Functional anatomy of the avian centrifugal visual system

Although first described over a century ago, the centrifugal visual system (CVS) projecting to the retina still remains somewhat of an enigma with reg ard to its functional role in visually-guided behavior. The highly develope d avian CVS has been the most extensively investigated and the anatomical o rganization of its two component centrifugal structures, the n. isthmo-opti cus (NIO) and ectopic neurons (EN), including its afferent brainstem projec tions is reviewed. The results of double-labeling studies combining axonal tracing techniques and immunohistofluorescence have demonstrated GABA immun oreactivity (-ir) of interneurons within the neuropilar zone of the NIO, ch oline acetyltransferase (ChAT)-ir and nitric oxide synthase (NOS)-ir in the centrifugal cells of the NIO and EN as well as in the afferent projection neurons of layers 9/10 of the optic tectum. The data are discussed in terms of neurochemical and excitatory/inhibitory mechanisms within the different components of the avian CVS in relation to hypotheses which have implicate d this system in visual attention and ground-feeding behavior. (C) 1999 Els evier Science B.V. All rights reserved.