In several vertebrates spinal neural networks are capable of generating motor output. In studies on isolated brain-spinal cord preparations of a lower vertebrate, the lamprey, we show that dynamic brain-spinal cord interactions alter the locomotor rhythm that is generated by the distributed spinal pattern generators. Our analyses, which utilize wavelet and novel time-varying covariance methods, indicate that a brain-spinal cord interactions have an inhibitory influence on the frequency of the rhythm. However, they contribute to higher variability in motor output. Variability is assessed from the coefficient of variance (CV) and decay in autocovariance. We find that neural activity recorded from rostral segments closer to the brain shows higher variability, i.e. variability is site specific. Since the rostral to caudal gradient persists after suppression of brain-spinal cord communication, it may reflect asymmetries within the spinal neural organization.
