Dependence of FC on power and age
The dependence of FC on age and change in power was further explored
with a robust linear regression model applied over all the healthy
subjects (refer Methods section) (Figure 6). Alpha, slow gamma, and fast
gamma FC was individually regressed with subject age and the change in
band power as independent variables, separately for each seed electrode
group. Figure 6a shows the scatter plot of alpha, slow gamma, and fast
gamma FC versus subject age, while Figure 6b shows the same versus
change in power, for the left electrode group (similar results were
obtained for other groups; data not shown). The coefficient in the
fitted model and its significance (p-value) are also shown in the plots.
The p-value relates to the t-statistic of the two-sided hypothesis test\((H:b=0)\), given the other terms in the model. Consistent with
previous results, alpha FC depended only on the age factor
(\(b_{\text{age}}\) = -0.002; p-value = 0.002) and not on the change in
power (\(b_{\text{power}}\) =0.004; p-value = 0.380). This can also be
observed in the median alpha FC values binned in 5-year intervals as
shown in black traces in Figure 6a, which decreased with age (left
plot).
In contrast, regression analysis for slow gamma revealed additional
dependencies not observed earlier. In particular,
slow gamma FC was found to depend
on both, age (\(b_{\text{age}}\) = -0.002; p-value = 0.0003) and change
in power (\(b_{\text{power}}\) =-0.026; p-value = 0.0001). Unexpectedly,
FC was negatively correlated with change in power (potential reasons are
discussed below). The dependence of slow-gamma FC on age can also be
observed in the median FC values binned over 5-year intervals (black
trace in Figure 6a). This dependence was weaker in our previous analysis
because we had partitioned the subjects into the two age groups,
middle-aged and elderly. Indeed, when we performed the same regression
analysis but after replacing the age with age category (0 for middle
aged and 1 for elderly) and excluding the power dependence, the
resulting coefficient barely reached significance in the slow gamma band
(\(b_{\text{age}}\) = -0.022; p-value = 0.03) even though it remained
significant for the alpha band (\(b_{\text{age}}\) = -0.024; p-value =
0.0028). Results for fast gamma FC were largely consistent with previous
analyses, showing no dependence on the age factor (\(b_{\text{age}}\) =
-0.001; p-value = 0.0816). In addition, there was a weak dependence on
the change in power (\(b_{\text{power}}\) =-0.013; p-value = 0.0185),
similar to the dependence observed for slow gamma.
The MCI subjects shown in red showed the expected results: these points
were scattered across either side of the black traces for alpha and
fast-gamma (left and right plots in Figure 6a), but most of the points
lay below the black trace for slow-gamma (middle plot).