Spatial summation and the cortical magnification of perimetric profiles

M-scaling of the conventional spot targets of clinical perimetry at low photopic adaptation levels, such as that of the Octopus automated perimeter, does not result in the expected isosensitive profile using the current equations for humans. This disparity has been attributed to variations in the ganglion cell characteristics across the retina, most notably that of spatial summation. The hypothesis was further investigated by M-scaling the perimetric sensitivity recorded under conditions favouring reduced spatial summation, namely an increased adaptation level and a longer stimulus duration afforded by the Humphrey Field Analyzer. The M-scaled data exhibited a paracentral reduction in sensitivity relative to the theoretical isosensitive profile and an increased sensitivity beyond an eccentricity of 12 degrees. This indicates that for perimetric spot stimuli, the current human M-scaling equations under represent the fovea at the visual cortex. The implications for the design of perimetric routines are discussed.