Optimizing touchscreen measures of rodent cognition by eliminating image bias

For the last twenty years, the Bussey-Saksida touchscreen-based operant conditioning platform has evolved in close parallel alongside the Cambridge Neuropsychological Test Automated Battery (CANTAB) to produce batteries of tests for studying complex cognitive functions in rodents that are increasingly analogous to human diagnostic tests and greatly narrow the translational gap in cognition research. Naturally, with this increasing usefulness comes increasing use, particularly by non-experts. This necessitates a greater understanding of, and a better controlling for, confounding factors that may limit the systems ability to optimally detect cognitive deficits when used as a widely accessible and commercially available standardized task. In the present study, we show a strong image preference bias in a standard pairwise discrimination task with a widely used spider-plane image pairing in a putative animal model for intellectual disability. This bias greatly influenced the performance of our experimental mice, significantly affecting the length of time it took mice to complete the task, their progress over time, and several accessory measures usefully recorded by the Bussey-Saksida touchscreen system. We further show that this bias can be corrected by using more similar image pairings without sacrificing the animals ability to learn to distinguish the stimuli. This approach eliminated all significant stimuli specific differences seen with the spider-plane pairing. We then analyzed the pixel composition of the various stimuli to suggest that the bias is due to a difference in image brightness. These findings highlight the importance of carefully modulating paired touchscreen stimuli to ensure equivalence prior to learning and the need for more studies of visual perception in mice, particularly as it relates to their performance in cognitive assays.