Thesis - Instituto de Telecomunicações

4.2. ELECTRODERMAL ACTIVITY 81the estimated amplitude, a, of the detected events.Comparing the acquired EDA signal (upper continuous line in figure 4.6) with the SCRsdetected (bottom curves), we can see that the latter follow the shape of the EDA signalclosely, thus revealing a good morphological match to the proposed model.We now address the temporal properties related to the first and second order derivativezeros. We based the EDA model on the assumption that those zeros were evenly spaced intime (see equation (4.26)). In order to evaluate the validity of such assumption, we analyzedthe detected SCR events, observing the signals in two distinct zones: the rise zone (fromt 0 to t 2 - see figure 4.4); and the peak zone (from t 1 to t 3 ). In both cases, the signal wasnormalized in time and amplitude. In the rise zone, the [t o ,t 2 ] time interval was normalizedto the [0, 1] interval for each of the detected events. The normalization in amplitude wasperformed by subtracting the signal by x t0 and dividing by x t2 . A similar normalizationwas performed for the peak zone, now in relation to the time intervals [t 1 ,t 3 ].All the obtained normalized signals were used to produce a mean SCR signal, and wecomputed an error distance measure (the root mean square) of each signal to the meanwave. We selected the SCR signal segments that had an error below the mean error (onestandard deviation), since some overlapping of SCR signals would corrupt the waves. Thisselection eliminated approximately half of the sample waves.For the rise zone of the SCRs, from the initial 3484 events, 1472 waves were selectedaccording to the 1 standard deviation selection criterion above. From these, a final meanwave was computed. Figure 4.7(a) shows the resulting mean wave. By determining the zerosin the second order derivative for each signal segment, we obtained estimates of normalized t 1values. The corresponding histogram is plotted in figure 4.7(b). Averaging the 1472 samples,we got t 1 =0.513 ± 0.0196. According to the evenly spaced time intervals assumption, t 1should be in the middle of the [t 0 ,t 2 ] segment, and therefore it should have the normalizedvalue 0.5. Experimental results thus confirm the underlying hypothesis.For the peak zone of the SCRs, 1515 waves were selected according to the previouslydefined criterion. The resulting normalized mean wave is shown in figure 4.8(a). Thehistogram for all the computed normalized t 2 values (determined from the zeros of the signalfirst order derivative) is depicted in figure 4.8(b), exhibiting a narrow Gaussian distribution,with mean value µ ≈ 0.5 (t 2 =0.512 ± 0.017).