Role of the cerebellum and motor cortex in the regulation of visually controlled locomotion

An account is given of the current state of knowledge of the contributions of the cerebellum and the forelimb motor cortex (MC) to the neural control of walking movements in the cat. The main emphasis is on information 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 visual input and for which the integrity of MC is essential. Evidence from 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 overground walking (i.e., when less constraint exists over the locus of footfall) and that this increase is greatest in late swing - early stance in the contralateral forelimb, consistent with one role of MC being to help 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 variable. Recordings from Purkinje cells and cerebellar nuclear neurones show that during ladder walking step-related activity is widespread in the vermal, paravermal, and crural regions of cortex and in the interposed and dentate nuclei. Nuclear cell activity is so timed that it could be contributing to producing the locomotor rhythms evident in MC cells, although this is not yet proven. Results are also presented and discussed relating to MC and cerebellar neuronal responses that occur when a step onto an unstable rung results in an unexpected external perturbation of the forelimb step cycle. MC responses begin with onset latency as short as 20 ms so that MC may assist spinal reflex mechanisms to produce a post hoc compensatory change in motor output. However, work in progress suggests that corresponding responses in paravermal cerebellum are weak and infrequent, so provisionally it seems that the MC responses are initiated via pathways that do not pass through the cerebellum. By contrast, current work involving a paradigm in which a ladder rung is motor driven to a new position as the animal approaches (thereby providing a visual cue that an adaptive change in gait will soon be required) is revealing in lateral cerebellar neurones, including dentate neurones, changes in discharge that are time locked to the execution of an adapted pace. In addition, there are prominent earlier responses, 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 ''primes for action'' other more directly motor regions of the central nervous system.