The detection of light scattered from stimuli in impaired regions of the visual field.

'What is the most intense stimulus I should use when studying an impaired region of the visual field?' This is a question which frequently faces the clinical investigator who is studying the function of a severely impaired region of the visual field, but it is a question to which answers cannot be found in the literature. The question arises because there are two distinct ways in which an intense stimulus, directed into a defective region of the field, may be detected. First, the stimulus may be seen simply because it lies above the threshold for the defective region. On the other hand, it is known that any light passing through the intraocular media suffers a small degree of scattering and illuminates a wide area of retina. If a stimulus is unusually intense, this scattered light may be detected by unimpaired regions of the field, even though the stimulus itself is still below threshold for the defective region in which it lies. In studying the function of an impaired region of the field one must, therefore, be sure that the patient is not detecting the light scattered from the stimulus. The only way to ensure this, is to keep the stimulus intensity below that at which scattered light is potentially detectable-that is, below the scattered-light threshold for the defective region under consideration. A number of studies of various aspects of intraocular light scatter have been reported (Stiles, 1928; Brindley and Willmer, 1952; Boynton and Clarke, 1962; Wolf and Morandi, 1962). Other authors have noted artefacts resulting from intraocularly scattered light-for example, Boynton and Riggs (1951), and Hubel and Wiesel (1960). Unfortunately, none of these authors were primarily concerned with the parameters determining the visibility of scattered light, and their data do not provide the scattered-light thresholds which the clinical investigator needs. It is the purpose of this paper to present, in a