LUMINANCE DISCRIMINATION OF BRIEF FLASHES UNDER VARIOUS CONDITIONS OF ADAPTATION.

Abstract

J. Physiol. (1965), 176, pp. 294-310 With 6 text-figures Printed in Great Britain LUMINANCE DISCRIMINATION OF BRIEF FLASHES UNDER VARIOUS CONDITIONS OF ADAPTATION BY T. N. CORNSWEET AND H. M. PINSKER From the Department of Psychology, University of California, Berkeley, U.S.A. (Received 25 May 1964) Weber's law states that the just-noticeable change in luminance is a fixed fraction of the base luminance. The classic studies of luminance discrimination have suggested that Weber's law holds only as a rough approximation over an intermediate range of luminances, and clearly breaks down at both low and high luminances (e.g. Barlow, 1957; Pirenne, 1962). However, most of these studies have been conducted under con- ditions that are somewhat complicated relative to our present under- standing of visual processes. We shall argue that luminance discrimination data to be presented here have been collected under conditions easier to interpret in terms of photochemical and neural processes, as well as signal detection theory. Under these simplified conditions, Weber's law is shown to hold exactly over the entire range of luminances from just above absolute threshold to at least five log units above absolute threshold. METHODS Apparatus. A schematic diagram of the apparatus is shown in Fig. 1. The source was an automobile tail light bulb run from a constant voltage d.c. power supply. Its radiance was constantly monitored by a photocell, and any changes in radiance were compensated for by adjustments in the current through the bulb. Lens L1 formed an image of the filament in the plane of the rotating disk shutter, and lens L2 then collimated the rays. The rays that passed through the beamsplltters BS1 and BS2 formed the main path, and fell upon an opaque stop S2, in which two holes had been placed one above the other. This stop deter- mined the stimulus configuration as it appeared to the observer in Maxwellian view (see inset, Fig. 1). The stimulus disks fell upon an area of the retina containing both rods and cones. Lens L3 was at its focal distance from 82 and from the pupil of the observer's eye, and therefore collimated the rays from the plane of S2 and formed a filament image in the plane of the pupil. This filament image was 2 mm in diameter, and thus changes in the diameter of the observer's pupil did not appreciably change the retinal illuminance. The rays that were reflected from BS1 and from mirrors M1 and M2 formed the side path, and passed through a special stop S, before being recombined with the main path by BS2. SI consisted of two vanes mounted on a shaft that could be rotated silently about a vertical axis (see inset, Fig. 1). The filament image was small enough, and the rays were well enough collimated that each of the vanes completely blocked the light from the side path at one of the holes in stop S9 and did not at all block the light at the other hole. Thus, by rotating S1,