The role of motor inhibition during covert speech production

Abstract

Covert speech is accompanied by a subjective multisensory experience with auditory and kinaesthetic components. An influential hypothesis states that these sensory percepts result from a simulation of the corresponding motor action that relies on the same internal models recruited for the control of overt speech. This simulationnist view raises the question of how it is possible to imagine speech without executing it. In this perspective, we discuss the possible role(s) played by motor inhibition during covert speech production. We suggest that considering covert speech as an inhibited form of overt speech maps naturally to the purported progressive internalisation of overt speech during childhood. However, we argue that the role of motor inhibition may differ widely across different forms of covert speech (e.g., condensed vs. expanded covert speech) and that considering this variety helps reconciling seemingly contradictory findings from the neuroimaging literature.

Motor inhibition prevents motor execution during typing imagery: evidence from an action‐mode switching paradigm

“…Results from this analysis showed that motor inhibition may have different effects on patterns of RTs and MTs, suggesting that it may affect different aspects of the latent activation function (i.e., the shape of the activation function or the motor execution threshold). More precisely, the shortening of RTs (jointly observed with the lengthening of MTs) following imagined trials is compatible with a decrease in the motor execution threshold (as reported in Table 3), whereas the lengthening of both RTs and MTs following imagined trials (as observed in previous studies) is compatible with a delay in the peak time of the activation function (as previously suggested by Nalborczyk et al, 2024).…”

“…Parameter estimates obtained by fitting the algorithmic model to these data suggest that these differences may be due to different underlying inhibitory mechanisms. More precisely, the shortening of RTs (jointly observed with the lengthening of MTs) following imagined trials is compatible with a decrease in the motor execution threshold (as reported in Table 3), whereas the lengthening of both RTs and MTs following imagined trials is compatible with a delay in the peak time of the activation function (as previously suggested by Nalborczyk et al, 2024).…”

“…However, these raw contrasts may be inconclusive about the mechanistic implementation. For instance, motor imagery could slow down the initiation of subsequent movements either by upregulating the motor execution threshold or by delaying the spread of excitatory inputs (Nalborczyk et al, 2024). We developed an algorithmic model intended to capture alternative inhibitory mechanisms, and to distinguish between modulations of the motor execution threshold and modulations in the shape of the underlying activation function (Nalborczyk et al, 2024).…”

“…From a psychological perspective, 𝐴 − 𝑇 may be related to the vividness of motor imagery percepts, 𝜇 may be related to the speed at which these mental percepts are established, and 𝜎 may be related to their duration (Nalborczyk et al, 2024). In this model, the reaction time is defined as the time at which the activation function crosses the threshold for motor imagery and the movement time is defined as the time "spent" above the threshold (Figure 9) (similar to what has been proposed for conscious access, e.g., Pereira et al, 2022).…”

“…However, these raw contrasts may be inconclusive about the mechanistic implementation. For instance, motor imagery could slow down the initiation of subsequent movements either by upregulating the motor execution threshold or by delaying the spread of excitatory inputs (Nalborczyk et al, 2024). In order to disentangle potentially distinct underlying inhibitory mechanisms, we fitted a novel algorithmic model of motor imagery allowing to infer the effect of inhibition in the previous trial on the underlying timecourse of motor activation during typing imagery in the current trial (see the modelling section).…”