Rod and cone contributions to the a-wave of the electroretinogram of the macaque

The electroretinogram (ERG) of anaesthetised dark-adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod- and cone-driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone-driven component was isolated by delivering the stimulus shortly after a rod-saturating background had been extinguished. The rod-driven component was derived by subtracting the cone-driven component from the mixed rod-cone ERG. The initial part of the leading edge of the rod-driven a-wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low-pass filter elements. Addition of a simple static saturating non-linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non-linearity or that any significant low-pass filter followed the non-linear stage of the model. A negative inner-retinal component contributed to the later part of the rod-driven a-wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a-wave up to the time of the first zero-crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod-bipolar cell response. The negative inner-retinal component essentially cancelled the early part of the rod-bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a-wave. The leading edge of the cone-driven a-wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod-suppressing background or by the glutamate analogue, cis-piperidine-2,3-dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a-wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ins after the stimulus. The leading edge of the cone-driven a-wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non-linear stage in the cone model was subject to a low-pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod- and cone-driven a-waves of the dark-adapted ERG.