The human brain mediates all human behavior ? aggression, violence, fear, ideology ? indeed, all human emotional, behavioral, cognitive and social functioning. This three pound mass of 100 billion neurons and 1000 billion glial cells is infinitely complex. Yet certain principles of brain organization and function can lead to insights regarding neurological factors involved in violence and aggression.
The brain has a hierarchical organization, from the lower, more simple areas to the more complex higher cortical areas. Simple, regulatory functions (e.g., regulation of respiration, heartrate, blood pressure, body temperature) are mediated by the 'lower' parts of the brain (brainstem and midbrain) and the most complex functions (e.g., language and abstract thinking) by cortical structures.
The brain's impulse-mediating capacity is related to the ratio between the excitatory activity of the lower, more-primitive portions of the brain and the modulating activity of higher, sub-cortical and cortical areas (Cortical Modulation Ratio). Any factors which increase the activity or reactivity of the brainstem (e.g., chronic traumatic stress, testosterone, dysregulated serotonin or norepinephrine systems) or decrease the moderating capacity of the limbic or cortical areas (e.g., neglect) will increase an individual's aggressivity, impulsivity, and capacity to display violence.
As the brain develops and the sub-cortical and cortical areas organize, they begin to modulate and 'control' the more primitive and 'reactive' lower portions of the brain. With a set of sufficient motor, sensory, emotional, cognitive and social experiences during infancy and childhood, the mature brain develops ? in a use-dependent fashion ? a mature, humane capacity to tolerate frustration. A frustrated three year old will have a difficult time modulating the reactive, brainstem-mediated state of arousal ? he will scream, kick, bite, throw and hit. However, the older child when frustrated may feel like kicking, biting and spitting, but has the capacity to modulate those urges. Loss of cortical function through any variety of pathological process (e.g., stroke, dementia, head injury, alcohol intoxication) results in regression ? simply, a loss of cortical modulation of arousal, impulsivity, motor hyperactivity, and aggressivity ? all mediated by lower portions of the central nervous system (brainstem, midbrain). Deprivation of key developmental experiences (which leads to underdevelopment of cortical, sub-cortical and limbic areas) will necessarily result in persistence of primitive, immature behavioral reactivity, and, thereby, predispose an individual to violent behavior.
The most dangerous children are created by a malignant combination of experiences. Developmental neglect and traumatic stress during childhood create violent, remorseless children. This is characterized by sensitized brainstem systems (e.g., serotonergic, noradrenergic and dopaminergic systems). Dysregulated brainstem functions (e.g., anxiety, impulsivity, poor affect regulation, motor hyperactivity) are then poorly modulated by poorly organized limbic and cortical neurophysiology and functions (e.g., empathy, problem-solving skills) which are the result of chaotic, undersocialized development. This experience-based imbalance predisposes to a host of neuropsychiatric problems and violent behavior.
As we search for solutions to the plagues of violence in our society, it will be imperative that we avoid the False God of Simple Solutions. The neurobiology of complex, heterogeneous behaviors is complex and heterogeneous. In the end, paying attention to the neurobiological impact of developmental experiences ? traumatic or nurturing ? will yield great insight for prevention and therapeutic interventions.
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