Measuring Mental Workload Variations in Office Work Tasks using fNIRS

•We studied whether fNIRS could differentiate between different levels of difficulty in everyday representations of reading and writing tasks of information workers.•We used personalised tasks to each participant, with simulated interruptions to increase ecological validity of task within a laboratory study.•Subjective self-reports showed differences for both reading and writing tasks, where the hard dual-task level was significantly different from easy and medium levels.•fNIRS over the prefrontal cortex was able to discern the same difference for reading, but not for writing.•These results could be explained by the neural areas in which mental workload was presented for both tasks.•fNIRS detected some differences in brain activity which corresponded to the interruptions, and the findings can be understood in terms of spare capacity models.