Analysis of the circuit for visual information and its function on the avian retinotectal system

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We can conclude the results obtained in three points as follows. 1) Retinotectal fiber terminals and the dendrites that receive the terminals formed interdigitation each other to shape large synaptic complexes. (a) Compared to the optic fiber terminals in the superior colliculus (cat, rabbit, hamster), arborization of preterminal branches of optic fibers in the chick tectum was more complex. (b) Synaptic complexes of the optic nerve terminals in the chick tectum were more complex than the glomerulus in the mammalian lateral geniculate body. (c) Sizes of the synaptic complexes of the optic fiber terminals in the tectum were more constant compared to those in the superior colliculus. (d) Shapes and sizes of dendritic postsynaptic elements were more variable than those in the superior colliculus. 2) Substance-P and acetylcholine receptor immunoreactive neurons in the retinal ganglion cell layer sent axons to the layer B and layer F of the chick tectum, respectively. Ganglion cells in the former has small somata with simple and narrow dendritic arborization. In the latter, more than two subtypes of ganglion cells were observed. One type of them had large somata with simple and broad dendritic arborization. 3) We studied the binocular vision by behavioral methods in chicks that received destruction of one eye on embryonic day 4. There was no significant difference in the rate of hit of black beads between the operated and normal chicks. The results suggest that the avian binocular vision would not necessarily require both eyes. Probably, the binocular vision in birds is carried out by the peculiar retinotectal system to birds.