Deciphering the reward-related impulsivity domains in rats: The big data study of historical control

Impulsivity is a lack of ability to control own impulses, and encompasses many subdomains. The variable-delay-to-signal (VDS) paradigm is behavioral procedure for assessing motor impulsivity and delay intolerance in rats, but it was unclear whether all parameters contributed to these domains. Therefore, the aim of this study was to uncover the relationship between impulsivity parameters in a large cohort. VDS adapted to a touchscreen environment was used to assess impulsivity in adult Sprague-Dawley rats. After 1 week of training, animals were tested in a 3-stage testing protocol. The first stage included 20 trials with 6s inter-trial interval (ITI6si) that suggested motor impulsivity. The second stage, with 60 randomly distributed trials of ITI9s or 15s, was interpreted as delay intolerance, whereas for the last stage (ITI6sf), which is similar to the first stage, it was unclear to which type of impulsivity it was associated. Principal component analysis (PCA) was used to determine the different behavioral domains. The results of 132 controls from 11 independent VDS experiments were analyzed. Based on the cumulative variance explained, scree plot, and eigenvalues, the main components were extracted whereby varimax rotation was used on factor loadings to extract the components. PCA with varimax rotation was performed in R studio. PCA revealed that 96.45% of the variance could be explained by 3 principal components (PCs). After varimax rotation, loadings for ITI9s and ITI15s were 0.8189 and 0.9419, respectively, for rotated PC1 (RC1), loading for ITI6sf was 0.9482 for RC2, and loading for 6si was 0.9183 for RC3. In the VDS paradigm, 3 different impulsivity domains could be determined. In addition to motor impulsivity and delay intolerance, it is suggested that reflection impulsivity can also be assessed as learning-based impulsivity.