ROLE OF THE CEREBELLUM AND MOTOR CORTEX IN THE REGULATION OF VISUALLYCONTROLLED LOCOMOTION

An account is given of the current state of knowledge of the contribut ions of the cerebellum and the forelimb motor cortex (MC) to the neura l control of walking movements in the cat. The main emphasis is on inf ormation obtained by recording from single MC and cerebellar neurones in chronically instrumented cats engaged in walking on the rungs of a horizontal ladder, a form of locomotion that is heavily dependent on v isual input and for which the integrity of MC is essential. Evidence f rom the authors' laboratory and from other studies is presented which establishes that MC neurones, including pyramidal tract neurones, show higher levels of activity during ladder walking than during overgroun d walking (i.e., when less constraint exists over the locus of footfal l) and that this increase is greatest in late swing - early stance in the contralateral forelimb, consistent with one role of MC being to he lp determine the locus of footfall. However, many MC neurones develop peak activity at other times in the step cycle, and a comparison with recordings during treadmill walking suggests MC may also help regulate stance duration when walking speed is an important performance variab le. Recordings from Purkinje cells and cerebellar nuclear neurones sho w that during ladder walking step-related activity is widespread in th e vermal, paravermal, and crural regions of cortex and in the interpos ed and dentate nuclei. Nuclear cell activity is so timed that it could be contributing to producing the locomotor rhythms evident in MC cell s, although this is not yet proven. Results are also presented and dis cussed relating to MC and cerebellar neuronal responses that occur whe n a step onto an unstable rung results in an unexpected external pertu rbation of the forelimb step cycle. MC responses begin with onset late ncy as short as 20 ms so that MC may assist spinal reflex mechanisms t o produce a post hoc compensatory change in motor output. However, wor k in progress suggests that corresponding responses in paravermal cere bellum are weak and infrequent, so provisionally it seems that the MC responses are initiated via pathways that do not pass through the cere bellum. By contrast, current work involving a paradigm in which a ladd er rung is motor driven to a new position as the animal approaches (th ereby providing a visual cue that an adaptive change in gait will soon be required) is revealing in lateral cerebellar neurones, including d entate neurones, changes in discharge that are time locked to the exec ution of an adapted pace. In addition, there are prominent earlier res ponses, which begin at short latency after the onset of rung movement. These apparently visual responses have characteristics that encourage the speculation that they may represent a cerebellar signal that ''pr imes for action'' other more directly motor regions of the central ner vous system.