Results

Figure 4.4.– DRT and EEG metrics (± 1 SE) as a function of task set.

Mean RTs as well as alpha and theta amplitudes are plotted in Figure 4.4. Mean DRT RTs of hits and alpha and theta amplitude levels can be found in Table 4.1 on the following page. Median HRs are given in Table 4.3 on page 90. As can be seen in Figure 4.4, longer DRT RTs can be observed for triple task scenarios relative to baselines as well as for the hard n-back condition relative to the easy n-back condition. Mean alpha amplitudes decrease in triple task scenarios relative to baselines. Additionally, higher mean theta amplitudes can be observed for SuRT easy relative to SuRT hard. In order to better assess DRT RTs, the mean RTs of each signal across the task duration is plotted in Figure 4.5 on page 88. As can be seen in Figure 4.5, the mean RTs in both baseline conditions vary less than the mean RTs of other more complex conditions. As the

BaselinesTDRT+Driving+ DrivingTDRTTDRT+ Driving0-Back2-BackSuRTEasySuRTHard MetricM(SD)M(SD) DRTRT–274.38 (42.58)377.91 (113.80)492.10 (148.83)722.56 (274.75)693.99 (173.67)699.25 (236.18) FMtheta–1.93 (.31)1.87 (.28)1.77 (.25)1.79 (.28)1.94 (.37)1.81 (.36) FMalpha–1.63 (.32)1.52 (.38)1.41 (.38)1.38 (.32)1.44 (.35)1.44 (.39) RSMalpha–1.51 (.28)1.45 (.32)1.33 (.29)1.28 (.27)1.31 (.25)1.30 (.30) Driving- SDLP.15 (.05)–.14 (.04).13 (.04).12 (.03).25 (.11).30 (.11) Driving- RMSE3.76 (1.30)–3.98 (1.07)4.50 (1.40)6.61 (4.04)7.97 (3.64)7.59 (2.97) Table4.1.–Meanperformancevaluespercondition:DRT(RTs[ms]);EEGvalues(waveamplitudes[µV];driving(SDLP[m]andRMSEofspeed[km/h]).

Figure 4.5.– Mean RTs per DRT signal over 2 minute scene duration for all participants (N = 12). Lines represent different task sets.

condition becomes more complex, in the triple task scenarios for example, the mean RTs also become more variable.

Driving SDLP and RMSE values are provided in Table 4.1 on the previous page, and n-back and SuRT performances are reported in Table 4.2; these data were not, however, used for hy-pothesis testing. Differences in the performance of these tasks were, however, observed. SDLP and RMSE remained similar across the two baselines. No large change between the SDLP during the easy and hard n-back conditions were observed and only a slightly higher SDLP was found for the SuRT hard condition relative to easy. RMSE increased for the triple task scenarios; this

Baselines TDRT + Driving +

Table 4.2.– Mean performance values for secondary tasks per condition: n-back (error percentage [%]) and SuRT (count of correctly solved trials).

value was higher for the difficult n-back task set, relative to easy, and little difference in the RMSE values between SuRT easy and hard was found. No errors occurred in the performance of the easy n-back task. Errors, however, occurred in the performance of the hard n-back task, which increased when performed together with the TDRT and driving task, relative to baseline.

For SuRT, the count of correctly solved trials recorded for baseline trials was higher than those performed together with the TDRT and driving task. Additionally, the easy variant yielded more correctly solved trials than the difficult task.

In the following sections, performance analyses are reported. Parametric analyses were per-formed despite normality violations as the ANOVA is relatively robust against such violations (see Schmider, Ziegler, Danay, Beyer, & Bühner, 2010). Four 6-way repeated measures ANOVA tests were run to evaluate whether mean DRT RTs, RSM and FM alpha, and FM theta ampli-tudes were sensitive to differences in task load. One additional non-parametric Friedman test was used to assess the effect of task load on DRT HR distributions. Significant pairwise com-parisons of interest, i.e., that between the easy and difficult levels of a ST, are elaborated with mean differences and 95% CI. Pairwise comparisons between STs and baseline measures (DRT for static conditions, DRT with simulated driving for dynamic conditions), are also reported.

Pearson correlations were planned to determine the strength of the relationship between the DRT and physiological metrics. However, a visual inspection of the data revealed no linear re-lationship between the variables and, therefore, correlations were not applicable.

4.3.1. DRT

RTs

Outliers were assessed by box plots. In the TDN and TDBE conditions, the values of 1 partici-pant were higher than others and resulted in outliers. As these values were considered reasonable (685.52 ms for TDN and 866.46 ms for TDBE), they were not omitted from the analysis. Normal-ity was also assessed. According to the Shapiro-Wilk test of normalNormal-ity, the data of all conditions, bar TDN (p = .007) and TDBE (p = .049), were normally distributed,p > .05. Mauchly´s

test indicated that the assumption of sphericity had been violated,χ²(14) = 38.17, p = .001, therefore, the Greenhouse-Geisser correction (ε = .45) was used. There was a main effect of task load,F(2.27,24.94) = 35.13, p < .001, partialη² =.76. Post-hoc pairwise comparisons with Bonferroni alpha adjustments indicated that static and dynamic baselines were significantly different than all other conditions,p≤.01. The static baseline yielded the fastest RTs, while the dynamic baseline had the second fastest RTs. In terms of differences in difficulty pairs, the n-back hard condition had significantly slower RTs than the easy condition with a mean difference of230.47 (95% CI[53.16to407.78])ms, p= .008. No difference for the SuRT condition diffi-culty pair was found.

Table 4.3.– TDRT HRs reported as:Mdn, minimum and maximum.

A Friedman test was run to determine if there were differences in TDRT HR distributions ac-cording to task load. Pairwise comparisons were performed (SPSS 22) with a Bonferroni correc-tion for multiple comparisons. TDRT HRs were significantly different according to task load, χ²(5) = 36.28, p < .001. Post hoc analysis revealed statistically significant differences in TDRT HR between TDBH (Mdn= .88) and TDBE (Mdn= 1.00),p =.019, TDBH and TNN (Mdn= 1.00),p = .004, TDBH and TDN (Mdn= 1.00), p = .004. All other comparisons were not statistically significant.

4.3.2. EEG Data

The data of two participants were identified as outliers (see Appendix B.1 on page 137 for box plots), however, were not excluded from the analysis as the values were not judged as extreme.

Baseline comparisons were also evaluated and can be found in Appendix B.2 on page 138.

Right Sensory Motor Alpha

Normality was assessed with the Shapiro-Wilk test: 2 conditions (TNN & TDVH) were nor-mally distributed,p > .05. The other 4 conditions were not normally distributed: TDN, TDBE, TDBH, and TDVE,p < .02. Mauchly´s test indicated that the assumption of sphericity had not been violated,χ²(14) = 19.32, p=.166. There was a main effect of task load,F(5,55) = 8.39, p < .001, partialη² =.43. Post-hoc pairwise comparisons with Bonferroni alpha adjust-ments indicated that RSM alpha in the TNN condition was significantly different than TDBE, TDBH, and TDVE,p < .05. Additionally, the TDN baseline RSM alpha was significantly dif-ferent than TDBH,p=.044. All other comparisons were not significantly different.

Frontal Medial Alpha

Normality was assessed with the Shapiro-Wilk test: 2 conditions (TNN & TDBH) were normally distributed,p > .05. The other 4 conditions were not normally distributed: TDN, TDBE, TDVE, and TDVH,p < .05. Mauchly´s test indicated that the assumption of sphericity had been vio-lated,χ²(14) = 24.93, p = .041, therefore, the Greenhouse-Geisser correction (ε = .54) was used. There was a main effect of task load,F(2.72,29.92) = 10.95, p < .001, partialη² =.50. Post-hoc pairwise comparisons with Bonferroni alpha adjustments indicated that TNN baseline FM alpha was significantly different than TDBE, TDBH, and TDVE,p < .02. The dynamic baseline FM alpha was significantly different than that of TDBE and TDBH,p < .05. All other comparisons were not significantly different.

Frontal Medial Theta

Outliers were assessed by box plots. Normality was assessed with the Shapiro-Wilk test: all con-ditions except 1, TDN,p=.017, were normally distributed,p > .05. Mauchly´s test indicated that the assumption of sphericity had not been violated,χ²(14) = 15.49, p=.362. There was a main effect of task load,F(5,55) = 2.75, p = .027, partialη² = .20. No post-hoc pairwise comparisons with Bonferroni alpha adjustments resulted as significant.