The impact of loss of the central visual field on OKN gain has been debated. Hood
21 described one patient with central scotoma who exhibited accelerated OKN. In other studies in patients with AMD, no significant differences or only slightly reduced gains have been found, compared with control subjects without scotoma.
8 9 This has been explained by assuming that peripheral retinal areas are sufficient to stimulate OKN. However, even small artificial central scotomas may lead to a large reduction in OKN gain. With central artificial scotomas, filling-in is usually not possible, because the scotoma itself is seen and OKN is suppressed through fixation.
15 22 23 24 25 26 27 28 29 In experiments with retinal stabilized scotomas, the scotoma itself can be seen to move and can become a stimulus for further movement or can be used to stare at while the moving stripes are neglected.
23 In studies of artificial scotomas the edge-effect of the mask used to create central field defect or the presence of lines in the central visual field are likely to contribute to suppression of the OKN. Schor and Narayan
27 found that reduction of the gain of OKN is due mainly, if not entirely, to the presence of sharp lateral boundaries fixed near the central visual field, rather than to the narrowing of the display. Murasugi et al.
28 found reduction in OKN gain when edges were within 10° to 15° of the fovea. When stationary edges were not parallel to the direction of the motion of the stimulus, reduction in OKN was eliminated. Similarly, inhibition of the OKN was reduced when stationary boundaries were eliminated by blurring edges, when central and peripheral regions were equated for contrast and the stimulus contained no high spatial frequencies, which are more visible in the central retina.
29 Our patients did not describe a positive scotoma with distinct edges. However, they had difficulties describing the scotomas. Therefore, suppression of OKN during periods of non–filling-in by fixating the scotoma or edges of the scotoma cannot be excluded. The range of scotoma size in our patients of between 15° and 18° correspond to the location of edges suppressing OKN used by Murasugi et al.
28 In this case, the peripheral retina alone could elicit the OKN, whereas during periods of non–filling-in the OKN may have been suppressed.