The Somatosensory System Physiology and Models

The terms somatosensory, tactual and haptic can be considered to be synonymous even though their precise definitions vary slightly. The most common of these terms is haptic, which has its origin in the Greek, meaning "lay hold of". The haptic system comprises two subsystems, the tactile and the kinesthetic. The tactile involves the sense of touch and the kinesthetic sense results from limb movement. The tactile sensors are located in the layers of the skin and therefore are frequently referred to as cutaneous sensors, while the kinesthetic sensors are located in the joint capsules as well as the muscles and tendons. While much less is known about the characteristics of these sensors and how they contribute to the total percept of motion, than the vestibular system. They can somewhat be described in engineering terms. The kinesthetic sensors, the muscle spindles and the Golgi tendon organs, provide the required feedback for the adaptive control behavior of the neuromuscular systems of the limbs. This control behavior interacts with the aircraft, or any vehicle, control system and has consequences for the control feel systems in both vehicles and simulators of those vehicles. Also considered is the effect of acceleration on the kinesthetic sensors for example in uncoordinated or high G flight. This paper presents a review of the state of knowledge as well as an engineering characterization of these systems. Mathematical models are included which indicate response of individual components to various inputs. Illustrations and data from the execution of these component models, integrated with more complex models of the human operator, such as the Hess model are also presented. Nomenclature