## 2 Literature Review
### Rationale for Selected Proficiency Indicators: HRV, TCT, and NASA-TLX  
The rationale for selecting HRV, TCT, and the NASA-TLX as primary indicators of proficiency is supported by empirical literature that highlights the relevance of these metrics for capturing the complexity of human performance in dynamic settings.  
* HRV is a well-established physiological metric that reflects the autonomic nervous system's activity, making it an important indicator of an individual's mental workload, stress, and overall arousal levels. Research indicates that HRV can be effectively used to assess cognitive load and emotional states, as it demonstrates sensitivity to variations in psychological stress and cognitive effort [21]. Studies have shown that HRV is associated with attentional control and emotional regulation, which are crucial for maintaining performance under pressure [44]. Time-domain and frequency-domain metrics, such as Standard Deviation of Normal-to-Normal intervals (SDNN) and Root Mean Square of Successive Differences (RMSSD) for time-domain analysis, and Low-Frequency (LF) and High-Frequency (HF) power for frequency-domain analysis, provide insights into autonomic balance and are relevant for understanding how HRV responds to cognitive demands [44].
* TCT serves as a direct and objective metric of performance efficiency. It is commonly utilized in studies related to skill acquisition. Research indicates that reductions in TCT over successive trials typically signify improvements in proficiency and learning, while increases or variability in TCT can indicate challenges, such as task difficulty or operator fatigue [23]. Studies in simulation environments confirm TCT's effectiveness as an objective tool for measuring skills and strategies, reinforcing its relevance to assessing proficiency in real-time systems [23].
* NASA-TLX is a validated measure of perceived mental workload, providing insights into the subjective experience of task demands through its multi-dimensional framework. Although it is not designed for continuous real-time monitoring, its inclusion in a multimodal assessment framework enhances the understanding of how operators perceive and manage workloads. Research illustrates that perceived workload can diverge from objective performance indicators, highlighting the importance of subjective assessments in human-centered evaluation approaches [45]. Moreover, empirical evidence shows that higher NASA-TLX scores are significantly associated with lower self-reported job satisfaction, indicating that the instrument can also serve as an indirect proxy for job-satisfaction levels when task demands are high [46].  
The combination of HRV, TCT, and NASA-TLX creates a balanced framework for assessing proficiency. HRV provides physiological insights into an operator's internal state, TCT offers a measure of task performance that is likely to link to higher-level productivity metrics, and NASA-TLX captures subjective perceptions of workload. This triad enables a comprehensive understanding of human proficiency, ensuring that critical aspects of performance are not overlooked, which is often the case in unimodal assessments [23; 44; 47].  
Although HRV, TCT, and NASA-TLX were selected because they are widely validated and practical across many domains, the framework is deliberately metric-agnostic: any appropriate trio of physiological (e.g., HRV, EEG, fNIRS), behavioral (e.g., TCT, error-rate, eye-movement latency), and subjective (e.g., NASA-TLX, SURG-TLX) indicators can be substituted to suit the constraints of a specific application, provided that all three information channels remain represented [48; 49].