Mouse paw preference: effects of variations in testing protocol.

Two experiments using two inbred strains of mice were conducted to study mouse paw preference. In the first study, 250 (50 x 5) paw reaches from each of 12 male and 12 female C57BL/6J mice were observed over an 8-week period using the Collins paw preference apparatus to investigate questions relating to the independence of reaches, the size of the tube into which the animals reach for flakes of food, and practise effects. Animals appeared to be much more highly lateralized when two independent reaches per day for 25 days (I protocol) were observed than when 50 reaches were observed in a single session (S protocol). Paradoxically, however, we found no evidence for the lack of independence of reaches when we examined the sequences of reaches performed by animals under the S protocol conditions. With practise, animals became slightly more lateralized, but there were no effects of sex, and approximately equal numbers of mice were left-pawed as were right-pawed. The size of the tube had no effect. The second study used 30 BALB mice, each of which was tested with both the I and S protocols, with order of testing counterbalanced. There was a significant protocol x order interaction such that those doing S first were more highly lateralized on the I task, but those doing I first were highly lateralized on both tasks.