Engineering Basis of PSYCHO-ANTAM, a Three-Degree-of-Freedom Servosphere for Unrestrained Behavioral Measurement of Small Terrestrial Organisms

This study presents the development and evaluation of PSYCHO-ANTAM, a three-degree-of-freedom (3-DoF) motion compensation system designed for tracking small terrestrial organisms while presenting precise multisensory stimuli. Unlike conventional 2D servospheres, this system independently controls pitch (rotation about the lateral axis), roll (rotation about the longitudinal axis), and yaw (rotation about the vertical axis), enabling accurate alignment of stimulus direction with the organism's heading. The system integrates three modules: heading detection via machine vision, PSR-based actuation, and trajectory measurement using a laser mouse sensor. Behavioral experiments with Armadillidium vulgare revealed significant differences in locomotion features—such as diffusion distance, turning angle, and Lévy exponent—between 2D and 3D environments, highlighting the device's potential for advanced behavioral studies. In particular, model selection based on the Humphries method and the Akaike information criterion supported exponential (Brownian) rather than power-law step-length distributions under both conditions, indicating that the Lévy exponent was used here as an index to test, rather than to confirm, the presence of Lévy-like movement.