1. M eth o d
1.1. Procedure and visual conditions
CN, acuity-control and normal control subjects were tested under two conditions: ‘eyes open’ and ‘eyes closed’. With eyes open, subjects were asked to fixate a small cross (1 x 1 cm) placed at the center of an earth-fixed window frame
(30 X 24 cm) at a distance of 50 cm from the eyes with a stationary background
100cm away from the foreground window frame. The visual environment was similar to that used in Experiment 1-5 in chapter two except that the room was normally illuminated. The order of presentation of the eyes open, eyes closed conditions was counterbalanced. In both conditions, subjects were instructed to ‘stand still and relaxed with hands at their side’. Each condition was presented for one minute, from which the last 50s were analyzed.
1.2. Data analysis
Postural equilibrium in the lateral direction was evaluated as sway path and also in the frequency domain. The sway path is the length of the path described by the COP or the head, and is defined as the sum of the distances between sequential points sampled during the analysis period (50s).
To calculate power spectra, the 50s epochs were detrended using a line of best fit and then windowed with a Hanning function. A fast Fourier transform (EFT) algorithm was then applied to the entire 6250 point signal (MATLAB •”•=). The resulting power spectrum had a frequency resolution of 0.02 Hz and bandwidth from 0 to 62.5 Hz. For subsequent analysis, only the 0 to 5 Hz range was considered. For statistical analysis the frequency components from 0.02 to 5 Hz were grouped into 20 bands, each spanning 0.25Hz with 12 or 13 (alternating) discrete frequency components per band. The Power in each of these bands was then calculated by summing the frequency components.
Chapter 5: Postural control and congenital nystagmus
Analysis of variance (ANOVA) was used to investigate the effects of ‘vision’ in the three subject groups. A two factor design was employed for sway path analysis (3 X 2) with ‘group’ as the between subject factor (CN, acuity-controls and normal controls) and ‘vision’ as the within subject factor (eyes closed versus eyes open). The Tukey test was used for post hoc comparison. A three factor design was employed for spectral analysis (3 x 2 x 20), with ‘frequency’ (0-0.25 Hz to 4.75-5 Hz) as a second within subject factor.
2. R esults (E xperim ent 1)
The sway path data measured from COP and head recordings for CN and the two control groups are given in Table 5.2.
Table 5.2. Mean sway path length (in cm) and standard deviations of both the COP and the head for the eyes closed and eyes open conditions in CN, acuity and normal controls. Eyes closed M SD Eyes open M SD Head CN subjects 67.8 13.2 49.1 1 2 . 1 Acuity-control subjects 56.5 1 2 . 2 35.9 9.5 Normal-control subjects 53.2 17.5 31.9 13.3 COP CN subjects 115.1 15.6 92.4 23.1 Acuity-control subjects 111.3 23.9 45.8 1 0 . 1 Normal-control subjects 128.1 55.1 50.3 11.5
Chapter 5: Postural control and congenital nystagmus
2.1. Effect o f vision on the COP
a. Sway path length
Comparisons between the sway path lengths measured in the eyes open and eyes closed conditions indicated that visual stabilization of the COP was more effective in the two control groups than in the CN subjects. The sway path of the COP with eyes open was reduced only by 19 % in the CN subjects as compared to 59 % and 57 % in the acuity control and normal-control subjects respectively. Figure 5.2 (left panel) illustrates the improvement of stability (%) with vision for each individual CN and acuity-control subject. The significant interaction between the ‘group’ factor (CN, acuity and normal controls) and the ‘vision’ factor (eyes closed versus eyes open) confirmed that vision was more stabilising in the two controls groups as compared to the CN group (ANOVA: Fij? =4. 1 p < 0.05). Post hoc comparison indicated that there was no difference between the three groups with eyes closed (p > 0.05). In the eyes open condition, the sway path was longer in CN subjects than in the other two control groups (p < 0.01). No differences of any kind were observed between the acuity- control and normal-control subjects (p > 0.05).
% COP 80 60 40- #o 2 0- o o o 0 o -2 0^ CN Acuity Controls % 60 40 20 Head CN Acuity Controls
Figure 5.2: Percentage improvement (%) of stability (sway path) with vision calculated for each subject with CN and acuity control for both the COP (left panel) and the head (right panel) in Experiment 1. The improvement was calculated as [(eyes closed score - eyes open score) / eyes closed score * 1 0 0].
Chapter 5; Postural control and congenital nystagmus
b. Power spectra
Spectral analysis of COP for the CN and the acuity-control subjects is shown in Figure 5.3. The frequency characteristics of the visual effect on COP displacements can be inferred from a comparison of the spectra of sway obtained with eyes closed versus eyes open (Dichgans et al. 1976; Lestienne et al. 1977). In CN, visual stabilization of the COP was restricted to frequencies lower than 1 Hz (Fig. 5.3). In contrast, for acuity-control subjects (and normal control subjects), vision had an effective stabilizing influence on COP throughout the frequency range 0 to 5 Hz (Fig. 5.3 for acuity-control subjects). Visual stabilization of the COP was significantly more effective in the control subjects (acuity control and normal control subjects) than in the CN subjects as reflected by the significant interaction between the ‘group’ factor and the ‘vision’ factor (ANOVA: ¥2,11 = 4.9 p < 0.05). ANOVAs
examining for within group effects indicated that the effect of vision in the CN subjects failed to reach significance either as a main effect (Fi,s = 3.7 p = 0.11) or in interaction with the frequency factor (F 19,95 = 0.9 p = 0.58). The effect of ‘vision’
was similar in the two control groups with a significant main effect of vision (p <0.05) and no interaction with the ‘frequency’ factor ‘ (p > 0.05).
Chapter 5: Postural co n tro l and c o n g en ita l nystagm u s