The Westheimer function as an indicator of fixation locus.

The dimensions of the Westheimer function are stable across populations and in time and vary with the eccentricity of fixation. The function is thus useful to assess whether an individual is fixing centrally and if not to estimate the degree of eccentricity of fixation. We identified both summation and inhibition-like regions in a 66-year-old woman with senile macular degeneration and estimated that the extent of eccentric fixation was approximately 4 degrees.