Evidence of top-down modulation of the Brentano illusion but not of the glare effect by transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) has been widely used for modulating sensory, motor and cognitive functions, but there are only few attempts to induce and change illusory perception. Visual illusions have been the most traditional and effective way to investigate visual processing through the comparison between physical reality and subjective reports. Here we used tDCS to modulate two different visual illusions, namely the Brentano illusion and the glare effect, with the aim of uncovering the influence of top-down mechanisms on bottom-up visual perception in two experiments. In Experiment 1, to a first group of subjects, real and sham cathodal tDCS (2 mA, 10 min) were applied over the left and right posterior parietal cortices (PPC). In Experiment 2, real and sham cathodal tDCS were applied to the left and right occipital cortices (OC) to a second group of participants. Results showed that tDCS was effective in modulating only the Brentano illusion, but not the glare effect. tDCS increased the Brentano illusion but specifically for the stimulated cortical area (right PPC), illusion direction (leftward), visual hemispace (left), and illusion length (160 mm). These findings suggest the existence of an inhibitory modulation of top-down mechanisms on bottom-up visual processing specifically for the Brentano illusion, but not for the glare effect. The lack of effect of occipital tDCS should consider the possible role of ocular compensation or of the unstimulated hemisphere, which deserves further investigation

Hebbian associative plasticity in the visuo-tactile domain: A cross-modal paired associative stimulation protocol

We developed and assessed the effects of a novel cross-modal protocol aimed at inducing associative (Hebbian-like) plasticity in the somatosensory cortical system through vision. Associative long-term potentiation can be induced in the primary somatosensory cortex (S1) by means of paired associative stimulation (PAS), in which a peripheral electrical stimulation of the median nerve is repeatedly paired with a transcranial magnetic stimulation (TMS) pulse over S1. Considering the mirror proprieties of S1, the cross-modal PAS (cm-PAS) consists of repetitive observation of bodily tactile stimulations, paired with TMS pulses over the contralateral S1. Through three experiments in healthy participants, we demonstrate that the cm-PAS is able to induce excitatory plastic effects with functional significance in S1, improving somatosensory processing at both behavioral (tactile acuity) and neurophysiological (somatosensory-evoked potentials) levels. The plastic effects induced by cm-PAS depend on the interval (20 m s) between the visual stimulus and the magnetic pulse, the targeted cortical site (S1), and the tactile content of the visual stimulus, which must represent a touch event. Such specificity implies the recruitment of cross-modal, mirror-like, mechanisms in S1, which are able to visually promote associative synaptic plasticity in S1 likely through the recruitment of predictive coding processe

Touch anticipation mediates cross-modal Hebbian plasticity in the primary somatosensory cortex

Paired associative stimulation (PAS) protocols can be used to induce Hebbian plasticity in the human brain. A modified, cross-modal version, of the PAS (cross-modal PAS, cm-PAS) has been recently developed. The cm-PAS consists in the repetitive pairings of a transcranial magnetic stimulation (TMS) pulse over the primary somatosensory cortex (S1) and a visual stimulus depicting a hand being touched; a 20 ms of inter-stimulus interval (ISI) is required to affect S1 plasticity, in turn modulating tactile acuity and somatosensory evoked potentials. The present study explores the role of anticipatory simulation in the cm-PAS efficacy, which could be responsible for such a short ISI. To this aim, we compared the effect of the original, fixed-frequency, cm-PAS to that of a jittered version, in which the time interval between trials was not steady but jittered, hence avoiding the anticipation of the upcoming visual-touch stimulus. Moreover, in the jittered PAS, the ISI between the paired stimulations was varied: it could match the early, somatosensory-driven, activation of S1 (20 ms), or the mirror recruitment of S1 by touch observation (150 ms). Results showed that tactile acuity is enhanced by the fixed-frequency cm-PAS, with an ISI of 20 ms between paired stimulation (visual-touch stimulus and TMS pulse over S1), and also by the jittered cm-PAS but only if the ISI is of 150 ms. These findings suggest that the cm-PAS with a jittered frequency, by preventing an anticipatory pre-activation of S1, delays the timing of the interaction between the visual-touch stimulus and the cortical pulse. On a broader perspective, our study highlights the possible involvement of sensory anticipation, likely through mirror-like simulation mechanisms, in tactile mirroring, as well as its influence of the optimal interval between the afferent and the magnetic pulse during PAS protocols

A tool to induce cross-modal Hebbian-like plasticity within the primary somatosensory cortex

Background In recent years neuroscientific evidence highlighted the cross-modal, mirror-like, properties of the primary somatosensory cortex (S1): this area is activated not only by one’s own somatosensation, but also by the observation of tactile stimulation of another person’s body. We explored whether the activity of S1, and the associated tactile processing, could be enhanced by activating Hebbian plasticity-induction mechanisms through their visual, mirror-based, recruitment. To this aim, we developed a novel, cross-modal, Paired Associative Stimulation protocol (cm-PAS). Methods Cm-PAS consists in the repetitive, time-locked pairing of a visual stimulus depicting a hand being touched and a Transcranial Magnetic Stimulation (TMS) pulse over S1. In four experiments we tested the i) timing ii) cortical area iii) visual stimulation selectivity of the cm-PAS, and iv) the role of predictive coding mechanisms. Cm-PAS effects were assessed by measuring tactile acuity (behavioural index) and Somatosensory Evoked Potentials (SEPs, neurophysiological index in experiment 3). Results Results show an enhancement of tactile acuity only with strict delay between the TMS pulse and the visual touch stimulus; an increase of SEPs following only S1 stimulation paired with a visuo-tactile stimulus. Moreover, we found that preventing the anticipation of the visual touch changes the time-course of cortical-visual interactions

Cortical dynamics underpinning the self-other distinction of touch: A TMS-EEG study

Touch supports processes crucial to human social behaviour, adding a bodily dimension to the perception and understanding of others' feelings. Mirror cortical activity was proposed to underpin the interpersonal sharing of touch, allowing an automatic and unconscious simulation of others' somatic states. However, recent evidence questioned the existence of a tactile shared representation in the primary somatosensory cortex (S1), and the neural correlates of self-other distinction in the somatosensory system remains unknown. We address these issues by exploring S1 reactivity, and the associated neural network oscillations and connectivity, to self and others' touch. Transcranial Magnetic Stimulation combined with Electroencephalography (TMS-EEG) recordings were performed during tactile perception and observation, looking for differences in cortical activation and connectivity between felt and seen touch. The sight of a touch directed to a human body part, but not to an object, triggered an early activation of S1 as a felt touch did, which, in both conditions, propagated to fronto-parietal regions. Critically, touch perception and observation shared an effective connectivity network generated in the beta band, which is typically associated to unconscious tactile processing. Conversely, alpha band connectivity, a marker of conscious tactile processing, was detected only for real tactile stimulation. Alpha connectivity within a fronto-parietal pathway seems to underpin the ability to distinguish self and others' somatosensory states, controlling and distinguishing shared tactile representations in S

Behavioural and emotional features of brain-damaged patients with abnormal social space boundaries

Introduction: The Interpersonal space (IPS) is the area surrounding the body that works as a defensive zone, and it is considered personal or private(1). The extension of this “comfort” space is flexible and modulable both by social and biological factors. These factors also include empathic abilities and personality traits such as impulsivity and behavioral regulation. It is commonly recognized that acquired brain lesions may lead to personality changes(2). Here we examine the alteration of the IPS in patients with acquired brain damage, and its association with empathy and behavior and personality changes featuring the frontal lobe syndrome. Methods: We measured IPS and empathic abilities in a group of patients (N=11) with acquired brain damage of traumatic or stroke aetiology and in a control group of age-matched healthy participants (N=40). IPS was assessed with the “stop-distance” paradigm, while cognitive and af ective emphatic abilities were assessed with the Interpersonal Reactivity Index (IRI). Moreover, in brain-damaged patients personality and behavioral changes were assessed through the Frontal Behavioral Index administered to patients’ caregivers. Noteworthy, patients’ selection was based on a clinical diagnosis of frontal lobe syndrome. Lesion size and its location were also analyzed. Results: Although the extension of the IPS in brain-damaged patients was overall comparable to that of controls, single case analyses showed that 2 out of 11 patients presented an enlargement of the IPS, which was associated with behavioral symptoms such as apathy, inattention, and emotional flatness. On the other hand, patients who showed a total loss of space boundaries exhibited impulsivity and inappropriateness. Our results also highlighted an overall reduction of empathic abilities in brain-damaged patients, as compared to healthy controls. Further single case analyses unveiled that the same patients exhibiting an IPS alteration have lower empathy. Patients exhibiting an alteration of IPS and empathy had brain lesions involving mostly frontal and temporal lobes. Discussion:The present study of ers a novel insight on the interplay between social space extension, personality traits and empathy: patients with brain damages causing clinical symptoms of frontal lobe syndrome may also present an alteration of IPS boundaries, along with behavioural dysregulation and empathic impairments. These findings support the view that the boundaries of the social, interpersonal, space are linked to empathy and behavioural regulation, likely sharing a common neural network