In the present study, we provided a first test for the idea that implicit motives are mediated by brain structures mediating motivation, an idea that was originally proposed by McClelland et al.
) and has, more recently, also been endorsed by LeDoux (2002
). We had predicted that individual differences in implicit power motivation would modulate activity in motivational brain areas in response to facial expressions signaling another person's anger or surprise (high incentive value), but not in response to neutral facial expressions (low incentive value).
In general, our findings support these predictions. In comparison to low-power participants, individuals high in nPower showed stronger bilateral activation in response to anger faces in the anterior caudate and the anterior insula, suggesting that they were more primed to recruit behavioral routines to cope with the dominance challenge inherent in encountering a threatening anger face and also may have experienced stronger bodily responses to it (Rolls, 1999
; Critchley et al.
). In addition, high-power participants had stronger lateral activation of the OFC in the left hemisphere than low-power participants, a finding that is consistent with the lateral OFC's propensity to represent negative incentives and to aid behavioral change for coping with them (Kringelbach, 2005
). Interestingly, high-power participants also responded with more activation of the left anteromedial OFC to angry faces relative to surprised faces. The medial OFC is dedicated to the representation of the reward values of a variety of incentives (Kringelbach, 2005
). We speculate that the coactivation of medial and lateral OFC in response to angry faces reflects high-power participants’ ambivalence about this emotional expression: on the one hand, it signals one's emotional impact on another person (rewarding); on the other, another person's anger also represents a dominance challenge the perceiver needs to cope with somehow (aversive). Clearly, however, angry faces turned out to be a potent stimulus for revealing activation differences between low- and high-power participants in key areas of the motivational brain.
Surprise faces were less effective for eliciting activation responses in high-power participants. High-power participants responded only with increased insula activation to this emotional stimulus, perhaps reflecting a somatosensory or ‘gut’ response (Damasio, 1994
). While this interpretation is consistent with the notion that surprise, like anger, should hold incentive value for power-motivated individuals, the lack of activation elicited by surprised faces in other motivational brain areas requires explanation. While we cannot rule out the possibility that anger is simply a stronger incentive for power-motivated individuals than surprise, two other factors may explain the differences in our findings for the two emotions. First, our previous research suggests that surprise is a potent reinforcer for power-motivated individuals only if it is being displayed by a sender of one's own gender, not if it is being displayed by a sender of the opposite gender (Schultheiss et al.
). The design of our present study did not allow us to separate effects of same-gender from opposite-gender surprise faces. Opposite-gender faces may, therefore, have diluted stronger effects that same-gender surprise faces may have had on high-power participants’ brain activation responses. The validity of this explanation could be tested in future studies by varying face gender within subjects in blocked or even event-related designs.
A second reason for the fewer effects found for surprised faces relative to angry faces may reside in the different motivational significance of these social signals for power-motivated individuals. Schultheiss et al.
) and Schultheiss and Hale (2007
) have argued that a surprised expression directed at the perceiver is a signal of the perceiver's attained
dominance (i.e. he/she has done something that the sender did not expect or have control over) and thus constitutes a reward for a power-motivated perceiver. In a sense, then, another's surprise is the outcome, rather than the start, of a motivational transaction with the environment and therefore does not necessitate any further action, mediated by activation of brain sites involved in response recruitment, such as the striatum. In contrast, an anger face expresses the sender's claim to dominance (e.g. Tiedens, 2001
) and signals a challenge to the perceiver (Schultheiss and Hale, 2007
). It thus not only constitutes a strong incentive for the high-power perceiver but also prompts some form of counteraction if he/she wants to maintain dominance. This interpretation is consistent with the observation that high-power participants showed specific activation in response to anger faces in the dorsal striatum, a brain area that is involved in the preparation of instrumental behavior (e.g. Delgado et al.
Contrary to our predictions, we did not observe nPower-dependent activation changes in amygdala and accumbens in response to emotional faces. This paucity of findings also extends to the results at the group level of analysis: we found amygdala activation only in response to neutral faces, relative to gray squares, but not in response to emotional faces, relative to neutral faces, and the accumbens remained silent in these analyses, too. One possible reason for this lack of activation findings in amygdala and accumbens may be insufficient resolution for detecting effects in these small-scale structures; the obvious remedy for this drawback would be to acquire images focusing only on limbic structures in future studies. Another possible reason for the lack of amygdala activation in the present study specifically may be that anger and surprise are less potent elicitors of amygdala activation than the emotional expression of fear (Murphy et al.
). Although no studies have been conducted yet on the effects of fearful expressions on power-motivated observers, further inquiry into the effects of nPower on amygdala activation could, therefore, fruitfully employ fearful faces as potent stimuli.