Object Geometry and Passive Sitting Influence the Measurement of Discrimination in the Displaced Object Recognition Task

Background: The Displaced object recognition (DOR) task is a widely used behavioral paradigm for evaluating spatial recognition memory in rodents. Standard protocols assume that any physical proximity to an object reflects active cognitive investigation; however, behavioral topographies such as passive sitting on objects may confound exploration metrics depending on object geometry.

Methods: Fourteen male C57BL/6 mice were tested in a DOR paradigm using either climb-resistant conical objects or flat-topped cylindrical objects. The discrimination index (DI) was quantified for both subjects using either raw or corrected (after mathematically subtracting the time that animal was passively sat on the subject) data.

Results: In familiarization and displacement-test phase, mean raw and corrected DI did not differ between cone and cylinder groups. However, comparing the raw data revealed a significant difference between familiarization and displacement test in the cone, but not cylindric group. Upon subtracting sitting time and calculating the corrected data, there was a significant difference between familiarization and displacement test in the cylindric group, too. There was a strongly and positively correlation between raw DI and the sitting time.

Conclusion: Object geometry significantly altered the behavioral repertoire of rodents. Flat-topped cylinder objects that facilitated passive sitting accounted for over 68% of the variance in raw DI during the test phase. Calculating the corrected data may unmask the real spatial memory in rodents.