SUMMARY AND CONCLUSIONS

1. We report here about the modulation of DSCT activity by limb posture. In principle, DSCT activity could represent limb position in one of several ways. According to a classical notion of DSCT function, DSCT activity might be expected to correlate with changes in individual joint angles. However, given the evidence for extensive polysynaptic convergence onto DSCT units, it is reasonable to propose that DSCT activity represents more global variables such as the orientation of limb segments or the length and orientation of the whole limb. 2. In 6 anesthetized cats we recorded the activity of 96 antidromically identified DSCT neurons while a robot arm passively positioned the left hindfoot in 20 positions distributed in the sagittal plane, holding each position for 8 s. For each position, we measured the joint angles, limb segment angles and the length and orientation of the limb axis (defined as the line connecting the hip joint to the hindpaw). We used regression statistics to quantify: a) possible relationships among geometrical variables of the hindlimb; b) relationships between DSCT firing rate and limb variables. 3. First, we found a statistically significant relationship among the joint angles which could be described by a covariance plane accounting for about 70 percent of the total variance. Thus the 3 degrees of freedom represented by the joint angles in the sagittal plane are effectively reduced to only 2 by the coupling between joints. This finding resembles that described for the behaving cat during stance. However, the correlation between the hip and ankle angles in the passively displaced hindlimb was just opposite to that observed during active stance. Moreover we observed that the length and the orientation of the limb axis is determined simply by a linear combination of the three joint angles. 4. Most of the DSCT neurons (82/96) were significantly modulated by changes in foot position (one way ANOVA, p<0.001). For those cells, we explored systematically how their activity was related to limb geometrical variables. We found mostly linear relationships between individual joint or limb segments angles and DSCT firing rates. However, while these relationships were statistically significant, the random variance was often quite high. Moreover, about 70% of the cells were modulated by more than one joint or limb segment angle, suggesting that a model incorporating global geometrical variables might explain a larger fraction of the variance in the neural data. 5. Consequently we tested how well DSCT activity was modulated by the length and the orientation of the limb axis using a linear regression model with length and orientation (or the equivalent linear combination of joint angles) as predictors. We found that this model explained a larger fraction of the variability in the firing pattern of nearly every modulated cell than did any of the single joint models tested. 6. We also attempted to account for the effect of the mechanical joint covariance on this result by accounting for correlated independent variables in the analysis. We used a regression model incorporating all three joint or limb segment angles and performed a backward elimination of insignificant or redundant variables. The result was that 67% of the neurons were independently modulated by at least 2 joint angles, indicating that the modulation did not necessarily reflect the biomechanical constraint of joint angles covariation, but rather a central convergence of sensory information from more than a single joint. 7. From these results we conclude that the firing rates of a majority of DSCT neurons encode the position of the hindfoot relative to the hip joint. We also propose, on the basis of behavioral studies on the control of cat stance, and also from previous neurophysiological data, that the representation of hindfoot position by DSCT neurons could be in the polar coordinate system defined by the limb axis length and orientation.

Received 6 December 1995; accepted in final form 29 February 1996. APS Manuscript Number J818-5. Article publication pending J. Neurophysiol. ISSN 1080-4757 Copyright 1996 The American Physiological Society. Published in APStracts on 27 March 96