Differential effects of synchronous and asynchronous multifinger coactivation on human tactile performance

<p>Abstract</p> <p>Background</p> <p>Repeated execution of a tactile task enhances task performance. In the present study we sought to improve tactile performance with unattended activation-based learning processes (i.e., focused stimulation of dermal receptors evoking neural coactivation (CA)). Previous studies show that the application of CA to a single finger reduced the stationary two-point discrimination threshold and significantly increased tactile acuity. These changes were accompanied by an expansion of the cortical finger representation in primary somatosensory cortex (SI). Here we investigated the effect of different types of multifinger CA on the tactile performance of each finger of the right hand.</p> <p>Results</p> <p>Synchronous and asynchronous CA was applied to all fingers of a subject's dominant hand. We evaluated changes in absolute touch thresholds, static two-point discrimination thresholds, and mislocalization of tactile stimuli to the fingertips. After synchronous CA, tactile acuity improved (i.e., discrimination thresholds decreased) and the frequency of mislocalization of tactile stimuli changed from directly neighboring fingers to more distant fingers. On the other hand, asynchronous CA did not significant improve tactile acuity. In fact, there was evidence of impaired tactile acuity. Multifinger CA with synchronous or asynchronous stimulation did not significantly alter absolute touch thresholds.</p> <p>Conclusion</p> <p>Our results demonstrate that it is possible to extend tactile CA to all fingers of a hand. The observed changes in mislocalization of tactile stimuli after synchronous CA indicate changes in the topography of the cortical hand representation. Although single-finger CA has been shown to improve tactile acuity, asynchronous CA of all fingers of the hand had the opposite effect, suggesting the need for synchrony in multifinger CA for improving tactile acuity.</p

Combined Induction of Rubber-Hand Illusion and Out-of-Body Experiences

The emergence of self-consciousness depends on several processes: those of body ownership, attributing self-identity to the body, and those of self-location, localizing our sense of self. Studies of phenomena like the rubber-hand illusion (RHi) and out-of-body experience (OBE) investigate these processes, respectively for representations of a body-part and the full-body. It is supposed that RHi only target processes related to body-part representations, while OBE only relates to full-body representations. The fundamental question whether the body-part and the full-body illusions relate to each other is nevertheless insufficiently investigated. In search for a link between body-part and full-body illusions in the brain we developed a behavioral task combining adapted versions of the RHi and OBE. Furthermore, for the investigation of this putative link we investigated the role of sensory and motor cues. We established a spatial dissociation between visual and proprioceptive feedback of a hand perceived through virtual reality in rest or action. Two experimental measures were introduced: one for the body-part illusion, the proprioceptive drift of the perceived localization of the hand, and one for the full-body illusion, the shift in subjective-straight-ahead (SSA). In the rest and action conditions it was observed that the proprioceptive drift of the left hand and the shift in SSA toward the manipulation side are equivalent. The combined effect was dependent on the manipulation of the visual representation of body parts, rejecting any main or even modulatory role for relevant motor programs. Our study demonstrates for the first time that there is a systematic relationship between the body-part illusion and the full-body illusion, as shown by our measures. This suggests a link between the representations in the brain of a body-part and the full-body, and consequently a common mechanism underpinning both forms of ownership and self-location

Improvement of sensorimotor functions in old age by passive sensory stimulation

Sensorimotor functions decrease in old age. The well-documented loss of tactile acuity in elderly is accompanied by deterioration of haptic performance and fine manipulative movements. Physical training and exercise can maintain sensorimotor fitness into high age. However, regular schedules of training require discipline and physical fitness. We here present an alternative interventional paradigm to enhance tactile, haptic, and fine motor performance based on passive, sensory stimulation by means of tactile coactivation. This approach is based on patterned, synchronous tactile stimulation applied to the fingertips for 3 hours. The stimulation drives plastic reorganizational changes in somatosensory cortex that affect perception and behavior: We demonstrate that following 3 hours of coactivation tactile acuity as well as haptic object exploration and fine motor performance are improved for at least 96 hours. Because this kind of intervention does not require active participation or attention of the subjects, we anticipate that coactivation is a prime candidate for future therapeutic interventions in patients with impaired sensorimotor abilities. It can be assumed that the maintenance and restoration of sensorimotor functions can ensure and preserve independence of daily living. Further optimizing of the stimulation protocol can be assumed to strengthen both the range and durability of its efficacy

Reshaping the full body illusion through visuoelectro-tactile sensations

The physical boundaries of our body do not define what we perceive as self. This malleable representation arises from the neural integration of sensory information coming from the environment. Manipulating the visual and haptic cues produces changes in body perception, inducing the Full Body Illusion (FBI), a vastly used approach to exploring humans’ perception. After pioneering FBI demonstrations, issues arose regarding its setup, using experimenter-based touch and pre-recorded videos. Moreover, its outcome measures are based mainly on subjective reports, leading to biased results, or on heterogeneous objective ones giving poor consensus on their validity. To address these limitations, we developed and tested a multisensory platform allowing highly controlled experimental conditions, thanks to the leveraged use of innovative technologies: Virtual Reality (VR) and Transcutaneous Electrical Nerve Stimulation (TENS). This enabled a high spatial and temporal precision of the visual and haptic cues, efficiently eliciting FBI. While it matched the classic approach in subjective measures, our setup resulted also in significant results for all objective measurements. Importantly, FBI was elicited when all 4 limbs were multimodally stimulated but also in a single limb condition. Our results behoove the adoption of a comprehensive set of measures, introducing a new neuroscientific platform to investigate body representations

Visual perceptual stability and the processing of self-motion information: neurophysiology, psychophysics and neuropsychology

While we move through our environment, we constantly have to deal with new sensory input. Especially the visual system has to deal with an ever-changing input signal, since we continuously move our eyes. For example, we change our direction of gaze about three times every second to a new area within our visual field with a fast, ballistic eye movement called a saccade. As a consequence, the entire projection of the surrounding world on our retina moves. Yet, we do not perceive this shift consciously. Instead, we have the impression of a stable world around us, in which objects have a well-defined location. In my thesis I aimed to investigate the underlying neural mechanisms of the visual perceptual stability of our environment. One hypothesis is that there is a coordinate transformation of the retinocentric input signal to a craniocentric (egocentric) and eventually even to a world centered (allocentric) frame of reference. Such a transformation into a craniocentric reference frame requires information about both the location of a stimulus on the retina and the current eye position within the head. The physicist Hermann von Helmholtz was one of the first who suggested that such an eye-position signal is available in the brain as an internal copy of the motor plan, which is sent to the eye muscles. This so-called efference copy allows the brain to classify actions as self-generated and differentiate them from being externally triggered. If we are the creator of an action, we are able to predict its outcome and can take this prediction into consideration for the further processing. For example, if the projection of the environment moves across the retina due to an eye movement, the shift is registered as self-induced and the brain maintains a stable percept of the world. However, if one gently pushes the eye from the side with a finger, we perceive a moving environment. Along the same lines, it is necessary to correctly attribute the movement of the visual field to our own self-motion, e.g. to perform eye movements accounting for the additional influences of our movements. The first study of my thesis shows that the perceived location of a stimulus might indeed be a combination of two independent neuronal signals, i.e. the position of the stimulus on the retina and information about the current eye-position or eye-movement, respectively. In this experiment, the mislocalization of briefly presented stimuli, which is characteristic for each type of eye-movement, leads to a perceptual localization of stimuli within the area of the blind spot on the retina. Yet, this is the region where the optic nerve leaves the eye, meaning that there are no photoreceptors available to convert light into neuronal signals. Physically, subjects should be blind for stimuli presented in this part of the visual field. In fact, a combination of the actual stimulus position with the specific, error-inducing eye-movement information is able to explain the experimentally measured behavior. The second study in my thesis investigates the underlying neural mechanism of the mislocalization of briefly presented stimuli during eye-movements. Many previous studies using animal models (the rhesus monkey) revealed internal representations of eye-position signals in various brain regions and therefore confirmed the hypothesis of an efference copy signal within the brain. Although these eye-position signals basically reflect the actual eye-position with good accuracy, there are also some spatial and temporal inaccuracies. These erroneous representations have been previously suggested as the source of perceptual mislocalization during saccades. The second study of my thesis extends this hypothesis to the mislocalization during smooth pursuit eye-movements. We usually perform such an eye movement when we want to continuously track a moving object with our eyes. I showed that the activity of neurons in the ventral intraparietal area of the rhesus monkey adequately represents the actual eye-position during smooth pursuit. However, there was a constant lead of the internal eye-position signal as compared to the real eye-position in direction of the ongoing eye-movement. In combination with a distortion of the visual map due to an uneven allocation of attention in direction of the future stimulus position, this results in a mislocalization pattern during smooth pursuit, which almost exactly resembles those typically measured in psychophysical experiments. Hence, on the one hand the efference copy of the eye-position signal provides the required signal to perform a coordinate transformation in order to preserve a stable perception of our environment. On the other hand small inaccuracies within this signal seem to cause perceptual errors when the visual system is experimentally pushed to its limits. The efference copy also plays a role in dysfunctions of the brain in neurological or psychiatric diseases. For example, many symptoms of schizophrenia patients could be explained by an impaired efference copy mechanism and a resulting misattribution of agency to self- and externally-produced actions. Following this hypothesis, the typically observed auditory hallucinations in these patients might be the result of an erroneously assigned agency of their own thoughts. To make a detailed analysis of this potentially impaired efference copy mechanism possible, the third study of my thesis investigated eye movements of schizophrenia patients and tried to step outside the limited capabilities of laboratory setups into the real world. This study showed that results of previous laboratory studies only partly resemble those obtained in the real world. For example, schizophrenia patients, when compared to healthy controls, usually show a more inaccurate smooth pursuit eye-movement in the laboratory. Yet, in the real world when they track a stationary object with their eyes while they are moving towards it, there are no differences between patients and healthy controls, although both types of eye-movements are closely related. This might be due to the fact that patients were able to use additional sources of information in the real world, e.g. self-motion information, to compensate for some of their deficits under certain conditions. Similarly, the fourth study of my thesis showed that typical impairments of eye-movements during healthy aging can be equalized by other sources of information available under natural conditions. At the same time, this work underlined the need of eye-movement measurements in the real world as a complement to laboratory studies to accurately describe the visual system, all mechanisms of perception and their interactions under natural circumstances. For example, experiments in the laboratory usually analyze particularly selected eye-movement parameters within a specific range, such as saccades of a certain amplitude. However, this does not reflect everyday life in which parameters like that are typically continuous and not normally distributed. Furthermore, motion-selective areas in the brain might play a much bigger role in natural environments, since we generally move our head and/or ourselves. To correctly analyze the contribution to and influences on eye-movements, one has to perform eye-movement studies under conditions as realistic as possible. The fifth study of my thesis aimed to investigate a possible application of eye-movement studies in the diagnosis of neuronal diseases. We showed that basic eye-movement parameters like saccade peak-velocity can be used to differentiate patients with Parkinson’s disease from patients with an atypical form of Parkinsonism, progressive supranuclear palsy. This differentiation is of particular importance since both diseases share a similar onset but have a considerably different progression and outcome, requiring different types of therapies. An early differential diagnosis, preferably in a subclinical stage, is needed to ensure the optimal treatment of the patients in order to ease the symptoms and eventually even improve the prognosis. The study showed that mobile eye-trackers are particularly well-suited to investigate eye movements in the daily clinical routine, due to their promising results in differential diagnosis and their easy, fast and reliable handling. In conclusion, my thesis underlines the importance of an interaction of all the different neuroscientific methods such as psychophysics, eye-movement measurements in the real world, electrophysiology and the investigation of neuropsychiatric patients to get a complete picture of how the brain works. The results of my thesis contribute to extent the current knowledge about the processing of information and the perception of our environment in the brain, point towards fields of application of eye-movement measurements and can be used as groundwork for future research

Enhanced tactile acuity through mental states

Bodily training typically evokes behavioral and perceptual gains, enforcing neuroplastic processes and affecting neural representations. We investigated the effect on somatosensory perception of a three-day Zen meditation exercise, a purely mental intervention. Tactile spatial discrimination of the right index finger was persistently improved by only 6 hours of mental-sensory focusing on this finger, suggesting that intrinsic brain activity created by mental states can alter perception and behavior similarly to external stimulation

Voluntary self-touch increases body ownership

Experimental manipulations of body ownership have indicated that multisensory integration is central to forming bodily self representation. Voluntary self touch is a unique multisensory situation involving corresponding motor, tactile and proprioceptive signals. Yet, even though self-touch is frequent in everyday life, its contribution to the formation of body ownership is not well understood. Here we investigated the role of voluntary self touch in body ownership using a novel adaptation of the rubber hand illusion (RHI), in which a robotic system and virtual reality allowed participants self-touch of real and virtual hands. In the first experiment, active and passive self-touch were applied in the absence of visual feedback. In the second experiment, we tested the role of visual feedback in this bodily illusion. Finally, in the third experiment, we compared active and passive self-touch to the classical RHI in which the touch is administered by the experimenter. We hypothesized that active self-touch would increase ownership over the virtual hand through the addition of motor signals strengthening the bodily illusion. The results indicated that active self-touch elicited stronger illusory ownership compared to passive self-touch and sensory only stimulation, and show an important role for active self-touch in the formation of bodily self

HIGH-INTENSITY EXERCISE AND MOTOR IMAGERY TRAINING PROGRAM: A THERAPEUTIC APPROACH IN NON-SPECIFIC CHRONIC LOWER BACK PAIN.

A dor lombar crónica não específica (DLC) tem, nos últimos anos, sido um dos flagelos da humanidade, apresentando elevadas taxas de prevalência e incidência, com reflexo nas altas taxas de absentismo, nos grandes custos sociais e sobrecarga de utilização dos cuidados de saúde. Esta doença não encontrou ainda a resposta terapêutica eficaz, seja na vertente farmacológica, seja na não farmacológica. Trata-se de uma doença de carácter global, e não apenas local, resultado das múltiplas disfunções que promove no indivíduo. Uma destas reflete-se a nível do sistema nervoso central e do sistema nervoso periférico, influenciando negativamente as conexões neuronais, a espessura cortical e o volume da substância cinzenta em determinadas áreas do encéfalo. A presença crónica da dor tem, por isso, repercussões na organização cerebral e no desempenho desta sobre a capacidade de modular e inibir a dor. Como reflexo, estão descritas alterações de sensibilidade discriminativa localizadas à região lombar dolorosa, no entanto, não se sabe ainda se a restante região do dorso é, ou não, também afetada. Dada a importância clínica desta informação para a correção postural e reforço muscular, o objetivo do primeiro trabalho foi o de averiguar a capacidade de discriminação sensorial táctil superficial e dolorosa em todo o dorso e compará-la com indivíduos saudáveis. Os resultados permitiram concluir que os doentes DLC apresentam menor capacidade de discriminação sensorial em todo o dorso, com diferenças acentuadas entre várias regiões. É sabido que programas de exercício físico têm sido os instrumentos de maior sucesso para diminuir os sintomas da DLC, pelo aumento do limiar de percepção da dor e da capacidade física destes doentes. Como exercício físico, particularmente o de alta intensidade, parece ter repercussões orgânicas favoráveis, periféricas e centrais, colocouse a hipótese que o treino físico de alta intensidade melhora a capacidade discriminativa da sensibilidade táctil superficial e dolorosa destes doentes e diminui a intensidade da sua dor, hipótese essa que foi testada no segundo trabalho desta tese. Os resultados obtidos confirmaram a hipótese de estudo, concluindo-se que o treino físico de alta intensidade melhora a discriminação sensorial no dorso e reduz a intensidade da dor referida assim como a extensão da área dolorosa. Sabe-se que, dependendo da intensidade do exercício físico, há produção de um vasto conjunto de neurotrofinas, fundamentais para a criação de uma nova rede de conexões cerebrais com a ativação do sistema de modulação de inibição descendente e com a proliferação de novas conexões cerebrais. A criação de novas conexões cerebrais está também associada ao treino da imagética motora, cuja eficácia terapêutica tem sido testada, de forma isolada, na DLC. Considerando este facto, assim como os resultados do segundo estudo desta tese, colocou-se a hipótese que o treino de imagética motora conjugado com o treino físico de alta intensidade possui melhores resultados comparativamente aos do treino físico quando aplicado de forma isolada. Esta hipótese foi testada no terceiro e quarto trabalhos desta tese, tendo os resultados confirmado a hipótese de trabalho, com melhoria na intensidade e na área da dor dos doentes. Os resultados dos quatro estudos realizados permitem concluir que a DCL tem um componente de afeção central, com repercussões periféricas na intensidade e extensão da área da dor, na discriminação da sensibilidade do dorso e na capacidade funcional. O treino físico de alta intensidade, especialmente quando associado com o treino de imagética motora, minimiza estas consequências.The non-specific chronic lower back pain (CLBP) has been for the last few years one of the mankind "scourges", with high prevalence and incidence rates, reflected in high rates of absenteeism, high social costs and overburden of the healthcare systems. This disease is yet to find an effective therapeutic response, either in the pharmacological or non-pharmacological treatments. It is a systemic condition, not just a local one, considering all the multiple dysfunctions it entails. One of these is reflected on the central nervous system and the peripheral nervous system, negatively influencing the neuronal connections, the cortical thickness and the volume of the gray matter in certain areas of the brain. The chronic pain has therefore repercussions on brain organization and its performance on the ability to modulate and inhibit pain. As a result, changes in discriminatory sensitivity located in the painful lumbar region are described, however, it is not yet known whether the remaining region of the dorsum is also affected. Given the clinical importance of this information for postural correction and muscle strengthening, the purpose of the first work of this thesis was to investigate the ability for superficial and painful tactile sensory discrimination throughout the dorsum of CLBP patients and compare it with healthy individuals. The results led to the conclusion that CLBP patients have less capacity for sensitive discrimination throughout the dorsum, with marked differences between various regions. It is known that physical exercise programs have been the most successful instruments to decrease CLBP symptoms by raising the threshold of pain perception and physical capacity of these patients. Given that physical exercise, specially of high-intensity seems to have favourable physiological responses, either peripheral and central, the second work of this thesis tested the raised hypothesis that high-intensity physical training improves the discriminative ability of these patients' superficial and painful tactile sensitivity and decreases the intensity of their pain. The results supported the hypothesis, leading to the conclusion that high-intensity physical training improves sensitive discrimination in the dorsum, reduces the intensity of referred pain as well as the extent of the painful area. It is known that, depending on the intensity of physical exercise, there is production of a vast set of neurotrophins, crucial for the creation of a new network of brain connections with the activation of the modulation system of descending inhibition and with the growth of new brain connections. The creation of new brain connections is also associated with the training of motor imaging, whose therapeutic efficacy has been tested, in isolation, in CLBP. Considering this, as well as the results of the second study, it was hypothesized that motor imaging training combined with high-intensity physical training would have better results compared to physical training when applied on its one. This hypothesis was tested in the third and fourth works of this thesis, and the results confirmed the working hypothesis, with improvement in the intensity and pain area of the patients. The results of the four studies carried out allow us to conclude that CLBP has a central affection component, with peripheral repercussions on the intensity and extent of the pain area, on the discrimination of the sensitivity of the dorsum and on the functional ability. High-intensity physical training, especially when associated with motor imaging training, minimizes these consequences

Perceptual stability during saccadic eye movements

Humans and other primates perform multiple fast eye movements per second in order to redirect gaze within the visual field. These so called saccades challenge visual perception: During the movement phases the projection of the outside world sweeps rapidly across the photoreceptors altering the retinal positions of objects that are otherwise stable in the environment. Despite this ever-changing sensory input, the brain creates the percept of a continuous, stable visual world. Currently, it is assumed that this perceptual stability is achieved by the synergistic interplay of multiple mechanisms, for example, a reduction of the sensitivity of the visual system around the time of the eye movement ('saccadic suppression') as well as transient reorganizations in the neuronal representations of space ('remapping'). This thesis comprises six studies on trans-saccadic perceptual stability