Fig. 8.

Period of elevated activity (period of elevated firing [PEF]), shortens as demand on accuracy of stepping increases. A and B: activity of a pyramidal tract projecting neuron during simple locomotion is presented as a raster of 35 step cycles (A) and as a histogram (B). In the raster, the duration of step cycles is normalized to 100% and the raster is rank-ordered according to the duration of the swing phase. The beginning of the stance phase in each stride is indicated by an open triangle. In the histogram, the horizontal dashed line shows the level of activity at rest. The horizontal continuous line shows the level above which the activity was considered “elevated” (see methods); and the duration of PEF is expressed as a portion of the step cycle that it encompasses, 70%. C and D: activity of the same neuron during ladder-18 locomotion. PEF has the same duration as that during simple locomotion. E and F: activity of the same neuron during ladder-12 locomotion. The portion of step cycle that includes PEF decreased to 60%. G and H: activity of the same neuron during ladder-5 locomotion. The portion of step cycle that PEF encompassed decreased further to 50%. I: durations of PEFs across locomotion tasks with different accuracy demands of all 19 neurons in which they progressively decreased as the width of ladder's crosspieces decreased.