Retinal afferent and efferent connections in congenitally monophthalmic chicks

The visual projections of the remaining eye of posthatch congenitally monophthalmic chicks were examined using wheat germ agglutinin-horseradish peroxidase. The morphology of the primary visual centres and their retinal projections contralateral to the injected eye were similar to those of normal chicks. The ipsilateral primary visual centres were smaller and less organized, yet all received retinal input. These ipsilateral retinal projections differ from those found in normal posthatch chicks [O'Leary et al.: Devl. Brain Res. 10: 93-109, 1983] in that they are more extensive and occupy some centres not previously reported to receive input. In the case of the ipsilateral isthmo-optic projection to the retina there was a substantial increase in the number of cells compared with normal chicks [O'Leary and Cowan: Devl. Brain Res. 12: 293-310, 1984]. A comparison of the extent of ipsilateral retinal afferents with that of normal chicks suggests that following loss of an eye two responses occur within the visual centres: in some centres there is a massive increase in the amount of ipsilateral terminals, whereas in others there is only a small increase. We propose that these responses are related to the intrinsic retinotopy within the visual centres. That is, highly retinotopic visual centres do not normally contain ipsilateral fibres, but following eye removal fibres from the ipsilateral eye are able to substantially innervate these regions. Presumably this effect is due to loss of the overriding influence of contralateral input, which would normally recognize and eliminate inappropriate ipsilateral fibres. In poorly retinotopic regions ipsilateral fibres are able to persist in both normal and monophthalmic chicks, as recognition cues may not be as precise as in highly retinotopic regions. Thus, the greater the retinotopic precision the finer the cues able to recognize ipsilateral fibres.