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Experiment 1

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Figure 4.2: Individual’s averaged sway responses to unpredictable versus actively controlled motion of a visual scene. The records show average position of the COP and the head (in mm) computed for each subject in both the unexpected and active conditions of Experiment 1. Upward deflections indicate deviation in the direction of stimulus motion.

Chapter 4: Action and predictability

In the active condition, stimulus motion induced a small postural response in the same direction in most subjects for both the COP and the head, with latencies of approximately 350 ms and 400 ms respectively. This postural adjustment had a smaller amplitude than that observed in the unpredictable condition, and it was followed by a corrective adjustment during stimulus motion, which shifted both the COP and the head back to the baseline level, or even (in some subjects), in the opposite direction to the stimulus. No clear anticipatory postural response (in the opposite direction to the expected stimulus) was observed during the 2.5 seconds preceding stimulus motion (baseline). One sample t-tests indicated that the mean response during stimulus motion was not different from baseline (zero), neither for the COP (r(7) = 0.5 p > 0.05) nor for the head (r(7) = 0.54 p > 0.05). Comparison between COP and head displacements during stimulus motion showed that the active

condition responses were significantly different from those in the unpredictable

condition (paired t-test, COP: r(7) = 3 p < 0.05; head t(7) = 2.9 p < 0.05).

No postural displacement was observed in the control condition with a stationary visual display (p > 0.05).

In summary, results showed that when the displacement of the visual scene was under the active control of the subjects, visually induced body sway measured both as head and COP displacement, was markedly reduced, in comparison with the response to unpredictable stimuli.

Chapter 4: Action and predictability

E. EXPERIMENT 2: Predictability versus action

Results of Experiment 1 showed that the postural readjustment induced by the displacement of a moving scene was markedly reduced when subjects were controlling actively that displacement. The rational of Experiment 2 was to test whether this inhibition is due to the active control per se or to the expectation- predictability which is inherent in the active control.

1. Procedure and visual conditions

Experiment 2 involved four conditions:

1) "unpredictable' condxiion: identical to that of Experiment 1.

2) 'A ctive’ condition: subjects triggered the computer-generated stimulus by pushing one of two buttons (for rightward or leftward scene motion). After the verbal instruction ‘button’ from the experimenter, the subject had to press within a few seconds. Subjects were asked to alternate right and left between trials.

3) 'predictable’ condition: subjects were told that the scene would move to the right or to the left 3 to 5s before the onset of motion. They were also given an audible beep (50 ms duration, 500Hz tone) 500 ms before stimulus onset. Thus, in the

'predictable’ condition, subjects knew when and in which direction the scene would move.

4) Control condition: no displacement of the visual scene.

The stimulus was the same in the three motion conditions, consisting of a displacement of the visual scene leftwards or rightwards at a velocity of 1 cm/s for 3 seconds. Each subject underwent 10 trials in each condition (with equal numbers of visual scene displacement rightward and leftward). The 40 trials for each subject were divided into two blocks of 2 0 trials, presented successively with a rest period of

Chapter 4: A ction and predictability

10 minutes between the two. Within each block the order of stimulus presentation was pseudo-randomized.

Prior to the experiment, we tested whether the act of pressing the button by itself had a significant postural effect. After being informed that the visual scene would not move, subjects pressed the right or left button alternatively (n=10). Results indicated that the active push (left or right) had no effect on the COP / head position (p >0.05).

2. R esults (E x p erim en t2)

No significant differences were observed in the anterior-posterior direction of sway, between the experimental conditions with a moving visual display and the control conditions with a stationary display. Therefore, only data concerning lateral sway will be reported. The averaged head sway in the lateral direction for each subject in the 'unpredictable ’, predictable' and 'active' conditions are given in Figure 4.3. Mean induced head/COP responses over the 8 subjects are reported in

table 4.2.

As in Experiment 1, unpredictable displacement of the visual scene induced a postural re-adjustment in the direction of motion. Except for the early transient COP response (latency of -280 ms) opposite to stimulus motion, COP displacement followed a similar trend to that of the head. In contrast to the unpredictable condition, no significant postural readjustment was observed when the displacement of the visual scene was triggered by the subjects themselves (active condition) neither for the head nor for the COP (p > 0.05). It is however interesting to notice that approximately 2.5sec after stimulus onset, a displacement of the head (and COP) in the direction of stimulus motion is initiated (see Fig 4.3). A resetting towards the initial baseline position is observed soon after stimulus offset. This late initiation of postural correction indicates that the beneficial effect of self-triggering the stimulus is limited to the first 2-2.5sec of stimulation.