One of the most socially significant of human behaviors is expression of emotions on the face. Accordingly, scientific interest in facial expressions has a long history (Darwin, 1872
) and is as alive today as ever (Gross, 1998
; Niedenthal, 2007
). Much of this interest has concerned the role of emotional expressions as social signals of internal states, and the mechanisms by which we recognize these expressions in others (e.g. Adolphs, 2002
; Ekman, Friesen, & Ancoli, 2001
; Olsson & Phelps, 2007
). Comparatively less attention has been paid, however, to the possibility that facial expressions are not only external manifestations of internal states, but can themselves trigger or modulate emotional experiences. First championed by William James over a century ago (James, 1894
), this view is now commonly known as the Facial Feedback Hypothesis (FFH) (Tomkins, 1962
Though it has been actively pursued since that time, support for the FFH has been somewhat mixed. For example, the FFH predicts two complementary effects: posing a facial expression should increase the intensity of emotional experience, and inhibiting facial expressions should decrease it. However, only the former effect has been found consistently (e.g. Gross, 1998
; Keillor, Barrett, Crucian, Kortenkamp, & Heilman, 2002
; Strack, Martin, & Stepper, 1988
) (for reviews see McIntosh, 1996
; Soussignan, 2004
). Additionally, strong and weak versions of the FFH have been presented, in which facial feedback either completely determines, or merely influences, emotional experience. Only the weak version has received support (Keillor et al., 2002
In many of these studies, participants were instructed to hold their faces still, to flex facial muscles for ostensibly non-emotional reasons, or to maintain a specific expression. This leads to two potential sets of problems. First, explicit instructions regarding facial movement or expression could influence participants’ perceptions (and reports) of their emotional experience (Strack et al., 1988
). Such instructions can also influence emotional reports through other channels. They may be distracting (Davis, In Press
; Davis, Senghas, & Ochsner, In Press
), and there is the potential that participants might use cognitive strategies of emotion regulation in order to help limit facial expressivity, even if they do not believe the study hypothesis pertains to emotional expressivity (Davis et al., In Press
). Furthermore, any results obtained could be attributed either to a) the mental processes engaged in producing an expression and/or in flexing facial muscles, or b) the feedback from the face once a facial movement occurs or an expression is formed. That is, they may be either productive or reactive.
These problems were partially avoided in a case study of a patient with Guillain-Barre syndrome (Keillor et al., 2002
). Despite total facial paralysis, this patient reported normal emotional experience. However, it is difficult to generalize from this finding to typical functioning because the disorder may involve other, more widespread effects (NINDS, 2007
). Furthermore, the patient’s pre-paralysis emotional status is not known. Consequently, any changes in emotional experience from before to after the onset of the syndrome would not have been detected.
A more controlled way to investigate the connection between facial expression and emotional experience would be to examine individuals who transiently lose the ability to move facial muscles related to the expression of emotion. This approach was taken in two studies of individuals who received injections of BOTOX® (Allergan, Inc., California, USA) into the corrugator supercilii for cosmetic treatment of glabellar frown lines between the eyes. The active ingredient of BOTOX (botulinum toxin type A, BoNT-A) is a neurotoxin that paralyzes the muscle into which it is injected (Dolly & Aoki, 2006
). In the first study, ten patients with ongoing treatment resistant major depression were given BoNT-A injections (Finzi & Wasserman, 2006
). Two months after treatment nine of ten participants were no longer clinically depressed, suggesting that BoNT-A injections – and facial feedback – can affect mood. This conclusion is tempered, however, by the facts that participants in this study were aware of the hypothesis, there was no control group, and these findings pertain to people with clinical diagnoses and to the alleviation of their clinical condition, which can include non-emotional as well as emotional changes. The second study used functional imaging to compare brain activity during imitation of angry and sad expressions in groups who either did or did not receive BoNT-A injections (Hennenlotter et al, 2009
). They found that during imitation of anger, BoNT-A decreased activity in the amygdala and its coupling with brainstem nuclei involved in autonomic control. Although this ingenious approach establishes a link between the inability to voluntarily contract specific muscles and neural systems implicated in triggering emotional responses, it provides only indirect evidence in favor of the FFH because it did not measure changes in emotional experience.
The present study built on and attempted to address some of the methodological limitations of prior work by investigating the connection between facial expression and emotional experience by comparing healthy participants’ self-reports of emotional experience before and after they received one of two types of cosmetic facial injections used to treat facial wrinkles. One group received injections of BOTOX®. The second group received injections of Restylane® (Medicis Pharmaceutical Corp., Arizona, USA) whose active ingredient is hyaluronic Acid (HA). The mechanism of action of HA differs critically from that of BoNT-A in that HA is a filler and has no effect on facial muscles (Brandt & Cazzaniga, 2007
We predicted that if facial feedback can influence emotional experience, then individuals who received BoNT-A injections would show a drop in self-reported emotional experience relative to any change shown by individuals who received HA injections. To test this prediction, we assessed emotional response to positive and negative video clips before and after treatment.