Afferent and efferent projections of the mesopallium in the pigeon (Columba livia).

The mesopallium is a thick cell plate occupying a substantial portion of the avian dorsal pallium, but its hodology is incompletely known. In pigeons we examined fiber connections of the frontodorsal (MFD) and frontoventral mesopallium (MFV), the ventrolateral mesopallium (MVL), the lateral (MIVl) and medial (MIVm) parts of the intermediate ventral mesopallium, and the caudal mesopallium (MC). MFV, MIVl, and MC connect reciprocally with secondary centers of the trigeminal, tectofugal, and auditory systems, respectively. MVL forms reciprocal connections with both the entopallial core and belt. MFV, MIVl, MVL, and MC receive thalamic inputs different from those of primary sensory pallial regions and have reciprocal connections with the caudolateral nidopallium (NCL) or arcopallium. MIVm has a strong reciprocal connection with the intermediate medial nidopallium. It receives afferents from the visual Wulst, rostral MC, posterior dorsointermediate thalamic nucleus, and caudal part of the posterior dorsolateral thalamic nucleus, is connected reciprocally with the arcopallium, and projects to NCL. MFD has reciprocal connections with the medial frontal nidopallium, arcopallium, posterior pallial amygdala, dorsolateral corticoid area, and projects to the medial part of medial striatum and hypothalamus. These results indicate that six subdivisions of the mesopallium have strong connections with corresponding portions of the nidopallium. In particular, the sensory mesopallial components of MFV, MIVl, MVL, and MC form parallel pathways to the one-way sensory streams in the nidopallium and make either feedback or feedforward circuits to the secondary centers of the nidopallium.