A TMS study on non-consciously triggered response tendencies in the motor cortex

Motor potentials evoked by transcranial magnetic stimulation: interpreting a simple measure of a complex system

Transcranial magnetic stimulation (TMS) is a non-invasive technique that is increasingly used to study the human brain. One of the principal outcome measures is the motor-evoked potential (MEP) elicited in a muscle following TMS over the primary motor cortex (M1), where it is used to estimate changes in corticospinal excitability. However, multiple elements play a role in MEP generation, so even apparently simple measures such as peak-to-peak amplitude have a complex interpretation. Here, we summarize what is currently known regarding the neural pathways and circuits that contribute to the MEP and discuss the factors that should be considered when interpreting MEP amplitude measured at rest in the context of motor processing and patients with neurological conditions. In the last part of this work, we also discuss how emerging technological approaches can be combined with TMS to improve our understanding of neural substrates that can influence MEPs. Overall, this review aims to highlight the capabilities and limitations of TMS that are important to recognize when attempting to disentangle sources that contribute to the physiological state-related changes in corticomotor excitability.

Working memory load reduces corticospinal suppression to former go and trained no-go cues

Environmental cues associated with an action can prime the motor system, decreasing response times and activating motor regions of the brain. However, when task goals change, the same responses to former go-associated cues are no longer required and motor priming needs to be inhibited to avoid unwanted behavioural errors. The present study tested whether the inhibition of motor system activity to presentations of former go cues is reliant on top-down, goal-directed cognitive control processes using a working memory (WM) load manipulation. Applying transcranial magnetic stimulation over the primary motor cortex to measure motor system activity during a Go/No-go task, we found that under low WM, corticospinal excitability was suppressed to former go and trained no-go cues relative to control cues. Under high WM, the cortical suppression to former go cues was reduced, suggesting that the underlying mechanism required executive control. Unexpectedly, we found a similar result for trained no-go cues and showed in a second experiment that the corticospinal suppression and WM effects were unrelated to local inhibitory function as indexed by short-interval intracortical inhibition. Our findings reveal that the interaction between former response cues and WM is complex and we discuss possible explanations of our findings in relation to models of response inhibition.

Motor Memory: Revealing Conditioned Action Tendencies Using Transcranial Magnetic Stimulation

Action tendencies can be elicited by motivationally salient stimuli (e.g., appetitive rewards) or objects that support utilization behaviors. These action tendencies can benefit behavioral performance through speeded RTs in response tasks and improve detection accuracy in attentional capture tasks. However, action tendencies can be counterproductive when goals change (e.g., refraining from junk foods or abstaining from alcohol). Maintaining control over cue-elicited action tendencies is therefore critical for successful behavior modification. To better understand this relationship, we used transcranial magnetic stimulation to investigate the neural signatures of action tendencies in the presence of previously trained response cues. Participants were presented with a continuous letter stream and instructed to respond quickly to two target letters using two different response keys. Following this training phase, the target letters were embedded in a new task (test phase), and we applied transcranial magnetic stimulation to the motor cortex and measured motor evoked potentials as an index of corticospinal excitability (CSE). We found that CSE could be potentiated by a former response cue trained within a single experimental session, even when participants were instructed to withhold responses during the test phase. Critically, attention to the previously trained response cue was required to elicit the primed modulation in CSE, and successful control of this activity was accompanied by CSE suppression. These findings suggest that well-trained response cues can come to prime a conditioned action tendency and provide a model for understanding how the implementation of cognitive control can override action automaticity.

Parallel processing streams for motor output and sensory prediction during action preparation

Sensory consequences of one's own actions are perceived as less intense than identical, externally generated stimuli. This is generally taken as evidence for sensory prediction of action consequences. Accordingly, recent theoretical models explain this attenuation by an anticipatory modulation of sensory processing prior to stimulus onset (Roussel et al. 2013) or even action execution (Brown et al. 2013). Experimentally, prestimulus changes that occur in anticipation of self-generated sensations are difficult to disentangle from more general effects of stimulus expectation, attention and task load (performing an action). Here, we show that an established manipulation of subjective agency over a stimulus leads to a predictive modulation in sensory cortex that is independent of these factors. We recorded magnetoencephalography while subjects performed a simple action with either hand and judged the loudness of a tone caused by the action. Effector selection was manipulated by subliminal motor priming. Compatible priming is known to enhance a subjective experience of agency over a consequent stimulus (Chambon and Haggard 2012). In line with this effect on subjective agency, we found stronger sensory attenuation when the action that caused the tone was compatibly primed. This perceptual effect was reflected in a transient phase-locked signal in auditory cortex before stimulus onset and motor execution. Interestingly, this sensory signal emerged at a time when the hemispheric lateralization of motor signals in M1 indicated ongoing effector selection. Our findings confirm theoretical predictions of a sensory modulation prior to self-generated sensations and support the idea that a sensory prediction is generated in parallel to motor output (Walsh and Haggard 2010), before an efference copy becomes available.

The negative compatibility effect with relevant masks: a case for automatic motor inhibition

For many years controversy has surrounded the so-called "negative compatibility effect" (NCE), a surprising phenomenon whereby responses to a target stimulus are delayed when the target is preceded by an unconscious, response-compatible prime. According to proponents of the "self-inhibition" hypothesis, the NCE occurs when a low-level self-inhibitory mechanism supresses early motor activations that are no longer supported by perceptual evidence. This account has been debated, however, by those who regard the NCE to be a stimulus-specific phenomenon that can be explained without recourse to a self-inhibitory mechanism. The present study used a novel reach-to-touch paradigm to test whether unconscious response priming would manifest as motor activation of the opposite-to-prime response (supporting mask-induced priming accounts), or motor inhibition of the primed response (supporting the notion of low-level self-inhibition). This paper presents new findings that show the emergence of positive and negative compatibility effects as they occur in stimulus processing time. In addition, evidence is provided suggesting that the NCE is not driven by the activation of the incorrect, "opposite-to-prime" response, but rather might reflect automatic motor inhibition.

Subliminal priming with nearly perfect performance in the prime-classification task

The subliminal priming paradigm is widely used by cognitive scientists, and claims of subliminal perception are common nowadays. Nevertheless, there are still those who remain skeptical. In a recent critique of subliminal priming, Pratte and Rouder (Attention, Perception, & Psychophysics, 71, 1276-1283, 2009) suggested that previous claims of subliminal priming may have been due to a failure to control the task difficulty between the experiment proper and the prime-classification task. Essentially, because the prime-classification task is more difficult than the experiment proper, the prime-classification task results may underrepresent the subjects' true ability to perceive the prime stimuli. To address this possibility, prime words were here presented in color. In the experiment proper, priming was observed. In the prime-classification task, subjects reported the color of the primes very accurately, indicating almost perfect control of task difficulty, but they could not identify the primes. Thus, I conclude that controlling for task difficulty does not eliminate subliminal priming.

The negative compatibility effect: A case for self-inhibition

In masked priming, a briefly presented prime stimulus is followed by a mask, which in turn is followed by the task-relevant target. Under certain conditions, negative compatibility effects (NCNCEs) occur, with impaired performance on compatible trials (where prime and target indicate the same response) relative to incompatible trials (where they indicate opposite responses). However, the exact boundary conditions of NCEs, and hence the functional significance of this effect, are still under discussion. In particular, it has been argued that the NCE might be a stimulus-specific phenomenon of little general interest. This paper presents new findings indicating that the NCE can be obtained under a wider variety of conditions, suggesting that it reflects more general processes in motor control. In addition, evidence is provided suggesting that prime identification levels in forced choice tasks - usually employed to estimate prime visibility in masked prime tasks - are affected by prior experience with the prime (Exp. 1) as well as by direct motor priming (Exp. 2 & 3).

Mask- and distractor-triggered inhibitory processes in the priming of motor responses: an EEG study

Masked stimuli presented near the threshold of conscious awareness may act as primes, affecting responses to subsequent targets. With arrows as primes and targets, the prime-mask-target sequence has been shown to evoke triphasic lateralized readiness potentials (LRP) with two phases of imbalance between hemispheres preceding the target-related contralateral preponderance of negativity: first a negative, then a positive preponderance contralateral to prime direction. The present article provides evidence that the second wave is related to mask presentation and reflects inhibitory processing independent of reductions in prime visibility, even being evoked by nonmasking distractors that leave the prime fully accessible for consciousness. Of all hypotheses put forward to account for inverse effects in masked priming, this finding is most compatible with the mask-triggered inhibition hypothesis suggested by P. Jaśkowski (2007).

Modifying the Cortical Processing for Motor Preparation by Repetitive Transcranial Magnetic Stimulation

To investigate the effects of repetitive transcranial magnetic stimulation (rTMS) on the central processing of motor preparation, we had subjects perform a precued-choice reaction time (RT) task. They had to press one of two buttons as quickly as possible after a go signal specifying both the hand to be used and the button to press. A precue preceding this signal conveyed full, partial, or no advance information (hand and/or button), such that RT shortened with increasing amount of information. We applied 1200 to 2400 pulses of 1-Hz rTMS over various cortical areas and compared the subjects' performances at various times before and after this intervention. rTMS delayed RT at two distinct phases after stimulation, one within 10 min and another with a peak at 20 to 30 min and lasting for 60 to 90 min, with no significant effects on error rates or movement time. The effect was significantly larger on left- than on right-hand responses. RT was prominently delayed over the premotor and motor cortices with similar effects across different conditions of advance information, suggesting that preparatory processes relatively close to the formation of motor output were influenced by rTMS. In contrast, the effect of rTMS over the supplementary motor area and the anterior parietal cortex varied with the amount of advance information, indicating specific roles played by these areas in integrating target and effector information. The primary motor area, especially of the left hemisphere, may take over this processing, implementing motor output based on the information processed in other areas.