Singularities in the inverse modeling of 2AFC contrast discrimination data

Analytical calculations show that two-alternative force-choice data are not always suitable for specifying the parameters of the underlying discrimination model. Experimentally, we show here that in the case of contrast discrimination in humans, a variety of models spanning a large range of parameters can explain the data within an experimental error. Monte-Carlo simulations indicate that the number of trials in psychophysical experiments is not the limiting factor in estimating the parameters in contrast discrimination. These results can therefore explain the contradictory conclusions made by different groups about the relationship between the response to contrast and the noise amplitude.