Topography of the multifocal electroretinogram.

The functional topography of the human retina was characterized using the multifocal electroretinogram (ERG), with particular attention to the form of the decline in response with retinal eccentricity. Population response variability was examined and compared to standard full field ERG variability. Burian-Allen contact lens electrodes were used to record the cone multifocal ERG from 50 young eyes (28.3 years +/-5.9 years). Responses were recorded in 8 min from 103 retinal locations within the central +/-22 degrees. The spatial distribution of local responses showed an exponential fall-off with eccentricity. The exponential slope parameter was highly similar across individuals. Excluding responses to the central element, the fall off with eccentricity approximated a power function with an exponent of -0.6, which compares to the -0.74 exponent for the human cone density profile. The inter-individual variance in response density is greatest at the central fovea, reducing towards more peripheral locations. The logarithm of response density, however, shows approximately equal variance across eccentricity, making log density a more appropriate way to view response topography. The population range (+/-2 S.D.) of response density is 0.42 log unit, similar to that of standardized ganzfeld electroretinography. The response exponential decay provides a potentially useful addition to element-by-element comparison, in deciding whether an eye's response is within normal limits.