Phase coherence of auditory steady-state response reflects the amount of cognitive workload in a modified N-back task

The auditory steady-state response (ASSR) is an oscillatory brain activity evoked by repetitive auditory stimuli. Previous studies have reported that the power and phase locking index (PLI) of ASSR could be modulated by the degree of workload. However, those studies used different physical stimuli for tasks of differing difficulty, and the effect of the internal workload itself has not been clearly understood. In this study, we employed the modified N-back task as a visual working memory task in order to vary the degree of difficulty while keeping the physical stimulus constant. The experiment consisted of four types of tasks: No-Load (NL), 1-back, 2-back, and 3-back tasks. The auditory stimulus was a 40 Hz click sound to induce ASSR. Sixteen healthy subjects participated in the present study and magnetoencephalogram responses were recorded using a 148-channel magnetometer system. The hit rate decreased and the reaction time increased according to the task difficulty. Grand averaged phase coherence activities showed the 40 Hz ASSR reductions accompanying an increase in the task difficulty even with the identical external stimuli. In particular, the phase coherence activities in 3-back task were significantly lower than that in the NL and 1-back tasks. Our results suggest that the ASSR can be a useful indicator for the amount of workload in the brain.