## 3 Results
### Physiological Assessment of Mental Workload
#### 3.6.1 Central Nervous Measures
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Spectral Power Band** & **Interpretation** & **Employed in** \\ \hline Alpha & Linked to relaxation and idle mental states. Alpha power decreases as cognitive & [32; 36; 42; 46; 49] \\ \hline Theta & Related to cognitive control, working memory, and sustained attention. Frontal & \\  & theta power increases with higher cognitive demand. & [36; 37; 46; 49] \\ \hline \multirow{2}{*}{Betheha} & Associated with active cognitive processing, stress, and alertness. Beta power & [32; 38] \\  & increases in response to heightened cognitive load and task complexity. & \\ \hline \multirow{2}{*}{Gamma} & Linked to complex cognitive functions, memory processing, and high attentional & \\  & states. Gamma oscillations correlate with high mental effort and concentration. & [32; 37; 41] \\ \hline \multirow{2}{*}{Alpha/Beta} & Higher Alpha/Beta ratio = relaxed mind & [37; 41] \\  & Lower Alpha/Beta ratio = alert state of mind & [37; 41] \\ \hline \multirow{2}{*}{Beta/Alpha} & Higher Beta/Alpha ratio = alert state of mind & [38] \\  & Lower Beta/Alpha ratio = relaxed mind & [38] \\ \hline \multirow{2}{*}{Alpha/theta} & Higher Alpha/Theta = focused and alert state of mind & [37; 41] \\  & Lower Alpha/Theta = more relaxed and meditative states & \\ \hline \multirow{2}{*}{Theta/Alpha} & Higher Theta/Alpha = more relaxed and meditative states & [36; 47] \\  & Lower Theta/Alpha = focused and alert state of mind & [47] \\ \hline \multirow{2}{*}{Theta ratio} & Associated with working memory and attentional control. & [47] \\ \hline \multirow{3}{*}{Gamma/Theta} & Research has shown that the gamma/theta ratio is higher during states of & \\  & focused attention, such as when performing a visual or auditory task. & \\ \cline{1-1}  & The Gamma/Theta ratio has also been linked to memory processing, with higher & [37; 41] \\ \cline{1-1}  & ratios observed during successful encoding and retrieval of memories. & \\ \hline \multirow{2}{*}{Beta/Alpha + Theta} & Reflects mental effort, vigilance, attention, alertness, and task engagement. & [47] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Power ratio interpretation (adapted from [41] and expanded by the authors). phases, suggesting a decrease in cognitive load over time [38]. The Gamma/Theta ratio (\(\gamma\)/\(\theta\)), associated with working memory and concentration, exhibited peaks during the most cognitively demanding task phases, particularly during the initial assembly and repositioning of the plate. This pattern indicates heightened cognitive demands, increased attentional focus, and active memory processing [37; 41]. The Theta/Alpha ratio (\(\theta\)/\(\alpha\)), an indicator of cognitive effort, exhibited an increase as workload levels intensified [47]. In collaborative settings, the presence of a robot significantly reduced \(\theta\)/\(\alpha\), suggesting a lower mental workload. This reduction was further supported by its strong correlation with mental demand scores in the NASA-TLX questionnaire [47]. Additionally, the Mental Workload Index proposed by [36], which is calculated as the ratio of frontal theta power (Fz) to parietal alpha power (Pz), peaked during the most cognitively demanding phases of the assembly task. Similarly, ref. [41] observed that the Alpha/Theta ratio increased during assembly, indicating a heightened state of focus and attention. The Theta/Beta ratio (\(\theta\)/\(\beta\)), associated with working memory and attentional control, showed that an increase in this index may signal reduced attentional control in high-demand tasks [47]. Lastly, besides extracting information from frequency domain bands, ref. [32] also extracted information from the time domain, namely mean, peak location, and kurtosis. Ref. [37] analyzed the variations in amplitude and fluctuations over time under different workload conditions. Ref. [46], in turn, used error-related potential (ERP) to detect errors. ERP consists of a time-locked negative peak, or error-related negativity, around 200 ms after the error occurrence.