We averaged contrast response functions across the left and right eyes of our control participants. The blue data in
Figure 2a show how the amplitude at the target frequency (10 Hz) increased as a function of target contrast. This is consistent with previous work.
26,27 When a 12-Hz mask was shown to the other eye at high contrast, this slightly reduced the response to the target (
Fig. 2a, green data), particularly at 26% target contrast. This gain control effect is similar to those reported previously,
26,27 though somewhat weaker.
A two-way repeated measures ANOVA on the normalized amplitudes (which satisfied the assumption of sphericity) revealed a significant effect of target contrast (F4,16 = 52.3, P < 0.001, partial η2 = 0.93), but no significant effect of mask contrast (F1,16 = 6.94, P = 0.58) and no interaction between the two variables (F4,16 = 1.94, P = 0.15). However, since we would expect to see no effect of mask contrast at low target contrasts (where the driven target response is negligible) or perhaps at high contrasts (because of saturation) it may be that inclusion of the full range of target contrasts explains this lack of a statistical effect of mask contrast. This was confirmed by running a paired samples t-test comparing the mask versus no-mask data at 26% target contrast, which did reveal a significant difference (t = 3.63, df = 4, P = 0.022).
Similar monotonic contrast response functions to the target stimuli were obtained in our amblyopic observers (
Fig. 2b). However, the response in the amblyopic eye (red data) was much weaker than that in the fellow eye (dark blue), and appeared to be shallower in slope when plotted on a linear
y-axis. There was no appreciable masking in either the fellow eye (light blue) or the amblyopic eye (orange). Indeed, there appeared to be a slight increase in response in the amblyopic eye when the mask was added (orange data). We consider possible explanations for this below.
We performed a three-way repeated measures ANOVA on the amblyope data, with eye (fellow or amblyopic), mask contrast (0% or 26%), and target contrast (five levels) as factors. Data showed significant deviations from sphericity for the contrast condition (W = 0.056, P = 0.013) and eye*contrast interaction (W = 0.038, P = 0.005), so we report Greenhouse-Geisser corrected values for these effects. There were highly significant effects of eye (F1,9 = 28.3, P < 0.001, partial η2 = 0.76) and target contrast (F2.05,18.4 = 59.1, P < 0.001, partial η2 = 0.87), and a significant interaction between the two (F1.8,15.8 = 33.7, P < 0.001, partial η2 = 0.79). There was no significant effect of mask contrast (F1,9 = 0.67, P = 0.43), and no other interactions were significant (all P > 0.05).
As an index of the amount of binocular imbalance, we first calculated the ratio (nondominant/dominant eye) of amplitudes for responses at the highest target contrast for each observer. These are plotted in
Figure 3a against the ratio of acuities (nondominant/dominant eye). A significant relationship is apparent (
r = 0.76,
P < 0.001, calculated using log acuity ratios), such that the greater the acuity difference between the eyes, the greater the reduction in neural response to stimuli shown in the weaker eye (this reduced to
r = 0.40,
P = 0.25 when the control data were omitted, largely due to the reduction in power).
We also fitted our data with a descriptive gain control equation
25 (
resp =
Rmax*
C1.4/(
Z1.4 +
C1.4) +
B) that had two free parameters (
Rmax and
Z) for each 5-point contrast response function (example fits are shown in
Fig. 2), and an overall baseline parameter (
B) common across all functions for a given observer. The
Rmax parameter proved uninformative, remaining relatively constant across conditions (for the normalized data, even setting it to unity for all functions and refitting with only the
Z and
B parameters free made no difference to our findings). The saturation constant (
Z) tracked the response reduction in the amblyopic eye. In
Figure 3b we plot the ratio of fitted saturation constants across the eyes against the acuity ratio, in the same format as
Figure 3a. This was highly correlated with the acuity ratio (
r = 0.79,
P < 0.001, calculated using log ratios), though again the correlation reduced when the control observers were omitted (
r = 0.56,
P = 0.09). Overall, the neural measurements were associated with real world visual ability and the magnitude of the amblyopic deficit.
The mask had a somewhat weaker effect than expected from the model predictions (
Fig. 1b) and previous work.
26,27 To confirm that the mask was exciting neural responses, we measured the amplitude of the 12-Hz component of the EEG signal. These are presented in
Figure 4 in the same format as
Figure 2. There was a strong response to the mask when presented to the control observers (
Fig. 4a, green data), which declined slightly as the target contrast increased (see Busse et al.
25 and Baker and Vilidaitė
27). A strong response was also observed for the fellow eye of the amblyopes (
Fig. 4b, orange data). There were significant effects of mask contrast for both control (
F1,4 = 14.0,
P < 0.001, partial
η2 = 0.78) and amblyopic (
F1,9 = 32.7,
P < 0.001, partial
η2 = 0.78) observers.
When the mask was shown to the amblyopic eye, it evoked a very weak response (
Fig. 4b, light blue data), barely above the baseline levels of noise when it was absent (
Fig. 4b, red and dark blue data). The weak response to the mask in the amblyopic eye explains the lack of a masking effect on the target shown by the light blue data in
Figure 2b. However, it is not clear why a mask in the fellow eye did not suppress the target response in the amblyopic eye (
Fig. 2b, compare red and orange data), as it was clearly exciting sufficient neurons to drive a strong response (
Fig. 4b, orange data).
To further address this issue, we calculated the phase variance of the responses. This is a measure of (inverse) coherence, caused by stronger inputs leading to greater phase locking of the SSVEP signal, and can provide a more sensitive measure than raw amplitude.
27 The phase variance of responses at the target frequency is shown in
Figure 5. These data replicate the main features of the raw amplitudes (
Fig. 2) but inverted. The masking effect in the control observers was clearer (left panel), but was still not evident in the amblyopes (right panel).
Lastly, we examined the intertrial variance of the amplitude at the target frequency. Consistent with previous reports,
27 the variance increased with amplitude. We regressed variance against amplitude for control participants, and both the amblyopic and fellow eyes of the amblyopes. The regression intercept and slope were very similar in all data sets, suggesting that noise was not substantially increased in the responses from amblyopic eyes. To support this, we computed the Fano factor (variance/mean) for each participant at each target contrast level. An ANOVA comparing Fano factors across groups was not significant (
F2,299 = 0.11,
P = 0.895), and there was no clear effect of target contrast. In summary, we do not find evidence of increased noise in amblyopia when using this EEG technique.