Abstract
Purpose::
Many people with visual impairment report that their scotomas appear filled-in with surrounding spatial structure. Such filling-in defeats wide field screening (campimetry) that could otherwise be used to detect blind spots. Visual fields must therefore be mapped with time-consuming microperimetry. Artificial scotomas (a mean luminance patch embedded in dynamic spatial structure) are also filled in normally-sighted people. Paradoxically, artificial scotomas (where physical structure is absent) give rise to illusory afterimages in homogenous fields. We mapped the afterimages generated by both real and artificial scotomas and assess the potential of the former, as a type of rapid visual perimetry.
Methods::
Retinal scotomas were mapped with SLO or Nidek microperimetry in people with central or paracentral visual field loss. Subjects maintained fixation on a prominent cross with their fovea or preferred retinal locus. An adaptation phase -consisting of a large field (32 deg by 24 deg) of high contrast dynamic (15Hz) white noise lasting 10 seconds - was cycled with a test phase, consisting of a blank mean luminance field lasting 10 seconds. During the test phase, subjects traced around any regions of inhomogenous appearance using a touch sensitive screen.
Results::
During the adapting phase, subjects with lesions were unaware of any missing texture at the locations of their scotomas. During the test phase they reported anomalous structure in areas of the display that corresponded closely to the known locations of their scotomas. Subjects could easily trace around these areas and there was excellent repeat reliability on their locations. Visually healthy control subjects did not report the presence of any structure in the test phase. On trials in which an artificial scotoma was simulated with a 2 deg mean luminance patch, healthy subjects perceived illusory noise at the location of the artificial scotoma.
Conclusions::
Perceptual completion within real and artificial retinal scotomas is an active process with similar properties in visually impaired and healthy observers. After-images can be induced to appear at the the filled-in locations and mapping these regions provides rapid, wide-field campimetry. Such testing could be self-administered and has the potential for early screening of visual field loss and to guide detailed microperimetry.
Keywords: low vision • visual fields • perimetry