The functional organization of the isthmo-optic nucleus in the pigeon.

1. Extracellular single unit records were taken from the isthmo-optic nucleus of the pigeon, locating the nucleus by the antidromic field potential resulting from electrical stimulation of the isthmo-optic tract.2. Visual receptive fields were investigated for 248 units, and were classed into three types: directionally selective, directionally non-selective, and posterior minimum. They ranged from 3 to 10 degrees in extent.3. The topographical representation of the visual field in the nucleus was investigated by histological verification of electrode tracks, and a correlation with the receptive field sequences. Vertical descent through the nucleus resulted in an upwards progression of receptive field locations. Lateromedial passage through the nucleus resulted in a forwards progression of receptive field locations.4. The representation of the lower visual field is greater than that of the superior visual field: that for the central parts of the inferior field is greater than that for the periphery, and the representation of the anterior inferior quadrant is greater than that of the posterior inferior quadrant.