The Orofacial Somatosensory System Is Modulated During Speech Planning and Production

Abstract

Purpose In our previous studies, we showed that the brain modulates the auditory system, and the modulation starts during speech planning. However, it remained unknown whether the brain uses similar mechanisms to modulate the orofacial somatosensory system. Here, we developed a novel behavioral paradigm to (a) examine whether the somatosensory system is modulated during speech planning and (b) determine the somatosensory modulation's time course during planning and production. Method Participants (N = 20) completed two experiments in which we applied electrical current stimulation to the lower lip to induce somatosensory sensation. In the first experiment, we used a staircase method (one-up, four-down) to determine each participant's perceptual threshold at rest (i.e., the stimulus that the participant detected on 85% of trials). In the second experiment, we estimated each participant's detection ratio of electrical stimuli (with a magnitude equivalent of their perceptual threshold) delivered at various time points before speaking and during a control condition (silent reading). Results We found that the overall detection ratio in the silent reading condition remained unchanged relative to the detection ratio at rest. Approximately 536 ms before speech onset, the detection ratio in the speaking condition was similar to that in the silent reading condition; however, the detection ratio in the speaking condition gradually started to decrease and reached its lowest level at 58 ms before speech onset. Conclusions Overall, we provided compelling behavioral evidence that, as the speech motor system prepares speech movements, it also modulates the orofacial somatosensory system in a temporally specific manner.

Figure 1.

This study consisted of two experiments, and the blocks of the two experiments were distributed throughout the experimental session (A). In the first experiment (B), we examined the stability of each participant's somatosensation of electrical current stimuli (i.e., perceptual threshold). We used a standard staircase method (one-up, four-down) to find each participant's 85% perceptual threshold during a rest condition. The average of the perceptual thresholds in the first two blocks of the first experiment was used in the second experiment. The goal of the second experiment was to examine somatosensory modulation before speaking and during silent reading (C). Each block of the second experiment consisted of speaking and silent reading trials (in random order). In each trial of the second experiment, a participant-specific electrical stimulus (D) was applied at a randomly selected time point (0.7, 0.85, 1,1.15, 1.3, or 1.45 s), and participants were instructed to indicate whether or not they perceived the stimulus.

Figure 2.

Average group and individual participants' perceptual thresholds at rest across all five blocks of the first experiment. The perceptual threshold in the last block was statistically significantly higher (p = .021) than that in the first block. All other pairwise comparisons between different blocks were not statistically significant (p > .228 in all cases). We used the average of perceptual thresholds of the first two blocks as the stimulation level in the speaking and reading conditions. Therefore, we averaged the data in the first two blocks and reanalyzed the reduced data set using a linear mixed-effects model. Our new analysis did not reveal a statistically significant effect of block (p = .062), suggesting that the measured perceptual threshold in the first two blocks remained stable throughout the study (“n.s.” corresponds to nonsignificant).

Figure 3.

Average group and individual participants' detection ratios in the silent reading condition and the speaking condition at various time points relative to the presentation of the target words. Detection ratios in the silent reading condition were similar at various time points and close to the detection threshold at rest (i.e., 85%); however, detection ratios during speech planning gradually decreased, with the largest decrease occurring during speech movement production. The green histogram shows the distribution of speech onset, and the pink histogram shows the distribution of speech offset. The distributions are based on all speaking trials of all participants. The p values correspond to between-condition comparisons at various time points (corrected for multiple comparisons), and “n.s.” corresponds to nonsignificant. Error bars correspond to ±1 standard error.

Figure 4.

To more accurately examine the time course of the modulation of the somatosensory system (i.e., decrease in detection ratio in the speaking condition) during speech planning, we recalculated detection ratios relative to the speech onset in each speaking trial. The average group and individual participants' detection ratios in the speaking condition are represented with different colors at different time points relative to speech onset. The pink distribution corresponds to the distribution of speech offset calculated based on all speaking trials of all participants. We found that the detection ratio at the first time point (farthest time point before speech onset; M = 79.366%, SD = 14.610%) was statistically significantly greater than detection ratios at all other time points except the second time point (p < .001 in all cases). The p values correspond to a comparison between detection ratios at each time point and the detection ratio at rest (corrected for multiple comparisons), and “n.s.” corresponds to nonsignificant. Error bars correspond to ±1 standard error.