EEG, Pupil Dilations, and Other Physiological Measures of Working Memory Load in the Sternberg Task

Recent evidence shows that physiological cues, such as pupil dilation (PD), heart rate (HR), skin conductivity (SC), and electroencephalography (EEG), can indicate cognitive load (CL) in users while performing tasks. This paper aims to investigate physiological (multimodal) measurement of CL in a Sternberg memory task as the difficulty level increases in both maintenance and probe phases. For this purpose, we designed a Sternberg memory test with four levels of difficulty determined by the number of letters in the words that need to be remembered. Our behavioral performance results show that the CL of the task is related to the number of letters in non-semantic words, which confirms that this task serves as an appropriate metric of CL (the task difficulty increases as the number of letters in words increases). We were interested in investigating the suitability of multimodal physiological measures as correlates of four CL levels for both the maintenance and probe phases in the Sternberg memory task. Our motivation was to: (1) design and create four levels of task difficulty with a gradual increase in CL rather than just high and low CL, (2) use the Sternberg test as our test bed, (3) explore both the maintenance and probe phases for measurement of CL, and (4) explore the correlation of physiological cues (PD, HR, SC, EEG) with CL in both phases. Testing with the system, we found that for both the maintenance and probe phases, there was a significant positive linear relationship between average baseline corrected PD and CL. We also observed that the average baseline corrected SC showed significant increases as the number of letters in the words increased for both the maintenance and probe phases. However, the HR analysis did not show any correlation with an increase in CL in either of the maintenance or probe phases. An additional analysis was conducted to investigate the correlation of these physiological signals for high (seven-letter words) versus low (four-letter words) CL loads. Our EEG analysis for the maintenance phase found significant positive linear relationships between the power spectral density (PSD) and CL for the upper alpha bands in the centrotemporal, frontal, and occipitoparietal regions of the brain and significant positive linear relationships between the PSD and CL for the lower alpha band in the frontal and occipitoparietal regions. However, our EEG analysis of the probe phase did not show any linear relationship between the PSD and CL in any region. These results suggest that PD, SC, and EEG could be used as suitable metrics for the measurement of cognitive load in Sternberg memory tasks. We discuss this, limitations of the study, and directions for future work.