Psychophysics toolbox

Following previous sample sizes of MEG studies that used MI to study speech tracking [2 (link),22 ], as well as previous recommendations [71 (link),72 (link),73 (link)], 20 healthy, native British participants took part in the study (9 female, age 23.6 ± 5.8 years [mean ± SD], age range: 18 to 39 years). All participants were right-handed [Edinburgh Handedness Inventory; 74 (link)], had normal hearing [Quick Hearing Check; 75 (link)], and normal or corrected-to-normal vision. Furthermore, participants had no self-reported current or previous neurological or language disorders.
MEG was recorded with a 248-magnetometer, whole-head MEG system (MAGNES 3600 WH, 4-D Neuroimaging, San Diego, CA) at a sampling rate of 1 KHz. Head positions were measured at the beginning and end of each run, using 5 coils placed on the participants’ heads. Coil positions were codigitised with head shape (FASTRAK, Polhemus Inc., Colchester, VT). Participants sat upright and fixated at a fixation point projected centrally on screen with a DLP projector. Sounds were transmitted binaurally through plastic earpieces, and 3.7 m–long plastic tubes connected to a sound pressure transducer. Stimulus presentation was controlled with Psychophysics toolbox [76 (link)] for MATLAB (The MathWorks, Inc., Natick, MA).

While the transcribed story was presented word by word on a noncumulative display, participants had the task to read each word once at a comfortable pace and quickly press a button to reveal the next word as soon as they had finished reading. A timeout of 6 s was implemented. The time interval between word appearance and button press was logged as the reading time. After each run, participants answered three four-option multiple-choice questions on the plot of the story (performance: Ra = 58.33 to 100% correct, M = 79.17%, and SD = 10.87%) and took a self-paced break. In total, each participant completed four of eight runs, which were randomly selected and presented in chronological order. Throughout the reading task, we recorded movement and pupil dilation of the participants’ left eye at a sampling rate of 250 Hz in one continuous shot with an eye tracker (EyeLink 1000, SR Research). Eye tracking data were not analyzed in the present study.
The experiment was controlled via the Psychophysics Toolbox (84 (link)) in MATLAB (R2017b, MathWorks). All words were presented 20% off from the left edge of the screen in white Arial font on a gray background with a visual angle of approximately 18°. Participants used a response pad (URP48, The Black Box ToolKit) to navigate the experiment with their right index finger. The experimental session took approximately 40 min.

Participants sat in a soundproof, dimly lit room with their heads supported by a chin and head rest affixed 42 cm in front of a gamma-calibrated LCD monitor (24” Iiyama Prolite B2481HS LED monitor, Iiyama Corporation, Tokyo, Japan; 60 Hz refresh rate, 1,920 × 1,080 resolution). Fixation during the task was checked using ViewPoint Eyetracker (MIU03 Monocular, Arrington Research, Inc., Scottsdale, AZ, United States; 220 Hz) which was mounted toward the left side of the chin and head rest. All stimuli were generated at runtime in MATLAB (The MathWorks, Inc.) using Psychophysics Toolbox (Brainard, 1997 (link); Pelli, 1997 (link); Kleiner et al., 2007 ). The stimulus PC interfaced with the Eyetracker PC via an Ethernet connection using the ViewPoint Client App and ViewPoint MATLAB toolbox (v2.8.5), providing runtime access to the Eyetracker data.

The stimuli had previously been used in other studies, and a complete description of them can be found elsewhere (Petrini et al., 2009a (link),b (link); Love et al., 2013 (link)). They comprised dynamic audiovisual movies (3 s) containing the point-light representation of a drummer playing a swing groove at 120 beats per minute, with an accent on the second beat (Figure 1). Audio and visual cues were shifted relative to each other to produce stimuli with different cue onset asynchrony (COA). The video was shifted to begin either after the audio (−333, −267, −200, −133, and −67 ms) or before the audio (+333, +267, +200, +133, and +67 ms), producing a total of 10 asynchronous stimuli to be used in the pre-fMRI experiment. Negative and positive numbers will be used to refer to audio-leading and video-leading COA levels respectively, and 0 COA will refer to the synchronous condition. To prevent participants from having to stay in the MRI scanner for an uncomfortably long time only 4 COA levels were used during the fMRI experiment: two asynchronous (−333, +333 COA) and two “synchronous” (0 COA and the individually defined PSS). The −333, 0 and +333 COA conditions are provided as Supplementary Videos 1–3, respectively.
Stimuli were presented using MATLAB 2007b (MATHWORKS Inc., Natick, MA) and the Psychophysics Toolbox (Brainard, 1997 (link); Pelli, 1997 (link)).

All experimental sounds were sinusoidal pure tones, with 5 ms rise/fall time and 44100 Hz sampling rate, generated using MATLAB (MathWorks, Natick, MA) with the Psychophysics toolbox [28 (link)–30 ]. Auditory stimuli used for the pitch discrimination task were chosen through pilot testing and consisted in a standard pitch sound of 500 Hz which was to be compared to 504, 508, 512, 515 or 518 Hz pitch test sounds. All sounds were played for 250 ms via headphones. The standard stimulus was randomly presented to the left ear or the right ear and the test sound to the opposite ear. The first sound was always presented to the left ear. A fixation cross appeared prior to the sound to signal the beginning of each trial.