An advantage in detecting the emotional valence of negative (compared to positive) words was found in all three experiments. In an emotional categorization task that required that participants decide whether a word was neutral or emotional, Experiment 1 showed better accuracy (hit rates) a well as higher sensitivity (measured with criterion-free d` scores) for briefly presented negative (compared to positive) words. Experiment 2 extended this result to conditions under which participants reported they were guessing and so showed no subjective awareness of the words’ valence. The negative-word advantage was found despite positive and negative words being equated for extremeness of emotional valence (as assessed by the participants’ postexperiment ratings), and despite comparing the negative words with positive words of the same or higher lexical frequency. An effect of negative valence on a measure of response bias was found under the long-duration presentation conditions of Experiment 1, but not under the short-duration conditions of that experiment or the reported guessing conditions of Experiment 2. Finally, Experiment 3 verified that an advantage for detecting emotional valence in negative (compared to positive) words is evident even when differences in the arousal induced by negative and positive words are ruled out as an alternative account: Higher accuracy was still found for negative words when controlling for individual participants’ arousal (in addition to valence) ratings, and despite negative and positive words appearing in the same blocks, ruling out both differences in individual words’ arousal ratings and differences in the overall levels of arousal in different blocks as alternative accounts for the negative valence detection advantage.
The intermixed block design of Experiment 3 also allows us to rule out another potential alternative account for the effects of negative valence. When valence was blocked (as in Experiments 1 and 2) one might suggest that the enhanced accuracy and sensitivity found for negative valence was in fact the result of emotionally neutral words being better categorized as such when the choice was between them and negative, rather than positive, words. If neutral words were more distinct from negative than from positive words, this implies that in the context of negative words they were perceived as being further away from that category—that is, more positive; in contrast, in the context of positive words they were not perceived as more negative (or if they were, this difference was not as extreme as the difference in a negative-word context). This, however, cannot explain the negative valence advantage found in Experiment 3, in which neutral words were intermixed with positive and negative words in the same block. Furthermore, any general (noncontext driven) bias toward perceiving neutral words as more positive (and for such words to therefore stand out more among negative than positive words) may have also been manifested in the subjective valence ratings that our participants provided at the end of the experiment. Across all three experiments, however, all our participants consistently rated all of the neutral words as neutral (rather than as somewhat more positive). Our exclusion procedure, based on the individual subjective valence ratings, also ensured that neither the positive nor negative valence categories used in our analysis was closer to the neutral category. We note, however, that as Experiment 3 required detection of emotional valence without specifying valence identity (positive or negative), our conclusion, when arousal is controlled for, is limited to the presence of emotional valence being better detected in negative than in positive words. It remains for future work to clarify whether, when controlling for arousal, identification of valence identity is better for negative than for positive words.
The present findings resolve the previous discrepancies in the results of previous studies investigating the effects of word valence on perceptual processing (Dijksterhuis & Aarts, 2003
; Snodgrass & Harring, 2005
; Zeelenberg et al., 2006
). Using a large corpus of words (more than five times as many words as were used in each condition than in any of the previous studies), and ruling out alternative accounts in terms of idiosyncratic valence ratings, words frequency, uniqueness and arousal, a clear negative valence detection advantage emerges.
Our findings are consistent with previous demonstrations that negative words (Anderson, 2005
; Anderson & Phelps, 2001
; Keil & Ihssen, 2004
; Ogawa & Suzuki, 2004
), as well as nonverbal, fearful face stimuli (e.g., Milders, Sahraie, Logan, & Donnellon, 2006
) are more likely than the corresponding neutral stimuli to escape the attentional blink (Shapiro et al., 1997
), indicating that negative stimuli may have preferential access to processing resources. Similarly, in a different task requiring word naming, negative words were found to have a lower duration threshold for accurate naming than neutral words (Gaillard et al., 2006
). The question of whether this advantage was due to the negative valence or whether it could be accounted for by emotional valence in general (in which case a similar advantage should be found for positive word valence as well) has not been answered conclusively by attentional blink studies: Ogawa and Suzuki (2004)
found no advantage for positive over neutral words, whereas Anderson (2005)
did, but showed that this advantage was attributable to arousal. Importantly, note that none of the above studies compared positive and negative words within the same experiment. Recently, Kihara and Osaka (2008)
equated the arousal of negative and positive words within the same attentional blink experiments, and found an advantage (i.e., the attentional blink was reduced) for negative, compared to positive, words. Though differential arousal rates induced by different valences may contribute to differences in successful processing of emotional stimuli, the results of Kihara and Osaka (2008)
, as well as those of Experiment 3 of the present study, suggest that words with negative valence are preferentially processed, compared to positive words, independently of any effect of arousal. This clearly establishes an advantage for negative word processing rather than a general effect of emotional valence.
It is interesting to note that when Gaillard et al. (2006)
found better naming accuracy for negative words, they also found that participants’ naming errors were more likely than would be predicted by chance to consist of naming a (wrong) negative word after a negative than after a neutral target. This implies that even if the target word had not been perceived at a level that enabled naming, its valence was nonetheless processed and influenced the subsequent response. The absence of positive words in that study, however, precludes any conclusions regarding the present issue of sensitivity to negative valence rather than to any emotion. Like Gaillard et al. (2006)
, in the present study we also find that negative valence can be extracted from a written word even if participants claim to be guessing what the target valence was. Unlike Gaillard et al. (2006)
, however, we used positive words as well and found that the ability to extract valence information, distinguishing an emotional word from a neutral one, is specific to negative words.
Further research will have to elucidate the mechanism underlying the present findings. Speculatively, the present results suggest that there may be a relationship between word valence and the speed of information accrual, leading to differences in valence detection for different valences. Categorization of stimuli as negative or positive is made based on very little information (e.g., Murphy & Zajonc, 1993
). Assuming that the amount of information available about a stimulus is a monotonic function of exposure duration, our findings suggest that negative (compared to positive) valence either increases the rate of information accrual or requires less information to be available for correct categorization. This conjecture is indirectly supported by the finding that attention speeds perceptual information processing (Carrasco & McElree, 2001
) coupled with research showing that emotional stimuli attract attention (e.g., in emotional Stroop tasks; McKenna & Sharma, 1995
; Pratto & John, 1991
). For orthographic stimuli, the attention-grabbing effect of emotional stimuli may be limited to tasks requiring semantic analysis (rather than phonological or graphic analysis; Huang et al., 2008
), implicating the lexical-semantic system in mediating the negative-word advantage. In the present study, however, attention was not manipulated and therefore the involvement of attentional processes in the negative-word advantage cannot be directly inferred from our results.
Alternative (though nonexclusive) possibilities include the idea that in accordance with the importance of swift judgments regarding potentially threatening information, the lexical system is organized so that negative words are more easily accessed than positive (or neutral) words. Postulating this, however, is insufficient to account for the present results as it still does not explain how valence information can be extracted from words even when full semantic information is not, as in Experiment 2 of the present study. A different possibility is that negative words are consolidated into working memory faster than positive or neutral ones; the stronger memory trace may lead to a greater likelihood of reporting negative emotional valence, even if the word itself is rapidly forgotten after very brief presentations.
The present findings may also have implications for understanding the neural mechanisms of emotion perception. An ongoing debate (Pessoa, 2005
) concerns the question of whether emotional stimuli are processed automatically, and whether their processing may not even require awareness. Whereas some neuroimaging studies have found that emotional stimuli caused activation in brain regions known to process emotional information (e.g., the amygdala) regardless of attentional manipulations and even when participants were not aware of the stimuli (Etkin et al., 2004
; Morris, Ohman, & Dolan, 1998
; Vuilleumier, Armony, Driver, & Dolan, 2001
; Vuilleumier, Armony, Clarke, Husain, Driver, & Dolan, 2002
; Whalen et al., 1998
), others have not found such activity in the absence of awareness and have in fact shown that the availability of attention is required for such activity to arise (Pessoa, McKenna, Gutierrez, & Ungerleider, 2002
; Pessoa, Japee, Sturman, & Ungerleider, 2006
). Here we have shown that emotional valence information can affect guessing behavior such that performance can exceed chance despite participants claiming they are unaware of the stimulus valence. The apparent discrepancy between these results and those demonstrating a dependence of emotional processing on the availability of attentional resources may be resolved by recent findings from our lab, which showed that attention can affect both neural activity (Bahrami, Lavie, & Rees, 2007
) and behavioral measures (Bahrami, Carmel, Walsh, Rees, & Lavie, 2008a
) induced by stimuli that participants were entirely unaware of. This suggests that the negative words of the present study may have captured attention even in the absence of awareness, leading to enhanced processing of the relevant stimulus dimension—valence. Future work will have to address this possibility by manipulating the attention directed at unconscious emotional stimuli.