2.5 Measurement of Components using Single Trial Peaks Approach
The Single Trial Peaks (STP) approach involved passing the
stimulus-locked ERP trials through a custom-built MATLAB program created
by the authors to capture the trial-by-trial amplitude and latency data
for each component of interest. The trial-by-trial peak-picking process
for each trial segment for each participant was conducted in two steps,
a forward and a backward pass, to optimize the accuracy in the detection
and measurement of each component peak. During the forward pass ,
all components measured at averaged ERP level were measured at the trial
level beginning with the component closest to the stimulus onset time
(i.e., P1), and proceeding in successive order until the component
furthest from onset time was reached. For each trial segment, successive
time windows for the peak-picking of each component were developed in a
recursive manner. For the first time window for the first component, P1,
the left boundary of the time window was time of stimulus onset (i.e., 0
ms) and the right boundary of the time window was the peak latency of
the second component, N1, obtained from the participant’s averaged ERP.
Then the next component at the trial-level, N1 was measured in the time
window with the left boundary defined by the peak latency of P1 from the
segment being scored and the right boundary defined by peak latency
measure of P2 from the participant’s averaged ERP. This recursive
procedure continued until all components measured in the averaged ERP
were measured in the trial segment. In order to measure the P3 at the
trial-level, the latency of N3 was estimated by adding 250 ms to the
participant’s averaged ERP latency for P3. If the right boundary latency
was equal to the averaged ERP latency, then an adjustment was completed
to narrow the window. This ensured that the picked amplitudes were not
on a slope and were the min/max of the averaged window. Within a time
window, the maximum voltage for positive components or the minimum
voltage for negative components in the window was recorded, along with
the corresponding latency.
The second processing step in the STP procedure, the backwardspass, was designed for the purpose of finding individual peaks that lay
outside the window obtained from the averaged stimulus-locked ERPs that
was used in the forward pass. Starting with the P3 and moving backwards
to P1, the program used the obtained latency values from the forward
pass to recalculate the max/min value between the two obtained latencies
for that trial. For the special case of the P1 latency being found to
equal to 0 ms after the first pass, the left boundary of the window was
adjusted to 50 ms with the right boundary being the latency of N1. This
adjustment was made to capture a peak in the most common window in the
literature (Gruber, Klimesch, Sauseng, Doppelmaryr, 2004; Zhang & Luo,
2011). If P1 did not equal 0, then the window was adjusted to 10 ms for
the left boundary in the backward pass check.
Additionally, peak-to-peak (p2p) measures for N1 p2p, P2p2p, N2 p2p, and P3 p2pwere determined at each trial the STP by subtracting the
baseline-to-peak value of the component preceding from the
baseline-to-peak (b2p) value component of interest. For example,
N1p2p was determined by subtracting
P1b2p from N1b2p (i.e.,
N1p2p = N1b2p - P1b2p).
Peak-to-peak measures have been found to be a more stable component in
the single-trial ERP analyses (Jongsma, et al., 2006) as well as in
averaged ERPs (Gavin, et al. 2019).