Abstract In three monkeys trained to finely grade grip force in a visuomotor step-tracking task, the effect of the context on neuronal force correlates was quantitatively assessed. Three trial types, which differed in force range, number, and direction of the force steps, were presented pseudo-randomly and cued with the color of the cursor serving as feedback of the exerted force. Quantitative an- alyses were made on 85 neurons with similar discharge patterns in the three trial types and significant linear pos- itive (54 cells) or negative (31 cells) correlation coeffi- cients between firing rate and force. An analysis of cova- riance (ANCOVA) showed that the population slopes for 2-step were steeper than for 3-step trials. Another ANC- OVA at the population level, computed on the differ- ences in firing rate and force between force steps, persis- tently disclosed a significant effect of trial type. For the first two force steps, the differences in firing rate were significantly larger in the 2-step than in the 3-step in- crease trials. Further analyses revealed that neither the force range nor the number of steps was a unique factor. A small group of neurons was tested in an additional tri- al series with a uniform cue for all three trials, leading to either a loss of context-dependency or to unexpected changes in firing rate. This demonstrates that the cue color was an important instruction for task performance and neuronal activity. The most important findings are that the context-dependent changes were occurring “on- line”, and that neurons displaying context-dependency were found in all three lateral premotor cortex hand re- gions and in the primary motor cortex. Finger muscle ac- tivity did not show any context dependency. The con- text-dependent effect leads to a normalization of the cor- tical activity. The advantage of normalization is dis- cussed and mechanisms for the gain regulation are pro- posed. Key words Precision grip · Context-dependency · Force · Finger representation · Motor cortex · Premotor cortex Introduction Neural correlates of static and dynamic force in the pri- mary motor cortex (M1) have been investigated for more than 30 years (Evarts 1968, 1969; Schmidt et al. 1975; Thach 1978; Cheney and Fetz 1980; Hoffman and Luschei 1980; Evarts et al. 1983; Porter and Lemon 1993; Ashe 1997; for a review, see Hepp-Reymond 1988). Force coding has been demonstrated under vari- ous force conditions (isotonic, isometric, and auxotonic) for single-joint movements, such as wrist or elbow, and jaw movements. In the majority of the studies, monoton- ic relations between neuronal activity and static force were found, being linear over a restricted force range. Departures from linearity often occurred at high and more rarely at low forces. For multijoint movements in 2D space, a relation of firing rate to force has also been shown by Kalaska et al. (1989) under static conditions and by Georgopoulos et al. (1992) for dynamic force. Recently, the latter data have been expanded to the 3D space by Taira et al. (1996), leading to the conclusion that, for a majority of M1 arm neurons, the neuronal ac- tivity mainly or preferentially correlates with the direc- tion of force rather than its magnitude. The precision grip has been considered to serve as an appropriate model to investigate neuronal correlates of force in a natural situation, as the modulation of the force exerted between the thumb and index finger is re- quired by every fine manipulation. This force is the re- sultant of the coordinate activity of many muscles, and M.-C. Hepp-Reymond · M. Kirkpatrick-Tanner · L. Gabernet Brain Research Institute, University Zurich-Irchel, Zurich, Switzerland M.-C. Hepp-Reymond ( ✉ ) · M. Kirkpatrick-Tanner · L. Gabernet Institute for Neuroinformatics, University Zurich-Irchel, Winterthurerstr. 190, CH-8057 Zurich, Switzerland, e-mail: mchr@ini.phys.ethz.ch H-X. Qi Department of Psychology, Vanderbilt University, USA B. Weber Department of Neurology, University Hospital, Zurich, Switzerland Exp Brain Res (1999) 128:123–133 © Springer-Verlag 1999 RESEARCH ARTICLE M-C. Hepp-Reymond · M. Kirkpatrick-Tanner L. Gabernet · H-X. Qi · B. Weber Context-dependent force coding in motor and premotor cortical areas Received: 10 November 1998 / Accepted: 13 March 1999