EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.

time over which the average is computed, the longer the interval the lower
the inflation.
The results of these analyses are that a minimum of a 500 millisecond
difference is required when using EEG biofeedback in order to compute an
accurate estimate of coherence or coupling between two time series. With a
500 millisecond average then the amount of inflation is relative low (e.g.,
0.2 or 20%) and as long as the same interval of time of averaging is used
with a normative database, then the Z scores of real-time coherence will be
valid and accurate. As seen in Fig. 19 a sample rate of 1,000 produces even
lower inflation, however, a 1 second difference between a brain event and
the feedback signal may be too long for connection formation in a
biofeedback setting.
Figure 19. Mean coherence (y-axis) and the integration window size in
milliseconds (x-axis). Top left is sample rate = 512 Hz, top right sample
rate = 256 Hz, bottom left sample rate = 128 Hz and bottom left sample rate
= 64 Hz. The amount of averaging from 125 msec. to 1,000 msec is the
critical variable in minimizing “inflation” and not the sample rate.