Neural underpinnings of the ‘agent brain’: new evidence from transcranial direct current stimulation

The sense of agency in near and far space

The sense of agency is the ability to recognize that we are the actors of our actions and their consequences. We explored whether and how spatial cues may modulate the agency experience by manipulating the ecological validity of the experimental setup (real-space or computer-based setup) and the distance of the action-outcome (near or far). We tested 58 healthy adults collecting explicit agency judgments and the perceived time interval between movements and outcomes (to quantify the intentional binding phenomenon, an implicit index of agency). Participants show greater implicit agency for voluntary actions when there is a temporal and spatial action-outcome contingency. Conversely, participants reported similar explicit agency for outcomes appearing in the near and far space. Notably, these effects were independent of the ecological validity of the setting. These results suggest that spatial proximity, realistic or illusory, is essential for feeling implicitly responsible for the consequences of our actions.

Facilitation of sensorimotor temporal recalibration mechanisms by cerebellar tDCS in patients with schizophrenia spectrum disorders and healthy individuals

Core symptoms in patients with schizophrenia spectrum disorders (SSD), like hallucinations or ego-disturbances, have been associated with a failure of internal forward models to predict the sensory outcomes of self-generated actions. Importantly, forward model predictions must also be able to flexibly recalibrate to changing environmental conditions, for example to account for additional delays between action and outcome. We investigated whether transcranial direct current stimulation (tDCS) can be used to improve these sensorimotor temporal recalibration mechanisms in patients and healthy individuals. While receiving tDCS on the cerebellum, temporo-parietal junction, supplementary motor area, or sham stimulation, patients with SSD and healthy control participants were repeatedly exposed to delays between actively or passively elicited button presses and auditory outcomes. Effects of this procedure on temporal perception were assessed with a delay detection task. Similar recalibration outcomes and faciliatory effects of cerebellar tDCS on recalibration were observed in SSD and healthy individuals. Our findings indicate that sensorimotor recalibration mechanisms may be preserved in SSD and highlight the importance of the cerebellum in both patients and healthy individuals for this process. They further suggest that cerebellar tDCS could be a promising tool for addressing deficits in action-outcome monitoring and related adaptive sensorimotor processes in SSD.

Neural correlates of an illusionary sense of agency caused by virtual reality

Sense of agency (SoA) is the sensation that self-actions lead to ensuing perceptual consequences. The prospective mechanism emphasizes that SoA arises from motor prediction and its comparison with actual action outcomes, while the reconstructive mechanism stresses that SoA emerges from retrospective causal processing about the action outcomes. Consistent with the prospective mechanism, motor planning regions were identified by neuroimaging studies using the temporal binding (TB) effect, a behavioral measure often linked to implicit SoA. Yet, TB also occurs during passive observation of another's action, lending support to the reconstructive mechanism, but its neural correlates remain unexplored. Here, we employed virtual reality (VR) to modulate such observation-based SoA and examined it with functional magnetic resonance imaging (fMRI). After manipulating an avatar hand in VR, participants passively observed an avatar's "action" and showed a significant increase in TB. The binding effect was associated with the right angular gyrus and inferior parietal lobule, which are critical nodes for inferential and agency processing. These results suggest that the experience of controlling an avatar may potentiate inferential processing within the right inferior parietal cortex and give rise to the illusionary SoA without voluntary action.

Effect of left temporoparietal transcranial direct current stimulation on self-bias effect and retrospective intentional binding paradigm: A randomised, double-blind, controlled study

BACKGROUND

Self-bias effect is expressed as a preferential selection and accelerated perception of self-related sensory information. Intentional binding (IB) is a related phenomenon where the sensory outcome from a voluntary action and the voluntary action itself are perceived to be closer to each other in time in both predictive (voluntary action predicting sensory consequence) and retrospective (sensory consequence features triggering self-related inference) contexts. Recent evidence indicates that self-related visual stimuli can affect retrospective intentional binding (rIB). We aimed to 1) replicate rIB in the auditory context, and 2) investigate the potential role of left temporoparietal junction (l-TPJ), a crucial node for the self-monitoring process, in self-bias effect and intentional binding effect by manipulating l-TPJ activity with neuromodulation [using High-Definition Transcranial Direct Current Stimulation (HD-tDCS)]. We anticipated self-bias and rIB effects to increase with anodal stimulation of l-TPJ in comparison to cathodal-stimulation of l-TPJ.

METHODS

Fourteen, right-handed, healthy participants performed sound-label matching (matching tones to self-and-other labels) and rIB (estimating time interval between a button press and a self/other labelled tone) tasks. Each participant underwent both anodal and cathodal stimulation of l-TPJ in separate sessions (at least 72 h apart). Assignment of HD-tDCS type was random and counter-balanced across participants. Behavioural data was collected at three time points: once at baseline (no-stimulation), and twice after stimulation with HD-tDCS.

RESULTS

Strong self-bias effect was observed across all experimental conditions. Neuromodulation of l-TPJ affected processing of other-labelled tone in the sound-label matching task. rIB was noted in baseline and anodal-HD-tDCS conditions where participants exhibited stronger binding for self-associated stimuli compared to other-associated stimuli.

CONCLUSION

l-TPJ may potentially play a critical role in self-other distinction. This may have possible implications for disorders of self-disturbances like psychosis.

A neurophenomenological fMRI study of a spontaneous automatic writer and a hypnotic cohort

PURPOSE

To evaluate the neurophenomenology of automatic writing (AW) in a spontaneous automatic writer (NN) and four high hypnotizables (HH).

METHODS

During fMRI, NN and the HH were cued to perform spontaneous (NN) or induced (HH) AW, and a comparison task of copying complex symbols, and to rate their experience of control and agency.

RESULTS

Compared to copying, for all participants AW was associated with less sense of control and agency and decreased BOLD signal responses in brain regions implicated in the sense of agency (left premotor cortex and insula, right premotor cortex, and supplemental motor area), and increased BOLD signal responses in the left and right temporoparietal junctions and the occipital lobes. During AW, the HH differed from NN in widespread BOLD decreases across the brain and increases in frontal and parietal regions.

CONCLUSIONS

Spontaneous and induced AW had similar effects on agency, but only partly overlapping effects on cortical activity.

The impact of cerebellar transcranial direct current stimulation (tDCS) on sensorimotor and inter-sensory temporal recalibration

The characteristic temporal relationship between actions and their sensory outcomes allows us to distinguish self- from externally generated sensory events. However, the complex sensory environment can cause transient delays between action and outcome calling for flexible recalibration of predicted sensorimotor timing. Since the neural underpinnings of this process are largely unknown this study investigated the involvement of the cerebellum by means of cerebellar transcranial direct current stimulation (ctDCS). While receiving anodal, cathodal, dual-hemisphere or sham ctDCS, in an adaptation phase, participants were exposed to constant delays of 150 ms between actively or passively generated button presses and visual sensory outcomes. Recalibration in the same (visual outcome) and in another sensory modality (auditory outcome) was assessed in a subsequent test phase during which variable delays between button press and visual or auditory outcome had to be detected. Results indicated that temporal recalibration occurred in audition after anodal ctDCS while it was absent in vision. As the adaptation modality was visual, effects in audition suggest that recalibration occurred on a supra-modal level. In active conditions, anodal ctDCS improved sensorimotor recalibration at the delay level closest to the adaptation delay, suggesting a precise cerebellar-dependent temporal recalibration mechanism. In passive conditions, the facilitation of inter-sensory recalibration by anodal ctDCS was overall stronger and tuned to larger delays. These findings point to a role of the cerebellum in supra-modal temporal recalibration across sensorimotor and perceptual domains, but the differential manifestation of the effect across delay levels in active and passive conditions points to differences in the underlying mechanisms depending on the availability of action-based predictions. Furthermore, these results suggest that anodal ctDCS can be a promising tool for facilitating effects of temporal recalibration in sensorimotor and inter-sensory contexts.

Sense of agency disturbances in movement disorders: A comprehensive review

Sense of agency refers to the experience that one's self-generated action causes an event in the external environment. Here, we review the behavioural and brain evidence of aberrant experiences of agency in movement disorders, clinical conditions characterized by either a paucity or an excess of movements unrelated to the patient's intention. We show that specific abnormal agency experiences characterize several movement disorders. Those manifestations are typically associated with structural and functional brain abnormalities. However, the evidence is sometimes conflicting, especially when considering results obtained through different agency measures. The present review aims to create order in the existing literature on sense of agency investigations in movement disorders and to provide a coherent overview framed within current neurocognitive models of motor awareness.

Predicting the Sensory Consequences of Self-Generated Actions: Pre-Supplementary Motor Area as Supra-Modal Hub in the Sense of Agency Experience

Sense of agency refers to the feeling that one's self-generated action caused an external environment event. In a previous study, we suggested that the supplementary motor area (SMA), in its anterior portion (pre-SMA), is a key structure for attributing the sense of agency for the visual consequences of self-generated movements. However, real-life actions can lead to outcomes in different sensory modalities, raising the question of whether SMA represents a supra-modal hub for the sense of agency. Here, we compared the agency experience for visual and auditory outcomes by taking advantage of the intentional binding effect (IB). We observed discrete time-windows for the agency manifestation across different sensory modalities: While there was an IB at 200 ms delay between the action and the visual outcome, a time compression was observed when the auditory outcome followed the action by 400 ms. The magnitude of the IB was mirrored by meaningful brain activity in the pre-SMA but only at the specific delay when a sizeable IB was seen. We conclude that attributing consequences of self-generated movements to one's action is based on similar mechanisms across sensory modalities and that those mechanisms are related to the functioning of the motor system.

Transcranial direct current stimulation improves action-outcome monitoring in schizophrenia spectrum disorder

Patients with schizophrenia spectrum disorder often demonstrate impairments in action-outcome monitoring. Passivity phenomena and hallucinations, in particular, have been related to impairments of efference copy-based predictions which are relevant for the monitoring of outcomes produced by voluntary action. Frontal transcranial direct current stimulation has been shown to improve action-outcome monitoring in healthy subjects. However, whether transcranial direct current stimulation can improve action monitoring in patients with schizophrenia spectrum disorder remains unknown. We investigated whether transcranial direct current stimulation can improve the detection of temporal action-outcome discrepancies in patients with schizophrenia spectrum disorder. On 4 separate days, we applied sham or left cathodal/right anodal transcranial direct current stimulation in a randomized order to frontal (F3/F4), parietal (CP3/CP4) and frontoparietal (F3/CP4) areas of 19 patients with schizophrenia spectrum disorder and 26 healthy control subjects. Action-outcome monitoring was assessed subsequent to 10 min of sham/transcranial direct current stimulation (1.5 mA). After a self-generated (active) or externally generated (passive) key press, subjects were presented with a visual outcome (a dot on the screen), which was presented after various delays (0-417 ms). Participants had to detect delays between the key press and the visual consequence. Symptom subgroups were explored based on the presence or absence of symptoms related to a paranoid-hallucinatory syndrome. In general, delay-detection performance was impaired in the schizophrenia spectrum disorder compared to the healthy control group. Interaction analyses showed group-specific (schizophrenia spectrum disorder versus healthy control group) and symptom-specific (with/without relevant paranoid-hallucinatory symptoms) transcranial direct current stimulation effects. Post hoc tests revealed that frontal transcranial direct current stimulation improved the detection of long delays in active conditions and reduced the proportion of false alarms in undelayed trials of the passive condition in patients. The patients with no or few paranoid-hallucinatory symptoms benefited especially from frontal transcranial direct current stimulation in active conditions, while improvement in the patients with paranoid-hallucinatory symptoms was predominantly reflected in reduced false alarm rates in passive conditions. These data provide some first evidence for the potential utility of transcranial direct current stimulation in improving efference copy mechanisms and action-outcome monitoring in schizophrenia spectrum disorder. Current data indicate that improving efference copy-related processes can be especially effective in patients with no or few positive symptoms, while intersensory matching (i.e. task-relevant in passive conditions) could be more susceptible to improvement in patients with paranoid-hallucinatory symptoms.

Multi-session Transcranial Direct Current Stimulation Over Primary Motor Cortex Facilitates Sequence Learning, Chunking, and One Year Retention

Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) can facilitate motor learning, but it has not been established how stimulation to other brain regions impacts online and offline motor sequence learning, as well as long-term retention. Here, we completed three experiments comparing the effects of tDCS and sham stimulation to the prefrontal cortex (PFC), M1, and the supplementary motor area complex to understand the contributions of these brain regions to motor sequence learning. In Experiment 1, we found that both left and right PFC tDCS groups displayed a slowing in learning in both reaction time and number of chunks, whereas stimulation over M1 improved both metrics over the course of three sessions. To better understand the sequence learning impairment of left PFC anodal stimulation, we tested a left PFC cathodal tDCS group in Experiment 2. The cathodal group demonstrated learning impairments similar to the left PFC anodal stimulation group. In Experiment 3, a subset of participants from the left PFC, M1, and sham tDCS groups of Experiment 1 returned to complete a single session without tDCS on the same sequences assigned to them 1 year previously. We found that the M1 tDCS group reduced reaction time at a faster rate relative to the sham and left PFC groups, demonstrating faster relearning after a one-year delay. Thus, our findings suggest that, regardless of the polarity of stimulation, tDCS to PFC impairs sequence learning, whereas stimulation to M1 facilitates learning and relearning, especially in terms of chunk formation.

Agency and responsibility over virtual movements controlled through different paradigms of brain-computer interface

KEY POINTS

Embodiment of a virtual body was induced and its movements were controlled by two different brain-computer interface (BCI) paradigms - one based on signals from sensorimotor versus one from visual cortical areas. BCI-control of movements engenders agency, but not equally for all paradigms. Cortical sensorimotor activation correlates with agency and responsibility. This has significant implications for neurological rehabilitation and neuroethics.

ABSTRACT

Agency is the attribution of an action to the self and is a prerequisite for experiencing responsibility over its consequences. Here we investigated agency and responsibility by studying the control of movements of an embodied avatar, via brain-computer interface (BCI) technology, in immersive virtual reality. After induction of virtual body ownership by visuomotor correlations, healthy participants performed a motor task with their virtual body. We compared the passive observation of the subject's 'own' virtual arm performing the task with (1) the control of the movement through activation of sensorimotor areas (motor imagery) and (2) the control of the movement through activation of visual areas (steady-state visually evoked potentials). The latter two conditions were carried out using a BCI and both shared the intention and the resulting action. We found that BCI-control of movements engenders the sense of agency, which is strongest for sensorimotor area activation. Furthermore, increased activity of sensorimotor areas, as measured using EEG, correlates with levels of agency and responsibility. We discuss the implications of these results for the neural basis of agency.

Externalization Errors of Olfactory Source Monitoring in Healthy Controls-An fMRI Study

Using a combined approach of functional magnetic resonance imaging (fMRI) and noninvasive brain stimulation (transcranial direct current stimulation [tDCS]), the present study investigated source memory and its link to mental imagery in the olfactory domain, as well as in the auditory domain. Source memory refers to the knowledge of the origin of mental experiences, differentiating events that have occurred and memories of imagined events. Because of a confusion between internally generated and externally perceived information, patients that are prone to hallucinations show decreased source memory accuracy; also, vivid mental imagery can lead to similar results in healthy controls. We tested source memory following cathodal tDCS stimulation using a mental imagery task, which required participants to perceive or imagine a set of the same olfactory and auditory stimuli during fMRI. The supplementary motor area (SMA) is involved in mental imagery across different modalities and potentially linked to source memory. Therefore, we attempted to modulate participants' SMA activation before entering the scanner using tDCS to influence source memory accuracy in healthy participants. Our results showed the same source memory accuracy between the olfactory and auditory modalities with no effects of stimulation. Finally, we found SMA's subregions differentially involved in olfactory and auditory imagery, with activation of dorsal SMA correlated with auditory source memory.

Who hit the ball out? An egocentric temporal order bias

Temporal order judgments can require integration of self-generated action events and external sensory information. We examined whether conscious experience is biased to perceive one's own action events to occur before simultaneous external events, such as deciding whether you or your opponent last touched a basketball heading out of bounds. Participants made temporal order judgments comparing their own touch to another participant's touch, a mechanical touch, or an auditory click. In all three manipulations, we find a robust bias to perceive self-generated action events to occur about 50 ms before external sensory events. We denote this bias to perceive self-actions earlier as the "egocentric temporal order" bias. Thus, if two players hit a ball nearly simultaneously, then both will likely have different subjective experiences of who touched last, leading to arguments.

Verbal long-term memory is enhanced by retrieval practice but impaired by prefrontal direct current stimulation

Retrieval practice involves repeatedly testing a student during the learning experience, reliably conferring learning advantages relative to repeated study. Transcranial direct current stimulation (tDCS) of the left dorsolateral prefrontal cortex (dlPFC) has also been shown to confer learning advantages for verbal memory, though research is equivocal. The present study examined the effects of retrieval versus study practice with or without left dlPFC tDCS on verbal episodic memory. Participants (N = 150) experienced either retrieval practice or study practice, and active anodal, active cathodal, or sham tDCS while encoding word lists, and then returned two days later for a final recall test. Three primary patterns emerged: first, during encoding, tDCS did not influence recall rates in the retrieval practice group. Second, during final recall, participants in the retrieval practice groups recalled more than those in the study practice groups. Finally, during final recall, anodal tDCS decreased recall relative to sham and cathodal stimulation, suggesting that it interfered with developing highly detailed memories that could be relied upon for subsequent recollection. Data support existing research demonstrating the effectiveness of retrieval practice as a learning strategy, but also suggest that anodal dlPFC stimulation can induce long-term negative impacts on verbal episodic memory retrieval.

The Agent Brain: A Review of Non-invasive Brain Stimulation Studies on Sensing Agency

According to philosophy of mind and neuroscientific models, the sense of agency can be defined as the sense that I am the one that is generating an action and causing its effects. Such ability to sense ourselves as causal agents is critical for the definition of intentional behavior and is a primary root for human interaction skills. The present mini-review aims at discussing evidences from non-invasive brain stimulation (NIBS) studies targeting functional correlates of different aspects of agency and evidences on the way stimulation techniques affect such core feature of human subjective experience. Clinical and brain imaging studies helped in defining a neural network mediating agency-related processes, which includes the dorsolateral prefrontal cortex (dlPFC), the cingulate cortex (CC), the supplementary and pre-supplementary motor areas (SMA and pre-SMA), the posterior parietal cortex (PPC) and its inferior regions and the cerebellum. However, while the plurality of those structures mirrors the complexity of the phenomenon, their actual roles with respect to different components of the experience of agency have been primarily explored via correlational techniques, without a clear evidence about their causal significance with respect to the integration of sensorimotor information, intentionalization, and action monitoring processes. Therefore, insights into the specific causal role of different cortical structures can be specified by using NIBS techniques, in order to provide improved understanding into the bases of our ability vs. inability to properly act in complex social contexts.

Hemispheric differences in the processing of visual consequences of active vs. passive movements: a transcranial direct current stimulation study

Perceiving the sensory consequences of one's own actions is essential to successfully interact with the environment. Previous studies compared self- (active) and externally generated (passive) movements to investigate the processing of voluntary action-outcomes. Increased temporal binding (intentional binding) as well as increased detection of delays between action and outcome have been observed for active compared to passive movements. Using transcranial direct stimulation (tDCS) it has been shown that left hemispheric anodal stimulation decreased the intentional binding effect. However, whether the left hemisphere contributes to delay detection performance between action and outcome is unknown. We investigated polarization-dependent effects of left and right frontoparietal tDCS on detecting temporal action-outcome discrepancies. We applied anodal and cathodal stimulation to frontal (F3/F4), parietal (CP3/CP4) and frontoparietal (F3/CP4) areas. After stimulation, participants were presented with visual feedback with various delays after a key press. They had to report whether they detected a delay between the key press and the feedback. In half of the trials the key press was self-initiated, in the other half it was externally generated. A main effect of electrode location indicated highest detection performance after frontal stimulation. Furthermore, we found that the advantage for active versus passive conditions was larger for left hemispheric anodal stimulation as compared to cathodal stimulation. Whereas the frontal cortex is related to delay detection performance in general, hemispheric differences seem to support the differentiation of self-initiated versus externally generated movement consequences.

Intentional binding as a marker of agency across the lifespan

The feeling of control over actions and their external effects is known as Sense of Agency (SoAg). People usually have a distinctive SoAg for events caused by their own actions. However, if the agent is a child or an older person, this feeling of being responsible for the consequences of an action may differ from what an adult would feel. The idea would be that children and elderly may have a reduced SoAg since their frontal lobes are developing or have started to loose their efficiency. The aim of this study was to elucidate whether the SoAg changes across lifespan, using the Intentional Binding (i.e., the temporal attraction between a voluntary action and its sensory consequence) as implicit measure. Data show that children and elderly are characterized by a reduced SoAg as compared to adults. These findings provide a fundamental step in the characterization of SoAg dynamics throughout individuals' lifetime.

Bioelectrical brain effects of one's own voice identification in pitch of voice auditory feedback

Control of voice fundamental frequency (F0) relies in part on comparison of the intended F0 level and auditory feedback. This comparison impacts "sense of agency", or SoA, commonly defined as being the agent of one's own actions and plays a key role for self-awareness and social interactions. SoA is aberrant in several psychiatric disorders. Knowledge about brain activity reflecting SoA can be used in clinical practice for these disorders. It was shown that perception of voice feedback as one's own voice, reflecting the recognition of SoA, alters auditory sensory processing. Using a voice perturbation paradigm we contrasted vocal and bioelectrical brain responses to auditory stimuli that differed in magnitude: 100 and 400 cents. Results suggest the different magnitudes were perceived as a pitch error in self-vocalization (100 cents) or as a pitch shift generated externally (400 cents). Vocalizations and neural responses to changes in pitch of self-vocalization were defined as those made to small magnitude pitch-shifts (100 cents) and which did not show differential neural responses to upward versus downward changes in voice pitch auditory feedback. Vocal responses to large magnitude pitch shifts (400 cents) were smaller than those made to small pitch shifts, and neural responses differed according to upwards versus downward changes in pitch. Our results suggest that the presence of SoA for self-produced sounds may modify bioelectrical brain responses reflecting differences in auditory processing of the direction of a pitch shift. We suggest that this modification of bioelectrical response can be used as a biological index of SoA. Possible neuronal mechanisms of this modification of bioelectrical brain response are discussed.

Inhibition of Pre-Supplementary Motor Area by Continuous Theta Burst Stimulation Leads to More Cautious Decision-making and More Efficient Sensory Evidence Integration

Decisions are made based on the integration of available evidence. The noise in evidence accumulation leads to a particular speed-accuracy tradeoff in decision-making, which can be modulated and optimized by adaptive decision threshold setting. Given the effect of pre-SMA activity on striatal excitability, we hypothesized that the inhibition of pre-SMA would lead to higher decision thresholds and an increased accuracy bias. We used offline continuous theta burst stimulation to assess the effect of transient inhibition of the right pre-SMA on the decision processes in a free-response two-alternative forced-choice task within the drift diffusion model framework. Participants became more cautious and set higher decision thresholds following right pre-SMA inhibition compared with inhibition of the control site (vertex). Increased decision thresholds were accompanied by an accuracy bias with no effects on post-error choice behavior. Participants also exhibited higher drift rates as a result of pre-SMA inhibition compared with the vertex inhibition. These results, in line with the striatal theory of speed-accuracy tradeoff, provide evidence for the functional role of pre-SMA activity in decision threshold modulation. Our results also suggest that pre-SMA might be a part of the brain network associated with the sensory evidence integration.

Physiology of free will

Free will is a perception that people have that they choose to make their movements. This perception includes a sense of willing the movement and self-agency that they are responsible for the movement. If there is a "free will force" that plays a role in movement selection, it should precede movement. There is no evidence for a driving force, and the perception of willing is not fully processed until after the movement. The perceptions of free will likely arise from an interaction between frontal and parietal areas. Free will might be considered to exist if a person's brain is functioning normally without coercion. Ann Neurol 2016;80:5-12.