Overall, our data showed that the MID performance was sensitive to contingencies in all three groups. A faster response to obtain a reward ($1) or to prevent a loss ($1) reflects enhanced vigilance energized by higher levels of motivation. Accuracy of performance was improved by contingencies, but the dichotomous nature of this variable, together with individual variability, reduced the power to detect significant group differences, and group by condition interactions.
With respect to reaction time, findings were consistent with predictions. Non-shy individuals demonstrated sensitivity to potential incentive by adapting their behavior to effectively both obtain reward and avoid punishment. Specifically, Reward and Punishment conditions in the Non-shy group elicited similarly enhanced performance relative to No-Incentive condition. The Non-shy group was also faster in the No-Incentive condition compared to the Shy and Control groups. Given the approach oriented nature of the MID task, and previous findings demonstrating activity in neural substrates of the reward system during the task, these findings suggest an overactive approach motivation system in this population that is either poorly modulated by contingencies, or functioning at a ceiling level. Non-shy individuals differed significantly from the shy individuals in their response to Punishment cues rather than Reward cues. This is consistent with previous work demonstrating shorter latencies following punishment in extraverted compared to introverted (shy-like) subjects, when the potential for both reward and punishment are present (Nichols & Newman, 1986
; Patterson et al., 1987
Shy individuals showed a large discrepancy between response to reward and response to punishment, as indexed by a significant reaction time difference between the two conditions. This discrepancy resulted from enhanced sensitivity to potential reward (shortest reaction time during Reward condition) rather than sensitivity to potential punishment (only slight reduction of reaction time during Punishment than during No-Incentive condition). Given prior data linking shyness to responses to socially rewarding stimuli (Davidson, Marshall, Tomarken, & Henriques, 2000
), the current and previous data suggest that various forms of rewards may be particularly powerful modifiers of behavior in shy individuals.
In the Punishment condition for our paradigm, subjects needed to approach stimuli signaling potential punishment in order to avoid the negative outcome. We hypothesized that shy individuals experience conflict from their predisposition to withdraw from negatively valenced stimuli, coupled with task demands requiring approach behavior (press button) to avoid negative consequences. Accordingly, we expected reaction time for the Shy group, but not other groups, to be slower during the Punishment compared to the Reward condition. Whereas performance scores are consistent with this idea, this interpretation remains speculative and cannot be clearly tested in the present paradigm. However, given that imaging data are available for the current task, we chose to begin our studies with the established MID task. Overall, the shy group showed a unique pattern of response to contingencies, characterized by a substantial discrepancy in strength of approach behavior in response to positive cues vs. negative cues, and a strong modulation of behavior by positive reinforcers.
This pattern of responses in shy individuals is consistent with proposals of an approach-withdrawal conflict in the social domain. It has been suggested that shy individuals exhibit normal motivation to approach and engage socially with others, while simultaneously finding themselves too fearful and withdrawal motivated to actually do so (Asendorpf, 1990
; Cheek & Buss, 1981
; Neal & Edelmann, 2003
). In a consistent line of study, Schmidt and Fox (1994)
reported patterns of electrophysiological and autonomic arousal in shy and sociable individuals just prior to a social encounter that support the idea of an approach-withdrawal conflict. While our current findings are consistent with these previous reports, they suggest that this conflict for shy individuals may not be specific to social contexts, and may generalize to less context specific types of rewarding or punishing stimuli.
These data may also carry implications for research on psychopathology. Considerable data document an association between perturbed reward-system function in the form of major depression, and perturbed regulation of anxiety in the form of extreme shyness. This includes data from both longitudinal and family-genetic studies documenting associations between social phobia and major depression (Lieb, Isensee, Hofler, Pfister, & Wittchen, 2002
; Stein et al., 2001
), as well as data from brain imaging studies implicating perturbed dopamine function specifically in both social phobia and major depression (Drevets, 2001
; Drevets et al., 2001
; Schneier et al., 2000
). The current data provide some of the first evidence to suggest that extreme shyness is associated with perturbed reward system function, as assessed using a task previously shown to engage relevant underlying neural circuitry.
Our findings extend theoretical accounts of personality that discuss outgoing social behavior only in terms of reward reactivity, and shy, anxious social behavior only in terms of punishment reactivity. Our findings suggest differences in the function or modulation of reward systems in shy and non-shy individuals, which can be tested using functional neuroimaging tools. Because of an increased sensitivity to potential reward in the shy and non-shy groups, a disproportionate involvement of the ventral striatum might be anticipated. The ventral striatum has been associated with reward processing (Di Chiara, 2002
) and therefore may be an area where both non-shy and shy individuals demonstrate differences during the processing of potential reward. Additionally, the amygdala has been particularly implicated in the processing of both potentially aversive stimuli and socially relevant stimuli (Adolphs, 1999
), and in stimulus-value associative learning (Baxter & Murray, 2002
). These roles suggest that the amygdala may be an area of functional divergence between non-shy and shy individuals during the processing of potential punishment, as well as, the processing of potential reward.
Results of the current study should be considered in light of some limitations. The sample for this study consisted of undergraduate university students that were not screened for psychiatric disorders. Given the nature of our social composite score, it is possible that cases of social anxiety or other anxiety disorders were included in the shy group. Additionally, this study did not have an objective measure of social behavior, such as those reported in previous studies (for example, Davidson, Marshall, et al., 2000
. Future studies exploring the relationship between social behavior and motivation processes would benefit from inclusion of psychiatric assessment by psychiatric interview, and objective measures confirming participant social behavior.
The current study serves as a valuable starting point for future research on the relationship between general motivation systems, and social behavior. Because the behavioral task employed in this study was initially developed for use in neuroimaging environments, the current findings can provide very useful guidance for future neuroimaging studies designed to explore neurophysiology involved in processing socially relevant stimuli and production of social behavior, and interaction with reward systems.