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Giszter - 1993 - Convergent force fields organized in the frog's spinal cord


Giszter SF, Mussa-Ivaldi FA, Bizzi E. Convergent force fields organized in the frog's spinal cord. J Neurosci. 1993 Feb;13(2):467-91. PUBMED

10 Word Summary

"Active" force fields give rise to observed frog behaviors.


Microstimulation of the gray matter of the frog's spinal cord was used to elicit motor responses. Force responses were recorded with the frog's ankle clamped while EMG activity was monitored. The collections of force patterns elicited at different leg configurations were summarized as force fields. These force fields showed convergence to an equilibrium point. The equilibrium paths were calculated from the force fields with the leg clamped. These paths predicted free limb motion in 75% of trials. The force fields were separated into active and prestimulation resting responses. The active force field responses had a fixed position equilibrium. These active force fields were modulated in amplitude over time, although the balance and orientations of forces in the pattern remained fixed. The active fields grouped into a few classes. These included both convergent and parallel fields. The convergent force fields (CFFS) could be observed in deafferented preparations. Motoneuron (MN) activity underlying the force fields was marked using sulforhodamine. The marked activity covered several segments. Several simulations and MN stimulations show that topography, limb geometry, and random activation could not account for the results. It is likely that propriospinal interneurons distribute the activity that underlies the responses observed here. Experiments showed that CFFs that resemble those elicited by microstimulation also underlie natural behaviors. The full variety of fields revealed by microstimulation was larger than the repertoire elicited by cutaneous stimulation. It was concluded that fixed-pattern force fields elicited in the spinal cord may be viewed as movement primitives. These force fields could form building blocks for more complex behaviors.


  • Virtual trajectory - trajectory of equilibrium point (path taken if no forces were to act on limb)
  • Frogs are immobilized and the spinalized frog is stimulated in the lateral neuropil of the gray matter. Endpoint forces at the ankle were recorded with the ankle fixed at different grid points.
  • Sulfur-rhodamine are used to determine the localization of the stimulation.
  • The forces measured were decomposed into a "resting force" and an "active force".
  • Different "groups" of muscles were activated depending on where the electrode was placed in the spinal column.
  • EMGs showed "rebound" (activity after stimulus had stopped)
  • Depth of electrode through gray matter changed the orientation of the endpoint force.
  • Orientation of endpoint force stays roughly constant temporally as magnitude increases.
  • The active field has a significantly different equilibrium point compared to the resting field.
  • It appears like the active field is nearly constant throughout the stimulation. 
    • Not too surprising as they are stimulating in a constant location and waveform.
  • Amplitude does not change field shape significantly.
  • Fields correspond to common behavioral motions.
    • Lumped into a few (~4) directions of force.
  • Similar fields were elicited from multiple locations in the spinal cord.
  • Comparison between predicted "virtual trajectory" and observed "free-limb" trajectory
    • Forward and return trajectories predicted
    • Predicted "pause phase" not observed in experiment
      • Could be because virtual trajectory computed from static data
  • No particular coordinate system showed a simplification of the force field structure.
    • Looking in joint coordinates a convergent force field was not necessarily observed.  In fact it showed that in one case the "equilibrium point" was equivalent to just turning on the hip flexor.
  • No obvious correlation between EMGs and force fields was discovered.
  • Force field convergence was still observed in deafferented frogs.
  • Rostrocaudal placement of stimulation resulted in unique activation of muscle groups and allowed for the decomposition of individual muscle force fields.
  • Loading of the limb does not disrupt the active force field.
  • Paper tried to show three things:
    • Stimulation of the spinal cord results in a finite number of force fields
    • Natural behaviors exhibit similar force fields to the "static" ones recorded
    • Divergent fields occur if individual motor neuron pools are activated, but these are not observed in natural behaviors.