The directional sensitivity of the photoreceptors in the human achromat.

1. The anatomical nature of the retinal photoreceptors in a typical, complete achromat was investigated by measuring their directional sensitivity. 2. A small (0.5 deg), brief (94 ms) test flash was placed at threshold by varying either its intensity (indirect method) or the intensity of a large (5 deg) adapting field (direct method). The dependent variable was the intensity of light required for the detection threshold as a function of the position of entry in the pupil. An infra-red viewing system was used to monitor the achromat's pupil and eye position. 3. At both scotopic and mesopic adapting field luminances, the complete achromat's receptors displayed only a small directional sensitivity effect. The effect was not wavelength dependent and could be attributed solely to the rods. 4. The results are consistent with other psychophysical evidence indicating that the complete achromat's retinae lack the post-receptoral function of the cone photoreceptors. Therefore they do not confirm previous reports that complete achromats have a second, high-intensity type of photoreceptor.