Abstract
1. This paper describes experiments carried out to record from output cells in the isthmo-optic nucleus.2. One-hundred and twenty-seven axonal responses were fired at fixed latency from the optic nerve-head.3. Ninety-nine cell responses were fired trans-synaptically from the optic nerve-head.4. Ninety-four cells were activated antidromically from the optic nerve-head.5. Tectal tracks could be recognized by the field potential profile of the N-wave, R-wave and P-wave, and by the occurrence of fixed latency axonal responses and trans-synaptically fired cells.6. Tectal tracks were verified histologically.7. Tracks yielding antidromically activated cells were traced histologically to the isthmo-optic nucleus.8. The antidromic A-wave could be recorded from the nucleus, corresponding in timing to the invasion of cell bodies.9. Somatic records in the nucleus could be recognized by their duration, conformation, and A-B blocking.10. When antidromic discharge was interacted with orthodromic firing, collision evidence could be provided, showing that the orthodromic impulse travels centrifugally to the retina.