Herman, Nolan, & Wallen, 2004; Junghöfer et al., 2005; Norris, Chen, Zhu, Small, & Cacioppo, 2004; Sabatinelli et al., 2005; Vuilleumier et al., 2001; Whalen et al., 1998), insular (e.g., Junghöfer et al., 2005; Simmons et al., 2004) and orbito-frontal cortex (e.g., Karama et al., 2002; Murphy, Nimmo-Smith, & Lawrence, 2003; Nitschke et al., 2004; Vuilleumier et al., 2001).
Fig. 1.4. Illustration of in-creased BOLD-activation in extrastriate cortex while viewing emotional arousing pictures (from Sabatinelli et al., 2004).
Electrophysiology of Emotion
Further studies used event-related brain potentials to determine the temporal brain dynamics in cortical processing of emotional stimuli. With regard to emotion theories, recent ERP research strongly relates to attentional processes mediating fast and accurate extraction of emotional/motivational information. Consistent with this view, a large number of studies demonstrated reliable modulations in electrocortical parameters evoked by the emotional stimulus significance. Specifically, the processing of emotional picture cues has been shown to be associated with several ERP waveforms differentiating from neutral cues. In the following sections, findings regarding most consistently observed differential processing patterns, an increased early posterior negativity (EPN), and enhanced late positive potential (LPP) are reviewed.
The Early Posterior Negativity (EPN)
Differentiating the processing of emotional as opposed to neutral visual stimulus materials, the EPN is a negative going waveform developing between ~100-150 ms and maximally pronounced ~200-300 ms after stimulus onset. The topography of the EPN is temporo-occipital with a less pronounced corresponding polarity reversal over fronto-central sensorsites (see Figure 1.5). Furthermore, estimates of the neural generators by current source density (CSD; Junghöfer, Elbert, Leiderer, Berg, & Rockstroh, 1997) and L2-minimum-norm procedures (L2-MMN; Hämäläinen & Ilmoniemi, 1994; Hauk, Keil, Elbert, & Müller, 2002) suggest EPN sources over occipito-temporal-parietal sites (Junghöfer, Bradley, Elbert, & Lang, 2001; Schupp, Stockburger, Codispoti et al., 2007;
Schupp et al., 2006). Of special interest, this early indicator of selective emotion processing co-varies with the level of emotional arousal. Specifically, pictures depicting highly arousing pleasant and unpleasant contents (e.g., erotica, mutilation) prompt more pronounced EPN amplitudes than same valenced stimuli rated lower in emotional arousal (e.g., romance, pollution; Junghöfer et al., 2001; Schupp et al., 2003a, 2003b; Schupp, Junghöfer, Weike, & Hamm, 2004). Further evidence, which suggests that the EPN component is driven by the emotional value of visual stimuli, refers to the antecedent and eliciting conditions. Besides naturalistic photographs of pleasant and unpleasant scenes, also facial expressions and even more abstract cues like emotion-related hand gestures and written words have been shown to be associated with selective processing patterns as indicated by the EPN (Flaisch, Schupp, Renner, & Junghöfer, 2009; Herbert, Junghöfer, &
Kissler, 2008; Kissler, Herbert, Peyk, & Junghöfer, 2007; Schupp, Öhman et al., 2004).
Fig. 1.5. (a) ERP waveforms for pleasant, neutral and unpleasant pictures and (b) topography plots of the EPN (from Schupp et al., 2003a).
Furthermore, the EPN occurs spontaneously in passive viewing conditions (Schupp et al., 2003b; Schupp, Junghöfer et al., 2004), and while participants hold unrelated task goals in mind (Schupp et al., 2003a; Schupp, Stockburger, Bublatzky et al., 2007, 2008).
Thus, indicative for stimulus-driven processing, the EPN effect appears relatively unaffected by stimulus repetition and presentation features such as varying exposure times (Junghöfer et al., 2001; Peyk, Schupp, Keil, Elbert, & Junghöfer, 2009; Schupp et al., 2005, 2006). Accordingly, rather than referring only to low-level perceptual stimulus characteristics (e.g., figure/ground compositions, picture size; cf. Bradley, Hamby, Löw, &
Lang, 2007; DeCesarei & Codispoti, 2006), the emotional arousal modulation appears
secondary to feature-based stimulus identification as indicated by P1-waveform (Schupp, Stockburger, Schmälzle et al., 2008).
However, the EPN appears not as a fully automated processing default. For instance, directing explicit attention to concurrently presented visual task stimuli (i.e., superimposed grid patterns) has been observed to reduce – albeit not abolish – EPN amplitude (Schupp, Stockburger, Bublatzky et al., 2007). Interestingly, indicating no crossmodal resource competition, explicit attention directed to the auditory modality did not markedly reduce EPN amplitude for concurrently presented emotional stimuli (Schupp, Stockburger, Bublatzky et al., 2008). In contrast to competition designs, paying explicit attention to emotional significant rather than neutral cues did not further potentiate the EPN (Schupp, Stockburger, Codispoti et al., 2007).
In summary, recent research suggested the EPN to reflect the facilitated processing of visual scenes depicting information of emotional and motivational significance (Schupp et al., 2006). Accordingly, motivational systems regulating motor output (i.e., favoring approach or avoidance dispositions) were suggested to modulate already early sensory encoding stages facilitating detection and categorization of significant stimuli (Schupp et al., 2003b).
The Late Positive Potential (LPP)
Subsequent to early perceptual encoding stages, pronounced late positive potentials (LPP) over parietal sensorsites have been consistently observed for emotional in comparison to neutral cues (maximum ~400-600ms; Cacioppo, Crites, Gardner, &
Berntson, 1994; Cuthbert et al., 2000; Del Cul et al., 2007; Keil et al., 2002; Palomba et al., 1997; Schupp et al., 2000, 2003b). Similar to the EPN modulation, the LPP varies as a function of affective arousal. Specifically, the higher a picture is evaluated in emotional
arousal, the stronger pronounced is the differentiation between emotional and neutral cues (see Figure 1.6; Schupp et al., 2000; Schupp, Cuthbert et al., 2004; Schupp, Junghöfer et al., 2004). Furthermore, the affective modulation of the LPP has been shown to be a robust phenomenon relatively unaffected by stimulus familiarity, physical characteristics (e.g., stimulus size, complexity) and terms of presentation (e.g., mixed-blocked presentation, exposure time; Bradley et al., 2007; Codispoti, Ferrari, & Bradley, 2006, 2007; DeCesarei
& Codispoti, 2006; Pastor et al., 2008). Extending the LPP time window up to several seconds, a prolonged positive slow wave over centro-parietal sensorsites differentiates between emotional and neutral cues (Cuthbert et al., 2000; Pastor et al., 2008). In contrast to the EPN modulation, the LPP amplitude has been observed to be enhanced by actively paying attention to affective stimuli (Schupp, Stockburger, Codispoti et al., 2007).
Fig. 1.6. ERP waveforms for a representative parietal sensor when viewing specific pleasant, neutral and unpleasant picture contents illustrating LPP and slow waves. Note positivity is plotted downwards (from Schupp, Cuthbert et al., 2004).
Seen from a broader perspective, emotionally arousing stimuli are assumed to activate motivational brain circuits, prompting a sustained attentional set (Bradley, 2000;
Lang et al., 1997; Hamm et al., 2003; Schupp, Cuthbert et al., 2004). Thought to display processes of stimulus evaluation subsequent to perceptual encoding, the LPP has been established as a sensitive measure of attention indicative for capacity-limited processing, representation in working memory and conscious recognition (Donchin & Coles, 1988;
Kranczioch, Debener, & Engel, 2003; Luck et al., 2000; Nieuwenhuis et al., 2005; Schupp et al., 2006).
An Integrative Perspective of Selective Attention
In terms of polarity, latency and topography, both the EPN and LPP component closely resemble findings from cognitive psychology on explicit selective attention processes. That is, paying attention to distinct stimulus features (e.g., color, shape) or higher-order categorizations based on semantic meaning (e.g., animal vs. non-animal) has been shown to be associated with a selection negativity (SN; occipital, ~120 ms poststimulus) and the P3 component (parietal, ~300-600 ms; Codispoti, Ferrari, Junghöfer,
& Schupp, 2006; Delorme, Rousselet, Mace, & Fabre-Thorpe, 2004; Donchin & Coles, 1988; Hillyard & Münte, 1984; Johnson, 1988; Kok, 2001; Nieuwenhuis et al., 2005;
Picton, 1992; Potts & Tucker, 2001; Schupp et al., 2003a, 2006; Smid, Jakob, & Heinze, 1999).
Integrating both the emotion and cognitive approach, implicit emotional and explicitly instructed stimulus significance are suggested to guide selective attention and draw processing resources towards meaningful environmental stimuli. Based on sensory and low-level perception (e.g., feature detection and integration; Treisman, 2006; Treisman
& Gelade, 1980), early perceptual-evaluative processes are suggested to extract potentially
significant stimuli from sensory input overflow. At this early transitory processing stage, ERP components such as EPN and SN may serve as a measure of stimulus evaluation in perceptual representation regions, i.e., striate and extrastriate visual cortex (Junghöfer et al., 2001; Schupp et al., 2003b, 2006). Significant information, already tagged for preferential processing, is suggested to access higher-order capacity limited processing stages (as reflected by LPP and P3 components) implicated in elaborate high-accuracy processing, stimulus consolidation and conscious recognition (Kranczioch, Debener, &
Engel, 2003; Öhman, 1986; Sergent, Baillet, Dehaene, 2005; Schupp, Stockburger, Codispoti et al., 2007; Schupp et al., 2006; Wickens, Kramer, Vanasse, & Donchin, 1983;
Vogel, Luck, & Shapiro, 1998). In transfer to a natural complex environment, emotionally significant stimuli presumably receive preferential processing throughout both early and late processing stages. Thus, accurate situational information based on motivationally guided attention, provides the fundament to efficiently organize appropriate behavior and action (Lang et al., 1997; Öhman et al., 2000).
Summary
Based on fundamental motivational systems (defensive and appetitive), emotions are considered as action dispositions providing the readiness to adequately act and respond in a given situation. As a method to elicit emotions and simulate environmental conditions in a laboratory setting, the picture viewing paradigm is introduced and related experimental research reviewed.
According to the motivated attention theory, emotional significant stimuli guide selective attention and receive enhanced processing. Suggested to reflect such attentional processes, the early posterior negativity and late positive potentials for emotional as opposed to neutral picture cues are discussed. Building upon facilitated processing of
emotional significant stimuli, physiological systems are primed to respond fast and accurately within an environmental context. This motivational priming account is supported by a broad array of data showing context-dependent modulation in defensive reflexes (e.g., startle response), autonomic (e.g., heart rate, skin conductance) and behavioral parameters (e.g., reaction time, avoidance behavior).