An Anatomical Investigation Of Higher Visual Structures In The Pigeon (\u3cem\u3eColumba livia\u3c/em\u3e)

Early visual processing in the avian brain has been studied extensively, the retina and midbrain in particular. However, a clear understanding of the higher visual centers in the forebrain (the telencephalon and thalamus) remains poor. Two structures located within the avian visual telencephalon, the entopallium (E) and the lateral portion of the intermediate nidopallium (NIL), merit extensive investigation based on their critical role in visual processing. The goal of the current study was to further clarify the anatomical characteristics of E and NIL. Visual information that reaches these telencephalic structures is mostly from the contralateral retina. Thus, blocking visual input on one side affects the opposite hemisphere, but leaves the hemisphere on the same side largely unaffected. This unique property of the avian visual system was used in order to emphasize neurochemical expression in the higher visual structures. After blocking visual input to one hemisphere either by monocular occlusion or unilateral lesion of the nucleus rotundus we examined the expression of specific neuroanatomical markers; namely cytochrome oxidase (CO) and the calcium binding protein, parvalbumin (PV) in E and ZENK protein expression in NIL. . Our results showed there were significantly more PV-ir cell bodies in the inner region of E compared to the outer region. In terms of PV-ir neuropil, regional differences within E were significant. In particular, the ventrolateral E tended to have a higher density of PV-ir neuropil than other regions except the most ventromedial area which was also high in PV-ir. Differential CO staining patterns were observed as well. However, unlike PV-ir neuropil the ventro-intermedial area showed a low level of CO staining compared to the other areas. Finally, there was a significant reduction of ZENK-ir cell bodies on NIL in the experimental hemisphere compared to control side. Based on this differential expression of ZENK, it was possible for the first time to visualize the location of NIL. The findings presented here are discussed in terms of their relevance to the identification of the size and extent of NIL and the heterogeneous nature of E