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An understanding of psychophysiological factors in complex human behaviors such as violence requires the study of how the brain functions. Brain function is a result of interactive influences from one’s genes and input from one’s environment. Although genes set the stage for individual potential and range of possible responses and behaviors, the environment helps to determine what particular form behavior will take in any given instance as well as ways in which behavior will fluctuate over time. Brain functions that give rise to emotions, moods, drives, memories, intelligence, personality, and much more are measurable by techniques that monitor psychophysiological responses by the brain and body to environmental input.
Differences in physiological activity of the nervous system, both peripheral (i.e., autonomic) and central (the brain), have been found between people with and without violence, and these differences often reflect an underlying dysfunction of neurotransmitter systems. Numerous studies suggest that stimulation-seeking, impulsivity, aggressiveness, hyperactivity, attention defici/hyperactivity disorder, lack of avoidance responses, and inability to empathize are behavioral correlates of serotonin and dopamine system abnormalities with measurable psychophysiological consequences. Most of the evidence supports the notion that individuals prone to violence and psychopathy have unusually low physiological levels of central nervous system (CNS) and autonomic nervous system (ANS) activity, which appear to be related to high levels of sensation seeking.
Behavior, Stress, and the Autonomic Nervous System
During a highly stressful or provocative situation, various physical responses occur that involve activation of the flight-or-fight mechanism, which is both a biochemical and a physiological process. During this activation, a chain reaction of bodily defenses is orchestrated by the release of several stress hormones that further reinforce the physiological processes that lead to increases in heart rate, blood pressure, and other measurable indices. As a result, energy becomes available to allow a determination of the best course of action, either fighting back or fleeing. Under conditions of severe stress, humans have been known to perform unusual feats of strength and endurance. Even under less severe conditions of stress, however, awareness and attention are heightened and physical strength increases.
According to the suboptimal arousal theory, this system allows humans to be conditioned by environment, for example, by avoiding behavior likely to result in a penalty or punishment or seeking stimulation that will provide pleasure or reward. Most individuals have been conditioned effectively enough to know not to steal or harm others simply due to the threat of punishment or negative consequence. When this stress system is hyperactive, however, it is too quick to respond or it responds without adequate provocation from the environment, such as in panic disorders. On the other hand, when this system is underactive, the individual does not experience sufficient physiological activation to produce discomfort. The result can be an underactive nervous system: The individual may not be conditionable because, in the absence of physiological and emotional discomfort, he or she will not respond appropriately to punishments or threats of punishments. Research suggests that such individuals cannot be effectively deterred from crime or high-risk behaviors merely with threats of punishment.
Electroencephalogram and Evoked Potentials
Physiological markers indicative of central nervous system instability have been repeatedly found in subjects with violent behavior, as reflected in electroencephalogram (EEG) and evoked potential (EP) differences. In particular, researchers have used the EEG to identify differences in brain activity between people with and without behavioral disorders. Individuals with a history of impulsive aggression tend to show relatively more slow wave activity in their EEG and delays in their EPs as compared to those without aggression. Individuals with a greater amount of the slow waves may not process information as efficiently or effectively; thus, such slowing may be related to cognitive deficits. Delays in EPs as they travel from the brainstem into the center of the brain to be registered also indicate an inefficiency in information processing. Relatively high levels of EEG slowing and EP delays in these subjects are thought to reflect arrested development of the brain and its function. These processes are often a function of irregularities in neurotransmitter systems that alter CNS arousal levels and thus may contribute to excessive stimulation needs, resulting in violence.
Skin Conductance and Heart Rate
Skin conductance (SC) measures the electrical activity in the skin and reflects CNS and peripheral nervous system function, which includes the autonomic nervous system. When SC responses to a stimulus are elevated, electrical activity within the skin is high. This reaction is a sign of the degree to which an individual is aroused and therefore is an indication of emotional state. Most studies of SC and its relationship to aggressiveness or antisocial behavior have focused on psychopaths. Investigators have consistently reported findings of low SC arousal in this population. Deficits in SC arousal are believed to be associated with low autonomic arousal levels that are, in turn, related to low emotionality, poor condition-ability, lack of empathy and remorse, and ability to lie easily. In psychopathic and aggressive individuals, such deficits in SC may be outwardly expressed as reduced or inappropriate emotional responses to socially meaningful stimuli. Put simply, individuals who do not respond to emotional stimuli (e.g., a bloody knife) with an emotional response (e.g., fear) are more difficult to deter by threats of punishment and more likely to seek out high levels of stimulation, increasing the likelihood of aggressive behaviors. Because both serotonin and dopamine play a regulatory role in the production of skin conductance in the ANS and frontal cortex, there is speculation that SC deficits result from a neurotransmitter imbalance.
Heart rate is another expression of nervous system function that reflects emotional state. During a resting state, low heart rate has been reliably found in antisocial and aggressive youngsters. These sorts of findings are consistent with the widely tested hypothesis that subjects with antisocial, psychopathic, and repeatedly violent behavior are more likely to be physiologically underaroused and, consequently, seek an unusual amount of stimulation in an inadvertent attempt to arouse their nervous systems.
Neuroimaging measures of brain physiology generally measure either glucose metabolism (the brain’s fuel) or blood flow, both reflective of the brain’s level or change in activity. Much of the neuroimaging work on violence focuses on psychopathy, a personality trait thought to characterize about 1% of the general population. Psychopathy is much more prevalent among individuals who engage in antisocial, criminal, and, at times, violent activities. The hallmark physiological traits that distinguish psychopathic individuals from those with secondary psychopathy (i.e., with underlying anxiety) and from other clinical and normal populations with similar behavioral outcomes involve the relative lack of autonomic and central nervous system arousal, as described above by studies using EEG, event-related potentials, skin conductance, and heart rate. There is much speculation and evidence that the relative attenuation of autonomic arousal in psychopaths provides insufficient stimulation to the nervous system and, thus, increases the need for sensation seeking. At the same time, low levels of arousability do not generate an adequate physiological response to produce discomfort, thereby deterring the individual from engaging in behavior that is normally associated with penalties or punishment. In nonpsychopathic subjects, violence or otherwise inappropriate behavior produces measurable increases in physiological responsivity, which may reinforce motivation to avoid behaviors previously associated with penalties (i.e., conditioned responses).
Recent neuroimaging studies using either position emission tomography or functional magnetic resonance imaging have provided additional evidence of neurophysiological involvement in psychopathy and other behavioral disorders often characterized by violent behavior. These studies implicate prefrontal dysfunction as a potential substrate of dysregulated cognitive, behavioral, and emotional expressions in psychopathy. A recent review of 17 neuroimaging studies revealed that low levels of activity in areas associated with aggressive and/or violent behavioral histories, particularly impulsive acts, are located in the prefrontal cortex and the medial temporal regions, a finding which can be explained in the context of negative emotion regulation.
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