Emotion, Etmnooi, or Emitoon? – Faster lexical access to emotional than to neutral words during reading

Abstract

Cortical processing of emotional words differs from that of neutral words. Using EEG event-related potentials (ERPs), the present study examines the functional stage(s) of this differentiation. Positive, negative, and neutral nouns were randomly mixed with pseudowords and letter strings derived from words within each valence and presented for reading while participants’ EEG was recorded. Results indicated emotion effects in the N1 (110–140 ms), early posterior negativity (EPN, 216–320) and late positive potential (LPP, 432–500 ms) time windows. Across valence, orthographic word-form effects occurred from about 180 ms after stimulus presentation. Crucially, in emotional words, lexicality effects (real words versus pseudowords) were identified from 216 ms, words being more negative over posterior cortex, coinciding with EPN effects, whereas neutral words differed from pseudowords only after 320 ms. Emotional content affects word processing at pre-lexical, lexical and post-lexical levels, but remarkably lexical access to emotional words is faster than access to neutral words.

Introduction

Visual processing of emotional words differs from visual processing of neutral words. Using electroencephalographic event-related potentials (ERP) emotional–neutral differences have been identified at various temporal stages following word onset in lexical-decision, evaluation, or reading tasks (for review see Kissler et al., 2006). Several ERP studies have described more negative-going ERPs for emotional words over occipital cortex between 200 and 300 ms (Herbert et al., 2008, Kissler et al., 2007, Kissler et al., 2009, Scott et al., 2009) or between 300 and 400 ms (Palazova et al., 2011, Schacht and Sommer, 2009a, Schacht and Sommer, 2009b) after word presentation. Collectively, these effects are referred to as emotion driven early posterior negativities (EPN). Morphologically analogous effects have been reported in emotional face (Schupp et al., 2004), scene (Junghofer et al., 2001), or gesture processing (Flaisch et al., 2011), and in explicit object-based attention tasks (Hillyard and Anllo-Vento, 1998, Schoenfeld et al., 2007), implying that early emotion effects in this time window reflect attentional highlighting of emotional stimuli in general (Schupp et al., 2006).

Larger parietal positivities arising around 500 ms after word onset have also been reported in emotion word processing (Fischler and Bradley, 2006, Herbert et al., 2006, Schacht and Sommer, 2009b), as well as in the processing of emotional scenes, faces, or gestures. Again, parallel effects are found in explicit attention tasks. Occasionally, effects of emotional content on the N400 component, a classic index of contextual semantic integration, have been found (Herbert et al., 2008, Kiehl et al., 1999), suggesting facilitated semantic integration of emotional words after a first initial attentional processing stage. Also, emotion effects arising earlier than 200 ms after word onset have been documented (Begleiter and Platz, 1969, Hofmann et al., 2009, Ortigue et al., 2004, Skrandies, 1998, Skrandies et al., 1998). These very early effects, in particular, support the view that emotional processing can operate even pre-attentively, outside the boundaries set by other information-processing mechanisms, as proposed by automatic vigilance (Pratto and John, 1991) or automatic evaluation models (Zajonc, 1980). In the context of language processing, such very early effects suggest the possibility of pre-lexical responses to emotional content.

At least under conditions of low to moderate competing cognitive load, emotional stimuli, including words, are preferentially processed. Still, it is debated whether ERP emotion effects are specific to a particular category or dimension of emotion. Whereas much data suggest a major impact of emotional intensity, i.e. stimulus arousal, on emotion word processing (Fischler and Bradley, 2006, Kissler et al., 2007, Schacht and Sommer, 2009b), some results indicate specific effects for negative (Bernat et al., 2001) or positive words (Kissler and Koessler, 2011). These discrepancies are reflected in different theoretical accounts. According to one model, rapid automatic allocation of attention specifically to negative stimuli is biologically adaptive in facilitating rapid withdrawal from potentially dangerous environments. Therefore this mechanism may by-pass other cognitive processes (Pratto and John, 1991). Alternatively, attentional orienting to emotionally arousing stimuli in general may be needed to mobilize appropriate approach or avoidance behavior (Lang et al., 1997). Finally, appraisal theory proposes a cascade of processing steps where sequential checks influenced by situational demands determine the patterning of emotional responses, allowing for more flexibility and perhaps reconciling some empirical discrepancies (Grandjean and Scherer, 2008, Scherer, 2009).

As outlined above, ERP effects of emotion in vision are often assumed to reflect spontaneous attentional highlighting of motivationally relevant emotional stimuli. This assumption is supported by analogous ERP effects in object-based selective attention tasks and in line with the general concept of emotionally motivated attention (Lang et al., 1997). Attention and emotion can affect the processing of perceptual objects, including words, at various temporal stages (Luck and Hillyard, 1999, Ruz and Nobre, 2008, Schupp et al., 2007, Vogel et al., 2005, Ziegler et al., 1997). Still, it is controversial, how much perceptual and cognitive processing has to be carried out, before a stimulus’ emotional significance is identified and how early in the processing stream emotion effects can occur. In other words, an important question still is: How much, if any, inference do preferences need?

In emotion word processing the question can be put as: When do emotion effects occur in relation to different processing stages in the mental lexicon? To answer this question, emotion effects can be pitched against a time-line of word recognition derived from models of visual word processing. According to classic serial word processing models (e.g. Coltheart et al., 2001, McClelland and Rumelhart, 1981), increasingly abstract information, including orthographic word form and phonological and lexical properties of a word, is extracted from the visual percept as activation travels from primary visual to higher order, multimodal, association areas of the brain, where information is combined to achieve full comprehension of a word's meaning (for review see Dien, 2009). Word-processing stages are assumed to partly overlap and to be organized in an interactive and cascaded fashion, but the extent of parallel processing is a matter of considerable debate (Barber and Kutas, 2007, Grainger and Holcomb, 2009, Pulvermuller et al., 2009). Traditional models agree that some perceptual invariance has to be extracted from the physical signal, before a word's meaning can be accessed. It is traditionally assumed that orthographic processes take place within the first 250 ms, followed by lexical and semantic access from about 300 ms after stimulus onset, culminating in full semantic contextual integration around 400 ms (Grainger and Holcomb, 2009). However, some models argue for a considerably higher speed of word recognition with parallel or near parallel processing of different attributes and present evidence for semantic processing even within the first 200 ms (e.g. Pulvermuller et al., 2009).

Because models and data differ regarding the speed and stages of word processing itself, identification of the locus of emotion effects along an empirically determined time-line of word processing is needed to inform models of word recognition and emotional processing. From a linguistic perspective, the extent to which emotional content can accelerate or by-pass other stages of word processing has implications for models of lexical access. Acceleration of word processing by emotion is suggested by consistent evidence of faster lexical decisions to emotional than to neutral words (Kousta et al., 2009, Schacht and Sommer, 2009b), but in lexical-decision emotional response facilitation may also play a role (Kissler and Koessler, 2011). Moreover, models such as the ‘automatic vigilance model’, developed from experiments with word stimuli, assume that emotion processing can operate pre-attentively (Pratto and John, 1991), suggesting the existence of short-cut routes, by-passing stages of perceptual analysis.

A linguistic challenge is to account for such effects within models of word recognition and to determine whether they are unique to emotion words or also occur for other semantic classes, and whether they could be mimicked by attention manipulations. From the perspective of emotion theory, data will provide information regarding the temporal evolution of emotion – cognition interactions and the direction of emotion effects at distinct processing stages. On-line measures such as ERPs can reveal such sequences that might be obscured in lexical-decision reaction times alone, which represent a compound measure, integrating the results of many different operations.

So far, two ERP studies investigated the pre-, peri-, or post-lexical status of emotion effects in word processing. Scott et al. (Scott et al., 2009) varied emotional content within high- and low-frequency words in a lexical-decision task. Since frequency effects in visual word processing are considered indicative of lexical access, word frequency by emotion interactions was identified to determine the functional status of emotion effects. Interactions occurred on the N1 around 150 ms and in the subsequent EPN window (200–300 ms). These N1 and EPN effects were assumed to indicate modulations of lexical access by emotional content. An earlier P1 amplitude reduction for negative words was interpreted as a pre-lexical perceptual defense mechanism.

Palazova et al. (2011) examined ERP emotion effects in high- and low-frequency adjectives, verbs, and nouns, likewise using a lexical-decision task. Results confirmed an early frequency effect around 100–150 ms which interacted with emotion. Word–pseudoword differences were found between 250 and 550 ms. Crucially, main effects of emotion were reflected by an EPN potential and a centro-parietal positivity (LPP), whose temporal onset largely coincided (EPN) or followed (LPP) word–pseudoword differentiation, supporting a lexical or post-lexical status of emotion effects.

The present study further investigates the status of emotion effects along the time-line of word processing using a silent reading paradigm: For adults, silent reading is a highly automatic and very natural process (e.g. Kahneman and Chajczyk, 1983). Simultaneously measuring on-going brain activity during reading can reveal emotional modulations irrespective of additional effects of response preparation and execution, which may differ from word-processing-related activity itself. Lexical-decision reaction times are consistently faster for emotion words, with some controversy surrounding the relative role of valence versus arousal (Estes and Adelman, 2008, Kousta et al., 2009). However, in lexical decision many different processes could be affected by emotion. Moreover, lexical decision, while highly successful in identifying word–non-word differences, draws explicit attention to a stimulus’ lexical status, thereby potentially affecting lexicality or emotion effects. In fact, Ziegler et al. (1997) demonstrated that the time course of lexical and semantic activation in word processing is task-dependent, with faster ERP word–pseudoword differentiation in a semantic categorization task than during letter search.

Here, emotion effects in reading are investigated in relation to effects of orthography and lexicality to determine the temporal and functional stage(s) of effects. We present positive, negative, and neutral nouns intermixed with letter strings and pseudowords to identify successively more refined processing stages. Under a serial model, differentiation of orthographically legal word forms (words or pseudowords) from illegal words forms (letter strings) should precede differentiation of real words (with a corresponding lexical entry) from pseudowords (without lexical entry). Emotion effects will be assessed along the time-line of such effects. Our previous studies have used the rapid serial visual presentation (RSVP) technique to study emotion effects in word processing. Although effects have been largely replicated in lexical-decision tasks (Hinojosa et al., 2010, Palazova et al., 2011, Schacht and Sommer, 2009a, Schacht and Sommer, 2009b, Scott et al., 2009), RSVP with its absence of baseline periods and inherent conceptual masking, may create a special experimental situation. Here, we examine emotion effects in single word reading outside RSVP. In particular, we address the timing of emotion effects in relation to orthographic word-form analysis and lexical access in positive, negative, and neutral words, investigating the possibility that emotional content may accelerate or by-pass some of these processes.