Is the caudomedial shell of the nucleus accumbens part of the extended amygdala? A consideration of connections.

The conceptual origins of two forebrain functional-anatomical systems, the ventral striatopallidum and extended amygdala, are reviewed briefly, and reductionist and constructionist doctrines pertaining to descriptions of forebrain organization are generally discussed briefly. The cortical, subcortical, and intrinsic connectional relationships of the caudomedial shell of the nucleus accumbens, an anomalous component of the ventral striatopallidum, are compared with those of the extended amygdala, leading to an attempt to formulate an assessment as to the extent to which the caudomedial shell might actually be regarded as a part of extended amygdala, or a transitional structure with a capacity to bridge extended amygdaloid and striatal mechanisms. The relatively distinct intrinsic circuitries and different outputs of the two are noted. Extended amygdala projects most strongly to the posterior half of the lateral hypothalamus and brainstem autonomic effector sites. Caudomedial shell and ventromedial ventral pallidum project throughout the lateral preoptico-lateral hypothalamic continuum and are major components in a basal ganglia-thalamocortical pathway to the prefrontal cortex. It is concluded that connectional features distinguishing the caudomedial shell/ventromedial ventral pallidum and extended amygdala are of sufficient magnitude that the two should most usefully be regarded as separate, interacting functional-anatomical entities.