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1. Unitary responses were recorded in the lateral tectum of the pigeon, with electrolyte-filled micropipettes after electrical stimulation of the optic nerve-head.

2. Optic nerve fibre spikes could be recognized by their conformation, fixed latency, brief recovery times, and location in the superficial tectum. Their action potentials were either triphasic with a prominent second phase, or monophasic positive.

3. The optic nerve consists of small myelinated fibres conducting at 5·3-8·0 m/sec. These axons probably have diameters in the order of 1·6-2·2 µ.

4. The fibre spikes were localized to the N-zone and R-zone. None was recorded deeper. Most of the fibre spikes preceded the tectal N-wave.

5. One hundred and fifty-six post-synaptically fired cells were recorded. These had a diphasic positive-negative conformation, and were fired at variable latency.

6. One hundred and forty of these cells fired a single spike to each stimulus to the optic nerve-head. Even the most stably fired cells could be proved to be trans-synaptically activated by the evidence of non-collision.

7. Sixteen of the 156 cells fired repetitively to single stimuli to the optic nerve-head.

8. Evidence could be obtained that afferent inhibition operates upon tectal cells.

9. Cells in the N-zone were fired earliest in the 3 msec interval, corresponding to the rising phase of the tectal N-wave. By comparison, cells in the P-zone were not fired in the 3 msec interval, and the proportion fired in the 4 msec interval was reduced. Cell firing in the P-zone must be produced by tectal interneurones.

10. Cells were present in the N-zone with recovery times below 5 msec. No cells in the P-zone had recovery times below 5 msec.

11. A clear correlation could be made between the distribution of fibre and cell spikes, and the field potential profile. A correlation could also be made between the timing and recovery time of cells in the N-zone and P-zone. The unitary records show that the tectum is activated radially by the retinotectal pathway.