Abstract
Transient event-related potentials (ERPs) and steady-state responses (SSRs) have been popularly employed to investigate the function of the human brain, but their relationship still remains a matter of debate. Some researchers believed that SSRs could be explained by the linear summation of successive transient ERPs (superposition hypothesis), while others believed that SSRs were the result of the entrainment of a neural rhythm driven by the periodic repetition of a sensory stimulus (oscillatory entrainment hypothesis). In the present study, taking auditory modality as an example, we aimed to clarify the distinct features of SSRs, evoked by the 40-Hz and 60-Hz periodic auditory stimulation, as compared to transient ERPs, evoked by a single click. We observed that (1) SSRs were mainly generated by phase synchronization, while late latency responses (LLRs) in transient ERPs were mainly generated by power enhancement; (2) scalp topographies of LLRs in transient ERPs were markedly different from those of SSRs; (3) the powers of both 40-Hz and 60-Hz SSRs were significantly correlated, while they were not significantly correlated with the N1 power in transient ERPs; (4) whereas SSRs were dominantly modulated by stimulus intensity, middle latency responses (MLRs) were not significantly modulated by both stimulus intensity and subjective loudness judgment, and LLRs were significantly modulated by subjective loudness judgment even within the same stimulus intensity. All these findings indicated that high-frequency SSRs were different from both MLRs and LLRs in transient ERPs, thus supporting the possibility of oscillatory entrainment hypothesis to the generation of SSRs. Therefore, SSRs could be used to explore distinct neural responses as compared to transient ERPs, and help us reveal novel and reliable neural mechanisms of the human brain.
Highlights
The presentation of a transient sensory event would disturb the spontaneous electroencephalographic (EEG) activity and evoke the event-related potentials (ERPs) that are time-locked and phaselocked to the sudden onset of the sensory stimulus [1,2]
To support the superposition hypothesis, it has been reported that the auditory SSRs evoked by stimulation of 40 Hz could be largely explained by the linear sum of transient auditory ERPs, e.g., auditory brainstem response and middle latency responses (MLRs) [14,17,24], and the visual SSRs elicited by a checkerboard stimulation reversing at different rates can be explained by the temporal superposition of transient visual ERPs [15]. This superposition hypothesis was challenged by the oscillatory entrainment hypothesis, which supported the idea that SSRs were the result of the entrainment of a neural rhythm driven by the periodic repetition of a sensory stimulus [19,20,21]
All these findings indicated that SSRs at high frequencies (e.g., 40 and 60 Hz) were markedly different with MLRs and late latency responses (LLRs) in transient ERPs
Summary
The presentation of a transient sensory event would disturb the spontaneous electroencephalographic (EEG) activity and evoke the event-related potentials (ERPs) that are time-locked and phaselocked to the sudden onset of the sensory stimulus [1,2]. SSRs are composed of a series of identical/similar temporal waveforms, and are normally identified in the frequency domain as peaks appearing at the frequency (and/or its harmonics) of the repeated stimulus, which are characterized by their power and scalp distribution [5]. For this reason, SSRs can be quantified unequivocally as compared to the transient ERPs, and capture several important advantages as summarized in Colon et al [6]. SSRs exhibit a high signal-to-noise ratio (SNR), indicating that a shorter time of data collection is needed to obtain reliable signals [6,7,8]
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