Intrinsic brain connectivity alterations despite intact pain inhibition in subjects with neuropathic pain after spinal cord injury: a pilot study

Endogenous pain modulation in humans is frequently investigated with conditioned pain modulation (CPM). Deficient pain inhibition is a proposed mechanism that contributes to neuropathic pain (NP) after spinal cord injury (SCI). Recent studies have combined CPM testing and neuroimaging to reveal neural correlates of CPM efficiency in chronic pain. This study investigated differences in CPM efficiency in relation to resting-state functional connectivity (rsFC) between 12 SCI-NP subjects and 13 age- and sex-matched healthy controls (HC). Twelve and 11 SCI-NP subjects were included in psychophysical and rsFC analyses, respectively. All HC were included in the final analyses. Psychophysical readouts were analysed to determine CPM efficiency within and between cohorts. Group differences of rsFC, in relation to CPM efficiency, were explored with seed-to-voxel rsFC analyses with pain modulatory regions, e.g. ventrolateral periaqueductal gray (vlPAG) and amygdala. Overall, pain inhibition was not deficient in SCI-NP subjects and was greater in those with more intense NP. Greater pain inhibition was associated with weaker rsFC between the vlPAG and amygdala with the visual and frontal cortex, respectively, in SCI-NP subjects but with stronger rsFC in HC. Taken together, SCI-NP subjects present with intact pain inhibition, but can be differentiated from HC by an inverse relationship between CPM efficiency and intrinsic connectivity of supraspinal regions. Future studies with larger cohorts are necessary to consolidate the findings in this study.

Implicit visual sensitivity towards slim versus overweight bodies modulates motor resonance in the primary motor cortex: A tDCS study

Motor resonance (MR) can be influenced by individual differences and similarity in the physical appearance between the actor and observer. Recently, we reported that action simulation is modulated by an implicit visual sensitivity towards normal-weight compared with overweight bodies. Furthermore, recent research has suggested the existence of an action observation network responsible for MR, with limited evidence whether the primary motor cortex (M1) is part of this. We expanded our previous findings with regards to the role of an implicit normal-weight-body preference in the MR mechanism. At the same time, we tested the functional relevance of M1 to MR, by using a transcranial direct current stimulation (tDCS) protocol. Seventeen normal-weight and 17 overweight participants were asked to observe normal-weight or overweight actors reaching and grasping a light or heavy cube, and then, at the end of each video-clip to indicate the correct cube weight. Before the task, all participants received 15 min of sham or cathodal tDCS over the left M1. Measures of anti-fat attitudes were also collected. During sham tDCS, all participants were better in simulating the actions performed by normal-weight compared with overweight models. Surprisingly, cathodal tDCS selectively improved the ability in the overweight group to simulate actions performed by the overweight models. This effect was not associated with scores of fat phobic attitudes or implicit anti-fat bias. Our findings are discussed in the context of relevance of M1 to MR and its social modulation by anti-fat attitudes.

Investigating the effects of tDCS on Visual Orientation Discrimination Task Performance: 'The possible influence of placebo'

The non-invasive neuromodulation technique tDCS offers the promise of a low cost tool for both research and clinical applications in psychology, psychiatry and neuroscience. However, findings regarding its efficacy are often equivocal. A key issue is that the clinical and cognitive applications studied are often complex and thus effects of tDCS are difficult to predict given its known effects on the basic underlying neurophysiology, namely alterations in cortical inhibition-excitation balance. As such, it may be beneficial to assess the effects of tDCS in tasks whose performance has a clear link to cortical inhibition-excitation balance such as the visual orientation discrimination task (ODT). In prior studies in our laboratory no practise effects were found during 2 consecutive runs of the ODT, thus in the current investigation, to examine the effects of tDCS, subjects received 10 minutes of 2mA occipital tDCS (sham, anode, cathode) between a first and second run of ODT. Surprisingly, subjects' performance significantly improved in the second run of ODT compared to the first one regardless of the tDCS stimulation type they received (anodal, cathodal, or sham-tDCS). Possible causes for such an improvement could have been due to either a generic 'placebo' effect of tDCS (as all subjects received some form of tDCS) or an increased delay period between the two runs of ODT of the current study compared to our previous work (10 minutes duration required to administer tDCS as opposed to ~2 minutes in previous studies as a 'break'). As such, we tested these two possibilities with a subsequent experiment in which subjects received 2 minutes or 10 minutes delay between the 2 runs (with no tDCS) or 10 minutes of sham-tDCS. Only sham-tDCS resulted in improved performance thus these data add to a growing literature suggesting that tDCS has powerful placebo effect that may occur even in the absence of active cortical modulation.

Effects of Acute and Subchronic Anodal Transcranial Direct Current Stimulation (tDCS) on Morphine-Induced Responses in Hotplate Apparatus

Background:

The endogenous opioid system plays a basic role in pain suppression. The opiate analgesia is the most powerful and useful technique for reducing severe pain in many medical conditions. Transcranial direct current stimulation (tDCS) is a neuromodulator technique by which the cerebral cortex is stimulated with a weak and constant electrical current by the painless and non-invasive method.

Materials and Methods:

In this experimental study, we investigated the effect of tDCS on morphine (1.25, 2.5 and 5 mg/kg)-induced pain responses; as we applied left prefrontal anodal stimulation with 0.2 mA intensity and 20 minutes.

Results:

our results revealed that the acute (One-time electrical stimulation 24 hours after the last administration of morphine three days) and subchronic (three times electrical stimulation; one session/day before each administration of morphine three days) left prefrontal anodal tDCS does not alter pain perception induced by different dose of morphine significantly.

Conclusion:

Finally, our data indicated that there is no potentiated effect between acute tDCS or subchronic tDCS and morphine administration with tested parameters significantly.

Transcranial direct current stimulation modulates implicit attitudes towards food in eating disorders

OBJECTIVE

Neuromodulation of regions involved in food processing is increasingly used in studies on eating behaviors, but results are controversial. We assessed the effects of anodal transcranial direct current stimulation (a-tDCS) on food and body implicit preferences in patients with eating disorders (EDs).

METHOD

Thirty-six ED patients and 36 healthy females completed three sessions with a-tDCS applied to the medial-prefrontal cortex (mPFC), the right extrastriate body area (rEBA) or in sham mode. Each participant then completed three Implicit Association Tests (IATs) on tasty/tasteless food, underweight/overweight body images, flowers versus insects as control. Differences in latency between incongruent and congruent blocks were calculated (D score).

RESULTS

The tDCS by group interaction was significant for the IAT-food D score, with patients showing weaker preference for tasty food than controls in sham, but not a-tDCS sessions. In particular, rEBA stimulation significantly increased patients' D score compared to sham. Moreover, a-tDCS over mPFC and rEBA selectively increased patients' reaction times in the incongruent blocks of the IAT-food.

DISCUSSION

A-tDCS on frontal and occipito-temporal cortices modulated food preferences in ED patients. The effect was specific for food images and selective in patients, but not in healthy participants. These findings suggest that neuromodulation of these regions could affect implicit food attitudes.

Visually induced analgesia during face or limb stimulation in healthy and migraine subjects

Background

Visually induced analgesia (VIA) defines a phenomenon in which viewing one’s own body part during its painful stimulation decreases the perception of pain. VIA occurs during direct vision of the stimulated body part and also when seeing it reflected in a mirror. To the best of our knowledge, VIA has not been studied in the trigeminal area, where it could be relevant for the control of headache.

Subjects and methods

We used heat stimuli (53°C) to induce pain in the right forehead or wrist in 11 healthy subjects (HSs) and 14 female migraine without aura (MO) patients between attacks. The subjects rated pain on a visual analog scale (VAS) and underwent contact heat-evoked potential (CHEP) recordings (five sequential blocks of four responses) with or without observation of their face/wrist in a mirror.

Results

During wrist stimulation, amplitude of the first block of P1–P2 components of CHEPs decreased compared to that in the control recording when HSs were seeing their wrist reflected in the mirror (p = 0.036; Z = 2.08); however, this was not found in MO patients. In the latter, the VAS pain score increased viewing the reflected wrist (p = 0.049; Z = 1.96). Seeing their forehead reflected in the mirror induced a significant increase in N2 latency of CHEPs in HSs, as well as an amplitude reduction in the first block of P1–P2 components of CHEPs both in HSs (p = 0.007; Z = 2.69) and MO patients (p = 0.035; Z = 2.10). Visualizing the body part did not modify habituation of CHEP amplitudes over the five blocks of averaged responses, neither during wrist nor during forehead stimulation.

Conclusion

This study adds to the available knowledge on VIA and demonstrates this phenomenon for painful stimuli in the trigeminal area, as long as CHEPs are used as indices of central pain processing. In migraine patients during interictal periods, VIA assessed with CHEPs is within normal limits in the face but absent at the wrist, possibly reflecting dysfunctioning of extracephalic pain control.

Transcranial direct current stimulation as a tool in the study of sensory-perceptual processing

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique with increasing popularity in the fields of basic research and rehabilitation. It is an affordable and safe procedure that is beginning to be used in the clinic, and is a tool with potential to contribute to the understanding of neural mechanisms in the fields of psychology, neuroscience, and medical research. This review presents examples of investigations in the fields of perception, basic sensory processes, and sensory rehabilitation that employed tDCS. We highlight some of the most relevant efforts in this area and discuss possible limitations and gaps in contemporary tDCS research. Topics include the five senses, pain, and multimodal integration. The present work aims to present the state of the art of this field of research and to inspire future investigations of perception using tDCS.

Viewing the body modulates both pain sensations and pain responses

Viewing the body can influence pain perception, even when vision is non-informative about the noxious stimulus. Prior studies used either continuous pain rating scales or pain detection thresholds, which cannot distinguish whether viewing the body changes the discriminability of noxious heat intensities or merely shifts reported pain levels. In Experiment 1, participants discriminated two intensities of heat-pain stimulation. Noxious stimuli were delivered to the hand in darkness immediately after participants viewed either their own hand or a non-body object appearing in the same location. The visual condition varied randomly between trials. Discriminability of the noxious heat intensities (d′) was lower after viewing the hand than after viewing the object, indicating that viewing the hand reduced the information about stimulus intensity available within the nociceptive system. In Experiment 2, the hand and the object were presented in separate blocks of trials. Viewing the hand shifted perceived pain levels irrespective of actual stimulus intensity, biasing responses toward ‘high pain’ judgments. In Experiment 3, participants saw the noxious stimulus as it approached and touched their hand or the object. Seeing the pain-inducing event counteracted the reduction in discriminability found when viewing the hand alone. These findings show that viewing the body can affect both perceptual processing of pain and responses to pain, depending on the visual context. Many factors modulate pain; our study highlights the importance of distinguishing modulations of perceptual processing from modulations of response bias.

Neuromodulation of Early Multisensory Interactions in the Visual Cortex

Merging information derived from different sensory channels allows the brain to amplify minimal signals to reduce their ambiguity, thereby improving the ability of orienting to, detecting, and identifying environmental events. Although multisensory interactions have been mostly ascribed to the activity of higher-order heteromodal areas, multisensory convergence may arise even in primary sensory-specific areas located very early along the cortical processing stream. In three experiments, we investigated early multisensory interactions in lower-level visual areas, by using a novel approach, based on the coupling of behavioral stimulation with two noninvasive brain stimulation techniques, namely, TMS and transcranial direct current stimulation (tDCS). First, we showed that redundant multisensory stimuli can increase visual cortical excitability, as measured by means of phosphene induction by occipital TMS; such physiological enhancement is followed by a behavioral facilitation through the amplification of signal intensity in sensory-specific visual areas. The more sensory inputs are combined (i.e., trimodal vs. bimodal stimuli), the greater are the benefits on phosphene perception. Second, neuroelectrical activity changes induced by tDCS in the temporal and in the parietal cortices, but not in the occipital cortex, can further boost the multisensory enhancement of visual cortical excitability, by increasing the auditory and tactile inputs from temporal and parietal regions, respectively, to lower-level visual areas.