Naturally occurring neuronal plasticity in visual wulst of the Baya weaver, Ploceus philippinus (Linnaeus, 1766)

The visual wulst (VW), the rostro-dorsal surface of the avian telencephalon extending from the midline to the lateral region of the brain, is a laminated "bulge" consisting in four histologically distinct rostro-caudally arranged laminae with a specific sequence: hyperpallium apicale, interstitial nucleus of hyperpallium apicale, hyperpallium intercalatum and hyperpallium densocellulare. The VW has been proposed to be the avian equivalent of the mammalian striate cortex. Various behavioral studies including lesion experiments have indicated the importance of the VW, which receives visual and/or auditory cues. We have investigated qualitatively and quantitatively the fascinating structural changes occurring in VW neurons of the seasonally breeding bird, Ploceus philippinus (Linnaeus, 1766). The Golgi method was used to study the seasonal fluctuations in the neuronal classes of the VW with regard to dendritic thickness, spine morphology and spine density during both the non-breeding and breeding periods of male Baya weaver birds. Significant variations in parameters studied among the various neuronal types located in the different well-demarcated regions of the VW are believed to contribute to the functional differences reported among the wulst regions. Thus, this study extends our view demonstrating naturally occurring neuronal plasticity in a seasonally dynamic avian brain of a bird that hones not only its learning and memorizing system but also its social and sexual system in preparation for the breeding season.