The aim of experiment 2 was to investigate the respective role of motion parallax and induced movement in the contra-directional postural response observed in Experiment 1. While induced movement is most effectively elicited when induced and inducing stimulus are as close as possible in all three dimensions of space (adjacency principle: Gogel & koslow 1972; Gogel & MacCracken 1979; Reinhardt- Ruthland 1988), motion parallax increases with increasing distance between two objects. As a consequence, if the contra-directional sway observed in foreground fixation condition (Experiment 1) was elicited by induced movement of the foreground, it should be even greater and more consistent when the earth fixed foreground is coplanar with the moving background than when the foreground and background are at 100 cm from each other.
1.
Method
1.1 Subjects
Eight normal subjects (24-50 years of age) with normal or corrected normal vision gave their informed consent to participate in the experiment. Three out of these eight subjects took part in Experiment 1.
1.2. Visual Conditions
Parameters of background motion were the same as in Experiment 1. Subjects were exposed to three visual conditions:
1) Background fixation: Subject were asked to look straight ahead at the background as in the background fixation condition in Experiment 1.
2) Foreground fixation: Subject had to fixate a laser point projected on a small earth-fixed black dot (1 cm of diameter) taped on a piece of perspex and adjusted at eye level. The fixation point was at 50 cm from the subject’s eyes, and at 100
Chapter 2: M otion parallax and visually induced body sway
cm from the background. In that visual environment, motion of the background or motion of the subject generate motion parallax.
3) Coplanar fixation: The subjects were asked to fixate an earth-fixed laser point projected directly onto the background. In that condition, motion of the background should induce an apparent movement of the earth fixed point in the opposite direction to background motion, but not motion parallax.
Each condition consisted of 21 trials, 7 with motion of the background to the right, 7 to the left and 7 with stationary background (control), divided in two blocks of 10 or 11 pseudo-randomized stimuli. The first three blocks (one per condition) followed by the second three blocks were presented in a Latin-square design. At the end of each block with the laser spot as a fixation point, subjects were asked to report whether they perceived this point as moving with respect to the background and in what direction. An analysis of variance (ANOVA) based on the mean induced response was performed on the three experimental conditions and LSD post hoc test was used for pairwise comparisons. A .05 significance level was adopted throughout.
2. Results and discussion
Subjective reports
In the coplanar condition, 7 out of the 8 subjects tested reported to see either systematically or from time to time, the earth-fixed laser point moving in the opposite direction of background motion (induced movement). In the foreground fixation condition, only one subject reported to see the laser point as moving.
Postural re-adjustments
When subjects were looking straight at the background (background condition), a lateral displacement of both the head and the COP in the direction of motion was elicited (see Fig. 2.6). In the foreground fixation condition with the fixation point placed 100 cm in front of the moving background, a lateral shift of both the head and the COP in the opposite direction of motion was observed (see Fig. 2.6).
C hapter 2; M o tio n parallax and v isu a lly in d u ced b o d y sw a y E xp erim en t 2 8 - Force plalfonn £ B c 0
1
0 c ce -8 - £ & e 3 c /5 C/5 £ 0 Cl O s § U -8 B ack g ro u n d fix atio n C o p la n a r fix atio n F o reg ro u n d fix atio nI [
/
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B ack g ro u n d m otio n 2 secF ig u re 2.6: G ran d m ean lateral tra n sla tio n o f th e h ea d (u p p e r p a n e l) and th e C O P (lo w e r pan el) e v o k e d by th e d isp lace m e n t o f th e b a c k g ro u n d in d iffe re n t visual co n d itio n s U p w a rd s d efle ctio n s in d ic a te a tra n sla tio n in th e d irec tio n o f m otion
C hapter 2: M o tio n parallax and v isu a lly in d u ced b od y sw a y
In the c o p la n a r co n d itio n , w hen the fix atio n p o in t w as p ro je c te d onto the m o v in g scene, no co n sisten t sw ay w as ob serv ed ; 59 % o f the re a d ju stm e n ts w ere in the d irec tio n o f m otion w h ile 41 % w ere in the o p p o site d irec tio n . W hen individual d ata w ere co m b in ed , a sm all shift in the d irectio n o f m o tio n em erg es, (see Fig. 2-6) b ut w as no t o b jectiv ated statistica lly (C O P; p = .1 2 ; H ead: t(l)= 0.8 p =.43). M ean h ea d in d u ced re sp o n ses in both the co p la n a r an d th e fo re g ro u n d fixation are p lo tte d in Fig. 2.7. In 7 out o f the 8 sub ject tested, the lateral d isp la c e m e n t o f the head (an d C O P ) in the o p p o site d irectio n to m otion w as m o re im p o rta n t than in the c o p la n a r co n d itio n . In terestin g ly , the su b ject fo r w hich th is w as no t true, had alread y a te n d e n c y to lean in the o p p o site directio n to m otion in the c o p la n a r co n d itio n .
b - 3
-6
- 9
C o p l a n a r c o n d i t i o n F o r e g r o u n d f i x a t i o n
F ig u re 2.7: M ean in d u ced re sp o n se o f the head in both the co n d itio n o f co p lan a r fix atio n (in d u ced m otion) and fo reg ro u n d fix atio n (m o tio n p arallax ). P o sitiv e values in d ic a te a d isp lace m e n t in the d irection o f b a c k g ro u n d m o tio n w h ile n eg a tiv e values in d icate a d isp lace m e n t in the o p p o site d irection.
T h e A N O V A in d icated that there w as a sig n ifican t e ffe c t o f the visual c o n d itio n both fo r the head (F 2,14= 17.2 p< .01) an d the C O P (F 2,14= 31.2 p< .01).
M ean c o m p a riso n (L S D ) re v ealed that the d iffe ren c e b etw e en the c o p la n a r co ndition (in d u c e d m o v em en t) and the fo reg ro u n d fix atio n c o n d itio n (m o tio n p arallax ) w as s ig n ific a n t both fo r head and C O P reco rd in g s (p< .05). F in a lly , a sig n ifican t
Chapter 2: Motion parallax and visually induced body sway
difference was observed between the condition of background fixation and the coplanar condition, both for the head and the COP (p<.05).
In summary, a consistent postural re-orientation in the opposite direction to background motion was elicited only in the presence of depth between the fixation point and the moving background (foreground condition) and not when the fixation point and the moving background were coplanar. This result suggests that motion parallax is a more relevant cue to elicit a contra-directional sway response than induced motion. In addition, the significant difference between the background and the coplanar conditions indicates that fixating a single stationary point projected onto the moving visual scene, often perceived as moving in the opposite direction of motion, was sufficient to reduce the driving effect of a moving visual surrounds.
Table 2.2 Mean induced head/COP response (average and standard deviation) in mm in the different visual conditions of Experiment 2.
Background Foreground Coplanar Stationary
fixation fixation fixation background
M SD M SD M SD M SD
Head sway 3.52 3.03 -3.68 2.27 0.68 2.31 0.64 1.04
COP 3.1 1.7 -2.8 1.53 1.08 1.7 0.42 0.6
Note - A positive value indicates a deviation in the direction of motion whereas a negative value indicates a deviation in the opposite direction to motion.
Chapter 2: Motion parallax and visually induced body sway