We recorded L- and M-cone isolating ERGs from human subjects using a silent substitution
technique at temporal rates of 12 and 30 Hz. These frequencies isolate the activity of cone-opponent
and non-opponent post-receptoral mechanis, respectively. ERGs were obtained using a
sequence of stimuli with different spatial configurations comprising; (1) circular stimuli of different
sizes which increased in 10 steps up to 70diameter, or (2) annular stimuli with a 70 outer diameter
but with different sized central ablations from 10 up to 60. L- and M-cone isolating ERGs were
obtained from five colour normal subjects using a DTL fibre electrode. Fourier ysis of the ERGs
was performed and we measured the amplitude of the first harmonic of the response. For 12 Hz
ERGs the L:M cone response amplitude ratio (L:MERG) was close to unity and remained stable
irrespective of the spatial configuration of the stimulus. The maintenance of this balanced ratio points
to the existence of cone selective input across the human retina for the L-M cone opponent
mechani. For 30 Hz the L:MERG ratio was greater than unity but varied depending upon which
region of the retina was being stimulated. This variation we consider to be a consequence of the
global response properties of M-cone ERGs rather than representing a real variation in L:M cone
ratios across the retina.
