Affective valence, stimulus attributes, and P300: Color vs. black/white and normal vs. scrambled images

Abstract

Pictures from the International Affective Picture System (IAPS) were selected to manipulate affective valence (unpleasant, neutral, pleasant) while keeping arousal level the same. The pictures were presented in an oddball paradigm, with a visual pattern used as the standard stimulus. Subjects pressed a button whenever a target was detected. Experiment 1 presented normal pictures in color and black/white. Control stimuli were constructed for both the color and black/white conditions by randomly rearranging 1 cm square fragments of each original picture to produce a “scrambled” image. Experiment 2 presented the same normal color pictures with large, medium, and small scrambled condition (2, 1, and 0.5 cm squares). The P300 event-related brain potential demonstrated larger amplitudes over frontal areas for positive compared to negative or neutral images for normal color pictures in both experiments. Attenuated and nonsignificant valence effects were obtained for black/white images. Scrambled stimuli in each study yielded no valence effects but demonstrated typical P300 topography that increased from frontal to parietal areas. The findings suggest that P300 amplitude is sensitive to affective picture valence in the absence of stimulus arousal differences, and that stimulus color contributes to ERP valence effects.

Introduction

The affective components of arousal (calm to exciting) and valence (unpleasant to pleasant) contribute to the quantity and quality of emotional experience (Bradley and Lang, 1994). For example, during free viewing, participants spend more time on arousing images compared to those rated as less arousing (Lang et al., 1993). Indeed, event-related potential (ERP) studies have suggested that highly arousing affective pictures elicit larger positive-going waveforms than less intense images (Maratos et al., 2000, Schupp et al., 2000). Differences in valence dimensions also can modulate ERP outcomes (Bernat et al., 2001), with positive and negative affective pictures producing overall larger positive-going ERPs than neutral control pictures (Cuthbert et al., 2000, Dolcos and Cabeza, 2002, Schupp et al., 2004). Furthermore, negative valence stimuli generate higher emotional intensity than those with positive valence—the so called “negative bias” effect (Ito et al., 1998, Lang et al., 1990). In most ERP studies, however, arousal and valence stimulus characteristics are not independent, so that valence differences may originate more from arousal than stimulus valence content (Olofsson et al., 2008).

Color may be a contributing factor in the affective picture processing since the manipulation of hue, saturation, and brightness can influence emotional responses (Hupka et al., 1997, Kaya and Epps, 2004). Subjects report feelings of pleasantness in conjunction with the brightening of an image, while darkening it produces unpleasant affect (Marston, 1927). Moreover, different colors elicit differential arousal levels: subjective ratings of green–yellow, blue–green, and green were reported to be the most arousing colors, whereas purple–blue and yellow–red were the least arousing (Valdez and Mehrabian, 1994). As arousal level contributes to the P300 and other ERP component amplitude, stimulus color may contribute to affective findings (cf. Cuthbert et al., 2000, Polich and Kok, 1995).

In addition to color, variables such as featural size can alter ERP measures (e.g., Covington and Polich, 1996, Polich et al., 1996). Indeed, the physical attributes such as overall complexity and featural composition that define affective content appear to help determine emotional reactivity (Bradley et al., 2007, Codispoti et al., 2006a, Codispoti et al., 2006b). Exactly how stimulus attributes interact with affective processes is uncertain, but evaluation of fundamental properties such as color and featural construction are likely to be involved (Dolcos and Cabeza, 2002, LeDoux, 1995). Physical stimulus characteristics may therefore contribute to the automatic and attentional processes that underlie early and late affective ERP responsivity (cf. Conroy and Polich, 2007, Öhman and Mineka, 2001, Rozenkrants et al., 2008, Schupp et al., 2003).

The present study was designed to address these issues by assessing affective processing of visual images that varied systematically in valence while keeping arousal level constant. Experiment 1 evaluated affective pictures presented in color and black/white. Control stimuli for each condition were constructed by rearranging square sections of each picture so that their visual image characteristics were distorted. Experiment 2 presented the same normal color pictures and three scrambled stimulus conditions that varied in section size to assess ERP valence effects and how the degree of distortion contributes to affective stimulus processing.