The multifocal ERG: unmasked by selective cross-correlation.

The purpose of this paper is to provide the reader with a better insight into the mechanisms of multifocal ERG (mfERG) recording. The construction of the first and second order mfERG responses were examined by recovering the response to specific pulse trains embedded in the m-sequence.A custom built pc based multifocal system driving a LED stimulator was used to record a 61 element mfERG and a global ERG. The global ERG recording was used to enable the recovery of different pulse trains embedded in the m-sequence. Summation of these individual pulse trains was performed and the results compared with the standard full cross-correlation. An isolated pulse response is defined as a flash of light that has no other flashes within two m-sequence base periods before or after the flash. This isolated pulse response was recovered from the raw data and this response input into a simple superposition model to predict the waveform shape for specific pulse trains. The superposition model was compared with the actual selective cross-correlation for a particular pulse train. The summations of the selective cross-correlation components give identical responses to the full cross-correlation. The superposition model also predicts the waveform shapes recovered by the selective cross-correlation procedure. The mfERG response is a complex composite response from a number of different pulse trains. Examination of the individual waveform shapes provides some insight into the origin of the mfERG waveform. The main contributions to the P1 component are the same as for an isolated response and as with the standard ERG this component is likely to be dominated by the mid retina. The N1 component is also likely to have similar origins to that of the isolated response but the amplitude is dominated by contributions from pulse trains where there is no change of state and therefore includes a component from the interaction between two consecutive stimuli. The N2 component is a composite response dominated by the interaction between two successive stimuli two base periods apart and the P1 component of a second stimulus delayed one frame from the first stimulus.