In addition to response differentiation the animal must also learn to discriminate the discriminative stimulus. This task can most clearly be seen when a number of stimuli can be presented to the animal only one of which is the true SD. mber of responses the animal makes to colour which differ slighIf the factor which distinguishes the SD is its colour then we soon see that the nutly from the SD colour is far fewer than would be the case if the SD did not have to be distinguished like this (see figure below).
