Workload in the human brain can be a useful marker of internal brain state. However, due to technical limitations, previous workload studies have been unable to record brain activity via conventional electroencephalography (EEG) and magnetoencephalography (MEG) devices in mobile participants. In this study, we used a wearable EEG system to estimate workload while participants walked in a naturalistic environment. Specifically, we used the auditory steady-state response (ASSR) which is an oscillatory brain activity evoked by repetitive auditory stimuli, as an estimation index of workload. Participants performed three types of N-back tasks, which were expected to command different workloads, while walking at a constant speed. We used a binaural 500 Hz pure tone with amplitude modulation at 40 Hz to evoke the ASSR. We found that the phase-locking index (PLI) of ASSR activity was significantly correlated with the degree of task difficulty, even for EEG data from few electrodes. Thus, ASSR appears to be an effective indicator of workload during walking in an ecologically valid environment.
Auditory induction is a continuity illusion in which missing sounds are perceived under appropriate conditions, for example, when noise is inserted during silent gaps in the sound. To elucidate the neural mechanisms underlying auditory induction, neural responses to tones interrupted by a silent gap or noise were examined in the core and belt fields of the auditory cortex using real-time optical imaging with a voltage-sensitive dye. Tone stimuli interrupted by a silent gap elicited responses to the second tone following the gap as well as early phasic responses to the first tone. Tone stimuli interrupted by broad-band noise (BN), considered to cause auditory induction, considerably reduced or eliminated responses to the tone following the noise. This reduction was stronger in the dorsocaudal field (field DC) and belt fields compared with the anterior field (the primary auditory cortex of guinea pig). Tone stimuli interrupted by notched (band-stopped) noise centered at the tone frequency, considered to decrease the strength of auditory induction, partially restored the second responses from the suppression caused by BN. These results suggest that substantial changes between responses to silent gap-inserted tones and those to BN-inserted tones emerged in field DC and belt fields. Moreover, the findings indicate that field DC is the first area in which these changes emerge, suggesting that it may be an important region for auditory induction of simple sounds.
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