Head contains both visual and vestibular systems. So, it is a very essential tool for controlling goal-oriented movements. It had been demonstrated that head is stabilised in space relative to transversal and frontal planes. In order to explore mechanisms which command this segment in locomotion relative to the horizontal plane, we had used a navigation task.
Six healthy subjects were asked to walk along a trajectory between two positions (initial and final) indicated by targets on the floor, bypassing an obstacle (a 1.8-meter-high tripod). The experiment was repeated in three conditions : 1) eyes blindfolded (dark) and eyes open (light), 2) with normal (spontaneous), fast and slow walking speed, 3) with right and left turn.
Instantaneous head and trunk orientation and walking direction in horizontal plane were calculated by means of a 4-camera ELITE® system operating at 100 Hz. It sampled the position of 4 markers aligned with antero-posterior axis of the two segments. Position of the turning point ("subjective corner") was assumed to be coincident with the maximum of curvature of the trajectory performed (both in darkness and light). We had measured the head peak rotation relative to the walking direction (theta) and his occurrence relative to the turning point.
The results showed that predictive head orienting movements occurred. The head started to rotate toward the final position before trunk and before the turning point of the trajectory was reached.
The head peak rotation is attained before (0.18±0.06 s) this turning point and its amplitude was about (33±8 ° ; very similar both in dark and light condition). Afterwards, the trajectory realigned to the head direction. The same pattern was observed in both conditions of vision.
By means of portable-EOG, we have tested if head anticipatory movement reflected gaze orientation (i.e. eyes in head + head in space movements). It was observed in six subjects that head and eyes turned almost at the same time.
Our study emphasises that predictive (anticipatory) eye-head synergy occurs during navigation task. Because of such synergy is also present in darkness, we suggest that the physiological mechanisms are implicated in both conditions of vision.
We suppose that those mechanisms could be assimilated with a pursuit of visual target. Prediction is a fundamental characteristic of those mechanisms tardily developed during evolution. It implies neocortical structures (medio-temporal areas MT et MST, oculomotor areas FEF and cerebellum).
We also hypothesise that in blindfolded condition, the target is memorised during trajectory planning. Inertial and proprioceptive cues generated during locomotion could provide the appropriate input to the predictive circuits controlling the gaze.
It is also an illustration of a "go where we look" strategy that we
support in our laboratory.
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