Biology subjects

Kikkert, S.

Publications and source records attributed to Kikkert, S..

12 recordsLinked to original sources

bioRxiv2026

Centrally generated movement-related signals engage ipsilateral S1 finger representations after tetraplegia

Ipsilateral primary somatosensory cortex (S1) exhibits somatotopically organized activity during unilateral hand movements, but whether this activity depends on peripheral input remains debated. We addressed this question using functional magnetic resonance imaging (fMRI) in 14 individuals with tetraplegia and 18 able-bodied controls. Participants performed or attempted individual finger movements during 3T fMRI. We quantified movement-related univariate activity and multivariate representational structure within ipsilateral S1. Univariate activity within the anatomically defined ipsilateral S1 hand area did not differ significantly between groups, whereas a continuous spatial analysis identified greater movement-related activity in individuals with tetraplegia in S1 bands lateral to the hand representation. Multivariate analyses revealed reliable separation of finger-specific activity patterns in both groups, with no significant group difference in mean inter-finger distance. The typicality of the overall representational geometry also did not differ significantly between groups. Finally, a person with complete hand paralysis and no measurable feeling or movement still showed clear, finger-specific brain activity patterns and a typical representation of the affected hand. Together, these findings indicate that centrally generated movement-related signals can engage finger-specific representations in ipsilateral S1 without overt movement or measurable hand-related peripheral input. Ipsilateral somatotopic activity therefore cannot be explained solely by sensory reafference from the moving hand.

neuroscience↗

bioRxiv2025

Revisiting face-to-hand area remapping in the human primary somatosensory cortex after a cervical spinal cord injury

Spinal cord injury (SCI) leads to profound disruptions in sensorimotor processing. Seminal research in non-human primates suggests this sensory deprivation causes functional remapping in the primary somatosensory cortex (S1), where somatotopic representations of deprived body parts, such as the hand in cervical SCI, become responsive to touch on intact body parts, such as the face. However, evidence for such remapping in humans remains inconclusive. We investigated face-to-hand remapping in 16 chronic cervical SCI patients and 21 able-bodied controls using two fMRI experiments. Experiment 1 employed a lip movement task, while Experiment 2 investigated the full architecture of S1 face reorganisation through vibrotactile stimulation of the forehead, lips and chin. We assessed (1) the level of face activity in the anatomical S1 hand area, (2) cortical shifts in peak face activity, (3) face-part separability in the S1 hand area and (4) correlations with clinical characteristics that may drive face-to-hand area remapping. Our results revealed no significant evidence in favour of face-to-hand area remapping in tetraplegic patients across markers of face-to-hand remapping during either lip movement or vibrotactile stimulation of face parts. Furthermore, our markers of remapping did not correlate with clinical characteristics. These findings suggest that cortical face-to-hand remapping is not apparent in human cervical SCI patients. Beyond providing insight into the limitations of cortical reorganisation in humans, this highlights the need to reassess rehabilitation strategies based on the assumption of large-scale face-to-hand reorganisation after an SCI.

neuroscience↗

bioRxiv2024

Motor imagery and execution activate similar finger representations that are spatially consistent over time

Finger representations in the sensorimotor cortex can be activated even in the absence of somatosensory input or motor output through mere top-down processes, such as motor imagery. While executed finger movements activate finger representations in the primary sensorimotor cortex that are spatially consistent over time within participants, the stability of top-down activated finger representations remains largely unexplored. Given the increasing use of top-down activated sensorimotor representations to both plan implantation of and control brain-computer interfaces, it is crucial to understand the stability of these representations. Here, we investigated the spatial consistency, and thereby reliability, of finger representations activated through motor imagery in the primary somatosensory and primary motor cortex over time. To assess this, participants performed imagined and executed individual finger movements in two 3T fMRI sessions that were [~]2 weeks apart. We observed highly consistent univariate finger-selective activity clusters and multivariate vertex-wise activity patterns within participants over time in both the motor imagery and motor execution task. Using a multivariate across-task decoding approach, we further found that motor execution and motor imagery activate similar finger representations in both the primary somatosensory and primary motor cortex. This demonstrates that motor imagery can be used to identify finger representations related to movement execution. Our findings not only validate the use of top-down processes for brain-computer interface planning and control, but also open up new opportunities for the development of sensorimotor training interventions that do not rely on overt movements.

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bioRxiv2024

Distinguishing the activity of adjacent somatosensory nuclei within the brainstem using 3T fMRI

Experimental evidence in animal models indicates that the brainstem plays a major role in sensory modulation. However, mapping functional activity within the human brainstem presents many methodological challenges. These constraints have deterred essential research into human sensory brainstem processing. Here, using a 3T fMRI sequence optimised for the brainstem, combined with uni- and multivariate analysis approaches, we investigated the extent to which functional activity of neighbouring somatosensory nuclei can be delineated in the brainstem, thalamus and primary somatosensory cortex (S1). Whilst traditional univariate approaches offered limited differentiation between adjacent hand and face activation in the brainstem, multivariate classification enabled above-chance decoding of these activity patterns across S1, the thalamus, and the brainstem. Our findings establish a robust methodological approach to explore signal processing within the brainstem and across the entire somatosensory stream. This is a fundamental step towards broadening our understanding of somatosensory processing within humans and determining what changes in sensory integration may occur in clinical populations following sensory deprivation.

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bioRxiv2024

TMS-based neurofeedback training of mental finger individuation induces neuroplastic changes in the sensorimotor cortex

Neurofeedback (NF) training based on motor imagery is increasingly used in neurorehabilitation with the aim to improve motor functions. However, the neuroplastic changes underpinning these improvements are poorly understood. Here, we used mental finger individuation, i.e., the selective facilitation of single finger representations without producing overt movements, as a model to study neuroplasticity induced by NF. To enhance mental finger individuation, we used transcranial magnetic stimulation (TMS)-based NF training. During motor imagery of individual finger movements, healthy participants were provided visual feedback on the size of motor evoked potentials, reflecting their finger-specific corticospinal excitability. We found that TMS-NF improved the top-down activation of finger-specific representations. First, intracortical inhibitory circuits in the primary motor cortex were tuned after training such that inhibition was selectively reduced for the finger that was mentally activated. Second, motor imagery finger representations in sensorimotor areas assessed with functional MRI became more distinct after training. Together, our results indicate that the neural underpinnings of finger individuation, a well-known model system for neuroplasticity, can be modified using TMS-NF guided motor imagery training. These findings demonstrate that TMS-NF induces neuroplasticity in the sensorimotor system, highlighting the promise of TMS-NF on the recovery of fine motor function.

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bioRxiv2022

Through the eye to the brain: Modulating LC-mediated arousal via pupil-based neurofeedback

The brains arousal state is controlled by several neuromodulatory nuclei known to substantially influence cognition and mental well-being. Here, we investigate whether human participants can gain volitional control of their arousal state using a pupil-based biofeedback approach. Our approach inverts a mechanism suggested by previous literature that links activity of the locus coeruleus (LC), one of the key regulators of central arousal, and pupil dynamics. We show that pupil-based biofeedback enables participants to acquire volitional control of pupil size. Applying pupil self-regulation systematically modulates activity of the LC and other brainstem structures involved in arousal control. Further, it modulates cardiovascular measures such as heart rate, and behavioural and psychophysiological responses during an oddball task. We provide evidence that pupil-based biofeedback makes the brains arousal system accessible to volitional control, a finding that has tremendous potential for translation to behavioral and clinical applications across various domains, including stress-related and anxiety disorders.

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bioRxiv2021

Finger representations in primary somatosensory cortex are modulated during vibrotactile working memory

It is well-established that several cortical areas represent vibrotactile stimuli in somatotopic maps. However, whether such somatotopic representations remain active during the delay period of working memory (WM) tasks, i.e. in the absence of any tactile stimulation, is unknown. In our experiment, participants had to compare two tactile stimuli with different vibration frequencies that were separated by a delay period (memory condition) or they were exposed to identical stimuli but did not have to solve a WM task (no memory condition). Importantly, both vibrotactile stimuli were either applied to the right index or little finger. Analyzing the delay period, we identified a well-known fronto-parietal network of brain regions involved in WM but we did not find WM specific activity in S1. However, using multi-voxel pattern analysis (MVPA) and representational similarity analysis (RSA), we found that S1 finger representations were more dissimilar during the delay period of the WM condition than during the control condition. These results indicate that WM processes modulate the representational geometry of S1 suggesting that some aspects of the tactile WM content are represented in a somatotopic fashion. HIGHLIGHTSO_LIMultivariate approaches were used to identify finger specific representational changes during vibrotactile frequency discrimination. C_LIO_LIVibrotactile working memory modulates somatotopic finger representations in contralateral S1 during the delay period, i.e. in the absence of any tactile stimuli C_LI

neuroscience↗

bioRxiv2021

Hand and face somatotopy shown using MRI-safe vibrotactile stimulation with a novel Soft Pneumatic Actuator (SPA)-Skin interface

The exact somatotopy of the human facial representation in the primary somatosensory cortex (S1) remains debated. One reason that progress has been hampered is due to the methodological challenge of how to apply automated vibrotactile stimuli to face areas in a manner that is: 1) reliable despite differences in the curvatures of face locations; and 2) MR-compatible and free of MR-interference artefacts when applied in the MR head-coil. Here we overcome this challenge by using soft pneumatic actuator (SPA) technology. SPAs are made of a soft silicon material and can be in- or deflated by means of airflow, have a small diameter, and are flexible in structure, enabling good skin contact even on curved body surfaces (as on the face). To validate our approach, we first mapped the well-characterised S1 finger layout using this novel device and confirmed that tactile stimulation of the fingers elicited characteristic somatotopic finger activations in S1. We then used the device to automatically and systematically deliver somatosensory stimulation to different face locations. We found that the forehead representation was least distant from the representation of the hand. Within the face representation, we found that the lip representation is most distant from the forehead representation, with the chin represented in between. Together, our results demonstrate that this novel MR compatible device produces robust and clear somatotopic representational patterns using vibrotactile stimulation through SPA-technology.

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bioRxiv2021

Finger representation in the cortex of the congenitally blind

Hand representation in the primary somatosensory cortex (S1) is thought to be shaped by experience. Individuals with congenital blindness rely on their sense of touch for completing daily tasks that in sighted people would be informed by vision, and possess superior tactile acuity. It has therefore been proposed that their S1 hand representation should differ from that of sighted individuals. Alternatively, it has been proposed that the improved tactile acuity in blind individuals is due to cross-modal plasticity, when regions in the occipital and temporal cortex are typically used for processing vision become activated by touch. We probed finger representation using psychophysics and 7T fMRI (1 mm3 resolution) in three individuals with bilateral anophthalmia, a rare condition in which both eyes fail to develop, as well as sighted controls. Despite anophthalmic individuals increased reliance on touch and superior tactile acuity, we found no evidence that they had more pronounced hand representation in S1. This is in line with recent research highlighting the stability of early sensory cortex, despite altered sensorimotor experience in adulthood. Unlike sighted controls, anophthalmic individuals activated the left human middle temporal complex (hMT+) during finger movement. This area did not express any hallmark of typical sensorimotor organisation, suggesting this and previously reported activity does not indicate low-level sensorimotor hand representation. However, left hMT+ contained some single finger information, beyond that found in sighted controls. This latter finding suggests that when the developmentally flexible area hMT+ is unaffected by retinal input, it can acquire novel cross-modal processes, which are potentially unrelated to the areas function in sighted people. As such, our findings highlight the opportunity for other organising principles, beyond domain specific plasticity, in shaping cross-modal reorganisation.

neuroscience↗

bioRxiv2021

Mental individuation of imagined finger movements can be achieved using TMS-based neurofeedback

Neurofeedback (NF) in combination with motor imagery (MI) can be used for training individuals to volitionally modulate sensorimotor activity without producing overt movements. However, until now, NF methods were of limited utility for mentally training specific hand and finger actions. Here we employed a novel transcranial magnetic stimulation (TMS) based protocol to probe and detect MI-induced motor activity patterns in the primary motor cortex (M1) with the aim to reinforce selective facilitation of single finger representations. We showed that TMS-NF training but not MI training with uninformative feedback enabled participants to selectively upregulate corticomotor excitability of one finger, while simultaneously downregulating excitability of other finger representations within the same hand. Successful finger individuation during MI was accompanied by strong desynchronisation of sensorimotor brain rhythms, particularly in the beta band, as measured by electroencephalography. Additionally, informative TMS-NF promoted more dissociable EEG activation patterns underlying single finger MI, when compared to MI of the control group where no such feedback was provided. Our findings suggest that selective TMS-NF is a new approach for acquiring the ability of finger individuation even if no overt movements are performed. This might offer new treatment modality for rehabilitation after stroke or spinal cord injury.

neuroscience↗

bioRxiv2021

Finger somatotopy is preserved after tetraplegia but deteriorates over time

Following spinal cord injury (SCI), the motor output flow to the limb(s) and sensory input to the brain is largely lost. While attempted movements with the paralysed and sensory deprived body part can still evoke signals in the sensorimotor system, this task-related "net" brain activity of SCI patients differs substantially from healthy controls. Such reorganised and/or altered activity is thought to reflect abnormal processing. It is however possible that this altered net sensorimotor activity in SCI patients conceals preserved somatotopically-specific representations of the paralysed and sensory deprived body parts that could be exploited in a functionally meaningful manner (e.g. via neuroprosthetics). In this cross-sectional study, we investigated whether a functional connection between the periphery and the brain is necessary to maintain somatosensory representations. We used functional MRI and an (attempted) finger movement task to characterise the somatotopic hand layout in the primary somatosensory cortex and structural MRI to assess spared spinal tissue bridges. We tested 14 tetraplegic SCI patients (mean age {+/-} s.e.m.=55 {+/-} 3.6; 1 female) who differed in terms of lesion completeness, retained sensorimotor functioning, and time since injury, as well as 18 healthy control participants (mean age {+/-} s.e.m.=56 {+/-} 3.6 years; 1 female). Our results revealed somatotopically organised representations of patients hands in which neighbouring clusters showed selectivity for neighbouring fingers in contralateral S1, qualitatively similar to those observed in healthy controls. To quantify whether these representations were normal in tetraplegic SCI patients we correlated each participants intricate representational distance pattern across all fingers (revealed using representational similarity analysis) with a canonical inter-finger distance pattern obtained from an independent sample. The resulting hand representation typicality scores were not significantly different between patients and controls. This was even true when considering two individual patients with no sensory hand functioning, no hand motor functioning, and no spared spinal tissue bridges. However, a correlational analysis revealed that over years since SCI the hand representation typicality in primary somatosensory cortex deteriorates. We show that somatosensory representations can be maintained for several years following SCI even in the absence of perhiperhal inputs. Such preserved cortical hand representations could therefore be exploited in a functionally meaningful way by rehabilitation approaches that attempt to establish new functional connections between the hand and the brain after an SCI (e.g. through neuroprosthetics). However, time since injury may critically influence the somatotopic representations of SCI patients and might thereby impact the success of such rehabilitation approaches.

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bioRxiv2020

Malleability of the cortical hand map following a finger nerve block

Individual fingers in the primary somatosensory cortex (S1) are known to be represented separately and adjacently, forming a cortical hand map. Electrophysiological studies in monkeys show that finger amputation triggers increased selectivity to the neighbouring fingers within the deprived S1, causing local reorganisation. Neuroimaging research in humans, however, shows persistent S1 finger representation of the missing hand, even decades after amputation. We aimed to resolve these apparently contrasting evidence by examining finger representation in humans following pharmacological amputation using single-finger nerve block and 7T neuroimaging. We hypothesised that beneath the apparent selectivity of individual fingers in the hand map, peripheral and central processing is distributed across fingers. If each finger contributes to the cortical representation of the others, then localised input loss will weaken finger representation across the hand map. For the same reason, the non-blocked fingers will stabilise the blocked fingers representation, resulting in persistent representation of the blocked finger. Using univariate selectivity profiling, we replicated the electrophysiological findings of local S1 reorganisation. However, more comprehensive analyses confirmed that local blocking reduced representation of all fingers across the entire hand area. Importantly, multivariate analysis demonstrated that despite input loss, representation of the blocked finger remained persistent and distinct from the unblocked fingers. Computational modelling suggested that the observed findings are driven by distributed processing underlying the topographic map, combined with homeostatic mechanisms. Our findings suggest that the long-standing depiction of the somatosensory hand map is misleading. As such, accounts for map reorganisation, e.g. following amputation, need to be reconsidered.

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