Results
Figure 1 illustrates the EEG power and FC analysis for a representative
subject. Figure 1a shows the topoplot of raw power in the slow gamma
range between 250-750 ms after stimulus onset, while Figure 1b shows the
change in power from baseline (-500 to 0 ms), revealing an increase in
power in the occipital area. As in our previous studies, all analyses
were performed on three groups of occipital electrodes (left, right and
back; see Methods for details). Figure 1c shows the change in average
power spectral density profile between stimulus and baseline periods for
the left (top) and right (bottom) electrode groups (these electrodes are
highlighted in Figure 1b), revealing a clear suppression in alpha and
elevation in slow and fast gamma bands. Consistent with our previous
studies, we limited analyses to the above listed frequency bands. To
estimate FC, we computed PPC between each of the electrodes in the left,
right or back group as “seed” and all other electrodes. Subsequently,
we averaged the PPC maps for all electrodes within a group to generate
an average PPC map for each group. Figure 1d shows the average PPC maps
with seed electrodes on the left (top) and right (bottom), which we aim
to contrast across different age groups and disease conditions.
The maps in Figure 1d show that FC falls with increasing inter-electrode
distance, as expected. However, some electrodes may have higher FC with
respect to the seed electrodes than what is expected based on their
inter-electrode distance. For example, left and right group of seed
electrodes may show enhanced FC due to potential inter-hemispheric
synchrony (Engel et al. , 1991). Figure 1e shows the FC for
different electrodes (frontal, central, temporal and occipital; see
inset for details) as a function of the inter-electrode distance (see
Methods for details) from each one of the seed electrodes in the left
(top) or right (bottom) electrode group. The FC decrease with
inter-electrode distance did not show any obvious deviations based on
the brain region. In particular, the homologous inter-hemispheric
occipital electrodes (yellow triangles and circles for top and bottom
plots, respectively) did not show greater FC as compared to
intra-hemispheric electrodes in the central region (red circles and
triangles for top and bottom plots, respectively). Further, different
seed electrodes within a group (different columns in Figure 1e) showed
similar trends. We therefore considered FC values between electrodes
pairs solely based on inter-electrode distance, ignoring which electrode
group they belonged to (i.e., ignoring the color and symbol in Figure
1e), and further pooled across different seed electrodes (pooling data
across the three columns in Figure 1e). Two more example subjects are
shown in Supplementary Figure 2 and 3.
To compare how FC varies with age, we divided the population into two
groups – middle-aged (50-64 years; N=91) and elderly (>64
years, N=127), as done in our previous study (Murty et al. , 2020)
and other studies (Sanderson & Scherbov, 2008). Splitting continuous
variables, like age in our present study, into artificial groups is
usually done for simplicity in interpretation and visualization but may
lead to problems such as reduced statistical power, inefficacy of
corrective measures for accounting confounding factors, incompatibility
across the studies, non-existent associations and unreliable risk
estimates (Naggara et al. , 2011). We therefore first show results
using grouped analysis to be comparable with previous studies (Figure
2-5), and subsequently confirm our results using regression analysis
(Figure 6). Indeed, as shown later, regression analysis reveals some
important associations that are not observed in grouped analysis.