2.1 Participants
We recruited 40 adults to participate in the experiment. Three participants were excluded due to excessive muscle artefacts or electric noise. Therefore, the data of the remaining 37 participants (age range: 19–26 years; mean age: 20.84 years; 19 females) were analyzed. Two of them were left-handed, while the others were right-handed. All participants were undergraduate students studying at the Chinese University of Hong Kong (CUHK). Written consent was obtained from each participant before the experiment began. Participants were compensated either with course credits or cash. This study was approved by The Joint Chinese University of Hong Kong–New Territories East Cluster Clinical Research Ethics Committee (The Joint CUHK-NTEC CREC) (reference no.: 2019.048).
2.2 Stimuli and Procedure
The experiment was designed and implemented using E-Prime 3.0 (Psychology Software Tools, Pittsburgh, PA). Stimuli consisted of pure sinusoidal tones spanning seven different frequencies (500 Hz, 550 Hz, 600 Hz, 650 Hz, 700 Hz, 750 Hz, and 800 Hz). Each tone had a duration of 70 ms, including 5 ms rise and fall times. The stimulus onset asynchrony (SOA) was fixed at 650 ms. All tones were generated and scaled to a sound pressure level (SPL) of 70 dB using Praat (Boersma, 2001).
The present study employed a roving paradigm, where a deviant stimulus becomes the subsequent standard. This design allows the MMN to accurately reflect inherent differences between the two types of stimuli, without being influenced by extraneous factors such as frequency or duration. The roving paradigm has been utilized in previous studies (e.g., Costa-Faidella et al., 2011a; Cowan et al., 1993; Garrido et al., 2008; Haenschel et al., 2005; Recasens et al., 2015).Top of Form
EEG data were collected using NetStation, employing a Net Amps 300 amplifier (Electrical Geodesics Inc.) and corresponding 128-channel nets with a reference electrode placed at Cz. The sampling rate was set to 500 Hz, and online filtering was applied with a high-pass cutoff of 0.1 Hz. Before the experiment commenced, the impedance of each electrode was verified to be below 50 kΩ. Data acquisition occurred within a sound-attenuated laboratory environment. Prior to the experiment, participants were informed that some sounds would be presented when they watched a silent movie, and they were asked to disregard these sounds. Also, participants were instructed to minimize body movements during the experiment to reduce muscle artifacts.
In the present experiment, the first tone was randomly selected from among seven types of tones with varying frequencies. Subsequently, there was an 85% probability that the following tone (standard) would match the frequency of the preceding tone, while there was a 15% chance that the next tone (deviant) would be different. This criterion remained consistent throughout the experiment to control participants’ expectations of the stimulus arrangement. However, an exception was made: if 30 tones were played consecutively, the next tone had to be a deviant. On average, this occurred 2.73 times on average (SD = 1.59 times; range: 0–5 times) across approximately 300 sequences. This restriction was implemented to ensure an adequate number of trials for analysis. Throughout the experiment, participants watched a silent movie, “Tom and Jerry”, on a laptop while the tones were played through two speakers positioned 80 cm away from them. The experiment ended after 2,000 tones had been presented, divided into two blocks with 1,000 tones each. A brief pause occurred between the two blocks to allow the experimenter to check the impedance of all electrodes and adjust if necessary. The duration of the experiment was approximately 20 minutes. Figure 1 illustrates the experimental paradigm.
After the EEG experiment, participants underwent a tone discrimination task using E-Prime 3.0 to confirm their tone discrimination ability. Details of this task are provided in Supplementary Material I.