C. Experimental design and procedures
This protocol is extracted from research article:
Speech compensation responses and sensorimotor adaptation to formant feedback perturbations
J Acoust Soc Am, Feb 17, 2021; DOI: 10.1121/10.0003440

The current study consisted of five sessions of 165 trials each with formant feedback perturbations. Trials in sessions 1, 3, and 5 included unpredictable formant perturbations, applied either at utterance onset or at mid-utterance, performed to examine within-trial online compensation responses. Trials in sessions 2 and 4 included consistent, predictable formant perturbations, performed to examine across-trial sensorimotor adaptation responses. For any given trial, participants were instructed to say either the word “head” (/hɛd/) or “hid” (/hId/), extending the vowel portion of the utterance for as long as the prompt word was displayed on the screen (approximately 2 s). The “hid” catch trials were infrequent (number of trials discussed below). These catch trials were added to (1) keep the experiment more engaging so as to prevent participants from becoming bored and inattentive to the speech production task, and to (2) encourage participants to make contrastive productions of these vowels and pay more attention to the acoustics of their production. Before the first session, a short practice session with experimenter (about 5–10 trials as needed) was done to ensure that the participant could hold out their word steadily as instructed. Within each session, there were random-length breaks (1.7–2.7 s) between trials as well as self-paced breaks after every 15 trials. Five-minute breaks were also administered in between sessions in an attempt to wash out possible carryover effects from previous sessions, during which the experimenter would verbally engage with the participant, for example, by asking questions relating to the current experiment.

Within-trial online compensation response data were collected from 495 trials evenly distributed across sessions 1, 3, and 5. Session 1 began with 15 familiarization phase trials included to acquaint participants to the experimental task and pace, with each trial being randomly chosen from any one of the trial types described below. The remaining 480 trials were distributed across the remainder of session 1 as well as sessions 3 and 5. These 480 trials consisted of 432 perturbation trials (360 “head” and 72 “hid” trials) and 48 unperturbed trials (30 “head” and 18 “hid” trials). Perturbation trials were unpredictable perturbations of the first formant (F1) of subjects' auditory feedback. Two types of unpredictable formant perturbations were applied: (1) unpredictable formant perturbations applied at mid-utterance, transiently for 400 ms with a 200–500 ms jittered delay from utterance onset [unpredictable mid-utterance perturbations, Fig. 1(a)], and (2) unpredictable formant perturbations at utterance onset applied for the entire trial duration [unpredictable whole utterance perturbations, Fig. 1(b)]. The 400 ms duration for the transient perturbations was chosen as it has often been used in previous mid-utterance pitch and loudness perturbation studies (Heinks-Maldonado and Houde, 2005; Kort et al., 2016). Four different F1 feedback perturbations were applied: −50 Hz, +50 Hz, −200 Hz, and +200 Hz for each mid- and whole-utterance perturbation, totaling to eight formant feedback perturbation conditions. These eight conditions were randomly distributed across the perturbation trials (see supplementary Table I).1 The choice of perturbation magnitudes of 50 and 200 Hz are well within the range of values that have been used in prior formant sensorimotor adaptation studies (Katseff et al., 2012; MacDonald et al., 2010).

Experimental paradigm. F1 perturbation applied at different timescales: (a) unpredictable transient mid-utterance perturbation (a 400 ms perturbation initiated after a 200–500 ms jitter delay from utterance onset); (b) unpredictable whole utterance perturbation, initiated at utterance onset and sustained for the whole utterance; and (c) consistent, predictable utterance-onset whole utterance perturbation applied over many trials (sensorimotor adaptation). RT = Reaction time light gray bars with dashed outline). Bars (light gray and white) with dashed outline indicate where no perturbation was applied, and solid orange bars indicate where perturbation was applied.

Across-trial sensorimotor adaptation response data were collected in session 2 and 4. In each of these sessions, the F1 perturbation was consistently applied over many trials arranged as a sequence of phases [Fig. 1(c)]: (1) a baseline phase of 42 trials (30 “head” trials and 12 “hid” trials), where feedback was unperturbed, (2) a hold phase of 81 trials (60 “head” trials and 21 “hid” trials), where F1 feedback was consistently perturbed by whole-trial perturbations of either +200 or −200 Hz (depending on the session), and (3) a washout phase of 42 trials (30 “head” trials and 12 “hid” trials), where feedback was again unperturbed. The session order for the two F1 perturbation directions was counterbalanced among participants, i.e., half of the participants received +200 Hz F1 feedback perturbation for session 2 and –200 Hz F1 feedback perturbation for session 4; for the other half, this order was reversed. The “hid” trials were again included as catch trials and not included in the analysis. The “head” and “hid” trials were randomized within each phase of each adaptation session.

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