The directional sensitivity of the retina and the spectral sensitivities of the rods and cones

Abstract

The primary object of this work was to determine the effect of direction of incidence on threshold sensitivity but the results obtained cover the wider problem of the dependence of threshold sensitivity on the condition of stimulation of the retina. The test stimulus was a patch of light of diam. 1[degree] and of wave-length [lambda], exposed for a fraction of a sec. every few secs. U[lambda], the smallest perceptible intensity of the test stimulus, was measured in energy units, the sensitivity then being defined as 1/U[lambda]. With parafoveal vision of the test stimulus by the dark-adapted eye, direction of incidence had little effect on sensitivity when X was less than 580 m[mu], but a pronounced effect for longer wavelengths. The dark-adapted fovea gave a pronounced directional effect at all wave-lengths. The condition of the retina was modified by making the subject view a patch of light of diam. 10[degree] (wave-length [mu], and intensity W[mu]) at whose centre the test stimulus was applied as an additional stimulus. For parafoveal vision, the directional effect showed a marked increase in passing from low to high intensities, and the curve connecting the threshold value U[lambda] and the intensity of the conditioning field W[mu] showed a corresponding change of law. These 2 connected effects were attributed to a change from rod vision at low intensities to cone vision at high intensities, the rods and cones being assumed to have different directional properties. For foveal vision, which involved the cones only, somewhat similar effects were observed and were explained by assuming 3 types of cones for which the relative spectral sensitivities were roughly detd. Normally the threshold value increased proportionally with the intensity of the conditioning field at high intensities (Weber''s Law) but a striking deviation from this rule was observed for a blue test stimulus on a red conditioning field. Increase in the intensity of the conditioning field beyond a certain value produced no corresponding increase in the threshold value. This result was ascribed to a special property of the ''blue'' cones. In the present investigation the hypothesis of 3 types of cone whose properties were approximately detd. was developed from measurements which were completely independent of colour-matching data.