Figure 1. A schematic of the experimental protocol.

Participants were instructed to fixate their eyes on the central red fixation line (Panel A). After a delay (1–2 s), the green trigger line, which was presented on the far right-hand side of the screen, and visible in participants’ peripheral vision, began to move smoothly across the screen in a leftwards direction at a speed of 6.5°/s (Panel B), such that after 3.75 s the trigger line overlapped with the fixation line. At this precise moment, dubbed the ‘sound-time’, two events occurred simultaneously (Panel C). Firstly, the participant was asked to imagine themselves producing a pre-defined phoneme in inner speech (either /ba/ or /bi/ or no inner phoneme). Secondly, an audible phoneme (either /BA/ or /BI/), produced by a male speaker, was delivered to the participant’s headphones. In Match trials (Panel D, top, blue), the inner phoneme was congruent with the audible phoneme (e.g., inner phoneme: /ba/; audible phoneme: /BA/). In Mismatch trials (Panel D, middle, red), the inner phoneme was incongruent with the audible phoneme (e.g., inner phoneme: /bi/; audible phoneme: /BA/). In Passive trials (Panel D, bottom, black), the participant did not produce an inner phoneme. Following the sound-time, the trigger line continued to move past the fixation line for an additional 1 s. The trial was then complete and the participant was asked to rate how successfully they managed to follow the instructions on that trial, on a scale from 1 (Not at all successful) to 5 (Completely successful).