Reactivity is a term that refers to the duration, type, and degree of response that any entity exhibits in response to a stimulus, as well as the threshold of stimulation required to elicit such a response. The essential questions concerning the measurement of reactivity in any system are: What is the range of fluctuation of the system at rest (background noise) and under stress, and how long does it take to recover? The response measured may be physiological, behavioral, or psychological. Reactivity is a term that is used in a broad sense, as well as in a more specific one. In the specific sense, reactivity refers to one particular temperamental quality (e.g., the degree of inhibition/harm avoidance in response to a noxious stimulus) as a response style. As a general term, we can talk about the reactivity of the heart rate, the reactivity of the immune system, the hormonal system, or even the reactivity of a shock absorber on a car!
The analogy of the shock absorber is quite, useful clinically: To determine if a shock absorber is functioning adequately, one can measure it with a ruler. If it measures below a certain measurement, it is defective. This is a static measurement, which will detect only the most defective shock absorbers. A dynamic, and more sensitive test of the reactivity of the shock absorber is to apply a stress (push down on the car) and observe the up and down motion of the car (the degree of reactivity). If the motion of the car fluctuates too widely and for too long, or too stiffly for too little time, the shock absorber, which measured a normal length on the static test, is deemed defective, or approaching that point. Biological systems function in the same way. In clinical practice patients may exhibit extremes of reactivity in numerous areas of function.
It is quite remarkable that leading researchers in various branches of the brain and behavioral sciences have independently come to recognize the importance of the concept of reactivity, each in his own area of study, be it biological, psychological, behavioral, or social.
Bowen, the father of family systems theory and therapy, observed a direct relationship between the increasing degrees of emotional reactivity of an individual and the failure to develop a sense of self separate from one’s family of origin (Kerr & Bowen, 1988). He called this concept, central to his theory of family systems, the differentiation of self. He considered the basic level of differentiation of an individual to be fixed, although structured effort or unusual life experiences could boost an individual to a higher level of differentiation.
Jerome Kagan (1994) is one of the primary researchers in the area of temperament. His work is focused on the ease and extent of arousal (reactivity) in infants and young children to various stimuli. Neurotransmitter systems normally operate within a limited range of activity. In abnormal states of depression these systems are highly variable, unstable, and inappropriately responsive to incoming information; they may even become disconnected from normal biological rhythms (Parry, 1995)!
In essence, these pioneers are saying the same thing: If your emotional “shock absorber” system works well, your emotions stay in a reasonable, consistent range, the boundary between self and non-self is relatively clear, and a sense of self can develop. Your nervous system and the nervous systems of those around you will not be shaken by every bump in the road of life. Explaining the concept of reactivity to clients has uniformly allowed clients to feel relief. The discussion would go something like this:
“Joan, if I were to measure the level of norepinephrine (NE) in the brain of a depressed person, the graph would look like this (point to the left side of the upper graph in figure 2.2, preceding the stress). Notice the variation in the baseline? Now if I were to put that person’s hand in ice water and measure the NE level, it would rise to some degree, and then take some time to return to the earlier noisy background. Now, if I were to measure NE in a non-depressed person, it would look like this (point to left side of bottom graph). Notice how stable the baseline is? If I then put that person’s hand in ice water, the NE response would be much greater, and then it would return to the stable baseline more quickly. This is a more efficient response.
“Joan, translated into your life, this means that internally you are always working with an emotional baseline of being somewhat upset. When a stress comes, you cannot react efficiently or effectively, and then it takes you a long while to recover. This is hard work. When you have this constant level of excess reactivity to deal with, it’s hard to deal with the world, and your self-esteem suffers. What’s wrong with me, you wonder. Why does everything bother me? Why am I so sensitive? Then you deal with this in any number of ways. You might stay away from people, try to control people and events, use drugs to calm yourself. But the bottom line is you become self-absorbed trying to deal with this internal reactivity. Everything around you bothers you, and you are so busy with this internal state that you can’t focus on others and learn about the world. Most of your strategies are aimed at trying to calm this internal reactivity. The non-depressed person responds efficiently, and then recovers quickly. Medication can help move your reactivity from here (upper graph) to here (lower graph).”
Temperament can be defined as emotional or behavioral qualities that appear early in life and are substantially determined by genetics. Temperament may be modified to a greater or lesser degree by environmental influences over time. Since temperamental characteristics are emotional traits, it is not surprising that they originate in large measure within the limbic system.
Under the umbrella of temperament, a small number of core traits has been identified and studied. As you read this section, be aware that researchers are in some disagreement about these core traits and often use different language to describe the same trait. These traits are probably independently inherited and can be thought of as four axes or dimensions that reflect an individual’s propensity toward
- Inhibition of behavior in response to a stimulus (also called harm avoidance, this limbic response involves intense reactivity to aversive stimuli).
- Impulsivity is not always considered to be a separate temperamental quality. Impulsivity seems to be mediated by serotonin neurons, as is harm avoidance. This common chemistry suggests that impulsivity is merely the opposite extreme of harm avoidance. Thus often only three temperamental qualities are identified. Because of its clinical importance, however, I will treat it as a separate temperamental quality.
- Maintenance of behavior that has already been established as pleasurable (reward dependence)
- Activation of behavior in response to potentially pleasurable stimuli (novelty seeking)
Recently, studies have been published indicating that identification of temperament may have predictive value. One controversial study (Bell et al., 1995) indicates that shyness, possibly in combination with depression, may help predict one’s risk of developing Parkinson’s disease! Thus, serotonin and dopamine dysregulation may manifest themselves differently over time: early in life as shyness and late in life as Parkinson’s disease.
Researchers have been struggling for years to identify some factor(s) that could be used to predict which medications will work, thereby decreasing the need for multiple drug trials by biological psychiatrists. Recently, a few studies (Ansseau et al., 1991; Joyce, Mudler, & Cloninger, 1994; Peselow, Fieve, & DiFiglia, 1992) have actually been able to do this. These researchers have found that different temperaments predicted which type of antidepressant would work (e.g., serotonergic vs. noradrenergic). Joyce found that severely depressed women high in fear (harm avoidance) with low need for approval (reward dependence) responded to desipramine (a noradrenergic antidepressant). High need for social approval (reward dependence) predicted clornipramine response (a serotonergic antidepressant). Depressed patients with an antisocial temperament (high novelty seeking, low reward dependence, low harm avoidance) did poorly with either drug. Patients with high levels of fear (harm avoidance) and need for approval (reward dependence) did well on either drug. If these findings are replicated, they will be quite valuable as clinical tools.