Characteristics of target-reaching in cats. I. Individual differences and intra-individual constancy.

Trajectory formation of unrestrained forelimb target-reaching was investigated in six cats. A Selspot-like recording system was used for three-dimensional recording of the position of the wrist every 3 ms with the aid of two cameras detecting infrared light emitted from diodes taped to the wrist. These measurements allowed reconstruction of movement paths in the horizontal and sagittal planes and velocity profiles in the direction of the cartesian x, y and z co-ordinates. Horizontal movement paths were smoothly curved, segmented or almost linear. Sagittal movement paths were sigmoid. The net velocity profile was usually bell-shaped with longer deceleration than acceleration, but for some slow movements the velocity profile had a plateau. When the net velocity profile was bell-shaped, the averaged sagittal movement paths and normalized x (protraction) and z (lifting) velocity profiles were virtually superimposable for fast and slow movements: thus, movement speed was changed by parallel scaling of protraction and lifting. Comparison of movement paths and velocity profiles amongst the different cats revealed considerable differences. The x profile was unimodal in one cat and double peaked in five cats: the second component was pronounced in two cats and small in the other three. The z profile was unimodal and, except for one cat, it had later onset and summit than the first component of the x profile. In contrast to the interindividual differences, there was a high degree of intraindividual constancy over 6-12 months. It is postulated that the interindividual variability depends on chance differences established early during learning of the task and that the imprinted pattern remains, resulting in intra-individual constancy.