Locomotion and head scanning initiated by hypothalamic stimulation are inversely related

Stimulation in the hypothalamus elicits locomotor stepping. Before stepping is initiated, head scanning movements occur. We determined the relationships between the latency of locomotor initiation and the number, extent and direction of the head scanning movements. Chronic stimulation electrodes were stereotaxically implanted in and around the hypothalamus of 29 rats. Under awake conditions, 38 locomotor sites were tested in a runway apparatus. Behaviors occurring between the onset of stimulation and the first step were recorded on videotape. Points on the rat were digitized at sampling rate of 6 Hz to produce measures of head angles in the vertical, horizontal, and sagittal planes. The priming paradigm was used with a current selected for each site that was minimally sufficient to produce reliable stepping. In trials at approximately 1-min intervals, a 5-s train of stimulation (the control) was followed by a second train (the test) delivered 5-20 s later. Initiation latency on control trains was strongly correlated with head movement measures. Vertical and lateral head movements were independent of one another. Together, their frequency and extent accounted for 85% of the variance in locomotor initiation latencies. In effective priming trials, when locomotor initiation latencies were reduced on the test train, the frequency and extent of vertical and lateral head movements were also reduced. In non-effective priming trials, when latencies were not reduced, head movements were not reduced. Head scanning and locomotor initiation reflect reciprocal processes.