Anticipatory Motor Control Research Articles

Softness discrimination with a tool.

The abilities of humans to discriminate the softness of rubber objects of differing compliance with a hand-held tool (a stylus) was measured under experimental conditions that differed as to how the tool was used and the kind of sensory information available. When the subject actively tapped or pressed the compliant objects, they discriminated softness as well by means of a stylus as they did by contacting the objects directly with the fingerpad. Discrimination with the stylus was unaffected by whether the stylus was controlled by one or two fingers. While tapping or pressing a stylus held in a precision grip, the grip force increased before, reached a maximum at the same time as, and decreased in parallel with the compressional force. This relationship was suggestive of anticipatory motor control based on an internal model of the motor system and the physical properties of the object. Discrimination was significantly better when tapping as opposed to pressing the objects with the stylus. This was hypothesized as due to the presence of tactile cues generated by the rapid increase in force rate as the stylus struck and indented the object during tapping. During tapping, the magnitude and rate of compressional force produced by the stylus against the object were greater, the harder the object. An additional cue, possibly kinesthetic, during pressing and tapping was the magnitude of indentation of the specimen by the stylus that was greater, the softer the object. Subjects could discriminate differences on softness by tactile cues alone in the absence of kinesthetic when compliant objects were tapped at approximately the same velocity by the experimenter against a stylus in contact with the subject's passive fingerpad. Discrimination deteriorated if the softer specimen of a pair was tapped with a slightly greater velocity than the harder and not possible if the specimens were pressed against the stylus without generating tactile cues of mechanical contact. In contrast, discrimination was possible during active pressing and unaffected by variations in velocity during active tapping. It is concluded that during active movements, kinesthetic information and knowledge of central efferent commands provide useful cues that are not present during passive touch. These cues allow the observer to discriminate differences in object compliance not confounded by differences in applied velocity.

Facilitation of neuronal activity in somatosensory and posterior parietal cortex during prehension.

In order to study prehension in a reproducible manner, we trained monkeys to perform a task in which rectangular, spherical, and cylindrical objects were grasped, lifted, held, and lowered in response to visual cues. The animal's hand movements were monitored using digital video, together with simultaneously recorded spike trains of neurons in primary somatosensory cortex (S-I) and posterior parietal cortex (PPC). Statistically significant task-related modulation of activity occurred in 78% of neurons tested in the hand area; twice as many cells were facilitated during object acquisition as were depressed. Cortical neurons receiving inputs from tactile receptors in glabrous skin of the fingers and palm, hairy skin of the hand dorsum, or deep receptors in muscles and joints of the hand modulated their firing rates during prehension in consistent and reproducible patterns. Spike trains of individual neurons differed in duration and amplitude of firing, the particular hand behavior(s) monitored, and their sensitivity to the shape of the grasped object. Neurons were classified by statistical analysis into groups whose spike trains were tuned to single task stages, spanned two successive stages, or were multiaction. The classes were not uniformly distributed in specific cytoarchitectonic fields, nor among particular somatosensory modalities. Sequential deformation of parts of the hand as the task progressed was reflected in successive responses of different members of this population. The earliest activity occurred in PPC, where 28% of neurons increased firing prior to hand contact with objects; such neurons may participate in anticipatory motor control programs. Activity shifted rostrally to S-I as the hand contacted the object and manipulated it. The shape of the grasped object had the strongest influence on PPC cells. The results suggest that parietal neurons monitor hand actions during prehension, as well as the physical properties of the grasped object, by shifting activity between populations responsive to hand shaping, grasping, and manipulatory behaviors.