Distribution of Calcium-Binding Proteins and Cytochrome Oxidase Activity in the Projective Zone (Wulst) of the Pigeon Thalamofugal Visual Pathway: A Discussion in the Light of Current Concepts on Homology between the Avian Wulst and the Mammalian Striate (Visual) Cortex

This polemic article focuses on the current concepts of the architectonics of the hyperpallial projective zone (Wulst) in the avian thalamofugal visual pathway and the distribution of calcium-binding proteins parvalbumin (PV) and calbindin (CB), as well as cytochrome oxidase (CO) activity, in these brain regions of birds, including our histochemical and immunohistochemical data obtained on the pigeon. In the principal thalamorecipient sensory area of the Wulst, n. interstitialis hyperpallium apicale/hyperpallium intercalatum (IHA/HI), three discrete zones are identified: the two, containing punctate PV-ir neuropil and a densely packed population of mainly small PV-ir stellate and granular cells, and a single zone formed by punctate CB-ir neuropil and mainly CB-ir cells. The IHA/HI area is distinguished by a high CO activity. In the hyperpallium apicale (HA) and hyperpallium densocellulare (HD), PV-ir and CB-ir neurons of varied morphotypes are scattered diffusely throughout neuropil which is moderately immunoreactive to both calcium-binding proteins. These areas are characterized by the presence of large pyramidal-like and multipolar PV-ir and CB-ir neurons with long branched dendrites forming a cellular/fibrous layer in the superficial HA area. It is concluded that the thalamofugal visual pathway in the pigeon has two, PV- and CB-specific, channels. We draw a comparative analysis of the variability of these channels in different avian species, which is supposed to be due to variable ecological and adaptive factors. Within the current idea about the organization and evolutionary origin of the avian Wulst and the mammalian striate (visual) cortex, the homology between these brain structures is discussed.