Using scalar products to refine the interpretative value of an orientation choice test.

Choice tests, which are often used to examine animal preferences, can be difficult to interpret when no clear choice has been made or when using very young animals which exclude test repetition. We present a new method to evaluate the behavior in a choice test based on the orientation of the animal and illustrate its use when facing those conditions. Using rat pups in an open field maze with a choice of odors, we obtained x,y coordinates of 2 markers (head and body center) using a video-tracking freeware. Two vectors were calculated: an animal orientation vector (body to head) and a perfect orientation vector (body to odor source). The angle between the 2 vectors in each frame was converted into a scalar product ranging from 1 (pup oriented directly towards the odor source) to -1 (facing the opposite direction). A mean scalar product was calculated for each odor source, with the difference between the 2 mean scalar products indicating degree of preference for an odor. The information provided by the mean scalar product difference (MSPD) could not be obtained from other measures, such as binary choice, velocity, or distance moved. The MSPD provides a single, noncategorical value for each animal to describe degree of preference in a choice test. This variable was more effective in differentiating animals, thus allowing a reduction in the number of animals or tests necessary to reach significance.