Suprathreshold responses of the visual system in normals and in demyelinating diseases

It is well-established that diseases affecting the visual pathway can result in the elevation of contrast thresholds. Little is known, however, about how people with decreased sensitivity to contrast perceive targets at suprathreshold levels of contrast. It is known that the normal visual response at suprathreshold levels cannot be linearly derived from the threshold contrast function. It may be expected that threshold abnormalities may not predict the quality of vision for high contrast images which prevail in normal operating conditions. In this investigation, the response to suprathreshold visual stimuli (vertical sine-wave gratings) in multiple sclerosis (MS) and optic neuritis patients with contrast sensitivity deficits was studied. Forty-eight normal eyes served as controls. Suprathreshold response was assessed with a matching procedure. Performance above threshold, for each individual, was estimated by a score which was the ratio of the contrast match to the contrast threshold for a given spatial frequency. The majority of the eyes studied showed that patients performed normally at high contrast levels. Indeed, in a number of cases, the scores implied that patients can perform better than normals. In some eyes, this enhancement of suprathreshold response was correlated with the threshold loss. These results suggest that a "compensation" mechanism helps make the perception of objects independent of the quality of detection at threshold for these patients. In other eyes, an abnormal decrease in the suprathreshold response measure was found, which was not related to the threshold deficit. Thus, suprathreshold responses of MS or optic neuritis patients cannot be predicted from their threshold contrast sensitivity.