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Essential tremor: the role of biobehavioral conditioning.


Essential Tremor [ET], the most common neurological movement disorder, has been described as a purely medical condition, with little consideration of how principles of behavior may come to exert joint control over tremor, related negative emotional arousal, and verbal behavior. A biobehavioral conditioning model is described that has been useful in accounting for these behaviors and in developing biobehavioral interventions.


Essential tremor (ET) is the most prevalent neurological movement disorder with approximately 14% of the population being affected by the disorder (Koller, Busenbark, Miner et al., 1994). ET is dominantly inherited with variable penetrance (Rautakorpi, Martilla, & Rinne, 1984). In such cases it is referred to as familial essential tremor. The term idiopathic essential tremor is used to classify the significant number of individuals who develop ET absent a family history of the disorder. Men and women are equally affected. Peak incidence occurs in the seventh (20.3%) decade of life (Koller et al., 1994). Hispanic and African American individuals are at greater risk of developing the disorder than other ethnic groups (Louis, Marder, Cote, et al., 1995).

ET can be distinguished from Parkinson's disease based on several characteristics. For example, among individuals with Parkinson's disease "cogwheel" rigidity or "ratcheting", observed when an upper limb is extended and flexed, bradykinesia (slowness of movement) and imbalance when ambulating are used as "soft neurological signs" to discriminate ET from Parkinson's disease (Elble & Koller, 1990). Finally, ET is pathologically distinct from Parkinson's disease, which is caused by a loss of dopaminergic cells in the substantia nigra. While various causal biological mechanisms have been imputed, the pathophysiology of ET appears to be linked to the thalamus (Tasker, 1998).

ET most often affects the hands and head; however, tremor of the lower limbs may also occur over time (Koller et al, 1994). Tremor involves continuous or intermittent oscillation resulting in horizontal, vertical and/or rotational movement (Lundervold, 1997). Tremor may also be defined according to three types of situations (Elble & Koller, 1990). Resting tremor occurs when the limbs are fully supported, as is the case when one is sitting in an armchair. Kinetic tremor occurs during the act of movement, such as reaching for an object. Postural tremor occurs when a posture is sustained against gravity, such as holding one's arms outstretched.

Individuals afflicted by ET are often significantly disabled in communication,

work, emotional adjustment, home management, and leisure activities (Bain, Findley, Atchinson, Behari, et al., 1993; Busenbark, Nash, Nash, Hubble, & Koller, 1991; Koller, Biary, & Cone, 1986; Koller et al., 1994). Approximately 50% of the individuals with ET have disability in performance of activities of daily living (ADL) involving use of the hands (Auff, Doppelbauer, & Fertl, 1991; Bain et al., 1993). Approximately 20% of individuals with ET report having such significant disability that they must leave their jobs or reduce their job responsibilities due to motor or anxiety-related disability (Metzer, 1992; Rautakorpi et al., 1984). Numerous clinical observations clearly point to the development of fear and avoidance behavior, such as social phobia secondary to tremor-related physical disability (Lundervold, 1997; Metzer, 1992; Wake et al., 1974).

The primary forms of intervention for ET are neurosurgery and medication. Neurosurgery may take two forms: severation of nerves of the brain thought to be responsible for the tremor or implantation of electrodes used in brain stimulation (Andrew, 1981; Tasker, 1998). Brain surgery is highly intrusive, expensive, and experimental limiting its application. Pharmacological treatment using Primidone, an antiseizure medication, or beta-blockers, such as Inderal, is not entirely satisfactory. Positive response rates to pharmacological intervention are approximately (40%) (Koller et al., 1994). This meager record of success for pharmacological intervention may in fact be an overestimate due to the use of unreliable measures and poor experimental method (Bain, 1993). Even when medication alters motor behavior, negative emotional arousal often remain (Koller et al., 1986; Metzer 1992).

It is well known that ET is exacerbated by stress, fatigue, or conditions where vigilance is required (e.g., eating). Social situations, such as being in public or being observed while performing ADLs worsen tremor, disability and emotional distress (Bain et al., 1993; Koller, 1984). These observations suggest that contextual factors and conditioning play an important, and, as yet unrecognized, role in the development of overt motoric and covert emotional responses related to tremor. The purpose of this paper is to describe a biobehavioral conditioning model of tremor and related emotional behavior.

Behavior analysis of complex human behavior

Human behavior may be observed to occur in four major response domains, motoric, verbal, visceral, and observational (Poppen, 1989; 1998). Relatively simple behavior may involve just one category, while complex behavior involves responding across two or more domains. When dealing with problematic behavior, it is useful to analyze the domains involved in order to devise effective alternatives. Each category of behavior can be defined in terms of both its functional attributes and its structural (or physiological) basis. In addition, behavior in each category may occur overtly, that is, available to observation by another person, or covertly, observable only to the behaving individual. General descriptions of these four domains are given below, with an application of this system to ET.

Motoric behavior functions to move one's body through space and to manipulate objects. Structurally, motoric behavior involves the skeletal muscular system. Overt examples include walking or buttoning a shirt. Covert examples include muscle tension in the neck or privately rehearsing a sequence of movements before engaging in a skilled action, such as a one-and-a-half gainer. ET is comprised in large part of overt motoric behavior. Covert components may include muscle tension in a limb or feelings of incipient tremor prior to overt occurrence.

Verbal behavior functions to maneuver in or manipulate one's social environment. Physiologically it includes the vocal musculature, though other motoric behavior may be employed in the service of verbal behavior, such as writing with a pen or signing with ASL. Overt verbal behavior includes speaking and writing, while covert aspects include 'talking' silently to oneself. ET often includes negative verbal statements, either public or private, about one's performance or other people's perceptions.

Visceral behavior functions to adjust the internal environment to meet the demands of daily life. Physiologically it includes the autonomically innervated organ systems. Overt examples include sweating or respiration, while covert examples include cardiac acceleration or salivation. ET may include visceral arousal characteristic of stress, such as shallow respiration or facial flushing.

Finally, observational behavior functions to seek and select relevant features in all of the above-listed environments. Physiologically it involves the sensory systems. Overt observation involves stimuli accessible to others, such as looking at a picture or smelling a rose. Covert observation involves attending to private sensations, such as a toothache, or engaging in imagery. ET often includes close attention to one's motoric performance or other's reactions.

Biobehavioral conditioning model

The primary focus of our research has been on the development of a biobehavioral conditioning model of ET based on current medical research and employing contemporary principles of behavior analysis and respondent conditioning (Forsythe & Eifert, 1998; Forsythe & Chorpita, 1996; Poppen, 1998).


As can be seen in Figure 1, Panel A, ET is hypothesized to initially result from a disturbance in biologic functioning at the central nervous system, particularly the thalamus (Elble & Koller, 1990). This biologic perturbation functions as an interoceptive unconditioned stimulus (US), which elicits an unconditioned motor response (UR), tremor. ET is the product of disrupted recruitment patterns of motor units, such that the discharge of other motor units becomes synchronized resulting in greater bursts of neuromuscular activity. Preliminary research by Elble (1989) reports that as tremor amplitude increases, the number of desynchronized neural pathways decreases. The occurrence of the tremor functions to further elicit and potentiate the original US, creating an interoceptive feedback loop (Razran, 1961; Figure 1, Panel B). Tremor like many other responses, also has stimulus properties. In this case, it is the tremor that functions as a US, which elicits observational and visceral responses, leading to further potentiation of the tremor. (Figure 1, Panel C). Tremor can also evoke other responses including emotional arousal, verbal responses, (motoric) avoidance behavior, and increased self-observation and awareness.

Tremor occurs in a social context. Previously neutral contextual antecedents, for example, social conversation or eating in the presence of others, begin to function as conditioned stimuli (CS) after being repeatedly associated with the occurrence of a biologic US and resulting tremor. (See Figure 2 below). The CS, a specific social contextual event, when presented by itself now elicits tremor. Each of these events, biologic dysfunction (US) by itself and/or conditioned contextual events (CS) elicits central and autonomic nervous system arousal, leading to tremor.

Central to contemporary models of respondent conditioning is the expanded concept of the conditioned response (CR; Forsythe & Eifert, 1998). Respondently conditioned behaviors are not limited to autonomic (visceral) functioning, but include motoric and verbal response systems (Staats & Eifert, 1990). Moreover, these same CRs may also function as discriminative stimuli for behavior under the control of operant consequences (Allen, 1998; Blackman, 1977).


Covert verbal responses, i.e., exclamations of negative emotions, e.g., "Damn, I just spilled all over my pants!" initially established through respondent conditioning, function as discriminative stimuli which influence the occurrence of other behavior, for example, performance of motor responses and self-observation. Engaging in these behaviors is followed by environmental consequences (positive or negative reinforcement) thus establishing operant control of the previously respondently conditioned verbal behavior. For example, the overt verbal response, "There's nothing that I can do about my tremor" is a discriminative stimulus for others to provide sympathy and help (positive reinforcement), which increases the likelihood that such verbal behavior and tremor occur in the future in that setting. In addition, this same verbal stimulus signals impending aversive stimulation (i.e., tremor, disability) and sets the occasion for avoidance behavior, for example, avoiding performance of ADLs and associated aversive conditions such as spilling, or failure. Avoidance behavior is thus negatively reinforced and increases in probability. As can be seen in Figure 3, a response within any of these systems then feeds back to central and autonomic nervous system mechanisms producing a self-activating feedback loop (Lundervold, 1997; Lundervold & Poppen, 1995).

Utility of theory

To be truly functional, theoretical models must lead to the development of assessment and intervention procedures that are testable and productive in terms of generating empirical data, and new research questions. Ultimately, the findings need to be disseminated leading to general application of the interventions.

In this phase of model building we have focused on the development and evaluation of biobehavioral assessment and interventions that are theory-driven. Multibehavior-multimethod assessment procedures, including clinical and self-ratings of tremor severity and emotional arousal, and electromyographic activity (EMG) are used. EMG serves as an objective, quantitative measure of tremor severity. Behavioral Relaxation Training (BRT) is used to decrease negative emotional arousal and lessen tremor severity (Poppen, 1998; Lundervold, Belwood, Craney, & Poppen, 2000). During analogue training situations individuals practice relaxed behaviors while performing ADLs. Participants are then instructed to perform relaxation while engaging in ADLs in vivo. Dynamic EMG biofeedback is used to further decrease tremor and refine motor behavior during ADL performance. Audio feedback, provided contingent on decreased EMG during dynamic performance of the ADL, functions as a reinforcer for altered covert motoric behavior (Lundervold & Poppen, 2000). BRT has also been used with imaginal exposure to conditioned aversive stimuli, instruction in coping self-statements, and praise to decrease avoidance behavior and negative emotional arousal (Lundervold, 1997). Coping self-statements are taught as replacement verbal responses that function as discriminative stimuli for engaging in relaxed behaviors and effective ADL performance. Finally, single-case (N = 1) research designs have been employed in the techniques-building stages of the research (Lundervold & Belwood, 2000).


The biobehavioral conditioning model described has enabled us to better understand the complex and interactive processes related to ET, to develop multibehavior-multimethod assessment procedures, and functional interventions addressing ET. Further research is needed to evaluate the model and the generalizability of the biobehavioral interventions used.


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Duane A. Lundervold

Central Missouri State University


Roger Poppen

Southern Illinois University-Carbondale

Please address correspondence to Duane A. Lundervold, Ph.D., Department of Psychology and Counselor Education, Lovinger 1016, Central Missouri State University, Warrensburg, MO 64093.
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Author:Lundervold, Duane A.; Poppen, Roger
Publication:The Behavior Analyst Today
Article Type:Disease/Disorder overview
Date:Jun 22, 2000
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