An Operant-based Touchscreen Morph Discrimination Task Does Not Detect Age-related Mnemonic Similarity Deficits in Rats

Abstract

The Mnemonic Similarity Task (MST) is sensitive to age-related cognitive decline in humans and has been adapted for rodents using 3D objects, where aged animals show deficits discriminating similar lures. To improve translational alignment and increase automation, we developed a touchscreen-based rat analog using a morphed Object-Cued Spatial Choice task with 2D image stimuli. Young and aged male and female Fischer 344 × Brown Norway rats were trained in Bussey-Saksida touchscreen chambers and tested with image pairs that varied parametrically in feature overlap. In a subset of animals, we also assessed perirhinal cortical engagement using Arc expression following low- and high-overlap task epochs. Both age groups acquired the task, and morph testing showed that performance declined systematically as stimulus similarity increased, confirming that the task manipulated discrimination difficulty. However, young and aged rats performed similarly across overlap conditions. In the Arc experiment, discrimination accuracy was again reduced by greater stimulus overlap, but perirhinal cortical Arc expression did not differ reliably by age or overlap condition. These findings indicate that the touchscreen morph task is sensitive to stimulus similarity but does not detect age-related mnemonic discrimination deficits previously observed with 3D object-based rodent MST paradigms.