Ten healthy subjects were recorded (6 males, 4 females). Their ages ranged between 24 and 46 years (mean= 35.5 years). All had Snellen visual acuities of 6/6 or better, and none had a clinical history of visual problems.
A four-channel montage was used. The electrodes were placed on a transverse row 5cm above the inion and 5 cm apart, with one electrode on the midline and the others 5 cm either side o f the midline electrode. A further electrode was placed at the inion. All 4 occipital electrodes were referred to a common mid-frontal reference (Fz).
Subjects sat 1 metre from an oscilloscope which displayed a field subtending 24 degrees horizontally by 18.5 degrees vertically (Hewlett Packard, 1321 A, X-Y Display, P4 phosphor). Black and white checks were presented to the left half-field (0-12 degrees); the right half consisted of a uniform grey field of the same average luminance as the checkerboard. A small ring (diameter subtending 1.5 degrees) at the centre of the vertical border of the pattern/blank interface provided a fixation spot for all stimulus conditions. Five sizes of check were presented, subtending 6, 12, 20, 50 and 80 minutes of arc at the subject's eye. The luminance levels were 11.5cd/m^ for white squares, and 0.004cd/m^ for black squares, and were constant across checksizes.
Four sizes of experimental scotomata were used (subtending 0-1.5, 0-2, 0-3 and 0-4.5 degrees). The stimulus field on the left half of the screen was masked with one of the four scotomata which occluded the central zone of the field with a notch for a fixation ring at the vertical edge which always remained visible (Figure 4.1). The left eye only was stimulated, and the right eye was occluded with a pad. Responses for each of the five checksizes were recorded. Checksize and scotoma size were varied randomly between subjects.
F ig u r e 4 .1 : P h o to g r a p h s s h o w in g fo u r v ie w s o f th e sc r e e n w ith the fo u r cen tral h e m i-s c o to m a ta u se d in the e x p e r im e n ts . L e ft h a lf-fie ld 50 ' c h e c k s are o c c lu d e d b y s c o to m a ta m e a su r in g 0 - 1 .5 d e g r e e s (to p le ft), 0 - 2 d e g r e e s (to p rig h t), 0 - 3 d e g r e e s (b o tto m le ft) and 0 - 4 .5 d e g r e e s (b o tto m righ t)
The V E P stim ulus is identical to that described in the previous chapter w ith pattern- onset, -reversal and -offset averaged in a single epoch lasting 900m s. T he checkerboard appeared for 300m s, was replaced by the com plem entary pattern w hich stayed on for a further 300m s (i.e. reversal), and then disappeared for 300m s (pattern-offset). T he average o f 64 such sequences was recorded for each stim ulus condition. Ipsilateral and contralateral occipital responses were recorded from the electrodes 5cm to the left and to the right o f the m idline, respectively. C om ponent identification was based on their polarity and latency as
describes in chapter 2. Peak latency and peak-to-peak amplitude measurements were made for each component.
4.3 Results
A clear experimental effect was evident. Increasing the size of the experimental scotoma resulted in attenuation of the majority of the VEP components. Figures 4.2 and 4.3 show the group average ipsilateral and contralateral responses, for the 5 checksizes, with increasing scotoma size. Component attenuation was especially conspicuous with occlusion using the two largest scotomata (0-3® and 0-4.5®) (figure 4.3). It was also evident that the VEP changes were most marked when checksizes of 12' and 20' were used, with ipsilateral reversal components and the contralateral P I 05 components attenuating to the greatest extent with even the smallest scotoma (0-1.5®) (figure 4.2).
W hen larger checks (50' and 80') were used, the VEP degradation was not as marked with the smaller scotomata (0-1.5® and 0-2®). This is likely to reflect the fact that the primarily macular-derived components, which are enhanced by smaller checks (onset CII, reversal N80, N145 and offset N85), have already attenuated with the use of large checks.
Ip sila te ra l C hannel S c o t o m a N o n e 0-1.5* 0-2* C h e ck siz e 6' 1 2' 2 0' 50 ' 80 ' C o n tr a la te r a l C h a n n el C h e c k s iz e 6 ’ 1 0 uV _i— I______ I I I I I i_ 200 400 ooo moo m s -I 1--- «_ _1 1 1 1 1_ ■J 1--- 1--- 1--- 1_______ l_ 200 400 ooo ooo m s 1 2' 2 0' 80' ± J_ X X I 200 - I 1---1_______ L. m s
O nset R eversal O ffset
2 0 0 -400 e o o aoo
m s
O nset R eversal O ffset
-1— I— 1______ I i_ 2 00 400 o o o OOO
m s
O nset R eversal O ffset
F ig u re 4.2: G ro u p a v erag e ip sila teral and c o n tra la te ra l p a tte rn -o n set, -rev e rsal a n d -o ffse t V E P s w ith o u t sc o to m a and w ith s c o to m a ta o f 0 -1.5 a n d 0-2°
Ip s ila te ra l Channel S c o t o m a N o n e C h e c k s i z e 6 1 2' 20 50' 80' X C o n tr a la te r a l C hannel 1 0 u V 12 20' 50' 80' X X . O m s O nset Reversal 0-3* O ffset m s
O nset Reversal O ffset
0-4^*
400 OOO OOO
m s
~1---2______ L_
O nset Reversal O ffset
F ig u re 4.3: G ro u p a v e ra g e ip sila te ra l an d c o n tra la te ra l p a tte m -o n se t, -re v e rsa l a n d -o ffse t V E P s w ith o u t sco to m a an d w ith s c o to m a ta o f 3° an d 4.5°
MANOVA gave highly significant individual effects of scotoma size (p<0.0001) and checksize (p<0.0001) on VEP amplitudes. Surprisingly, there were no significant effects of checksize by scotoma size. The effects of checksize alone will not be discussed further as the trends were in line with those described in the previous chapter.
Examination of the univariate results showed that scotoma size effect was marked for all the ipsilateral (Cl: p<0.001; CH: p<0.002; CIII: p<0.0001), and contralateral (P105: p<0.0001) onset components, and the ipsilateral components only of pattem-reversal and offset (i.e., N80, PlOO and N145 for reversal, and N85, PlOO and N165 for offset - all at p<0.0001). Interestingly, the contralateral reversal N105 component and the offset N115 component did not show a significant change. There was a strong negative correlation between amplitudes of the majority of VEP components and scotoma size - as scotoma size increased, amplitudes decreased (Spearman's r, ranged between -0.22 and -0.48; p<0.001). However, neither the contralateral offset N115 (r= -0.14, p<0.03), nor the contralateral reversal N105 demonstrated a significant change (r= -0.12, p=0.118). These associations were evident for all checksizes (partial correlation). There were no significant correlations between VEP latencies and scotoma size for any onset or reversal components. Conversely, all offset components: N85 (r= -0.144, p<0.03), Pl l O (r= -0.186, p<0.004), N165 (r= -0.256, p< 0.0001) and contralateral N 115 (r= -0.211; p<0.002), showed significant negative correlations with increasing scotoma size.
M ANOVA of latency measures showed a significant effect of scotoma size (Pillais' test: p<0.03), but like the amplitude findings, it did not show a significant effect of checksize by scotoma size. The ipsilateral N145 of the reversal VEP (p<0.03) and all of the ipsilateral offset components: N85, Pl l O and N165 (p<0.0001) showed significant latency changes; however none of the onset latencies were significantly altered by scotoma.
using paired t-tests (corrected using a full Bonferroni adjustment). Attenuation of the VEP with scotoma was greatest for onset contralateral P I 05 and reversal ipsilateral PlOO and N 145 (all p<0.0001). These highly significant differences were found across all scotoma sizes (Figure 4.4). Ipsilateral onset CEQ, reversal N80, offset N85 and Pl l O showed significant attenuation (p<0.0001) for the two larger scotoma sizes of 0-3 deg. and 0-4.5 deg. only. CII amplitude on the other hand, showed attenuation only when the largest (0-4.5 deg.) scotoma was introduced. Onset Cl, reversal contralateral N105 and offset ipsilateral N165 and contralateral N i l 5 failed to show statistically significant changes for the different scotoma sizes.
The onset contralateral P I 05 showed the same broadening of the waveform for larger checks (50' and 80') as was found in the checksize experiments described in chapter 3. Interestingly, the sharply defined P I 05 obtained with small checks (6' and 12') were very conspicuously degraded with the introduction of even the smallest scotoma (0-1.5°). However, the broadened P I 05 obtained with larger checks appeared to be minimally influenced by small scotomata (0-1.5° and 0-2°), and even with the larger scotomata (> 0-3°), the waveform attenuation was much less marked than that obtained with small checks. Furthermore, the P I 05 component was one of the very few components still discernible when large scotomata were introduced. These observations lend further support to our suggestion that the altered morphology of the P I 05 obtained to large checksizes, reflects its different origins and mechanisms than the P I 05 obtained with small checksizes (the former reflecting paramacular origins and the latter macular origins).
O n s e t C l ( u V ) R e v e r s a l 12 10 8 6 4 2 0 ( u V ) O f f s e t N 8 5 ( u V ) N 8 0 1 0 IV