Visuo-vestibular mechanisms of bodily self-consciousness

Bodily self-consciousness is linked to multisensory integration and is particularly dependent on vestibular perception providing the brain with the main sensory cues about body motion and location in space. Vestibular and visual inputs are permanently balanced and integrated to encode the most optimal representation of the external world and of the observer in the central nervous system. Vection, an illusory self-motion experience induced only by visual stimuli, illustrates the fact that the visual and the vestibular systems share common neural underpinnings and a similar phenomenology. Optokinetic stimulation inducing vection and direct vestibular stimulation induce whole-body motion sensations that can be used to explore multisensory interactions. A failure in visuo-vestibular integration, artificially induced by the methods of cognitive psychology or in pathological conditions, has also been reported to altered own body perception and bodily self-consciousness. The respective contributions of the vestibular and visual systems to bodily self-consciousness amongst other polymodal sensory mechanisms, and the neural correlates of visuo-vestibular convergence, should be better understood. We first performed a neuroimaging study of brain regions where optokinetic and vestibular stimuli converge, using 7T functional magnetic resonance imaging in individual subjects. We identified three main regions of convergence: (1) the depth of supramarginal gyrus or retroinsular cortex, (2) the surface of supramarginal gyrus at the temporo-parietal junction, (3) and the posterior part of middle temporal gyrus and superior temporal sulcus. Then, we aimed to induce the embodiment of an external fake rubber hand through visuo-tactile conflict - the so-called rubber hand illusion paradigm, and studied how this integration is modulated by vection. Subjects experiencing vection in the direction of the rubber hand mislocalised the position of their real hand towards the rubber hand indicating that visuo-vestibular stimuli can enhance visuo-tactile integration. We also investigated if visuo-proprioceptive and tactile integration in peripersonal space could be dynamically updated based on the congruency of visual and proprioceptive feedback. A pair of rubber hands or feet provided visual feedback. Fake and real limbs were crossed or uncrossed. We showed that sensory cues were integrated in peripersonal space, dynamically reshaped but only for hands. Finally, we investigated a rare case of an illusory own body perception in an epileptic patient suffering from multiple daily disembodiments during seizures. Seizures were associated to a focal cortical microdysplasia juxtaposed to a developmental venous anomaly in the left angular gyrus, a brain region known to be important for visuo-vestibular integration and bodily self-consciousness. Our results characterize the inferior parietal lobule as a crucial structure in merging visual, vestibular, tactile and proprioceptive inputs, allowing the emergence of the global and unified experience of being âI.â Multisensory body representation can be reshaped transiently using visual and vestibular signals or in relation to a medical condition affecting the temporo-parietal junction. The integration of visual and vestibular signals, aims to adapt dynamically our internal representations to constant changes occurring in our environment

Characterizing the dynamics of vestibular reflex gain modulation using balance-relevant sensory conflict

Electrical vestibular stimulation (EVS) can be used to evoke reflexive body sways as a probe of vestibular control of balance. However, EVS introduces sensory conflict by decoupling vestibular input from actual body motion, prompting the central nervous system (CNS) to potentially perceive vestibular signals as less reliable. In contrast, light touch reduces sway by providing reliable feedback about body motion and spatial orientation. The juxtaposition of reliable and unreliable sensory cues enables exploration of multisensory integration during balance control. I hypothesized that when light touch is available, coherence and gain between EVS input and center of pressure (CoP) output would decrease as the CNS reduces the weighting of vestibular cues. Additionally, I hypothesized that the CNS would require less than 0.5 seconds to adjust weighting of sensory cues upon introduction or removal of light touch. In two experiments, participants stood as still as possible while receiving continuous stochastic EVS (with a frequency of 0-25 Hz, amplitude of ± 4 mA, and a duration of 200-300 seconds), while either: lightly touching a load cell (<2 N); holding their hand above a load cell; or intermittently switching between touching and not touching the load cell. Anterior-posterior (AP) CoP and linear accelerations from body-worn accelerometers were collected to calculate the root mean square (RMS) of AP CoP, as well as the coherence and gain between EVS input and AP CoP or acceleration outputs. Light touch led to a decrease in CoP RMS (mean 49% decrease) with and without EVS. Significant coherence between EVS and AP CoP was observed between 0.5 Hz and 24 Hz in the NO TOUCH condition, and between 0.5 Hz and 30 Hz in the TOUCH condition, with TOUCH having significantly greater coherence from 11 to 30 Hz. Opposite to coherence, EVS-AP CoP gain decreased in the TOUCH condition between 0.5-8 Hz (mean decrease 63%). Among the available acceleration data, only the head exhibited a significant increase in coherence above 10 Hz in the TOUCH condition, compared to the NO TOUCH condition. Light touch reduced CoP displacement, but increased variation in the CoP signal that can be explained by EVS input. Light touch may cause the CNS to attribute EVS signals to head movements and therefore up-weight vestibulocollic responses while downweighting vestibulospinal balance responses. Changes in coherence and gain started before the transition to the NO TOUCH condition and after the transition to the TOUCH condition. The loss of sensory information may be more destabilizing than addition, necessitating anticipatory adjustments. These findings demonstrate the ability of one sensory modality to modulate the utilization of another by the CNS, and highlight asymmetries in the timing of responses to the introduction and removal of sensory information, which may impact behavior.

Long-term memory encoding of facial information in humans: an EEG and tACS study

In recent years, the investigation of memory formation and retrieval has attracted increasing interest. As oscillatory activity plays a crucial role in neuroplastic processes, episodic memory is to a considerable extent attributable to synaptic changes, synchronization, and neurophysiological alterations through oscillating electric fields. Perception processes are part of episodic memory encoding. Human face perception and encoding arouse particular interest due to their fundamental relevance in social behavior. This study aimed to determine the causal role of brain dynamics in the encoding of facial episodic memory in humans. As recent studies revealed an enhancement in cognitive processes by the entrainment of internal brain oscillations, tACS stepped up as a new method of non-invasive brain stimulation to induce neuroplasticity (Antal et al. 2008; Antal and Herrmann 2016). It is a promising tool to test the role of brain oscillations on episodic memory encoding in humans and the potential for memory improvement. For the entire study, we developed a memory task that includes encoding, a Short-Term Memory Retrieval Part, maintenance, and a Long-Term Memory Retrieval Part. In the longterm face recognition, we assessed both the performance and the choice confidence on the 3-point scale. Two consecutive experiments were performed. For the first experiment (20 participants), we used 128-channel EEG to identify the region of the brain that is exclusively responsible for the long-term face encoding and the frequency of the brain rhythm that is associated with the successful subsequent recognition. Then, we conducted the tACS experiment (25 participants) based on the frequency and spatial data from the EEG experiment. We implemented a double-blinded, randomized, counterbalanced, crossover, and placebo-controlled study design. 20 minutes of 4 Hz-tACS at 3 mA peak-to-peak were applied during the encoding task to the identified right or to the left TPO area for active control. One more session included sham stimulation to one or the other area. The EEG study revealed a significant synchronization of brain oscillations during successful long-term facial memory encoding in the right TPO area at the low theta range (4 Hz). In complete agreement, the placebo-controlled tACS study showed a significant enhancement of long-term memory recognition performance after the low theta-stimulation of the right but not the left TPO area. The results indicate that low theta oscillations in the right TPO area are vital for successful episodic long-term memory encoding of facial stimuli. Secondly, we confirm that active low theta-tACS applied on this area during encoding improves the subsequent memory recognition performance. This supports the concept of lateralization for face processing in the right posterior brain region; moreover it puts forward this area as a crucial neocortical node in communication with the hippocampus for the long-term memory encoding (Pitcher et al. 2011; Geib et al. 2017). The results are in agreement with other studies that revealed a positive correlation between theta power and memory performance (Pahor and Jaušovec 2014; Clouter et al. 2017). However, the present work reveals a causal link between the empowered low theta in the right TPO area and enhanced subsequent long-term memory recognition. In summary, tACS is a highly suitable non-invasive tool to entrain local neocortical low theta activity and enhance long-term memory encoding, which is important in the clinical context for revealing novel therapeutic strategies in prosopagnosia and prosopamnesia.2021-09-1

Induction of plasticity in subcortical structures and its application in spinal cord injury

Ph. D. ThesisMost current non-invasive plasticity protocols target the motor cortex and its corticospinal projections. Approaches for inducing plasticity in sub-cortical circuits and alternative descending pathways such as the reticulospinal tract (RST) are less well developed. The overall aim of this thesis was to gain a better understanding of the extent to which corticospinal transmissions are altered after spinal cord injury (SCI) and to explore the mechanisms of non-invasive stimulation protocols at the cortical and subcortical level. In the first study, transcranial magnetic stimulation was used to elicit motor-evoked potentials (MEPs) in the biceps brachii using different coil orientations, which allows for preferential activation of different neural elements. Analysis of MEP latencies suggests that differences between MEPs elicited by specific coil orientations may not be fully preserved in humans with cervical SCI, both in the biceps and in more distal muscle groups. In a second study, we developed a novel associative stimulation paradigm, which paired loud acoustic stimuli with transcranial magnetic stimulation over the motor cortex in healthy participants and observed enhanced motor output after stimulus pairing ended. Electrophysiological measurements in humans and direct measurements in monkeys undergoing a similar protocol implicate corticoreticular connections as the most likely substrate for the plastic changes. Finally, we used a custom built device to deliver precisely paired auditory clicks with electric stimulation to the muscle. We observed changes in electrophysiological measurements consistent with the induction of sub-cortical plasticity in the biceps muscle. We then used the same protocol to target the triceps muscle in individuals with SCI over the course of 4 weeks. Notably, we did not observe the same changes as in the biceps muscle, suggesting that elbow flexors and extensors have a different potential for plasticity, perhaps due to a differential control of flexor and extensor motoneurons by corticospinal and reticulospinal pathways.International Spinal Research Trus

Vestibular Contributions to Human Memory

The vestibular system is an ancient structure which supports the detection and control of self-motion. The pervasiveness of this sensory system is evidenced by the diversity of its anatomical projections and the profound impact it has on a range of higher level functions, particularly spatial memory. The aim of this thesis was to better characterise the association between the vestibular system and human memory; while many studies have explored this association from a biological perspective few have done so from a psychological one. In Chapter 1, evidence was drawn from 101 neuro-otology patients to show that vestibular dysfunction can exert a direct negative effect on memory and allied cognitive processes, independently of age and comorbid psychiatric and fatigue symptoms. In Chapters 3 and 4, the separability of these cognitive, psychiatric and fatigue symptoms was further demonstrated in eight traumatic brain injury patients who, following a programme of daily vestibular stimulation, showed cognitive improvement and electrophysiological modulation in the absence of psychiatric or fatigue-related changes. Finally in Chapter 5, a set of normative experiments indicated that, beyond any generic arousal effect (unspecific to any particular cognitive process), visual memory can utilise temporally coincident vestibular activation to help individuate one memory from another. Together these findings help clarify the range of and manner in which vestibular signals interact with visual short-term memory and allied cognitive processes. The findings also have clinical implications for the diagnosis and management of vestibular, neuropsychiatric and amnesic conditions

Identification of residual descending pathways after human spinal cord injury.

Spinal Cord Injury (SCI) in humans is a heterogeneous diagnosis, resulting in variable paralysis and paresthesia based on the mechanism, rostro-caudal location, and severity of injury. Both neurophysiological and anatomical studies have suggested that subclinical residual supraspinal-spinal connectivity exists in a subset of individuals deemed to have motor and sensory complete injuries. Recent reports of volitional movement in chronic, motor complete individuals during epidural spinal stimulation have provided compelling evidence that these residual projections may be capable of mediating volitional movement when the functional state of spinal circuitry is electrically modulated. It was the goal of this project to identify subclinical and pathway-specific subliminal influences on spinal excitability after human SCI, and further to determine their relationship to volitional muscle activation after injury. Results demonstrated that volitional muscle activation can be identified and quantified in an objective manner via neurophysiological assessment, with greater resolution than current clinical measures. Electrophysiological studies probing the nervous system in subjects with chronic SCI, evidence was obtained of residual descending influence even in subjects classified as having motor and sensory complete injuries. Comparisons between volitional muscle activation and detection of descending modulation of multisegmental muscle responses in incomplete SCI participants revealed that interlimb modulation of a given motor pool was a strong predictor of predicted volitional movement, but was also observed in muscles without volitional activation, suggesting the pathways mediating the observed modulation may be necessary but not sufficient for volitional muscle activation after SCI

Aerospace Medicine and Biology: 1983 cumulative index

This publication is a cumulative index to the abstracts contained in the Supplements 242 through 253 of Aerospace Medicine and Biology: A Continuing Bibliography. It includes six indexes--subject, personal author, corporate source, contract number, report number, and accession number