Control Strategies Correcting Inaccurately Programmed Fingertip Forces: Model Predictions Derived From Human Behavior

Abstract

Fagergren, Anders, Ö rjan Ekeberg, and Hans Forssberg. Control strategies correcting inaccurately programmed fingertip forces: model predictions derived from human behavior. J Neurophysiol 89: 2904-2916, 2003. First published February 12, 2003 10.1152/jn.00939.2002. When picking up a familiar object between the index finger and the thumb, the motor commands are predetermined by the CNS to correspond to the frictional demand of the finger-object contact area. If the friction is less than expected, the object will start to slip out of the hand, giving rise to unexpected sensory information. Here we study the correction strategies of the motor system in response to an unexpected frictional demand. The motor commands to the mononeuron pool are estimated by a novel technique combining behavioral recordings and neuromuscular modelling.We first propose a mathematical model incorporating muscles, hand mechanics, and the action of lifting an object. A simple control system sends motor commands to and receives sensory signals from the model. We identify three factors influencing the efficiency of the correction: the time development of the motor command, the delay between the onset of the grip and load forces (GF-LF-delay), and how fast the lift is performed. A sensitivity analysis describes how these factors affect the ability to prevent or stop slipping and suggests an efficient control strategy that prepares and corrects for an altered frictional condition.We then analyzed fingertip grip and load forces (GF and LF) and position data from 200 lifts made by five healthy subjects. The friction was occasionally reduced, forcing an increase of the GF to prevent the object being dropped. The data were then analyzed by feeding it through the inverted model. This provided an estimate of the motor commands to the motoneuron pool.As suggested by the sensitivity analysis the GF-LF-delay was indeed used by the subjects to prevent slip. In agreement with recordings from neurons in the primary motor cortex of the monkey, a sharp burst in the estimated GF motor command (NGF) efficiently arrested any slip. The estimated motor commands indicate a control system that uses a small set of corrective commands, which together with the GF-LF-delay form efficient correction strategies. The selection of a strategy depends on the amount of tactile information reporting unexpected friction and how long it takes to arrive. We believe that this technique of estimating the motor commands behind the fingertip forces during a precision grip lift can provide a powerful tool for the investigation of the central control of the motor system.

Motor symptoms in Parkinson’s disease: A unified framework

“…In a typical PG task, participants are required to use precision grip and lift an object to a designated height (Eliasson, Forssberg, Hung, & Gordon, 2006;Fagergren, Ekeberg, & Forssberg, 2003;Johansson, 1998;Johansson & Cole, 1994;Johansson, Riso, Hager, & Backstrom, 1992;Johansson & Westling, 1984, 1988a, 1988bUlloa, 2004). The two primary forces generated by the fingers are load force (which lifts the fingers) and grip force (which couples the fingers and the object).…”

Brain Activity During Predictable and Unpredictable Weight Changes When Lifting Objects

“…The information stored is used to set the parameters of the motor program that generates distributed muscle commands to the muscles producing the fingertip forces. The importance of such sensorimotor memory representations for the retrieval of parameters relevant to motor commands has been demonstrated regarding the prediction of object's weight Westling 1984, 1988;Westling and Johansson 1987), shape (Jenmalm and Johansson 1997;Jenmalm et al 2000), and frictional characteristics (Edin et al 1992;Fagergren et al 2003;Johansson and Westling 1984). At the same time, the motor system uses an efference copy of the motor command (Von Holst and Mittelstaedt 1950) produced when the movement is made to predict the sensory inputs of the motor act (Wolpert et al 1995) and compares it with the sensory input produced by the movement (Fig.…”

Bio-inspired algorithms for tactile control of dexterous manipulation

“…The function Slip Correction is used to stop the slip, and to adjust the force ratio based on the intensity of the slip: Fagergren et al [10] studied human behavior, and found that the best control strategy to stop slips was a burst increase of the grip force. Therefore, this same strategy is used in the above function to stop slips.…”