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).