3.4 Do trial-to-trial measures display systematic change?
The Curve Fitting app and functions of the Curve Fitting Toolbox in MATLAB (2020b) were used to perform exploratory analyses to determine if any of the components’ ST amplitude or latency variability exhibit systematic change across successive trials. The curve fitting process for each component measure (e.g., ST baseline-to-peak P1 amplitude) started with averaging the measure across those participants with correct responses for the first trial and then averaging separately for each successive trial in a similar manner through trial number 480. This averaging procedure was also completed for participant response times (RT) for each correct trial. Scatterplots were then made with cross-participant mean amplitude (or latency) plotted on the y-axis as a function of trial number on the x-axis separately for each block (i.e., Trials 1-240 and Trials 241-480) and then with both blocks combined (i.e., Trials 1-480). Then to determine if these distributions of means contained any change in values as a function of trial number, the scatterplot of each ST component was evaluated using four Sum of Sine models where in each successive model the phase constant for the model’s sine wave term was incremented from 1 to 4 unless there was evidence of overfitting of the curve as depicted in Figure 1. Overlays of the single trial mean amplitudes derived from STP baseline-to-peak, STP peak-to-peak, and STW approaches are shown in Figure 2 for each component. For comparison purposes, the curve fit of the RT are also shown in Figure 2. Non-linear patterns were found in all components of STP baseline-to-peak and STW approaches with the most dramatic changes in those from the STP peak-to-peak approach. The single trial mean latencies for STP approach are shown in Figure 3 along with mean RT.
The amount of variance (i.e., R 2 values) accounted for by each fitted curve depicted in the figures are reported in Table 11. For the baseline-to-peak and peak-to-peak measures from STP approach, all were successfully fitted to sine wave value of 4 except for one. TheR2 values of the peak-to-peak measures were generally larger than those of the baseline-to-peak measures. In contrast, only two of the amplitude measures from the STW approach could be fitted to sine wave value of 4 and all but two of theR2 values of measures from the STW approach were lower than those from the STP approach.