Photoreceptor contributions to contrast sensitivity: Applications in radiological diagnosis

Electrophysiological and psychophysical observations are described which appear to have important implications in diagnostic image interpretation. While the interpretation of a complex radiographie image depends upon processing by higher order neural centers, this interpretation process is certainly influenced by the sensitivity of the visual system. We have focused our attention on two mechanisms which operate at the input stages of the visual system and which directly affect visual sensitivity: 1) the effects of background illumination on cone photoreceptor sensitivity, and 2) the image-forming properties of the eye (including small involuntary eye movements). Any visual information which is transmitted to higher neural centers must first be preprocessed by these mechanisms. We have constructed a model of the preprocessing which occurs in the eye by which light patterns viewed by a subject are transformed into patterns of neural activity of the cone photoreceptors in the subject's retina. The desensitization of cone photoreceptors by steady background illumination was studied using intracellular techniques. Background intensities desensitize both cone photoreceptors and the photopic visual system in a similar manner. This finding is consistent with the hypothesis that photopic visual threshold may be set at the very input to the visual system by the sensitivity of the cone photoreceptors. These two mechanisms characterize the behavior of the input stages of the visual system and appear to account for much of the visual system desensitizations caused by both uniform and stylized nonuniform backgrounds.