ORGANIZATION OF THE AVIAN CORTICOSTRIATAL PROJECTION SYSTEM - A RETROGRADE AND ANTEROGRADE PATHWAY TRACING STUDY IN PIGEONS

Birds have well-developed basal ganglia within the telencephalon, incl uding a striatum consisting of the medially located lobus parolfactori us (LPO) and the laterally located paleostriatum augmentatum (PA). Rel atively little is known, however, about the extent and organization of the telencephalic ''cortical'' input to the avian basal ganglia (i.e. , the avian ''corticostriatal'' projection system). Using retrograde a nd anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse include s the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL) , pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (s uch as the ''Limbic'' NCL, pyriform cortex, and ventral/caudal parts o f the archistriatum) project to such ventral striatal structures as th e olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such ''limbic'' pallial areas also projec t to medialmost LPO and lateralmost PA, while the hyperstriatum access orium portion of the Wulst, the PE, and the dorsal parts of the archis triatum were found to project primarily to the remainder of LPO (the l ateral two-thirds) and PA (the medial four-fifths) The available evide nce indicates that the diverse pallial regions projecting to the stria tum in birds, as in mammals, are parts of higher order sensory or moto r systems. The extensive corticostriatal system in both birds and mamm als appears to include two types of pallial neurons: 1) those that pro ject to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brains tem (i.e., those in the PE). The lack of extensive corticostriatal pro jections from either type of neuron in anamniotes suggests that the an amniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensor y and motor information for the striatum, possibly facilitating the ro le of the basal ganglia in movement control. (C) 1995 Wiley-Liss, Inc.