The relationship of visual threshold to retinal position and area

The absolute visual threshold of a field on zero background covering a given retinal area in the fully dark-adapted eye depends on at least three factors: the proportion of the incident light absorbed by the photosensitive pigment, the summation properties of the retinal area concerned, and the number of acting quanta necessary to evoke a visual response. This number may, of course, depend on the distribution, in space and time, of the quanta. The relationship between the areas and the thresholds of fields centred at a given retinal position has often been used as the basis for theories about the number of acting quanta required, but such deductions depend on assumptions-usually of uniformity of the retinal areas concernedabout the first two points. The present experiments were undertaken to test the validity of such assumptions, and to investigate the possibility of formulating a plausible theory of visual performance in threshold and simple acuity experiments on the basis of 'visual units', groups of receptors acting together. The experiments on acuity are described elsewhere (Hallett, 1961).