A failure to train the "same-different" differentiation of photic stimuli in dogs

Dogs were unable to learn "same-different" differentiation of pairs of photic stimuli when continuous light (CL) and pulsing light (PL) were presented in four combinations: CL-PL and PL-CL served as S(D) (positive instrumental conditioned stimulus), whereas CL-CL PL-PL were S delta (inhibitory stimulus). Also the dogs which have learned this task with tones were unable to transfer to photic stimuli. Differentiation of the single stimuli (CL and PL) as S(D) and S(delta) was quite easy and showed that the stimuli were readily discriminable.