By recording antidromic field potentials and unit responses generated in the retina by stimulation of the optic tract and optic disc, evidence was obtained which suggests the presence of four major conduction velocity groups in the cat's optic nerve. The axons from all peripheral retina appear to fall into two groups, fast and slow, which correspond to the two major velocity groups described by earlier workers. Evidence is presented that the axons which arise from the area centralis form two distinctly slower conduction velocity groups. For each conduction velocity group, and for 60 single units, conduction velocity was estimated for both the intraretinal (unmyelinated) and extraretinal (myelinated) segments of the axons. All axons encountered accelerated markedly on leaving the retina. An anatomical basis for the classification of conduction velocity groups is presented in an accompanying paper.
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