Abstract
The range of c. 1012 ambient light levels to which we can be exposed massively exceeds the <103 response range of neurons in the visual system, but we can see well in dim starlight and bright sunlight. This remarkable ability is achieved largely by a speeding up of the visual response as light levels increase, causing characteristic changes in our sensitivity to different rates of flicker. Here, we account for over 65 years of flicker-sensitivity measurements with an elegantly-simple, physiologically-relevant model built from first-order low-pass filters and subtractive inhibition. There are only two intensity-dependent model parameters: one adjusts the speed of the visual response by shortening the time constants of some of the filters in the direct cascade as well as those in the inhibitory stages; the other parameter adjusts the overall gain at higher light levels. After reviewing the physiological literature, we associate the variable gain and three of the variable-speed filters with biochemical processes in cone photoreceptors, and a further variable-speed filter with processes in ganglion cells. The variable-speed but fixed-strength subtractive inhibition is most likely associated with lateral connections in the retina. Additional fixed-speed filters may be more central. The model can explain the important characteristics of human flicker-sensitivity including the approximate dependences of low-frequency sensitivity on contrast (Weber’s law) and of high-frequency sensitivity on amplitude (“high-frequency linearity”), the exponential loss of high-frequency sensitivity with increasing frequency, and the logarithmic increase in temporal acuity with light level (Ferry-Porter law). In the time-domain, the model can account for several characteristics of flash sensitivity including changes in contrast sensitivity with light level (de Vries-Rose and Weber’s laws) and changes in temporal summation (Bloch’s law). The new model provides fundamental insights into the workings of the visual system and gives a simple account of many visual phenomena.
Highlights
We propose a simple model that accounts for these and other features of light adaptation seen in temporal contrast-sensitivity functions” (TCSFs) data
We have proposed a simple model of human light adaptation made up of a cascade of low-pass filters and two stages of subtractive inhibition
We can provide an excellent description of existing TCSF data by adjusting the time constants of the four earliest LP-stages in the direct path and the two in the inhibitory stages with light level and, at higher levels, the overall gain, g, but fixing the strength of inhibition, k, and the time constants of the final two LP-stages
Summary
A primary goal of visual sensitivity-regulation or light adaptation is to enable the visual system to perform effectively over light levels that can vary by more than 1012 despite the dynamic. Human light adaptation ranges of neurons in the visual pathway being limited to 103 or less In order to prevent later postreceptoral neurons from exceeding their limited dynamic range and saturating, light adaptation at moderate and high light levels must occur primarily at or before the synapses in the cone pedicles. The perceptual effects of such speed adjustments produce characteristic changes in our sensitivity to flickering light with changes in light level. We bring together 65 years of flicker-sensitivity measurements to provide a new model of light adaptation with just two leveldependent parameters. One parameter of the model controls speed, and the other controls gain
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