A latent 5-dimensional space for action representation: Geometric validity and dissociation from kinematics

A fundamental question in visual perception is how the human visual system transforms the rich kinematic information present in observed actions into coherent social meaning. We addressed this by validating a 5-dimensional action space model - defined by Formidableness, Friendliness, Locomotion, Abduction, and Environmental Interaction - as a perceptual representation of avatar-conveyed actions. Using Representational Similarity Analysis, we first demonstrated strong topological correspondence between the model's geometry and the structure of independent perceptual judgements, with cross-validated regression confirming the model as a generative framework that reliably predicts how observers evaluate novel actions. A morphing paradigm further revealed that perceptual ratings scaled approximately linearly with geometric distances along model dimensions, with each dimension selectively predicting its corresponding perceptual quality, satisfying the criteria for a valid psychological metric space. Critically, the 5D model showed substantially stronger alignment with semantic representations of actions than with their raw skeletal kinematics - an association robust to statistical control for kinematic similarity. This dissociation suggests that higher-order social-evaluative dimensions of action perception are largely invariant to low-level motion statistics, consistent with a hierarchical visual processing architecture in which kinematic input is progressively abstracted into a compact, semantically organised representational space optimised for social inference.