3.1 The Feasibility of Single Trial Measures
Signal to Noise Ratios: SNR values for each component in a participant’s averaged ERP were calculated in a manner recommended by Spencer (2005), Debener et al. (2006) and Hu et al. (2010) where the “signal” was the peak amplitude of the component, and the “noise” was the standard deviation of the amplitude of all data points in the baseline period (i.e., -200 ms to 0 ms). For both the STP and STW approaches, SNR values were first calculated at the trail level using the same definitions of “signal” and “noise”. Additionally, for direct comparison purposes to the SNR derived each individual participant’s averaged ERPs, the SNRs at the trial level were then averaged for each participant. For a brief summary, means across participants and their corresponding standard deviations for each component are shown in Table 1.
As expected, and in keeping with the justification for averaging ERPs often found in the literature, the mean SNR for all components was the largest when the components were measured from averaged ERPs. However, the general assumption that the “signal” of interest is too small to be measured in single trials may not be true for at least the paradigm used in this study as the mean SNRs for the STP approach, while lower than SNRs of the averaged ERPs, did range from 1.62 to 2.9 and were larger than the SNRs of the STW approach. More importantly, the within-group variability (i.e., SD s) of the SNRs for the STP approach is considerably smaller than the variability of the SNR of the averaged ERPs but larger than those for the STW approach. A more direct comparison of the three approaches can be made by computing their coefficient of variation (CV ). The lower the coefficient of variation value the better, as it denotes that the spread of data values are low relative to the mean of the values. When the differences in the variability of the approaches are viewed in relation to their mean amplitudes as shown in Table 1, other than for P1, the STP and STW approaches are nearly the same regarding finding a “signal” within “noise”, and both have much smaller CV s than the averaged ERPs.
Comparison of Measurement Approaches: To allow for direct comparison between the STP (baseline-to-peak), STW, and averaged ERP approaches, the ST data for each participant were first averaged across trials then these averages were summarized across participants to produce descriptive statistics; the means, standard deviations, minimum and maximum values. These descriptive statistics for the amplitude measures derived from the averaged ERP, STW, and STP approaches are provided in Table 2. Descriptive data for the latencies for the averaged ERP and STP approach are provided in Table 3. In general, the STP approach produced the largest within-group mean amplitudes along with having the largest standard deviation across all of the components measured. With the exception of P1, the averaged ERP approach produced considerably larger with-group means but only slightly larger standard deviations than the STW approach. However, when these approaches are viewed in relation to their coefficient of variations (Table 4), the STP approach had considerably smaller CV values than the other two approaches for all components. The STW approach had larger absoluteCV values than the averaged ERP approach for all components except for the P1 component.