Potentials in the amphibian tectum, evoked by stimulation of the optic nerve, were recorded extracellularly. Four discrete potentials, referred to as the m
1
, m
2
, u
1
and u
2
waves, occur at different latencies after stimulation. We have shown that these waves represent summed post-synaptic potentials generated by synchronous activation of four different groups of optic nerve fibres. The m
1
and m
2
waves are generated by two classes of myelinated optic nerve fibres, previously characterized as ‘dimming’ and ‘event’ fibres. The maximum amplitude of the m
2
wave occurs just below, and of the m
2
wave just above, cell layer 8 of P. Ramón. The u
1
and u
2
waves are generated by ‘edge’ and ‘convexity’ fibres, respectively. The maximum amplitude of the u
1
wave occurs near the surface of the tectum, and of the u
2
wave some 100 μm below it. Postsynaptic cell bodies for all four classes of optic fibre are located in layer 8.
The Electric Stimulation of the Concentrating (Adrenergic) and the Dispersing (Cholinergic) Nerve-Fibres of the Melanophores in the Catfish
