Among the most striking features of human emotion is the variability that is apparent across individuals in the quality and intensity of dispositional mood and emotional reactions to similar incentives and challenges. The broad ranges of differences in these varied affective phenomena has been referred to as “affective style†(Davidson, 1998). Differences among people in affective style appear to be associated with temperament (Kagan, Reznick, & Snidman, 1988), personality (Gross, Sutton & Ketelaar, 1998) and vulnerability to psychopathology (Meehl, 1975). Moreover, such differences are not a unique human attribute but appear to be present in a number of different species (see, e.g., Davidson, Kalin, & Shelton, 1993; Kalin, 1993).
The next section of this chapter will introduce conceptual distinctions among the various components of affective style and will highlight methodological challenges to their study. The third section will present a brief overview of the anatomy of two basic motivational/emotional systems—the approach and withdrawal systems. Then the fourth section will consider individual differences in these basic systems and indicate how such differences might be studied. The fifth section will address the relation between such individual differences and psychopathology. It is our intuition that some of the individual differences in basic processes of affective style are central to determining either resilience or vulnerability. Such differences can be conceptualized as diatheses that affect an individual's response to a stressful life event. Finally, the last section will consider some of the implications of this perspective for assessment, treatment, and plasticity.
The Constituents of Affective Style
Many phenomena are subsumed under the rubric of affective style. A concept featured in many discussions of affective development, affective disorders, and personality is “emotion regulation†(Thompson, 1994). Emotion regulation
refers to a broad constellation of processes that serve to amplify, attenuate, or maintain the strength of emotional reactions. Included among these processes are certain features of attention that regulate the extent to which an organism can be distracted from a potentially aversive stimulus (Derrybeny & Reed, 1996) and the capacity for self-generated imagery to replace emotions that are unwanted with more desirable imagery scripts. Emotion regulation can be both automatic and controlled. Automatic emotion regulation may result from the progressive automization of processes that initially were voluntary and controlled and have evolved to become more automatic with practice. We hold the view that regulatory processes are an intrinsic part of emotional behavior and rarely does an emotion get generated in the absence of recruiting associated regulatory processes. For this reason, it is often conceptually difficult to distinguish sharply between where an emotion ends and regulation begins. Even more problematic is the methodological challenge of operationalizing these different components in the stream of affective behavior.
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refers to a broad constellation of processes that serve to amplify, attenuate, or maintain the strength of emotional reactions. Included among these processes are certain features of attention that regulate the extent to which an organism can be distracted from a potentially aversive stimulus (Derrybeny & Reed, 1996) and the capacity for self-generated imagery to replace emotions that are unwanted with more desirable imagery scripts. Emotion regulation can be both automatic and controlled. Automatic emotion regulation may result from the progressive automization of processes that initially were voluntary and controlled and have evolved to become more automatic with practice. We hold the view that regulatory processes are an intrinsic part of emotional behavior and rarely does an emotion get generated in the absence of recruiting associated regulatory processes. For this reason, it is often conceptually difficult to distinguish sharply between where an emotion ends and regulation begins. Even more problematic is the methodological challenge of operationalizing these different components in the stream of affective behavior.
When considering the question of individual differences in affective behavior, one must specify the particular response systems in which the individual differences are being explored. It is not necessarily the case that the same pattern of individual differences would be found across response systems. Thus, for example, an individual may have a low threshold for the elicitation of the subjective experience (as reflected in self-reports) of a particular emotion but a relatively high threshold for the elicitation of a particular physiological change. It is important not to assume that individual differences in any parameter of affective responding will necessarily generalize across response systems,
within the same emotion. Equally important is the question of whether individual differences associated with the generation of a particular specific emotion will necessarily generalize to other emotions. For example, are those individuals who are behaviorally expressive in response to a fear challenge also likely to show comparably high levels of expressivity in response to positive incentives? While systematic research on this question is still required, initial evidence suggests that at least certain aspects of affective style may be emotion-specific, or at least valence specific (e.g., Wheeler, Davidson, & Tomarken, 1993).
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within the same emotion. Equally important is the question of whether individual differences associated with the generation of a particular specific emotion will necessarily generalize to other emotions. For example, are those individuals who are behaviorally expressive in response to a fear challenge also likely to show comparably high levels of expressivity in response to positive incentives? While systematic research on this question is still required, initial evidence suggests that at least certain aspects of affective style may be emotion-specific, or at least valence specific (e.g., Wheeler, Davidson, & Tomarken, 1993).
In addition to emotion regulation, there are likely also intrinsic differences in certain components of emotional responding. For example, there may be individual differences in the threshold for eliciting components of a particular emotion, given a stimulus of a certain intensity. Thus, some individuals are likely to produce facial signs of disgust upon presentation of a particular intensity of noxious stimulus, whereas other individuals may require a more intense stimulus for the elicitation of the same response at a comparable intensity. This suggestion implies that dose-response functions may reliably differ across individuals. Unfortunately, systematic studies of this kind have not been performed, in part because of the difficulty of creating stimuli that are graded in intensity and designed to elicit the same emotion.
There are also likely to be individual differences in the peak or amplitude of the response. Upon presentation of a series of graded stimuli that differ in intensity, the maximum amplitude in a certain system (e.g., intensity of a facial contraction, change in heart rate, etc.) is likely to differ systematically across subjects. Some individuals will respond with a larger amplitude peak compared with others. Again, such individual differences may well be quite specific to particular systems and will not necessarily generalize across systems, even within the same emotion. Thus, the individual who is in the tail of the distribution in heart rate response to a fearful stimulus will not necessarily be in the tail of the distribution in facial response.
Another parameter that is likely to differ systematically across individuals is the rise time to peak. Some individuals will rise quickly in a certain response system, while others will rise more slowly. There may be an association between the peak of the response and the rise time to the peak within certain systems for particular emotions. Thus, it may be the case that for anger-related emotion, those individuals with higher peak vocal responses also show a faster rise time, but to the best of my knowledge, there are no systematic data related to such differences.
Finally, another component of intrinsic differences across individuals is the recovery time. Following perturbation in a particular system, some individuals recover quickly and others recover slowly. For example, following a fear-pro-voking encounter, some individuals show a persisting heart rate elevation that might last for minutes, while other individuals show a comparable peak and rise time, but recover much more quickly. As with other parameters, there are likely to be differences in recovery time across different response systems. Some individuals may recover rapidly in their expressive behavior, while recovering slowly in certain autonomic channels. As is noted in a later section, individual differences in recovery time may be particularly important for identifying individuals vulnerable to mood and anxiety disorders.
These specific parameters of individual differences describe affective chronometry—the temporal dynamics of affective responding. Very little is known about the factors that govern these individual differences and the extent to which such differences are specific to particular emotion response systems or generalize across emotions (e.g., is the heart rate recovery following fear similar to that following disgust?). Moreover, the general issue of the extent to which these different parameters that have been identified are orthogonal or correlated features of emotional responding is an empirical question that has yet to be answered. I hope to show that affective chronometry is a feature of affective style that is methodologically tractable and can yield to experimental study of its neural substrates.
We also hold that affective style is critical in understanding the continuity between normal and abnormal functioning and in the prediction of psychopathology and the delineation of vulnerability. On the opposite side of the spectrum, such individual differences in affective style will also feature centrally in any comprehensive theory of resilience. The fact that some individuals reside “off the diagonal†and appear to maintain very high levels of psychological wellbeing despite their exposure to objective life adversity is likely related to their affective style (Ryff & Singer, 1998). Some of these implications will be discussed at the end of this chapter.
We first consider some of the neural substrates of two fundamental emotion systems. This provides the foundation for a consideration of individual differences in these systems and the neural circuitry responsible for such differences.
The Circuitry of Approach-and Withdrawal-Related Emotion
Although the focus of my empirical research has been on measures of prefrontal brain activity, it must be emphasized at the outset that the circuit instantiating
emotion in the human brain is complex and involves a number of interrelated structures. Preciously few empirical studies using modern neuroimaging procedures that afford a high degree of spatial resolution have been performed (see George et al., 1995; Paradise et al., 1997, for examples). Therefore, hypotheses about the set of structures that participate in the production of emotion must necessarily be speculative and based to a large extent on the information available from the animal literature (e.g., LeDoux, 1987) and from theoretical accounts of the processes involved in human emotion.
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emotion in the human brain is complex and involves a number of interrelated structures. Preciously few empirical studies using modern neuroimaging procedures that afford a high degree of spatial resolution have been performed (see George et al., 1995; Paradise et al., 1997, for examples). Therefore, hypotheses about the set of structures that participate in the production of emotion must necessarily be speculative and based to a large extent on the information available from the animal literature (e.g., LeDoux, 1987) and from theoretical accounts of the processes involved in human emotion.
Based upon the available strands of theory and evidence, numerous scientists have proposed two basic circuits each mediating different forms of motivation and emotion (see, e.g., Gray, 1994; Davidson, 1995; Lang, Bradley, & Cuthbert, 1990). The approach system facilitates appetitive behavior and generates certain types of positive affect that are approach related, e.g., enthusiasm, pride, and so on (see Depue & Collins, 1999, for review). This form of positive affect is usually generated in the context of moving toward a desired goal (see Lazarus, 1991, and Stein & Trabasso, 1992, for theoretical accounts of emotion that place a premium on goal states). The representation of a goal state in working memory is hypothesized to be implemented in dorsolateral prefrontal cortex. The medial prefrontal cortex seems to play an important role in maintaining representations of behavioral-reinforcement contingencies in working memory (Thorpe, Rolls & Maddison, 1983). In addition, output from the medial prefrontal cortex to nucleus accumbens (NA) neurons modulates the transfer of motivationally relevant information through the NA (Kalivas, Churchill, & Klitenick, 1993). The basal ganglia are hypothesized to be involved in the expression of the abstract goal in action plans and in the anticipation of reward (Schultz, Apicella, Romo, & Scarnati, 1995; Schultz, Romo, Ljungberg, Mirenowicz, Hollerman & Dickinson, 1995). The NA, particularly the caudomedial shell region of the NA, is a major convergence zone for motivationally relevant information from a myriad of limbic structures. Cells in this region of the NA increase their firing rate during reward expectation (see Schultz, Apicella, et al., 1995). There are likely other structures involved in this circuit which depend upon a number of factors including the nature of the stimuli signaling appetitive information, the extent to which the behavioral-reinforcement contingency is novel or overlearned, and the nature of the anticipated behavioral response.
It should be noted that the activation of this approach system is hypothesized to be associated with one particular form of positive affect and not all forms of such emotion. It is specifically predicted to be associated with pre-goal attain ment positive affect, the form of positive affect that is elicited as an organism moves closer toward an appetitive goal. Post-goal attainment positive affect represents another form of positive emotion that is not expected to be associated with activation of this circuit (see Davidson, 1994, for a more extended discussion of this distinction). This latter type of positive affect may be phenomenologically experienced as contentment and is expected to occur when the prefrontal cortex goes off-line after a desired goal has been achieved. Cells in the NA have also been shown to decrease their firing rate during post-goal consummatory behavior (e.g., Henriksen & Giacchino, 1993).
Lawful individual differences can enter into many different stages of the approach system. Such individual differences and their role in modulating vulnerability to psychopathology will be considered in detail later. For the moment, it is important to underscore two issues. One is that there are individual
differences in the tonic level of activation of the approach system which alters an individual's propensity to experience approach-related positive affect. Second, there are likely to be individual differences in the capacity to shift between pre- and post-goal attainment positive affect and in the ratio between these two forms of positive affect. Upon reaching a desired goal, some individuals will immediately replace the just-achieved goal with a new desired goal, and so will have little opportunity to experience post-goal attainment positive affect, or contentment. There may be an optimal balance between these two forms of positive affect, though this issue has never been studied.
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differences in the tonic level of activation of the approach system which alters an individual's propensity to experience approach-related positive affect. Second, there are likely to be individual differences in the capacity to shift between pre- and post-goal attainment positive affect and in the ratio between these two forms of positive affect. Upon reaching a desired goal, some individuals will immediately replace the just-achieved goal with a new desired goal, and so will have little opportunity to experience post-goal attainment positive affect, or contentment. There may be an optimal balance between these two forms of positive affect, though this issue has never been studied.
There appears to be a second system concerned with the neural implementation of withdrawal. This system facilitates the withdrawal of an individual from sources of aversive stimulation and generates certain forms of negative affect that are withdrawal related. Both fear and disgust are associated with increasing the distance between the organism and a source of aversive stimulation. From invasive animal studies and human neuroimaging studies, it appears that the amygdala is critically involved in this system (e.g., LeDoux, 1987). Using functional magnetic resonance imaging (fMRI) we have recently demonstrated, for the first time, activation in the human amygdala in response to aversive pictures compared with neutral control pictures (Irwin et al., 1996). In addition, the temporal polar region also appears to be activated during with-drawal-related emotion (e.g., Reiman, Fusselman, Fox, & Raichle, 1989; but see Drevets, Videen, MacLeod, Haller, & Raichle, 1992). These effects, at least in humans, appear to be more pronounced on the right side of the brain (see Davidson, 1992, 1993, for reviews). In the human electrophysiological studies, the right frontal region is also activated during withdrawal-related negative affective states (e.g., Davidson, Ekman, Saron, Senulis & Friesen, 1990). At present it is not entirely clear whether this electroencephalogram (EEC) change reflects activation at a frontal site or whether the activity recorded from the frontal scalp region is volume-conducted from other cortical loci. The resolution of this uncertainty must await additional studies using positron emission tomography (PET) or fMRI, which have sufficient spatial resolution to differentiate among different anterior cortical regions. In addition to the temporal polar region, the amygdala and possibly the prefrontal cortex, it is also likely that the basal ganglia and hypothalamus are involved in the motor and autonomic components, respectively, of withdrawal-related negative affect (see Smith, DeVita, & Astley, 1990).
The nature of the relation between these two hypothesized affect systems also remains to be delineated. The emotion literature is replete with different proposals regarding the interrelations among different forms of positive and negative affect. Some theorists have proposed a single bivalent dimension that ranges from unpleasant to pleasant affect, with a second dimension that reflects arousal (e.g., Russell, 1980). Other theorists have suggested that affect space is best described by two orthogonal positive and negative dimensions (e.g., Watson & Tellegen, 1985). Still other workers have suggested that the degree of orthogonality between positive and negative affect depends upon the temporal frame of analysis (Diener & Emmons, 1984). This formulation holds that when assessed in the moment, positive and negative affect are reciprocally related, but when examined over a longer time frame (e.g., dispositional affect) they are orthogonal. It must be emphasized that these analyses of the relation between positive and negative affect are all based exclusively upon measures of self
report and therefore their generalizability to other measures of affect are uncertain. However, based upon new data to be described here, we believe that a growing corpus of data does indeed indicate that one function of positive affect is to inhibit concurrent negative affect.
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report and therefore their generalizability to other measures of affect are uncertain. However, based upon new data to be described here, we believe that a growing corpus of data does indeed indicate that one function of positive affect is to inhibit concurrent negative affect.
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