Research ReportAffective picture perception: Emotion, context, and the late positive potential
Introduction
Bioelectric measures of neural activity at the scalp using event-related potentials (ERPs) result in reliable differences when viewing affective or neutral pictures. In a number of different studies, emotionally arousing (pleasant and unpleasant) pictures elicit a larger late positive potential (LPP), compared to neutral pictures, which develops around 300–400 ms following picture onset, lasts for several hundred milliseconds and is maximal over centro-parietal sites (Cacioppo et al., 1993, Cacioppo et al., 1994, Cuthbert et al., 2000, Keil et al., 2002, Palomba et al., 1997, Schupp et al., 2000, Schupp et al., 2003, Schupp et al., 2004). Using a slow time constant, Cuthbert et al. (2000) also noted an extended centro-parietal positive slow wave that was significantly larger for affective than neutral pictures, and was sustained over a 6-s picture viewing period. These findings have been interpreted in terms of motivated attention, in which emotional pictures naturally engage attentional resources as a result of activation in fundamental motivational systems mediating appropriate survival behaviors (Lang et al., 1997).
Modulation of the late positive potential when viewing emotional, compared to neutral, pictures has proven to be a stable, replicable finding. Moreover, affective differentiation in the late positive potential remains intact despite multiple repetitions (up to 60) of the identical pleasant, neutral, and unpleasant pictures (Codispoti et al., 2006b, Codispoti et al., 2007), suggesting that the enhanced LPP during emotional picture processing may be obligatory, reflecting processing essential in initial semantic categorization. In the current study, we assessed ERPs to pleasant, neutral and unpleasant pictures that were presented in blocks of similarly valenced stimuli, and compared these ERPs to those elicited when viewing affective pictures intermixed with neutral stimuli. When pictures are presented in blocks of similarly valenced material, hedonic valence remains the same from trial to trial, presumably eliminating some of the evaluative processing. To the extent that the LPP reflects the result of an initial evaluative categorization (i.e., “good” or “bad”), its modulation by emotional arousal could be attenuated in a blocked context, in which this categorization is aided by previous stimulus information. On the other hand, if the LPP reflects an obligatory attention allocation to emotionally engaging stimuli, presentation context should not affect the magnitude of the LPP.
Previous studies have suggested that affective discrimination of this late ERP component is relatively unaffected by a variety of stimulus and context manipulations. For example, Cuthbert et al. (1995) found similar ERP modulation when either passively viewing pictures, or making explicit evaluative ratings. Relatedly, Cardinale et al. (2005) found that affective modulation of the LPP was present in the context of making non-affective categorizations. Codispoti and coworkers also reported that modulation of the late positive potential by affect was unaffected when a simultaneous non-affective task was presented at the fovea (Codispoti et al., 2006a). Moreover, similar modulation by emotion is found regardless of stimulus size (De Cesarei and Codispoti, 2006), duration (Codispoti et al., 2007) or complexity (Bradley et al., 2007). Taken together, these findings suggest that affective modulation of the LPP is a robust phenomenon that is not strongly influenced by perceptual factors (e.g., stimulus size, complexity), familiarity, or task requirements. On the other hand, in all of these studies, information regarding picture affect was not available prior to presentation.
Several studies that have focused on earlier ERP components (150–300 ms) have found that emotional pictures also prompt less positivity over occipital sites, compared to neutral pictures (e.g., Codispoti et al., 2007, Schupp et al., 2003). This earlier component, however, is also strongly affected by perceptual complexity (Bradley et al., 2007) and is most pronounced when viewing pleasant, particularly erotic, pictures (De Cesarei and Codispoti, 2006, Schupp et al., 2004), which may be mediated by featural overlap. If so, we expected to find increased occipital positivity in this early time window for pleasant pictures in the current study as well.
Heart rate and skin conductance activity were measured as additional indices of processing that might inform any differences found as a function of the context of presentation. When pictures are mixed in hedonic valence, unpleasant pictures prompt significantly greater cardiac deceleration than either neutral or pleasant pictures, which we have interpreted as indexing heightened sensory intake (e.g., Bradley et al., 2001), drawing on previous data and theory linking cardiac deceleration to orienting activity (Graham, 1992). Previous studies have indicated that this cardiac component quickly disappears with picture repetition (Bradley et al., 1993), suggesting it may be related to initial evaluative processing required for determining hedonic valence. If so, we expected that differences in cardiac deceleration would be attenuated when pictures are presented in hedonically similar blocks.
Skin conductance magnitude typically varies with emotional arousal, with unpleasant or pleasant pictures eliciting larger responses than neutral stimuli in an intermixed presentation context (e.g., Lang et al., 1993, Bradley et al., 2001). To the extent that skin conductance changes are due to novelty, surprise or unexpectedness (e.g., suddenly seeing a picture of a mutilated body), which are also known to modulate electrodermal responsivity (e.g., Maltzman and Boyd, 1984), blocked presentation could attenuate the differences between emotional and neutral stimuli. On the other hand, if skin conductance reflects the intensity with which motivational systems are activated by the picture cue, as we have hypothesized (e.g., Lang et al., 1993), the same pattern of modulation was expected regardless of presentation context.
Section snippets
Event-related potentials
Fig. 1 illustrates ERP waveforms at representative fronto-central, centro-parietal, and occipital sensors when pictures were presented in a blocked or mixed presentation mode.
Early time window (150–300 ms)
In the early time window, Picture Content modulated ERPs over occipital, F(2,72) = 11.21, p < 0.001, and fronto-central sensors, F(2,72) = 8.94, p < 0.001, with pleasant pictures showing less positivity over occipital sites and less negativity over fronto-central sites compared to either neutral (Fs(1,36) > 8.96, ps < 0.005), or
Discussion
The present study replicates and extends prior ERP research during affective picture processing, manipulating the context of picture presentation. In general, the current results suggest that whether emotional pictures were presented in blocks of similarly valenced content, or in an intermixed presentation mode, ERPs measured during sustained picture viewing were strikingly similar. In both cases, pleasant and unpleasant pictures prompted a larger late positive potential (400–700 ms) than
Participants
Forty-one participants (21 men, 20 women; 18–22 years old) from a University of Florida General Psychology course participated as part of a class requirement. Half of the participants viewed pictures presented in blocks of similarly valenced stimuli (11 men, 10 women); the other half-viewed pictures in a mixed condition (10 men, 10 women), in which the hedonic valence of the picture varied from trial to trial. Some participants were excluded on the basis of artifacts in the physiological data.
Acknowledgments
This research was supported in part by grants from the National Institute of Mental Health (P50 MH 72850) to P. J. Lang, and the National Institute of Dental Research (DE 13956) to M.M. Bradley, and by post-doctoral fellowships from Generalitat Valenciana (CTESPB/2002/11) and Jaume I University (E-2004-14) to M.C. Pastor.
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