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Introduction & Aim: Mentally demanding tasks are often, but not always, those that place a high load on working memory. In fact, the N-back task is often relied on as the "ground truth" for evaluating brain-computer interfaces that seek to infer user experienced workload [Brouwer2011]. Manual production lines is a real-world example that requires workers to memorize complex assembly instructions, an aspect that is becoming more demanding in recent times as production lot sizes decrease. To assist manual assembly, in-situ displays have been developed that provide just-in-time instructions for assembly [Funk2016]. The utility of this in alleviating mental workload has been validated with measures such as questionnaires or semi-structured interviews, but never with neuroimaging. In this work, we employ electroencephalography (EEG) to evaluate the extent to which in-situ displays alleviate visuospatial working memory during manual assembly.
Methods: An Emotiv Epoc was used to record EEG data throughout the whole experiment. In a within-subject study design (N=12), participants were instructed to use either paper or projected in-situ instructions to assemble a Lego Duplo construction [Funk2016]. Paper instructions were printed on A4 sized sheets of paper. In-situ instructions were projected directly into the workplace (see Figure 1). Before the start of the assembly task, participants performed a one-minute eyes-opened/eyes-closed task followed by two N-back tasks (N=0 and N=2) to ensure a correct setup of the Emotiv Epoc through alpha desynchronization and to determine the individual bandwidth for alpha power. Afterward, participants started to assemble two different Lego Duplo starting with either paper or projected in-situ instructions based on the balanced Latin square. Ground truth for perceived workload is measured via NASA-TLX questionnaires filled out by participants after every N-back and assembly trial.
Results: Alpha power was derived and noise suppressed by using a spatio-spectral decomposition filter [Nikulin2011]. Our findings show significantly higher alpha desynchronization when conducting an N-back task with N=2 than N=0, F(1, 11) = 34.82, p < 0.001. By comparing projected in-situ and paper instructions, projected in-situ instructions showed significantly less alpha desynchronization than paper instructions, F(1, 11) = 14.92, p < 0.003. This converges with collected NASA-TLX questionnaires evaluating perceived workload levels. We also found that item selection errors have significantly increased when using paper instructions instead of projected in-situ instructions, F(1, 11) = 10.75, p < 0.007.
Discussion and Conclusion: Our study shows encouraging results of using a low-cost EEG to evaluate assembly instruction system. Possible reasons for paper instructions causing higher alpha desynchronization, and thus working memory load, might be that every step (e.g. picking and placing a brick) has to be remembered by participants while projected in-situ instructions obviate the need for remembering single steps. This is backed up by a reduced number of item selection errors and supported by NASA-TLX questionnaires. This provides insights for developers to evaluate the feasibility of their assistive system design on a cognitive basis.
