Purpose:
Visual–span profiles are plots of letter–recognition accuracy as a function of letter position left and right of the midline (Fig. 1). Previous research has shown that visual–span profiles are reduced in size in normal peripheral vision and the reduction is correlated with reading speed (1). It is hypothesized that a reduced visual span is a primary factor limiting reading speed in AMD patients with bilateral central scotomas. In this study, we measured the visual–span profiles in AMD subjects to test the prediction of an overall depression in visual–span profiles and to determine the effects of scotomas left or right of the point of fixation.
Methods:
Eleven subjects with AMD and 3 age–matched normals were tested. Visual–span profiles were measured using the trigram method described by Legge et al. (1). Letter size was approximately twice the subject’s critical print size and the exposure time for the letters was adjusted to achieve a maximum accuracy of 80%. The size of the visual span was computed as the area under the curve in units of bits of information transmitted (Fig 1). The binocular visual field was mapped by Tangent screen for AMD subjects to estimate the location and size of scotomas.
Results:
The overall visual–span profiles for AMD subjects were reduced in size when compared with normal controls. Area of the visual–span profile, expressed as total information transmitted was significantly reduced in subjects with AMD (p<0.01). Eight of the 11 AMD subjects had central scotomas within the central 20°. These subjects exhibited localized depressions in the visual–span profiles (Fig. 1), related to the location of their scotomas. The size of the visual span was strongly correlated with the size of the binocular field loss in steradians (rs=–0.9, p<0.01).
Conclusions:
As predicted, visual–span profiles for people with AMD were significantly reduced compared with normal controls. Localized depressions in the visual–span profiles were related to the location of scotomas measured with conventional perimetry.
References:
(1) Legge et al. Vision Res 2001;41: 725–743.
Keywords: low vision • reading • age-related macular degeneration