Invariance of the pattern electroretinogram evoked by psychophysically equivalent stimuli in human ageing.

1. The aim of this study was to assess the contribution of retinal ganglion cells to the decline in contrast sensitivity during human ageing. 2. After determination of the appropriate refraction for each subject, younger subjects were arranged to be exposed to a display luminance which was suprathreshold by the same amount as in older subjects wearing a 4.0 mm diameter artificial pupil with a neutral density filter. 3. In fifty-four subjects, aged 20-99 years, contrast sensitivities measured in response to phase-reversed grating patterns of 2, 5 and 8 cycles per degree declined significantly with increasing age at each spatial frequency studied. 4. Subjects were made psychophysically equivalent by setting the display contrast at x5 and x10 contrast threshold for each subject. The pattern electroretinogram (PERG) was recorded with a sterile silver thread (DLT) electrode placed in the lower canthus of one eye, with the indifferent electrode on the temple and the earth on the forehead. 5. For each contrast multiple at each spatial frequency, the PERG implicit time showed no significant change with age, indicating equivalence of the response across the age range. 6. Control experiments with two young and two elderly subjects established that the PERG implicit time decreased appreciably with increasing contrast, over a range of x2 to x20 contrast threshold. 7. Since the psychophysically equivalent stimulus displays had generated equivalent PERGs in terms of implicit time in young and elderly subjects, this was consistent with the equivalence of retinal ganglion cell function under these conditions. 8. Adverse changes within the retina were therefore inferred to play a major role in the decline in contrast sensitivity with age.