Effects of dopamine and opioid receptor antagonism on the neural processing of social and nonsocial rewards.

Rewards are a broad category of stimuli inducing approach behavior to aid survival. Extensive evidence from animal research has shown that wanting (the motivation to pursue a reward) and liking (the pleasure associated with its consumption) are mostly regulated by dopaminergic and opioidergic activity in dedicated brain areas. However, less is known about the neuroanatomy of dopaminergic and opioidergic regulation of reward processing in humans, especially when considering different types of rewards (i.e., social and nonsocial). To fill this gap of knowledge, we combined dopaminergic and opioidergic antagonism (via amisulpride and naltrexone administration) with functional neuroimaging to investigate the neurochemical and neuroanatomical bases of wanting and liking of matched nonsocial (food) and social (interpersonal touch) rewards, using a randomized, between-subject, placebo-controlled, double-blind design. While no drug effect was observed at the behavioral level, brain activity was modulated by the administered compounds. In particular, opioid antagonism, compared to placebo, reduced activity in the medial orbitofrontal cortex during consumption of the most valued social and nonsocial rewards. Dopamine antagonism, however, had no clear effects on brain activity in response to reward anticipation. These findings provide insights into the neurobiology of human reward processing and suggest a similar opioidergic regulation of the neural responses to social and nonsocial reward consumption.

Cortico-cortical paired associative stimulation (ccPAS) over premotor-motor areas affects local circuitries in the human motor cortex via Hebbian plasticity.

Transcranial magnetic stimulation (TMS) studies have shown that cortico-cortical paired associative stimulation (ccPAS) can strengthen connectivity between the ventral premotor cortex (PMv) and the primary motor cortex (M1) by modulating convergent input over M1 via Hebbian spike-timing-dependent plasticity (STDP). However, whether ccPAS locally affects M1 activity remains unclear. We tested 60 right-handed young healthy humans in two studies, using a combination of dual coil TMS and ccPAS over the left PMv and M1 to probe and manipulate PMv-to-M1 connectivity, and single- and paired-pulse TMS to assess neural activity within M1. We provide convergent evidence that ccPAS, relying on repeated activations of excitatory PMv-to-M1 connections, acts locally over M1. During ccPAS, motor-evoked potentials (MEPs) induced by paired PMv-M1 stimulation gradually increased. Following ccPAS, the threshold for inducing MEPs of different amplitudes decreased, and the input-output curve (IO) slope increased, highlighting increased M1 corticospinal excitability. Moreover, ccPAS reduced the magnitude of short-interval intracortical inhibition (SICI), reflecting suppression of GABA-ergic interneuronal mechanisms within M1, without affecting intracortical facilitation (ICF). These changes were specific to ccPAS Hebbian strengthening of PMv-to-M1 connectivity, as no modulations were observed when reversing the order of the PMv-M1 stimulation during a control ccPAS protocol. These findings expand prior ccPAS research that focused on the malleability of cortico-cortical connectivity at the network-level, and highlight local changes in the area of convergent activation (i.e., M1) during plasticity induction. These findings provide new mechanistic insights into the physiological basis of ccPAS that are relevant for protocol optimization.

Enhanced action performance following TMS manipulation of associative plasticity in ventral premotor-motor pathway.

Skillful goal-directed manual actions such as grasping and manipulating objects are supported by a large sensorimotor network. Within this network, the ventral premotor cortex (PMv) transforms visual information about objects into motor commands that are conveyed to the primary motor cortex (M1), allowing fine control of finger movements. However, it is unknown whether transcranial magnetic stimulation (TMS) of this PMv-to-M1 hierarchical pathway improves action performance. To fill in this gap, here, we used cortico-cortical paired associative stimulation (ccPAS) with the aim of manipulating synaptic efficacy in the human PMv-to-M1 pathway. We found that repeatedly pairing TMS of pre-and post-synaptic nodes of the PMv-to-M1 pathway (i.e., PMv-to-M1 ccPAS) increased motor excitability and enhanced performance on the 9-Hole Peg Test (9-HPT), which taps into PMv-M1 functioning. These effects were specific to the ccPAS protocol consistent with the direction of the PMv-to-M1 hierarchy, as no effects were observed when reversing the order of the paired TMS pulses (i.e., following a M1-to-PMv ccPAS) or when administering sham ccPAS. Additionally, the effect of PMv-to-M1 ccPAS appeared functionally specific, as no behavioral enhancement was observed in a visuomotor control task. We therefore provide novel causal evidence that the PMv-to-M1 pathway, which is instrumental to object-oriented hand actions, is sensitive to TMS manipulations of associative plasticity. Our study highlights the causal role of the PMv-to-M1 pathway in controlling skillful object-oriented hand actions and suggests that ccPAS might be a useful tool for investigating the functional relevance of directional connectivity in humans. These findings may have implications for designing novel therapeutic strategies based on the manipulation of associative plasticity in cortico-cortical networks.

Driving Hebbian plasticity over ventral premotor-motor projections transiently enhances motor resonance.

BACKGROUND: Making sense of others' actions relies on the activation of an action observation network (AON), which maps visual information about observed actions onto the observer's motor system. This motor resonance process manifests in the primary motor cortex (M1) as increased corticospinal excitability finely tuned to the muscles engaged in the observed action. Motor resonance in M1 is facilitated by projections from higher-order AON regions. However, whether manipulating the strength of AON-to-M1 connectivity affects motor resonance remains unclear. METHODS: We used transcranial magnetic stimulation (TMS) in 48 healthy humans. Cortico-cortical paired associative stimulation (ccPAS) was administered over M1 and the ventral premotor cortex (PMv), a key AON node, to induce spike-timing-dependent plasticity (STDP) in the pathway connecting them. Single-pulse TMS assessed motor resonance during action observation. RESULTS: Before ccPAS, action observation increased corticospinal excitability in the muscles corresponding to the observed movements, reflecting motor resonance in M1. Notably, ccPAS aimed at strengthening projections from PMv to M1 (PMv→M1) induced short-term enhancement of motor resonance. The enhancement specifically occurred with the ccPAS configuration consistent with forward PMv→M1 projections and dissipated 20 min post-stimulation; ccPAS administered in the reverse order (M1→PMv) and sham stimulation did not affect motor resonance. CONCLUSIONS: These findings provide the first evidence that inducing STDP to strengthen PMv input to M1 neurons causally enhances muscle-specific motor resonance in M1. Our study sheds light on the plastic mechanisms that shape AON functionality and demonstrates that exogenous manipulation of AON connectivity can influence basic mirror mechanisms that underlie social perception.

Neural Hyperresponsivity During the Anticipation of Tangible Social and Nonsocial Rewards in Autism Spectrum Disorder: A Concurrent Neuroimaging and Facial Electromyography Study.

BACKGROUND: Atypical anticipation of social reward has been shown to lie at the core of the social challenges faced by individuals with autism spectrum disorder (ASD). However, previous research has yielded inconsistent results and has often overlooked crucial characteristics of stimuli. Here, we investigated ASD reward processing using social and nonsocial tangible stimuli, carefully matched on several key dimensions. METHODS: We examined the anticipation and consumption of social (interpersonal touch) and nonsocial (flavored milk) rewards in 25 high-functioning individuals with ASD and 25 neurotypical adult individuals. In addition to subjective ratings of wanting and liking, we measured physical energetic expenditure to obtain the rewards, brain activity with neuroimaging, and facial reactions through electromyography on a trial-by-trial basis. RESULTS: Participants with ASD did not exhibit reduced motivation for social or nonsocial rewards; their subjective ratings, motivated efforts, and facial reactions were comparable to those of neurotypical participants. However, anticipation of higher-value rewards increased neural activation in lateral parietal cortices, sensorimotor regions, and the orbitofrontal cortex. Moreover, participants with ASD exhibited hyperconnectivity between frontal medial regions and occipital regions and the thalamus. CONCLUSIONS: Individuals with ASD who experienced rewards with tangible characteristics, whether social or nonsocial, displayed typical subjective and objective motivational and hedonic responses. Notably, the observed hyperactivations in sensory and attentional nodes during anticipation suggest atypical sensory overprocessing of forthcoming rewards rather than decreased reward value. While these atypicalities may not have manifested in observable behavior here, they could impact real-life social interactions that require nuanced predictions, potentially leading to the misperception of reduced interest in rewarding social stimuli in ASD.

Transcranial cortico-cortical paired associative stimulation (ccPAS) over ventral premotor-motor pathways enhances action performance and corticomotor excitability in young adults more than in elderly adults.

Transcranial magnetic stimulation (TMS) methods such as cortico-cortical paired associative stimulation (ccPAS) can increase the strength of functional connectivity between ventral premotor cortex (PMv) and primary motor cortex (M1) via spike timing-dependent plasticity (STDP), leading to enhanced motor functions in young adults. However, whether this STDP-inducing protocol is effective in the aging brain remains unclear. In two groups of young and elderly healthy adults, we evaluated manual dexterity with the 9-hole peg task before and after ccPAS of the left PMv-M1 circuit. We observed that ccPAS enhanced dexterity in young adults, and this effect was anticipated by a progressive increase in motor-evoked potentials (MEPs) during ccPAS administration. No similar effects were observed in elderly individuals or in a control task. Across age groups, we observed that the magnitude of MEP changes predicted larger behavioral improvements. These findings demonstrate that left PMv-to-M1 ccPAS induces functionally specific improvements in young adults' manual dexterity and an increase in corticomotor excitability, but altered plasticity prevents the effectiveness of ccPAS in the elderly.

Neurophysiological Markers of Premotor-Motor Network Plasticity Predict Motor Performance in Young and Older Adults.

Aging is commonly associated with a decline in motor control and neural plasticity. Tuning cortico-cortical interactions between premotor and motor areas is essential for controlling fine manual movements. However, whether plasticity in premotor-motor circuits predicts hand motor abilities in young and elderly humans remains unclear. Here, we administered transcranial magnetic stimulation (TMS) over the ventral premotor cortex (PMv) and primary motor cortex (M1) using the cortico-cortical paired-associative stimulation (ccPAS) protocol to manipulate the strength of PMv-to-M1 connectivity in 14 young and 14 elderly healthy adults. We assessed changes in motor-evoked potentials (MEPs) during ccPAS as an index of PMv-M1 network plasticity. We tested whether the magnitude of MEP changes might predict interindividual differences in performance in two motor tasks that rely on premotor-motor circuits, i.e., the nine-hole pegboard test and a choice reaction task. Results show lower motor performance and decreased PMv-M1 network plasticity in elderly adults. Critically, the slope of MEP changes during ccPAS accurately predicted performance at the two tasks across age groups, with larger slopes (i.e., MEP increase) predicting better motor performance at baseline in both young and elderly participants. These findings suggest that physiological indices of PMv-M1 plasticity could provide a neurophysiological marker of fine motor control across age-groups.

Anticipatory and Consummatory Responses to Touch and Food Rewards: A Protocol for Human Research.

Understanding the neural basis of reward processing is a major concern, as it holds the key to alleviating symptoms of addiction and poor mental health. However, this goal seems difficult to attain as long as research on reward processing cannot easily be compared across species and reward types, due to methodological differences and the presence of confounding factors. We recently developed an experimental paradigm that allows monitoring anticipatory and consummatory responses to matched social (touch) and nonsocial (food) rewards in adult humans. The following protocol describes in detail the materials and the paradigm, which measures reward wanting and liking with a real effort task and subjective ratings. It can also be used in combination with facial electromyography (EMG), brain imaging (e.g., fMRI), and pharmacological interventions. It is our firm belief that the field will profit greatly from more research being conducted on reward processing using this and similarly controlled paradigms, which allow for cross-species comparison.

Increasing interhemispheric connectivity between human visual motion areas uncovers asymmetric sensitivity to horizontal motion.

Our conscious perceptual experience relies on a hierarchical process involving integration of low-level sensory encoding and higher-order sensory selection.1 This hierarchical process may scale at different levels of brain functioning, including integration of information between the hemispheres.2-5 Here, we test this hypothesis for the perception of visual motion stimuli. Across 3 experiments, we manipulated the connectivity between the left and right visual motion complexes (V5/MT+) responsible for horizontal motion perception2,3 by means of transcranial magnetic stimulation (TMS).4,5 We found that enhancing the strength of connections from the left to the right V5/MT+, by inducing spike-timing-dependent plasticity6 in this pathway, increased sensitivity to horizontal motion. These changes were present immediately and lasted at least 90 min after intervention. Notably, little perceptual changes were observed when strengthening connections from the right to the left V5/MT+. Furthermore, we found that this asymmetric modulation was mirrored by an asymmetric perceptual bias in the direction of the horizontal motion. Overall, observers were biased toward leftward relative to rightward motion direction. Crucially, following the strengthening of the connections from right to left V5/MT+, this bias could be momentarily reversed. These results suggest that the projections connecting left and right V5/MT+ in the human visual cortex are asymmetrical, subtending a hierarchical role of hemispheric specialization7-10 favoring left-to-right hemisphere processing for integrating local sensory input into coherent global motion perception.

Gradual enhancement of corticomotor excitability during cortico-cortical paired associative stimulation.

Cortico-cortical paired associative stimulation (ccPAS) is an effective transcranial magnetic stimulation (TMS) method for inducing associative plasticity between interconnected brain areas in humans. Prior ccPAS studies have focused on protocol's aftereffects. Here, we investigated physiological changes induced "online" during ccPAS administration. We tested 109 participants receiving ccPAS over left ventral premotor cortex (PMv) and primary motor cortex (M1) using a standard procedure (90 paired-pulses with 8-ms interstimulus interval, repeated at 0.1 Hz frequency). On each paired-pulse, we recorded a motor-evoked potential (MEP) to continuously trace the emergence of corticomotor changes. Participant receiving forward-ccPAS (on each pair, a first TMS pulse was administered over PMv, second over M1, i.e., PMv-to-M1) showed a gradual and linear increase in MEP size that did not reach a plateau at the end of the protocol and was greater in participants with low motor threshold. Participants receiving reverse-ccPAS (i.e., M1-to-PMv) showed a trend toward inhibition. Our study highlights the facilitatory and inhibitory modulations that occur during ccPAS administration and suggest that online MEP monitoring could provide insights into the malleability of the motor system and protocol's effectiveness. Our findings open interesting prospects about ccPAS potential optimization in experimental and clinical settings.

How culturally unique are pandemic effects? Evaluating cultural similarities and differences in effects of age, biological sex, and political beliefs on COVID impacts.

Despite being bio-epidemiological phenomena, the causes and effects of pandemics are culturally influenced in ways that go beyond national boundaries. However, they are often studied in isolated pockets, and this fact makes it difficult to parse the unique influence of specific cultural psychologies. To help fill in this gap, the present study applies existing cultural theories via linear mixed modeling to test the influence of unique cultural factors in a multi-national sample (that moves beyond Western nations) on the effects of age, biological sex, and political beliefs on pandemic outcomes that include adverse financial impacts, adverse resource impacts, adverse psychological impacts, and the health impacts of COVID. Our study spanned 19 nations (participant N = 14,133) and involved translations into 9 languages. Linear mixed models revealed similarities across cultures, with both young persons and women reporting worse outcomes from COVID across the multi-national sample. However, these effects were generally qualified by culture-specific variance, and overall more evidence emerged for effects unique to each culture than effects similar across cultures. Follow-up analyses suggested this cultural variability was consistent with models of pre-existing inequalities and socioecological stressors exacerbating the effects of the pandemic. Collectively, this evidence highlights the importance of developing culturally flexible models for understanding the cross-cultural nature of pandemic psychology beyond typical WEIRD approaches.

Large Gatherings? No, Thank You. Devaluation of Crowded Social Scenes During the COVID-19 Pandemic.

In most European countries, the first wave of the COVID-19 pandemic (spring 2020) led to the imposition of physical distancing rules, resulting in a drastic and sudden reduction of real-life social interactions. Even people not directly affected by the virus itself were impacted in their physical and/or mental health, as well as in their financial security, by governmental lockdown measures. We investigated whether the combination of these events had changed people's appraisal of social scenes by testing 241 participants recruited mainly in Italy, Austria, and Germany in an online, preregistered study conducted about 50 days after the beginning of the COVID-19 outbreak in Europe. Images depicting individuals alone, in small groups (up to four people), and in large groups (more than seven people) were rated in terms of valence, arousal, and perceived physical distance. Pre-pandemic normative ratings were obtained from a validated database (OASIS). Several self-report measures were also taken, and condensed into four factors through factor analysis. All images were rated as more arousing compared to the pre-pandemic period, and the greater the decrease in real-life physical interactions reported by participants, the higher the ratings of arousal. As expected, only images depicting large gatherings of people were rated less positively during, compared to before, the pandemic. These ratings of valence were, however, moderated by a factor that included participants' number of days in isolation, relationship closeness, and perceived COVID-19 threat. Higher scores on this factor were associated with more positive ratings of images of individuals alone and in small groups, suggesting an increased appreciation of safer social situations, such as intimate and small-group contacts. The same factor was inversely related to the perceived physical distance between individuals in images of small and large groups, suggesting an impact of lockdown measures and contagion-related worries on the representation of interpersonal space. These findings point to rapid and compelling psychological and social consequences of the lockdown measures imposed during the COVID-19 pandemic on the perception of social groups. Further studies should assess the long-term impact of such events as typical everyday life is restored.

Strengthening functionally specific neural pathways with transcranial brain stimulation.

Cortico-cortical paired associative stimulation (ccPAS) is a recently established offline dual-coil transcranial magnetic stimulation (TMS) protocol [1-3] based on the Hebbian principle of associative plasticity and designed to transiently enhance synaptic efficiency in neural pathways linking two interconnected (targeted) brain regions [4,5]. Here, we present a new 'function-tuning ccPAS' paradigm in which, by pairing ccPAS with the presentation of a specific visual feature, for example a specific motion direction, we can selectively target and enhance the synaptic efficiency of functionally specific, but spatially overlapping, pathways. We report that ccPAS applied in a state-dependent manner and at a low intensity selectively enhanced detection of the specific motion direction primed during the combined visual-TMS manipulations. This paradigm significantly enhances the specificity of TMS-induced plasticity, by allowing the targeting of cortico-cortical pathways associated with specific functions.

Long-latency interhemispheric interactions between motor-related areas and the primary motor cortex: a dual site TMS study.

The primary motor cortex (M1) is highly influenced by premotor/motor areas both within and across hemispheres. Dual site transcranial magnetic stimulation (dsTMS) has revealed interhemispheric interactions mainly at early latencies. Here, we used dsTMS to systematically investigate long-latency causal interactions between right-hemisphere motor areas and the left M1 (lM1). We stimulated lM1 using a suprathreshold test stimulus (TS) to elicit motor-evoked potentials (MEPs) in the right hand. Either a suprathreshold or a subthreshold conditioning stimulus (CS) was applied over the right M1 (rM1), the right ventral premotor cortex (rPMv), the right dorsal premotor cortex (rPMd) or the supplementary motor area (SMA) prior to the TS at various CS-TS inter-stimulus intervals (ISIs: 40-150 ms). The CS strongly affected lM1 excitability depending on ISI, CS site and intensity. Inhibitory effects were observed independently of CS intensity when conditioning PMv, rM1 and SMA at a 40-ms ISI, with larger effects after PMv conditioning. Inhibition was observed with suprathreshold PMv and rM1 conditioning at a 150-ms ISI, while site-specific, intensity-dependent facilitation was detected at an 80-ms ISI. Thus, long-latency interhemispheric interactions, likely reflecting indirect cortico-cortical/cortico-subcortical pathways, cannot be reduced to nonspecific activation across motor structures. Instead, they reflect intensity-dependent, connection- and time-specific mechanisms.

Empowering Reentrant Projections from V5 to V1 Boosts Sensitivity to Motion.

Evidence from macaques [1] and humans [2, 3] has shown that back projections from extrastriate areas to the primary visual area (V1) determine whether visual awareness will arise. For example, reentrant projections from the visual motion area (V5) to V1 are considered to be critical for awareness of motion [2, 3]. If these projections are also instrumental to functional processing of moving stimuli [4-8], then increasing synaptic efficacy in V5-V1 connections should induce functionally relevant short-term plastic changes, resulting in enhanced perception of visual motion. Using transcranial magnetic stimulation (TMS), we applied a novel cortico-cortical paired associative stimulation (ccPAS) protocol to transiently enhance visual motion sensitivity and demonstrate both the functional relevance of V5-V1 reentrant projections to motion perception and their plasticity. Specifically, we found that ccPAS aimed at strengthening reentrant connectivity from V5 to V1 (but not in the opposite direction) enhanced the human ability to perceive coherent visual motion. This perceptual enhancement followed the temporal profile of Hebbian plasticity [9-18] and was observed only when an optimal timing of 20 ms between TMS pulses [2, 3, 5, 6] was used, not when TMS pulses were delivered synchronously. Thus, plastic change is critically dependent on both the direction and timing of connectivity; if either of these requirements was not met, perceptual enhancement did not take place. We therefore provide novel causal evidence that V5-V1 back projections, instrumental to motion perception, are functionally malleable. These findings have implications for theoretical models of visual awareness and for the rehabilitation of visual deficits.

Long-latency modulation of motor cortex excitability by ipsilateral posterior inferior frontal gyrus and pre-supplementary motor area.

The primary motor cortex (M1) is strongly influenced by several frontal regions. Dual-site transcranial magnetic stimulation (dsTMS) has highlighted the timing of early (<40 ms) prefrontal/premotor influences over M1. Here we used dsTMS to investigate, for the first time, longer-latency causal interactions of the posterior inferior frontal gyrus (pIFG) and pre-supplementary motor area (pre-SMA) with M1 at rest. A suprathreshold test stimulus (TS) was applied over M1 producing a motor-evoked potential (MEP) in the relaxed hand. Either a subthreshold or a suprathreshold conditioning stimulus (CS) was administered over ipsilateral pIFG/pre-SMA sites before the TS at different CS-TS inter-stimulus intervals (ISIs: 40-150 ms). Independently of intensity, CS over pIFG and pre-SMA (but not over a control site) inhibited MEPs at an ISI of 40 ms. The CS over pIFG produced a second peak of inhibition at an ISI of 150 ms. Additionally, facilitatory modulations were found at an ISI of 60 ms, with supra- but not subthreshold CS intensities. These findings suggest differential modulatory roles of pIFG and pre-SMA in M1 excitability. In particular, the pIFG -but not the pre-SMA- exerts intensity-dependent modulatory influences over M1 within the explored time window of 40-150 ms, evidencing fine-tuned control of M1 output.