Emotion and motivated behavior: postural adjustments to affective picture viewing

Abstract

Thirty-six participants (18 female, 18 male) viewed affective pictures to investigate the coupling between emotional reactions and motivated behavior. Framed within the biphasic theory of emotion, the three systems approach was employed by collecting measures of subjective report, expressive physiology, and motivated behavior. Postural adjustments associated with viewing affective pictures were measured. Results indicated sex-differences for postural responses to unpleasant pictures; an effect not found for pleasant and neutral picture contents. Females exhibited increased postural movement in the posterior direction, and males exhibited increased movement in the anterior direction, for unpleasant pictures. Subjective report of valence and arousal using the self-assessment manikin (SAM), and the startle eye-blink reflex were collected during a separate session, which replicated previous picture-viewing research. Specifically, participants rated pleasant pictures higher in valence and exhibited smaller startle responses compared to unpleasant pictures. Females also reported lower valence ratings compared to males across all picture contents. These findings extend our knowledge of motivated engagement with affective stimuli and indicate that postural responses may provide insight into sex-related differences in withdrawal behavior.

Introduction

Darwin first described a relationship between emotion and posture in his observation of animals and humans (Darwin, 1872). Inherent in this observation is the functional link between varying motivational states of emotional reactivity and specific postural responses that reflect accompanying behavior. Contemporary theories of emotion subscribe to the concept that overt behavior is one of several reactions in an organism’s repertoire that reflect underlying motivational states (see Elliot and Covington, 2001 for review). Yet few researchers have explicitly examined the relation between emotion and posture.

Emotion theorists have long examined motivated behavior using an approach-withdrawal model from many different perspectives including: neuroanatomical (LeDoux, 1995), psychophysiological (Davidson et al., 1990), personality (Elliot and Thrash, 2002), goal attainment (Forster et al., 1998), attitudes (Cacioppo et al., 1993), and social (Zajonc, 1998). Collectively, these findings have led Elliot and Covington (2001) to contend, “approach-avoidance is not just an important motivational distinction, but that it is fundamental and basic, and should be construed as the foundation on which other motivational distinctions rest” (p. 74).

One contemporary theory of emotion, the biphasic theory of emotion (Lang, 1985, Lang, 2000, Lang et al., 1997), contends that emotion is fundamentally organized around two basic motivational systems: appetitive and defensive (Lang, 2000). The appetitive system is responsible for approach behaviors and involves preservative actions that underlie pleasant reactions (Bradley et al., 2001a). The defensive system is responsible for withdrawal or avoidance behavior that is activated in the context of threat and underlies unpleasant reactions (Bradley et al., 2001a, Lang, 2000). These two systems are mediated by brain circuits that have evolved to organize behavior for the purpose of survival (Lang, 2000).

In this context, emotions are considered to be action dispositions that organize behavior along an approach-withdrawal dimension. The direction (i.e., approach-withdrawal) of an organism’s reaction results from the affectively motivated significance of a stimulus with responses biphasically organized along two dimensions: hedonic valence (i.e., pleasant-appetitive motivation or unpleasant-defensive motivation) and arousal (degree of motivational activation; Bradley et al., 2001a, Lang, 1985). These responses reflect the organism’s underlying emotional valence evaluation of the stimulus, which, in turn, prompts approach or withdrawal reactions. Lang, 1968, Lang, 2000 proposed that emotions are motivational states of readiness that prime reactions through three different systems: expressive and evaluative language (i.e., verbal report), physiological changes mediated by the somatic and autonomic nervous system (e.g., expressive physiological changes such as facial electromyography—EMG), and behavioral sequelae. Although a preponderance of affective research has incorporated verbal report and expressive physiology, few studies have examined behavioral concomitants of emotional reactions.

Of the prior research that has examined the behavioral response system, two different approaches have been explored. One approach manipulated postural set to evoke emotional responses. Specifically, Riskind and Gotay (1982) instructed participants to assume either a slumped physical posture or an expansive physical posture, and Duclos et al. (1989) had participants adopt postures characteristic of fear, anger, and sadness. In both studies participants were instructed to report emotional responses using subjective report inventories during or immediately following the specific posture manipulations. Although related to the topic of emotion and posture, these studies are not directly related to the present focus on approach-withdrawal behaviors, since postural manipulation was used to examine changes in specific emotional states without regard for the approach-withdrawal dichotomy. Alternatively, Cacioppo et al. (1993) examined approach-withdrawal behavior by having participants engage in either isometric arm flexion (approach) or extension (withdrawal) while making attitudinal judgments. Collectively, results from these three studies suggested increased pleasant emotion when participants were placed in positive postural sets (e.g., arm flexion) and increased unpleasant emotion when participants assumed negative postural sets (e.g., arm extension).

The second approach examined reaction time (RT) to the onset of emotional pictures and viewing time (i.e., the length of time a participant voluntarily chooses to watch a picture) to study the relationship between emotional stimuli and motivated behavior. These studies have shown that when viewing novel pictures, participants exhibit delayed RT responses during more arousing relative to less arousing images, regardless of hedonic valence, suggesting that novel stimuli may utilize more attentional resources during encoding processes (Bradley et al., 1992, Lang et al., 1997). This effect was especially pronounced for highly arousing unpleasant stimuli (Lang et al., 1997). With regard to viewing time, participants have been observed to watch pleasant and unpleasant pictures longer than neutral ones (Hamm et al., 1995). Results from both studies indicate that these measures reflect behavioral concomitants of arousal, rather than hedonic valence.

In addition, sex-related differences in reactions to affective pictures have been reported (Bradley et al., 2001b). Generally, females react with a greater defensive set when viewing highly arousing unpleasant pictures (e.g., scenes of threat and mutilation), and males have displayed greater appetitive reactions to specific pleasant contents (e.g., erotica), when measured using expressive physiological and self-report measures. For example, in response to unpleasant pictures females exhibit greater modulation of the startle eye-blink reflex and report decreased valence and increased arousal, compared to males. Alternatively, males rate pleasant pictures higher in valence and arousal and respond with greater inhibition of the startle-blink reflex, relative to females (Bradley et al., 2001b). Sex-related differences have also been reported for other physiological measures such as skin conductance and facial EMG. Bradley et al. (2001b) point out that despite the fact that “cues that activate the appetitive and defensive systems should be potent for both men and women, who share survival risks … differential activation of these motivational systems could arise”. Accordingly, various biological and sociocultural hypotheses for sex-related differences in affective responding have been posited (Bradley et al., 2001b).

The purpose of the present study was to examine an alternative measure of behavior that has been used in the study of balance, changes in center of pressure (COP). COP serves as a summary measure of movement made above the surface of support. Balance is dependent upon the integration of information from proprioceptive, vestibular, and sensory systems (Redfern et al., 2001), and reflects the dynamic coupling of perception and action related to the regulation of movement (Bertenthal et al., 1997). COP is typically measured with a force platform. This device, usually a flat metal surface mounted over force transducers, measures changes in the forces exerted on the surface. These forces are used to calculate an overall COP. During quiet standing, COP has a limited range of movement (only a few cm) and is highly correlated with movement of the center of mass (COM). COP is traditionally examined separately for changes in movement in the anterior–posterior and medial–lateral directions. To date, few studies of postural control have examine postural responses to emotion-laden stimuli.

The goal of this study was to examine the influence that pictures, which varied in affective content, would have on postural behavior. It was hypothesized that COP would reflect approach-withdrawal behavior with unpleasant pictures prompting a shift in the COP away from the picture and pleasant pictures prompting a shift in the COP toward the picture, relative to neutral pictures. For the purpose of better understanding postural data, the startle eye-blink reflex and subjective reports of valence and arousal were also measured. Replicating previous research, it was hypothesized that pleasant pictures would elicit inhibition of the eye-blink reflex and higher valence and arousal ratings, and unpleasant pictures would elicit potentiation of the eye-blink reflex and lower valence and higher arousal ratings, compared to neutral pictures. Lastly, based on the findings of Bradley et al. (2001b), sex-differences were expected, such that females would exhibit responses across all measures that were indicative of greater withdrawal from unpleasant pictures and males would exhibit responses that were indicative of greater approach to pleasant pictures.