Visual system alterations in White Zebra Finches.

Visual system anomalies in albino mammals are generally seen to be caused by a lack of retinal pigment and misrouting of retinofugal optic fibers. This study shows that the central visual system of white zebra finches is physiologically different from normally colored (wild type) birds, although the eye pigmentation and the retinofugal projection appear to be normal. Ipsilaterally evoked potentials in our white birds are enhanced in comparison to wild type birds, whereas in albino mammals the ipsilateral component of visually evoked potentials is reduced. Picrotoxin-induced blockade of inhibitory synapses in the ectostriatum reveals remarkable differences between wild type and white zebra finches. In wild type zebra finches, a significant shift of ipsilateral to contralateral stimulus response ratios is observed. However, there is no detectable shift in the white morph. The data suggest that inhibition of ipsilateral stimulus processing, as observed in wild type zebra finches, is significantly reduced in the white morph. Our results indicate that the effects observed in white zebra finches cannot be explained by the theories that have been developed for albinotic animals. We assume that in white zebra finches a genetic defect, which causes the white plumage, is coupled with the demonstrated deviations of inhibitory mechanisms in the central visual system.