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Filling-in along horizontal and vertical meridians.

PURPOSE
Filling-in is not homogeneous across the visual field; rather, fading time (FT) varies with eccentricity and polar angle. The reasons for this are unclear. The authors investigated FT along horizontal and vertical meridians in central and peripheral visual fields for luminance-defined targets, comparing Weber contrast sensitivity (CSw). They also compared cone-mediated and rod-mediated filling-in with previously described photoreceptor densities. This represents the first investigation into rod-mediated filling-in.


METHODS
Fading times were recorded in nine volunteers using luminance-defined stimuli at eccentricities of 0 degrees , 2.5 degrees , 5 degrees , 10 degrees , and 20 degrees for high-contrast stimuli and 0 degrees , 1.25 degrees , 2.5 degrees , 3.75 degrees , and 5 degrees for low-contrast stimuli and were compared with CSw at each location. Fading times were also recorded at 0 degrees , 2.5 degrees , 5 degrees , 10 degrees , and 20 degrees using red-green stimuli and at 5 degrees , 10 degrees , and 20 degrees under dark-adapted conditions and compared with cone and rod densities in the nasal, temporal, superior, and inferior hemimeridians.


RESULTS
Consistent anisotropy was evident in central and peripheral fields for luminance-defined targets and for chromatic targets, with horizontal meridians taking longer to fill in. The same pattern was observed in CSw and the previously described cone density. Log FT and CSw were correlated irrespective of visual field location. The ratio of rod-mediated FT to rod density decreased with eccentricity.


CONCLUSIONS
Anisotropy between horizontal and vertical meridians for FT is consistent in central and peripheral fields, reflecting patterns in CSw and cone density. This is discussed in the context of cortical processes underlying filling-in. For rod-mediated filling-in, more peripheral eccentricities are characterized by reduced FTs in relation to rod density.