Elsevier

Psychiatry Research

Volume 157, Issues 1–3, 15 January 2008, Pages 77-85
Psychiatry Research

Is a neutral face really evaluated as being emotionally neutral?

https://doi.org/10.1016/j.psychres.2007.02.005Get rights and content

Abstract

Most of the functional neuroimaging studies on emotion have used neutral faces as a baseline condition. The aim of the present study was to explore whether prototypical neutral faces are evaluated as displaying neutral emotions. Twenty-one subjects performed the Extrinsic Affective Simon Task (EAST), a validated implicit task that measures the emotional evaluation of target stimuli. All stimuli consisted of two juxtaposed faces from standardized facial pictures. The attribute stimuli (positive vs. negative), which needed to be classified on the basis of extrinsic valence, were presented as black and white facial pictures. The target stimuli were color-filtered positive, negative, neutral, and positive/negative faces, and subjects were instructed to classify them on the basis of the filtered color (blue vs. green). The responses to the positive target faces were associated with the positive emotions and the responses to the negative target faces were associated with the negative emotions. For the neutral faces, the responses were similar to those of negative faces, while for the positive/negative stimuli, the responses were undifferentiated. These findings suggested that prototypical “neutral” faces may be evaluated as negative in some circumstances, which suggests that the inclusion of neutral faces as a baseline condition might introduce an experimental confound in functional neuroimaging studies.

Introduction

The ability to read emotions from the facial expressions of others is crucial to survival for humans (Darwin, 1872/1965). Some researchers have insisted that emotion is, at least in part, read into the face by the observer (Bruner and Tagiuri, 1954, Russell and Fehr, 1987). Certain configurations of facial features which result from specific patterns of facial muscle contractions have been reported to be universally evaluated as certain basic emotions (Ekman, 1992, Ekman, 1999). These basic emotions include happiness, which is a positive emotion, and sadness, anger, fear, and disgust, which are negative emotions, as well as other emotions such as surprise (Ekman, 1992, Ekman, 1999).

To date, most functional neuroimaging studies have conventionally used prototypical neutral faces as a baseline condition for comparing specific facial expressions (Sprengelmeyer et al., 1998, Kesler-West et al., 2001, Pessoa et al., 2002, Kilts et al., 2003; for review, see Phan et al., 2002, Wager and Smith, 2003). However, to our knowledge, no one has addressed whether the prototypical neutral faces are judged to be neutral by subjects during the emotional task of the neuroimaging studies. In most of these studies, the duration and speed of presentation of the emotional stimuli are brief (< 3 s) and fast (the usual interstimulus interval was 2–3 s). In addition, the tasks in numerous studies (Morris et al., 1998, Phillips et al., 1997, Phillips et al., 2001, Phillips et al., 2004) are emotionally implicit, as is indicated by the fact that the limbic regions of the brain are more activated when the task is emotionally implicit (such as gender discrimination) rather than explicit (Phan et al., 2002). In fact, one previous report (Kesler-West et al., 2001) revealed that post-scan ratings of the neutral faces were labeled or identified as happy, angry and fearful, as well as neutral.

Although prototypical neutral faces are judged to be neutral because the poser's facial muscles are relaxed (Ekman and Friesen, 1978, Young et al., 1997), the emotional recognition of prototypical neutral faces may be influenced by several factors such as the experimental context (Russell and Fehr, 1987, Surakka et al., 1999). Neutral faces tend to be evaluated as sad if the preceding expression was a happy expression and vice versa (Russell and Fehr, 1987). Self-report may be another influencing factor that is affected by a variety of distortions such as social norms or investigators’ expectations (Parrot and Hertel, 1999).

To avoid these pitfalls, implicit measures have been used including the priming method and Greenwald's Implicit Association Tests (IATs) (Fazio and Olson, 2003). Standard IATs assess the strength of associations between a bipolar target concept (flower vs. insect) and a bipolar attribute concept (good vs. bad) by comparing the response times for two differently combined discrimination tasks (Greenwald et al., 1998). The effects produced by IATs are typically much larger, and the reliability is much higher than those reported for priming methods (Egloff and Schmukle, 2002). However, IAT measures are limited to the assessment of the relative strength of associations between bipolar concepts (De Houwer, 2003, Teige et al., 2004). Unlike the IAT, the EAST (Extrinsic Affective Simon Task) (De Houwer, 2003) is a non-relative measure of automatic associations between two concepts, which makes it possible to measure the evaluation of multiple items (name of the participant, first name of previous participant, flower, insect, vs. XXXX) at the same time. The EAST is a variation of the Affective Simon Tasks (De Houwer and Eelen, 1998). In the Affective Simon paradigms, participants are asked to give a positive or a negative response to target stimuli on the basis of a specific stimulus feature (e.g. color) while ignoring the valence of the target stimulus. In an EAST experiment, the attribute stimuli (positive vs. negative) are presented as white words, and target stimuli (e.g. flowers vs. insects) are presented as colored words. Each stimulus appears one-by-one on a monitor. Depending on the word's color (blue vs. green), subjects are instructed to discriminate stimuli by pressing a left or a right response key according to different features. The basic assumption of the EAST is that subjects automatically associate the target stimuli (e.g. flowers) with attribute stimuli (e.g. positive), although they are instructed to concentrate exclusively on color for discrimination. Similar to IATs (usually Cohen's d > 0.6), the EAST has a reliable effect size (de Houwer, 2003).

Based on the above literature, we realized that it should be possible to investigate the emotional evaluations of neutral expressions by using the pictorial EAST. The pictorial EAST (as well as the word EAST) was expected to provide valid and direct information for effective evaluation (Huijding and de Jong, 2005). The aim of the present study was to determine whether prototypical neutral faces were evaluated as neutral. We measured the emotional evaluation performances of the stimuli of positive, negative and neutral faces with the EAST. Positive/negative stimuli with simultaneous positive and negative valences were added to verify whether the EAST reflects a separate estimate of the emotional evaluation of neutral as well as positive and negative stimuli, since positive and negative faces of positive/negative stimuli will compete and leave relative differences in positive and negative responses undifferentiated. We expected that the responses to the positive and negative faces would be associated with the positive and negative, respectively. We selected happy, neutral, and fearful faces from standardized facial pictures (Ekman and Friesen, 1976) and constructed the positive, negative, neutral, and positive/negative stimuli, which were prepared as juxtaposed happy–happy, fearful–fearful, neutral–neutral, and happy–fearful faces, in an EAST procedure. We hypothesized that this implicit approach would determine the subjective emotional valence of neutral faces better than a traditional approach that could be influenced by the subjects’ intentions or controls (Parrot and Hertel, 1999).

Section snippets

Subjects

Twenty-one healthy subjects (male 9, female 12) were recruited through a local newspaper advertisement. All subjects fulfilled the inclusion criteria, which consisted of their being aged 18 to 40, having more than 12 years of education, and being right-handed. Handedness was assessed with the revised version of Annett's Hand Preference Questionnaire (Annett, 1970, Briggs and Nebes, 1975). All subjects were subjected to a comprehensive psychiatric evaluation using the Structured Clinical

Verification of the emotional stimuli

The paired t-tests showed that the explicit valence ratings of happy and fearful faces for black and white trials were significantly different (mean valence score and standard deviation, 7.77 ± 1.13 for the happy faces vs. 2.64 ± 1.39 for the fearful faces, P < 0.001). For color-filtered trials, there were significant differences between three of the explicit valence ratings of happy faces (7.65 ± 1.08), fearful faces (2.56 ± 1.13) and neutral faces (4.50 ± 0.77) (all P < 0.001). There was no significant

Discussion

To determine whether the prototypical neutral faces are actually evaluated as neutral, we measured emotional evaluation performances of the positive, negative, neutral, and positive/negative facial stimuli using the EAST, an implicit measure. Upon explicit rating, as expected, the participants judged the positive, negative, and neutral faces as positive, negative, and neutral. In the EAST, positive (happy–happy) and negative (fearful–fearful) stimuli were evaluated as positive and negative,

Acknowledgement

This study was supported by a grant of the Korea Science & Engineering Foundation, interdisciplinary research (Contract grant number: R01–2005–000–10963–0).

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