In the study of human perception we often need to measure how sensitive is an observer to a stimulus variation, and how her/his sensitivity changes due to changes in the context or experimental manipulations. In many applications this can be done by estimating the slope of the psychometric function1, a parameter that relates to the precision with which the observer can make judgements about the stimulus. A psychometric function is generally characterized by 2-3 parameters: the slope, the threshold (or criterion), and an optional lapse parameter, which indicate the rate at which attention lapses (i.