Impulsivity in humans: neurobiology and pathology

Impulsivity refers to premature behaviours and rash, poorly considered decisions which are made in spite of negative consequences. It is not a singular entity, consisting of a number of sub-categories under the two broad headings of motor and decisional impulsivity. The subtypes of motor impulsivity are laid out first, discussing waiting impulsivity, also known as premature responding, which can be a predictor of addiction, and response inhibition impulsivity, which is the capacity to inhibit pre-potent response. The chapter then explores the forms of decisional impulsivity, such as delay discounting and reflection impulsivity, along with their relation to attention-deficit/hyperactivity disorder. Finally, the prevalence of impulsivity across disorders as wide-ranging as obsessive–compulsive disorder and Huntington’s disease is discussed, and a case study of Parkinson’s-related impulsivity is presented.

Anatomical and functional organization of the human substantia nigra and its connections

We investigated the anatomical and functional organization of the human substantia nigra (SN) using diffusion and functional MRI data from the Human Connectome Project. We identified a tripartite connectivity-based parcellation of SN with a limbic, cognitive, motor arrangement. The medial SN connects with limbic striatal and cortical regions and encodes value (greater response to monetary wins than losses during fMRI), while the ventral SN connects with associative regions of cortex and striatum and encodes salience (equal response to wins and losses). The lateral SN connects with somatomotor regions of striatum and cortex and also encodes salience. Behavioral measures from delay discounting and flanker tasks supported a role for the value-coding medial SN network in decisional impulsivity, while the salience-coding ventral SN network was associated with motor impulsivity. In sum, there is anatomical and functional heterogeneity of human SN, which underpins value versus salience coding, and impulsive choice versus impulsive action.

Anatomical and functional organization of the human substantia nigra and its connections

We investigated the anatomical and functional organization of the human substantia nigra (SN) using diffusion and functional MRI data from the Human Connectome Project. We identified a tripartite connectivity-based parcellation of SN with a limbic, cognitive and motor arrangement. The medial SN connects with limbic striatal and cortical regions and encodes value (greater response to monetary wins than losses during fMRI), while the ventral SN connects with associative regions of cortex and striatum and encodes salience (equal response to wins and losses). The lateral SN connects with somatomotor regions of striatum and cortex and also encodes salience. Connectivity within the value-coding medial SN network was associated with a measure of decisional impulsivity, while the salience-coding ventral SN network was associated with motor impulsivity. In sum, there is anatomical and functional heterogeneity of human SN, and separate SN networks underpin value versus salience coding, and impulsive choice versus impulsive action.