Regional differences in pigeon optic tract, chiasm, and retino-receptive layers of optic tectum

Electron-microscopic examination of the pigeon optic chiasm, tract, stratum opticum, and retino-receptive layers of the optic tectum revealed regional differences at each level. Axonal size in the fiber pathways paralleled that previously reported for pigeon optic nerve, with mean diameter values of 0.96 micrometer for optic chiasm and 1.06 micrometer for optic tract. The dorsolateral aspects of these pathways contained a heterogeneous population of fibers (mean diameter congruent to 1.44 micrometer) similar to that found in the nasal portion of optic nerve, while the ventromedial regions were occupied by a more homogeneous population of smaller fibers (mean diameter congruent to 0.82 micrometer) resembling those observed in the temporal portion of the nerve. The retino-receptive layers of anteroventral optic tectum (avT) differed ultrastructurally from those of posterodorsal tectum (pdT) with respect to the thickness of horizontal dendrites in layer 2-3, the size of optic terminals in layers 2-7, and the number of synaptic contacts per terminal. These findings point towards a regional variation in the processing of visual information throughout the retino-tectal system and suggest that neurons in avT vs. pdT should show differences in the way they modify the neurophysiological characteristics of their respective optic inputs.