Motor command inhibition and the representation of response mode during motor imagery

Inequivalent and uncorrelated response priming in motor imagery and execution

Introduction

Theoretical considerations on motor imagery and motor execution have long been dominated by the functional equivalence view. Previous empirical works comparing these two modes of actions, however, have largely relied on subjective judgments on the imagery process, which may be exposed to various biases. The current study aims to re-examine the commonality and distinguishable aspects of motor imagery and execution via a response repetition paradigm. This framework aims to offer an alternative approach devoid of self-reporting, opening the opportunity for less subjective evaluation of the disparities and correlations between motor imagery and motor execution.

Methods

Participants performed manual speeded-choice on prime-probe pairs in each trial under three conditions distinguished by the modes of response on the prime: mere observation (Perceptual), imagining response (Imagery), and actual responses (Execution). Responses to the following probe were all actual execution of button press. While Experiment 1 compared the basic repetition effects in the three prime conditions, Experiment 2 extended the prime duration to enhance the quality of MI and monitored electromyography (EMG) for excluding prime imagery with muscle activities to enhance specificity of the underlying mechanism.

Results

In Experiment 1, there was no significant repetition effect after mere observation. However, significant repetition effects were observed in both imagery and execution conditions, respectively, which were also significantly correlated. In Experiment 2, trials with excessive EMG activities were excluded before further statistical analysis. A consistent repetition effect pattern in both Imagery and Execution but not the Perception condition. Now the correlation between Imagery and Execution conditions were not significant.

Conclusion

Findings from the current study provide a novel application of a classical paradigm, aiming to minimize the subjectivity inherent in imagery assessments while examining the relationship between motor imagery and motor execution. By highlighting differences and the absence of correlation in repetition effects, the study challenges the functional equivalence hypothesis of imagery and execution. Motor representations of imagery and execution, when measured without subjective judgments, appear to be more distinguishable than traditionally thought. Future studies may examine the neural underpinnings of the response repetition paradigm to further elucidating the common and separable aspects of these two modes of action.

Distinct neural mechanisms support inner speaking and inner hearing

Humans have the ability to mentally examine speech. This covert form of speech production is often accompanied by sensory (e.g., auditory) percepts. However, the cognitive and neural mechanisms that generate these percepts are still debated. According to a prominent proposal, inner speech has at least two distinct phenomenological components: inner speaking and inner hearing. We used transcranial magnetic stimulation to test whether these two phenomenologically distinct processes are supported by distinct neural mechanisms. We hypothesised that inner speaking relies more strongly on an online motor-to-sensory simulation that constructs a multisensory experience, whereas inner hearing relies more strongly on a memory-retrieval process, where the multisensory experience is reconstructed from stored motor-to-sensory associations. Accordingly, we predicted that the speech motor system will be involved more strongly during inner speaking than inner hearing. This would be revealed by modulations of TMS evoked responses at muscle level following stimulation of the lip primary motor cortex. Overall, data collected from 31 participants corroborated this prediction, showing that inner speaking increases the excitability of the primary motor cortex more than inner hearing. Moreover, this effect was more pronounced during the inner production of a syllable that strongly recruits the lips (vs. a syllable that recruits the lips to a lesser extent). These results are compatible with models assuming that the primary motor cortex is involved during inner speech and contribute to clarify the neural implementation of the fundamental ability of silently speaking in one's mind.

The Role of Motor Inhibition During Covert Speech Production

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 simulationist 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 internalization of overt speech during childhood. We further 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.

Automatic effects of covert practice

Automatic behaviour is supposedly underlain by the unintentional retrieval of processing episodes, which are stored during the repeated overt practice of a task or activity. In the present study, we investigated whether covertly practicing a task (e.g., repeatedly imagining responding to a stimulus) also leads to the storage of processing episodes and thus to automatic behaviour. Participants first either responded overtly or covertly to stimuli according to a first categorization task in a practice phase. We then measured the presence of automatic response-congruency effects in a subsequent test phase that involved a different categorization task but the same stimuli and responses. Our results indicate that covert practice can lead to a response-congruency effect. We conclude that covert practice can lead to automatic behaviour and discuss the different components of covert practice, such as motor imagery, visual imagery, and inner speech, that contribute to the formation of processing episodes in memory.

Inhibitory mechanisms in motor imagery: disentangling different forms of inhibition using action mode switching

In motor imagery, probably several inhibitory mechanisms prevent actual movements: global inhibition, effector-specific inhibition, and inhibition retrieved during target processing. We investigated factors that may influence those mechanisms. In an action mode switching paradigm, participants imagined and executed movements from home buttons to target buttons. We analysed sequential effects. Activation (due to execution) or inhibition (due to imagination) in the previous trial should affect performance in the subsequent trial, enabling conclusions about inhibitory mechanisms in motor imagery. In Experiment 1, evidence for global and effector-specific inhibition was observed. Evidence for inhibition retrieved during target processing was inconclusive. Data patterns were similar when start and end of the imagined movements were indicated with an effector that was part of the imagined movement (hand) and with a different effector (feet). In Experiment 2, we ruled out that the use of biological stimuli (left/right hands in Experiment 1) to indicate the effector causes sequential effects attributed to effector-specific inhibition, by using uppercase letters (R, L). As in Experiment 1, evidence for effector-specific inhibition was observed. In conclusion, we could reliably disentangle several inhibitory mechanisms in motor imagery.

Probing the temporal dynamics of movement inhibition in motor imagery

Beyond the lack of overt movement in motor imagery (MI), MI is thought to be functionally equivalent to motor execution (ME). Two theories appear viable to explain the neural mechanism underlying the inhibition of movement in MI, with one suggesting the inhibition of movement in MI occurs early in the planning process, and the other suggesting it occurs after the planning for movement is compete. Here we sought to generate evidence related to the timing of movement inhibition in MI. Participants performed a motor task via MI and ME that had distinct preparation and performance phases, with brain activity obtained throughout. Analysis of sensor-level data was performed to isolate event related desynchrony (ERD) in the mu and beta frequency bands in both a sensorimotor and left parietal region of interest (ROI). The magnitude of ERD in the sensorimotor ROI was significantly greater in ME than MI during both the preparatory and performance phases. The reduced ERD in the mu and beta frequency bands in the sensorimotor ROI during the preparatory phase for MI, compared to ME, suggests that movement planning is inhibited (or at least reduced) in MI, contributing to the lack of movement. While past work has shown that the networks of functional brain activity underlying MI and ME are heavily overlapping, differences in the temporal dynamics of this activity suggest that MI and ME are not equivalent processes.

Motor imagery entails task-set inhibition

Motor imagery requires the covert execution of a movement without any overt motor output. Previous studies indicated that motor imagery results in the prolonged inhibition of motor commands. In the present study, we investigated whether motor imagery also leads to the inhibition of more abstract task representations. To do so, we investigated the effect of motor imagery on n - 2 repetition costs, which offer an index of the extent to which task representations are inhibited. Participants switched among three tasks and among two response modes: overt and covert responding (i.e., motor imagery). N - 2 repetition costs were present when the current trial required an overt response but absent when the current trial required a covert response. Furthermore, n - 2 repetition costs were more pronounced when trial n - 1 required a covert response rather than an overt response. This pattern of results suggests that motor imagery also leads to the inhibition of abstract task representations. We discuss our findings in view of current conceptualizations of motor imagery and argue that the inhibitory mechanism entailed by motor imagery targets more than motor commands alone. Finally, we also relate our findings to the mechanisms underlying the inhibition of task representations.