Fixed versus variable internal noise in contrast transduction: The significance of Whittle's data

A longstanding issue in vision research concerns whether the internal noise involved in contrast transduction is fixed or variable in relation to contrast magnitude. Previous attempts to resolve the issue have focused on the analysis of contrast discrimination data, despite the fact that the effects of internal noise on thresholds are necessarily compounded by the shape of the underlying transducer function. An alternative approach is to compare data obtained from a particular class of scaling experiment - one based on a comparison of perceived contrast differences - with data from discrimination experiments gathered across the full range of contrast. Data from two studies by the late Paul Whittle provide the basis for such an analysis, pointing to the conclusion that contrast internal noise is fixed not variable.