When studying animal perception and assessing welfare, researchers often use artificial stimuli (e.g., pictures, slides, videos and computer images) as substitutes for real animals or objects, as they are non-invasive and allow greater control over stimulus presentation. In the past, most studies have used cathode ray tube (CRT) screens; however, there is some evidence that these give different results to non-flickering thin film transistor (TFT) screens. Our previous research found domestic hens' critical flicker fusion frequency ranges between 80-90Hz, well above CRT screens which generally have a flicker rate of around 60Hz. Thus, stimuli presented on CRT screens may appear distorted to hens. This study aimed to investigate whether changing the flicker rate of CRT screens altered hens' discrimination. Six domestic hens were trained to discriminate between a black circle and a black cross presented on a TFT (flicker-less) screen using a forced-choice discrimination procedure (i.e., peck left key if shown circle, peck right key if shown cross) until they reached 85% correct over 5 sessions. Sessions were terminated after 40 minutes had elapsed or after 30 reinforcers had been obtained, whichever occurred first. The stimuli were then presented in test sessions on a CRT screen set at four different flicker rates (60, 75, 85, and 100Hz). The hens accuracy on the TFT screen and the CRT flickering at 100Hz was not different (Wilcoxon Signed Ranks test, z=-1.753, p>0.05), however, accuracy decreased systematically and significantly as the refresh rate of the CRT screen was decreased (Friedman's test, ?2(4)=14.6, p<0.05). These results imply that the change in flicker rate changed the appearance of the stimuli enough to affect the hens' discrimination and stimulus control breaks down for hens when stimuli appear to be flickering.