Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 140

comes from the observations, accumulated over years, of correlations between the
latency and amplitude of positive (P) and negative (N) deflections of the average
ERP and the behavioral or clinical state of the subject. According to the published
guidelines ([Picton et al., 2000], p. 141), “the simplest approach is to consider the
ERP waveform as a set of waves, to pick the peaks (and troughs) of these waves, and
to measure the amplitude and latency at these deflections.” These peak amplitude
measurements are not representing absolute values of electric brain activity, but are
obtained either relative to a pre-stimulus baseline (so called baseline to peak anal-
ysis) or sometimes to an immediately preceding or following peak (peak-to-peak
analysis). Example of an evoked field is shown in Figure 4.18.

latency

M100

− 200

0 200 400 600 800 1000 1200

Time [ms]

FIGURE 4.18: An example of auditory evoked field. The black line is the average
of 200 trials, the gray outline is the standard deviation of the mean.

The deflections appearing in the first tenths of a second are the easiest to interpret.
They reflect the first stages of sensory processing. For instance, the brainstem audi-
tory evoked potentials, (BAEP) consists of 7 waves with latencies between 1 and 12
ms generated in well defined locations in the auditory pathways. BAEPs can be used
as markers of the integrity of the auditory pathway.
The deflections appearing after the first hundred of ms are related to activity of spe-
cific brain systems, which, in some cases, have been delineated (e.g., [Giard et al.,
1995, Pineda, 1995, Nieuwenhuis et al., 2005]). Some of these deflections are re-
lated to cognitive operations. As such, their presence depends on the experimental