Reduction of GABAergic neurons within the tectofugal visual system of white zebra finches

The central visual system of white zebra finches is physiologically and anatomically different from normally coloured (wild type) animals. The main difference to normal birds is an enhanced response to ipsilateral stimulation in all areas of the tectofugal visual pathway. Previous experiments indicated that besides an enhancement of recrossing fibers, it might be a lack of inhibition which causes this effect. We show here that such an explanation can only be true for a part of the entopallium, the telencephalic station of this projection. Only within the so-called perientopallium, the number of GABAergic neurons is strongly reduced, while there is no significant difference between white and wild type birds in the other visual areas. It is speculated that these neurons in normal birds inhibit ipsilateral input conveyed by the second visual projection in birds, the thalamofugal pathway.