Biomedical Engineering Reference
In-Depth Information
function and the networks of genes responsible for the development and maintenance of
these neural circuits. A major challenge for future research will be to determine whether
there is strong or weak scale invariance at the multiple spatiotemporal scales of brain or-
ganization. A combined genetics, genomics, and integrative neuroscience approach has
the potential to redefine our conceptualization of neuropsychiatric illnesses with the im-
plementation of objective quantitative measures that can then be translated into probabil-
istic diagnoses for these complex diseases of the human brain.
1. INTRODUCTION
The broad questions posed by astrophysics and by motivation neuroscience
are conceptually similar but converse in their focus, one peering out at the uni-
verse and the other gazing into the brain. The former asks, "what is the nature of
what we perceive?" and "how is something created from nothing?" The latter
asks, "how do we perceive, control interpretation of perception, and exercise
free will?" The principal question of motivation neuroscience asks, "why is there
directed action?"
Motivation is the engine that allows organisms to make choices, direct their
behavior, or plan their actions across time. Motivated behavior can be defined
by goal-directed behavior that optimizes the fitness of an organism or social
group. It depends on input from evaluative processes regarding internal homeo-
static and socially acquired needs, potential goal-objects in the environment
meeting these needs, remembered consequences of previous behavior toward
goal-objects, and perceived needs in other cooperative or competitive organ-
isms. The combined neural systems that produce this directed action constitute
the neural basis of what we call motivation (263). These neural systems are
composed of multiple subsystems that have evolved to allow an organism to
assign a value to objects in the environment so that the organism works for "re-
wards" and avoids "sanctions" or aversive outcomes. Central to these neural
systems are a set of subcortical gray matter regions [nucleus accumbens (NAc),
caudate, putamen, hippocampus, amygdala, sublenticular extended amygdala of
the basal forebrain (SLEA), hypothalamus, and thalamus] (117) and components
of the paralimbic girdle [including the orbitofrontal cortex (GOb), insula, cingu-
late cortex, parahippocampus, and temporal pole] (174). Other networks across
the prefrontal cortex also appear to be engaged in the evaluative and decision
making components of motivated behavior. A number of these regions are
modulated by dopaminergic neurons in the substantia nigra, the retrotuberal
field, and the ventral tegmental area (henceforth jointly referred to as the ventral
tegmentum: VT) (228). Less is understood regarding the roles of non-dopa-
minergic neuromodulators (noradrenergic, serotonergic, cholinergic, steroid
hormones, and neuropeptides) that appear to alter the balance between excita-
tory and inhibitory synaptic neurotransmission during motivated behavior. In
1954, Olds and Milner (192) were the first to implicate a subset of these regions
in reward-mediated behavior. In subsequent decades, pioneering studies by oth-
ers contributed to our further understanding of these systems (89,98,148,265).
Search WWH ::




Custom Search