Who's flying the plane: Serotonin levels, aggression and free will

Abstract

The present paper addresses the philosophical problem raised by current causal neurochemical models of impulsive violence and aggression: to what extent can we hold violent criminal offenders responsible for their conduct if that conduct is the result of deterministic biochemical processes in the brain. This question is currently receiving a great deal of attention among neuroscientists, legal scholars and philosophers. We examine our current knowledge of neuroscience to assess the possible roles of deterministic factors which induce impulsive aggression, and the extent to which this behavior can be controlled by neural conditioning mechanisms. Neural conditioning mechanisms, we suggest, may underlie what we consider the basis of responsible (though not necessarily moral) behavior: the capacity to give and take reasons. The models we first examine are based in part upon the role played by the neurotransmitter, serotonin, in the regulation of violence and aggression. Collectively, these results would appear to argue in favor of the view that low brain serotonin levels induce impulsive aggression which overrides mechanisms related to rational decision making processes. We next present an account of responsibility as based on the capacity to exercise a certain kind of reason-responsive control over one's conduct. The problem with such accounts of responsibility, however, is that they fail to specify a neurobiological realization of such mechanisms of control. We present a neurobiological, and weakly determinist, framework for understanding how persons can exercise guidance control over their conduct. This framework is based upon classical conditioning of neurons in the prefrontal cortex that allow for a decision making mechanism that provides for prefrontal cortical control of the sites in the brain which express aggressive behavior that include the hypothalamus and midbrain periaqueductal gray. The authors support the view that, in many circumstances, neural conditioning mechanisms provide the basis for the control of human aggression in spite of the presence of brain serotonin levels that might otherwise favor the expression of impulsive aggressive behavior. Indeed if those neural conditioning mechanisms underlie the human capacity to exercise control, they may be the neural realization of reason-responsiveness generally.

The role of serotonin in impulsive aggression, suicide, and homicide in adolescents and adults: a literature review

“…Serotonin does not pass the blood-brain barrier and thus must be synthesized in the brain, in the raphe nuclei found in the brainstem (9). Tryptophan passes the blood-brain barrier through transport and is hydroxylated by tryptophan hydroxylase (TPH) into 5-hydroxytryptophan (5-HTP).…”

Neurobiología de la agresión y la violencia

“…Se ha encontrado que los niveles del neurotransmisor 5-HT en regiones corticales como la CPF y la CCA son inversamente proporcionales a los niveles de agresión impulsiva (Coccaro, Fanning, Phan y Lee, 2015;Siegel y Douard, 2011;Yanowitch y Coccaro, 2011).…”

“…Utilizando tareas en que los sujetos podían demostrar una conducta agresiva o no agresiva hacia otra persona, varios estudios han mostrado, tanto en hombres como en mujeres, que la reducción mediante la dieta de la 5-HT cerebral está asociada con la conducta agresiva impulsiva (Coccaro et al, 2015;Quadros, Takahashi y Miczek, 2010;Siegel y Douard, 2011) que se muestra ante estímulos nocivos, amenazantes o provocadores. Recíprocamente, si se aumenta la dosis de triptófano en la dieta se observa una irritabilidad y agresión disminuida en estudios experimentales en humanos.…”

Psicobiología de la agresión y la violencia

“…Los niveles de presencia de este neurotransmisor son inversamente proporcionales a los niveles de agresión impulsiva, así pues, la reducción mediante dieta de dicho neurotransmisor, está asociada con la conducta agresiva e impulsiva que se manifiesta ante estímulos nocivos, amenazantes o provocadores. De manera inversa, si se aumenta la dosis de triptófano en la dieta, se observa una irritabilidad y agresión disminuida; lo anterior de acuerdo a estudios experimentales llevados a cabo con humanos(Coccaro, Fanning, Phan, & Lee, 2015;Siegel & Douard, 2011;Yanowitch & Coccaro, 2011).En un reciente meta-análisis sobre estudios que han puesto en relación la serotonina con la agresión, se encontró que la serotonina explica sólo un poco más del 1% de la varianza en agresión, ira y hostilidad. Sin embargo afirman que la hipótesis de la deficiencia de serotonina permanece abierta al debate en tanto puede haber varias…”