A second group of 9 observers saw each image 10 times at a duration of 0.5 sec and 10 times at a duration of 2 sec. The sequence of 480 images was completely randomized separately for each observer. Data analysis Method. Internal stimulus values have a normal distribution with unit variance. Stimulus distributions for mixtures of stimuli have distances proportional to the ratios of the mixtures.
All subjects have the same pattern of distances between the stimulus distributions.
They differ only by a multiplicative subject sensitivity factor. Category boundaries vary across subjects, but are constant over stimuli. The scaling model has one image discriminability parameter for each of the 6 image sets and one sensitivity factor and 3 category boundaries for each of the observers. Observers tested with the two durations were allowed separate sensitivity factors. Parameters were estimated by the method of maximum likelihood.