User:Lewiscatton/HILAS Psychophysiology
Psychophysiology
[edit]Measures of the cardiovascular system include Heart Rate (HR), blood pressure and respiration. This system should be viewed as a whole, as changes on one variable affect others – for example a change in respiration rate has an effect on HR. The average HR reflects metabolic activity and has been found to be sensitive both to general alertness level and compensatory effort (De Waard, 1996; Mulder et al., 2005). In absence of physical effort, the main determinant of HR response in pilots is mental workload (Roscoe, 1992). Not only physical and mental effort affect HR level (e.g. Lee & Park, 1990), emotional factors, such as high responsibility or the fear of failing for a test, also influence mean HR (Jorna, 1993).
A feedback loop between the central nervous system and peripheral autonomic sensors causes irregularities in HR. Heart Rate Variability (HRV) reflects performance of this feedback system and has successfully been used as indicator of mental effort. Frequency analysis can decompose variability into components associated with biological control mechanisms (e.g. Kramer, 1991; Mulder, 1992). HR can be used as unobtrusive measure to reflect mental effort during task performance. The “profile technique” uses a moving average and can give an indication of high workload periods, i.e. moments where operators “have to try hard(er)”. Resolution of these periods is 10-15 seconds. Examples can be found in Hoogeboom & Mulder (2004), Mulder et al. (2005) and De Waard (1996, 2002).
Output
[edit]The output is a combination of readings from the different sensors which are analysed by programs such as HEART or CARSPAN to obtain frequency information. The outputs that are used for the psychophysiology analysis are:
- Electrocardiogram (ECG) measurements: The R-peaks (or inter-beat-interval times) are detected, time-stamped and stored, and can be converted to commonly known measures such as beats per minute. Data can then be analysed in the frequency domain to obtain information about mental effort. The ECG measures:
- Average heart rate (HR)
- Heart rate variability (HRV)
- Heart rate variability in the frequency domain
- Blood Pressure. Pulse transition time (PTT) is a time-measure, the time from an R-peak to a 50% photoplesmographic deflection.
- Respiration rate has a major impact on heart rate and has to be assessed to be able to control for it. The respiration frequency and amplitude were measured as part of the comparison to identify workload.
Methodology
[edit]The Electrocardiogram (ECG) is commonly measured with three small Ag-AgCl electrodes, attached to the chest. The ECG signals of both pilots were processed by two Vitaports (TEMEC Instruments), were sampled at 256 Hz and stored to disk. R-peaks were detected and artefacts corrected using the CARSPAN programme. With CARSPAN also the power in the different frequency bands was determined. It was expected that the ECG measurements did not have an impact on the pilots’ performance during the flight. Undesired influences such as speech, moving at the seat, etc. were reflected in the measurements. Synchronisation with performance was required, e.g. by time-stamping data. Respiration was measured with a flexible belt over the participant’s chest and also further analysed with CARSPAN. The respiration rate was used in the data analysis phase to correct artefacts that arose in the ECG signals.