Supplementary motor area plays a causal role in automatic inhibition of motor responses

BACKGROUND

The masked-priming paradigm is used to test unconscious inhibitory processes of the brain. A tendency towards responses that are incompatible with the prime, designated as negative compatibility effect (NCE), emerges when the perception of a priming visual stimulus is "masked" afterwards. This effect presumably stems from a subliminal inhibitory process against the masked-prime. Prior lesions as well as activation studies suggest a key role of SMA in this effect.

OBJECTIVE

This study was conducted to elucidate a causal role of SMA in the subliminal response inhibition represented by the NCE.

METHODS

Using a repeated-measures pre-post design with a group of healthy people, physiological measures (resting and active motor thresholds and motor evoked potential (MEP) amplitude) and behavioral ones (choice reaction time (CRT), positive compatibility effect (PCE) and NCE) were obtained before and after three quadripulse stimulation (QPS), namely sham, M1-QPS, and SMA-QPS, on different days. CRT and PCE served as indices for different aspects of motor execution.

RESULTS

Motor thresholds were not altered after any QPS, although the M1-QPS increased MEP amplitude. Neither CRT nor PCE was altered significantly after QPS protocols. NCE was abolished after the SMA-QPS.

CONCLUSIONS

Abolished NCE after the SMA-QPS in the absence of MEP changes suggests that (1) SMA plays a cardinal role in the NCE, and (2) the network involved in NCE is different from that of MEP generation.

The role of response readiness in subliminal visuomotor processes

The present study aims to examine the impact of response readiness on visuomotor processes triggered by subliminal stimuli using a mixed paradigm involving the masked prime paradigm and a foreperiod paradigm. Experiment 1 ensured that response readiness was successfully manipulated in the mixed paradigm. Importantly, Experiment 2 found that the negative compatibility effect (NCE; a behavioral indicator of subliminal visuomotor processes) disappeared and that response time lost its power to modulate the compatibility effect (CE) with reduced response readiness (due to temporal uncertainty). These results of CEs both independent of response latency and across different levels of response latency indicate that response readiness is a prerequisite for obtaining the NCE. The findings suggest that automatic processing of subliminal stimuli is susceptible to top-down control for reducing the interference of irrelevant information, which ensures a high degree of adaptability and flexibility of our cognitive system in interactions with the changing environment.

Unconscious priming dissociates 'free choice' from 'spontaneous urge' responses

Advances in neuroscience offer the exciting prospect of understanding 'free' choices - the subject of the free will debate in philosophy. However, while physiological techniques and analysis have progressed rapidly to meet this challenge, task design has not. The challenge is now to develop laboratory tasks that adequately capture 'free' picking or choosing. To isolate 'internally' generated intentions from those impelled by external stimulus, observers are asked to 'choose freely' or to wait for a felt 'urge'. However, no previous work has explicitly distinguished between instructions that refer to 'urges' versus to 'choosing'. The philosopher Alfred Mele (e.g., 2009; 2014) has argued that the distinction is of crucial conceptual importance, but the two have not yet been empirically distinguished. Here, we show that conscious and unconscious, task-irrelevant primes, bias observers' binary choices when they are instructed to 'choose freely', not when they 'wait for an urge', underscoring the practical importance of Mele's conceptual distinction. Neuroscience must incorporate this distinction if we are to understand processes underpinning free choice.

The availability of attentional resources modulates the inhibitory strength related to weakly activated priming

The current study investigated the role of attention in inhibitory processes (the inhibitory processes described in the current study refer only to those associated with masked or flanked priming) using a mixed paradigm involving the negative compatibility effect (NCE) and object-based attention. Accumulating evidence suggests that attention can be spread more easily within the same object, which increases the availability of attentional resources, than across different objects. Accordingly, we manipulated distractor location (with primes presented in the same object versus presented in different objects) together with prime/target compatibility (compatible versus incompatible) and prime-distractor stimulus onset asynchrony (SOA, 23 ms vs 70 ms). The aim was to investigate whether inhibitory processes related to weakly activated priming, which have been previously assumed to be automatic, depend on the availability of attentional resources. The results of Experiment 1 showed a significant NCE for the 70-ms SOA when the prime and distractor were presented in the same object (greater attentional resource availability); however, reversed NCEs were obtained for all other conditions. Experiment 2 was designed to disentangle whether the results of Experiment 1 were affected by the prime position, and the results indicated that the prime position did not modulate the NCE in Experiment 1. Together, these results are consistent with the claim that the availability of attentional resources modulates the inhibitory strength related to weakly activated priming. Specifically, if attentional resources are assigned to the distractor when it is presented in the same object as the prime, the strength of the inhibition elicited by the distractor may increase and reverse the activation elicited by the prime, which could lead to a significant NCE.

Does response link updating contribute to the negative compatibility effect?

Lleras and Enns (2004) argued that the negative compatibility effect (NCE) may partly originate from object updating (OU) between a prime and mask. This process could occur not only at the feature level, which facilitates target identification via feature updating (the updated feature is identical to the target in the feature), but also at the response level, which benefits target response via response link updating (an updated response link has an identical reaction to the target). This study aims to present experimental evidence for the latter hypothesis that response link updating is one process that triggers an NCE. The design used a 4 (stimuli) to 2 (responses) paradigm in which the left-hand response was assigned to "1" and "2" and the right-hand response was assigned to "3" and "4" (counterbalanced across the participants). Additionally, we manipulated the strength of OU (strong, "1" and "3" as primes versus weak OU condition, "2" and "4" as primes), response set of prime and target (e.g., same, "1" and "3" as both primes and targets versus different response set, "1" and "3" as primes or targets but "2" and "4" as targets or primes), and compatibility (compatible versus incompatible). The results showed a significant NCE in the strong OU (effective object updating) and different response set (separating response link updating from OU process) condition. Combined with the response time quartiles, which showed a special developmental course for this condition, the results suggest that response link updating is one process that triggers an NCE when the prime and target come from different response sets, and the processing of response link updating does not share identical developmental courses as the motor inhibition.

Perceptual and motor contributions to the negative compatibility effect

The current study focused on contributions to the negative compatibility effect (NCE) from object-updating at the perceptual level and self-inhibition (i.e., automatic motor inhibition) at the response (motor) level. We hypothesized that contributions to the NCE from these two levels were moderated by the strength of stimulus and reaction (S-R) link: object-updating should have greater impact on the NCE with weak S-R links, but both object-updating and self-inhibition should impact on the NCE when the S-R links became strong. To test this hypothesis, in two experiments we used a novel type of stimuli and manipulated mask relevance (relevant versus irrelevant mask) and prime/target compatibility. Participants performed two tasks over three successive days. Results showed that under equivalent masking effectiveness between the two conditions of mask, a significant NCE was observed only in the relevant mask condition when the S-R links were weak, but both close to equal NCE sizes were observed in the two mask conditions when the S-R links were acquired through practice. The results indicated that perceptual and motor contributions to the NCE were moderated by the strength of S-R links, if the strength of the links was too weak to trigger the following inhibition, the NCE primarily originated from object-updating at the perceptual level; if the strength of the links was reinforced by practice, which exceeded the inhibitory threshold, the NCE originated from both object-updating and self-inhibition, but the latter was primarily responsible.

Spatial primes produce dissociated inhibitory effects on saccadic latencies and trajectories

In masked priming, a briefly presented prime can facilitate or inhibit responses to a subsequent target. In most instances, targets with an associated response that is congruent with the prime direction speed up reaction times to the target (a positive compatibility effect; PCE). However, under certain circumstances, slower responses for compatible primes are obtained (a negative compatibility effect; NCE). NCEs can be found when a long pre-target delay is used. During the delay, inhibition is assumed to take place, and therefore an effect on saccade trajectories may also be expected. In a previous study, we found the effects of inhibition on response times and trajectories to be dissociated, but this experiment varied the timing of several aspects of the stimulus sequence and it is therefore unclear what caused the dissociation. In the present study, we varied only one aspect of the timing, but replicated the dissociation. By varying just the pre-target delay, we found a PCE for a short delay, and an NCE for a long delay, but saccade trajectories deviated away from prime directions in both conditions. This suggests dissociated inhibitory effects of primes on response times and saccade trajectories.

Subliminal action priming modulates the perceived intensity of sensory action consequences

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In theory, the motor system attenuates action-outcome perception to signal agency.

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We use subliminal motor priming to test for the predicted motor locus of this effect.

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The perceived intensity of an action-outcome is attenuated by compatible priming.

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Compatible priming is known to enhance explicit agency judgements.

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Sensory attenuation and agency inference depend on overlapping motoric signals.

The sense of control over the consequences of one’s actions depends on predictions about these consequences. According to an influential computational model, consistency between predicted and observed action consequences attenuates perceived stimulus intensity, which might provide a marker of agentic control. An important assumption of this model is that these predictions are generated within the motor system. However, previous studies of sensory attenuation have typically confounded motor-specific perceptual modulation with perceptual effects of stimulus predictability that are not specific to motor action. As a result, these studies cannot unambiguously attribute sensory attenuation to a motor locus. We present a psychophysical experiment on auditory attenuation that avoids this pitfall. Subliminal masked priming of motor actions with compatible prime–target pairs has previously been shown to modulate both reaction times and the explicit feeling of control over action consequences. Here, we demonstrate reduced perceived loudness of tones caused by compatibly primed actions. Importantly, this modulation results from a manipulation of motor processing and is not confounded by stimulus predictability. We discuss our results with respect to theoretical models of the mechanisms underlying sensory attenuation and subliminal motor priming.