REPRESENTATION OF PASSIVE HINDLIMB POSTURES IN CAT SPINOCEREBELLAR ACTIVITY

1. We report here about the modulation of dorsal spinocerebellar tract (DSCT) activity by limb posture. In principle, DSCT activity could re present 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 reasonab le 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 six anesthetized cats we recorded the activity o f 96 antidromically identified DSCT neurons while a robot arm passivel y positioned the left hindfoot in 20 positions distributed in the sagi ttal plane, holding each position for 8 s. For each position we measur ed the joint angles, limb segment angles, and the length and orientati on of the limb axis (defined as the line connecting the hip joint to t he hindpaw). We used regression statistics to quantify 1) possible rel ationships among geometric variables of the hindlimb and 2) relationsh ips between DSCT firing rate and limb variables. 3. First, we found a statistically significant relationship among the joint angles that cou ld be described by a covariance plane accounting for similar to 70 per cent of the total variance. Thus the 3 degrees of freedom represented by the joint angles in the sagittal plane are effectively reduced to o nly 2 by the coupling between joints. This finding resembles that desc ribed for the behaving cat during stance. However, the correlation bet ween the hip and ankle angles in the passively displaced hindlimb was just the opposite of that observed during active stance. Moreover, we observed that the length and the orientation of the limb axis is deter mined simply by a linear combination of the three joint angles. 4. Mos t of the DSCT neurons (82 of 96) were significantly modulated by chang es in foot position (1-way analysis of variance, P < 0.001). For those cells, we explored systematically how their activity was related to l imb geometric variables. We found mostly linear relationships between individual joint or limb segments angles and DSCT firing rates. Howeve r, although these relation ships were statistically significant, the r andom variance was often quite high. Moreover, similar to 70% of the c ells were modulated by more than one joint or limb segment angle, sugg esting that a model incorporating global geometric variables might exp lain a larger fraction of the variance in the neural data. 5. Conseque ntly we tested how well DSCT activity was modulated by the length and the orientation of the limb axis with the use of a linear regression m odel 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 eve ry modulated cell than did any of the single joint models tested. 6. W e also attempted to account for the effect of the mechanical joint cov ariance on this result by accounting for correlated independent variab les in the analysis. We used a regression model incorporating all thre e 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 two joint angles, ind icating that the modulation did not necessarily reflect the biomechani cal constraint of joint angle covariation, but rather a central conver gence of sensory information from more than a single joint. 7. From th ese results we conclude that the firing rates of a majority of DSCT ne urons encode the position of the hindfoot relative to the hip joint. W e also propose, on the basis of behavioral studies on the control of c at stance and also from previous neurophysiological data, that the rep resentation of hindfoot position by DSCT neurons could be in the polar coordinate system defined by the limb axis length and orientation.