Increment thresholds in a subject deficient in cone vision

Abstract

It is well recognized, particularly from the work of Hecht (1937), that measurements of visual function at different levels of illumination generally fall into two categories, a low intensity scotopic portion and a high intensity photopic portion. Thus in the time course of dark-adaptation, and in the curves where log. increment threshold, log. acuity or fficker fusion frequency is plotted against log. field intensity, the results are found to fall upon two distinct branches. The low-intensity branch has the spectral sensitivity of rhodopsin, and it is not seen in measurements made upon the fovea centralis; it is therefore considered to be due to rod function. The other branch has a-different spectral sensitivity, it is found upon the rod-free fovea and hence it is considered to be due to cone function. Now though there are considerable difficulties in distinguishing the functions of the various types of cone, it is easy to separate cone curves from rod curves over the whole intensity range of the cones. For not only may measurements be made upon the fovea where rods are absent, but even elsewhere it turns out that cones have an increment sensitivity and fusion frequency so much higher than rods that the plotted curves generally show the cone branch substantially in its entirety. It is otherwise with rod curves, for not only is there no part of the human retina which contains rods without cones, but the very features which make cone measurements outstanding in a mixed population make rod measurements unavailable. In fact, we can only measure rods at intensities below the cone threshold. This does not mean that rods are necessarily inactive at higher intensities, but their activity cannot be measured by ordinary threshold procedures, and we are left wondering what course the rod branch may take after it disappears behind the cone branch.This question could be answered if we were able to investigate a subject who had no cones but whose rods and rod pathways to the brain were * Instituto Neurologico, Via Celoria 11, Milano. t Present address: Physiological Laboratory, University of Cambridge.12-2

The initiation of excitation and light-adaptation in Limulus ventral photoreceptors

“…To find the action spectrum of light adaptation, we measured, at several wavelengths, the intensity of a steady adapting light needed to produce a criterion desensitization. (This strategy is similar to that used by Stiles, 1939;Fuortes et al, 1961;and Fain, 1976. ) The details of the procedure for determining the criterion action spectrum of excitation were as follows.…”

Sensitivity of toad rods: Dependence on wave‐length and background illumination.

“…It also occurs in man. The increment threshold curve of the human rod system turns upward from the WeberFechner line (Aguilar & Stiles, 1954), and at a somewhat brighter intensity the rods become unresponsive even to the brightest stimulus (Fuortes et al 1961). In all vertebrates so far investigated, the rods begin their approach to increment saturation at between 10-5 and 11-5 log quanta cm-2 sec-1.…”

Characteristics of anisometropic suppression: Simple reaction time measurements

“…Both background fields had the same luminance and gave a retinal illuminance of 1,720 td (based on a dilated pupil size of8 mm for each observer). This value ensured that observers were at least 2 log units into the region ofWeber adaptation (Blackwell, 1946;Fuortes, Gunkel, & Rushton, 1961).…”