Theoretical interpretation and derivation of flash-on-flash threshold parameters in visual system diseases

Visual increment thresholds measured on backgrounds flashed on simultaneously with the test flash exhibit saturation rather than following Weber's law. These flash-on-flash thresholds have been modeled with saturating nonlinearities similar to those used to describe intensity-response functions of retinal neurons. Recently, the flash-on-flash technique has been used to assess the mechanisms of threshold elevations in visual system diseases. However, the results were interpreted in a qualitative way. This paper extends the clinical application of flash-on-flash threshold measures by providing a theoretically derived algorithm for estimating nonlinear model parameters from flash-on-flash threshold data. The performance of the algorithm is evaluated with Monte Carlo techniques, and the theoretical interpretations of the model parameters aretested experimentally. Published clinical flash-on-flash threshold data are reanalyzed using the algorithm. A previously unappreciated decrease in the half-saturation constant of retinitis pigmentosa patients was revealed by this analysis.