Finger Force Matching and Verbal Reports: Testing Predictions of the Iso-Perceptual Manifold Concept

We used force matching and verbal reports of finger force to explore a prediction of the iso-perceptual manifold concept, which assumes that stable percepts are associated with a manifold in the afferent-efferent space. Young subjects produced various force magnitudes with the index finger, middle finger, or both fingers together. Further, they reported the force level using a verbal scale and by matching the force with fingers of the contralateral hand. Force matching, but not verbal reports, showed larger variable errors for individual fingers in the two-finger task compared to the single-finger tasks. We discuss possible differences in afferent and efferent contributions to force perception at low and high forces based on the idea of motor control with referent coordinates for the effectors. The differences between the force matching and verbal reports are possibly related to neural circuitry differences between perceiving without action and perceiving-to-act.