Functional correlate and delineated connectivity pattern of human motion aftereffect responses substantiate a subjacent visual-vestibular interaction

Angular distribution of fractal temporal correlations supports adaptive responses to wobble board instability

Contemporary dynamical models of human postural control propose an intermittent controller regulating the postural centre of pressure (CoP) about a stable saddle-shaped manifold along anatomical anteroposterior (AP) and mediolateral (ML) axes, releasing CoP in an outwards spiral when inactive. Experimental manipulations can evoke this saddle-type topology in fractal temporal correlations along the AP axis and reducing correlations along the ML axis. However, true effects of task demands may often manifest within angular space between anatomical AP and ML axes-a space not typically modelled explicitly. We tested how instability and attentional load influence postural control across the full angular range of fractal variability along the two-dimensional (2D) support surface. Forty-eight healthy young adults performed a suprapostural Trail Making Test (TMT) while standing on a wobble board, inducing continuous perturbations along the ML axis. Stable, quiet standing exhibited classic saddle-like topology, with stronger fractal temporal correlations in CoP displacements along AP axes. The attentional demand of the TMT did not affect angular variation or strength of fractal temporal correlations across the 2Dsupport surface. However, maintaining upright balance on the wobble board reshaped and reoriented the angular distribution of fractal temporal correlations, accentuating saddle-like angular variation and rotating the strongest fractal temporal correlations predominantly along the ML axis. Stabilizing posture in the face of wobble board instability prompted the saddle-type angular distribution of fractal temporal correlations. These findings challenge the traditional dependence of postural control theories exclusively on external force-plate axes and underscore the significance of multifractality in defining control parameters that govern postural stability across the full angular range of the 2D support surface.

Postural control in gymnasts: anisotropic fractal scaling reveals proprioceptive reintegration in vestibular perturbation

Dexterous postural control subtly complements movement variability with sensory correlations at many scales. The expressive poise of gymnasts exemplifies this lyrical punctuation of release with constraint, from coarse grain to fine scales. Dexterous postural control upon a 2D support surface might collapse the variation of center of pressure (CoP) to a relatively 1D orientation-a direction often oriented towards the focal point of a visual task. Sensory corrections in dexterous postural control might manifest in temporal correlations, specifically as fractional Brownian motions whose differences are more and less correlated with fractional Gaussian noises (fGns) with progressively larger and smaller Hurst exponent H. Traditional empirical work examines this arrangement of lower-dimensional compression of CoP along two orthogonal axes, anteroposterior (AP) and mediolateral (ML). Eyes-open and face-forward orientations cultivate greater variability along AP than ML axes, and the orthogonal distribution of spatial variability has so far gone hand in hand with an orthogonal distribution of H, for example, larger in AP and lower in ML. However, perturbing the orientation of task focus might destabilize the postural synergy away from its 1D distribution and homogenize the temporal correlations across the 2D support surface, resulting in narrower angles between the directions of the largest and smallest H. We used oriented fractal scaling component analysis (OFSCA) to investigate whether sensory corrections in postural control might thus become suborthogonal. OFSCA models raw 2D CoP trajectory by decomposing it in all directions along the 2D support surface and fits the directions with the largest and smallest H. We studied a sample of gymnasts in eyes-open and face-forward quiet posture, and results from OFSCA confirm that such posture exhibits the classic orthogonal distribution of temporal correlations. Head-turning resulted in a simultaneous decrease in this angle Δθ, which promptly reversed once gymnasts reoriented their heads forward. However, when vision was absent, there was only a discernible negative trend in Δθ, indicating a shift in the angle's direction but not a statistically significant one. Thus, the narrowing of Δθ may signify an adaptive strategy in postural control. The swift recovery of Δθ upon returning to a forward-facing posture suggests that the temporary reduction is specific to head-turning and does not impose a lasting burden on postural control. Turning the head reduced the angle between these two orientations, facilitating the release of postural degrees of freedom towards a more uniform spread of the CoP across both dimensions of the support surface. The innovative aspect of this work is that it shows how fractality might serve as a control parameter of adaptive mechanisms of dexterous postural control.

Cerebral Hemodynamic Responses to the Sensory Conflict Between Visual and Rotary Vestibular Stimuli: An Analysis With a Multichannel Near-Infrared Spectroscopy (NIRS) System

Sensory conflict among visual, vestibular, and somatosensory information induces vertiginous sensation and postural instability. To elucidate the cognitive mechanisms of the integration between the visual and vestibular cues in humans, we analyzed the cortical hemodynamic responses during sensory conflict between visual and horizontal rotatory vestibular stimulation using a multichannel near-infrared spectroscopy (NIRS) system. The subjects sat on a rotatory chair that was accelerated at 3°/s² for 20 s to the right or left, kept rotating at 60°/s for 80 s, and then decelerated at 3°/s² for 20 s. The subjects were instructed to watch white stripes projected on a screen surrounding the chair during the acceleration and deceleration periods. The white stripes moved in two ways; in the "congruent" condition, the stripes moved in the opposite direction of chair rotation at 3°/s² (i.e., natural visual stimulation), whereas in the "incongruent" condition, the stripes moved in the same direction of chair rotation at 3°/s² (i.e., conflicted visual stimulation). The cortical hemodynamic activity was recorded from the bilateral temporoparietal regions. Statistical analyses using NIRS-SPM software indicated that hemodynamic activity increased in the bilateral temporoparietal junctions (TPJs) and human MT+ complex, including the medial temporal (MT) area and medial superior temporal (MST) area in the incongruent condition. Furthermore, the subjective strength of the vertiginous sensation was negatively correlated with hemodynamic activity in the dorsal part of the supramarginal gyrus (SMG) in and around the intraparietal sulcus (IPS). These results suggest that sensory conflict between the visual and vestibular stimuli promotes cortical cognitive processes in the cortical network consisting of the TPJ, the medial temporal gyrus (MTG), and IPS, which might contribute to self-motion perception to maintain a sense of balance or equilibrioception during sensory conflict.

Rotating One's Head Modulates the Perceived Velocity of Motion Aftereffect

As a prominent illusion, the motion aftereffect (MAE) has traditionally been considered a visual phenomenon. Recent neuroimaging work has revealed increased activities in MT+ and decreased activities in vestibular regions during the MAE, supporting the notion of visual-vestibular interaction on the MAE. Since the head had to remain stationary in fMRI experiments, vestibular self-motion signals were absent in those studies. Accordingly, more direct evidence is still lacking in terms of whether and how vestibular signals modulate the MAE. By developing a virtual reality approach, the present study for the first time demonstrates that horizontal head rotation affects the perceived velocity of the MAE. We found that the MAE was predominantly perceived as moving faster when its direction was opposite to the direction of head rotation than when its direction was the same as head rotation. The magnitude of this effect was positively correlated with the velocity of head rotation. Similar result patterns were not observed for the real motion stimuli. Our findings support a 'cross-modal bias' hypothesis that after living in a multisensory environment long-term the brain develops a strong association between signals from the visual and vestibular pathways. Consequently, weak biasing visual signals in the associated direction can spontaneously emerge with the input of vestibular signals in the multisensory brain areas, substantially modulating the illusory visual motion represented in those areas as well. The hypothesis can also be used to explain other multisensory integration phenomena.

Non-linear Amplification of Variability Through Interaction Across Scales Supports Greater Accuracy in Manual Aiming: Evidence From a Multifractal Analysis With Comparisons to Linear Surrogates in the Fitts Task

Movement coordination depends on directing our limbs to the right place and in the right time. Movement science can study this central requirement in the Fitts task that asks participants to touch each of two targets in alternation, as accurately and as fast as they can. The Fitts task is an experimental attempt to focus on how the movement system balances its attention to speed and to accuracy. This balance in the Fitts task exhibits a hierarchical organization according to which finer details (e.g., kinematics of single sweeps from one target to the other) change with relatively broader constraints of task parameters (e.g., distance between targets and width of targets). The present work seeks to test the hypothesis that this hierarchical organization of movement coordination reflects a multifractal tensegrity in which non-linear interactions across scale support stability. We collected movement series data during a easy variant of the Fitts task to apply just such a multifractal analysis with surrogate comparison to allow clearer test of non-linear interactions across scale. Furthermore, we test the role of visual feedback both in potential and in fact, i.e., by manipulating both whether experimenters instructed participants that they might potentially have to close their eyes during the task and whether participants actually closed their eyes halfway through the task. We predict that (1) non-linear interactions across scales in hand movement series will produce variability that will actually stabilize aiming in the Fitts task, reducing standard deviation of target contacts; (2) non-linear interactions across scales in head sway will stabilize aiming following the actual closing eyes; and (3) non-linear interactions across scales in head sway and in hand movements will interact to support stabilizing effects of expectation about closing eyes. In sum, this work attempts to make the case that the multifractal-tensegrity hypothesis supports more accurate aiming behavior in the Fitts task.

Increased brain responsivity to galvanic vestibular stimulation in bilateral vestibular failure

In this event-related functional magnetic resonance imaging (fMRI) study we investigated how the brain of patients with bilateral vestibular failure (BVF) responds to vestibular stimuli. We used imperceptible noisy galvanic vestibular stimulation (GVS) and perceptible bi-mastoidal GVS intensities and related the corresponding brain activity to the evoked motion perception. In contrast to caloric irrigation, GVS stimulates the vestibular organ at its potentially intact afferent nerve site.

Motion perception thresholds and cortical responses were compared between 26 BVF patients to 27 age-matched healthy control participants. To identify the specificity of vestibular cortical responses we used a parametric design with different stimulus intensities (noisy imperceptible, low perceptible, high perceptible) allowing region-specific stimulus response functions. In a 2 × 3 flexible factorial design all GVS-related brain activities were contrasted with a sham condition that did not evoke perceived motion.

Patients had a higher motion perception threshold and rated the vestibular stimuli higher than the healthy participants. There was a stimulus intensity related and region-specific increase of activity with steep stimulus response functions in parietal operculum (e.g. OP2), insula, superior temporal gyrus, early visual cortices (V3) and cerebellum while activity in the hippocampus and intraparietal sulcus did not correlate with vestibular stimulus intensity. Using whole brain analysis, group comparisons revealed increased brain activity in early visual cortices (V3) and superior temporal gyrus of patients but there was no significant interaction, i.e. stimulus-response function in these regions were still similar in both groups. Brain activity in these regions during (high)GVS increased with higher dizziness-related handicap scores but was not related to the degree of vestibular impairment or disease duration. nGVS did not evoke cortical responses in any group.

Our data indicate that perceptible GVS-related cortical responsivity is not diminished but increased in multisensory (visual-vestibular) cortical regions despite bilateral failure of the peripheral vestibular organ. The increased activity in early visual cortices (V3) and superior temporal gyrus of BVF patients has several potential implications: (i) their cortical reciprocal inhibitory visuo-vestibular interaction is dysfunctional, (ii) it may contribute to the visual dependency of BVF patients, and (iii) it needs to be considered when BVF patients receive peripheral vestibular stimulation devices, e.g. vestibular implants or portable GVS devices. Imperceptible nGVS did not elicit cortical brain responses making it unlikely that the reported balance improvement of BVF by nGVS is mediated by cortical mechanisms.

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Responsivity to galvanic vestibular stimuli is increased in the visual and superior temporal Cortex of patients with bilateral vestibulopathy.

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Group differences correlated with clinical scores of disability.

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Dysfunctional visual-vestibular interaction is proposed.

Direction selective habituation of motion adaptation

Both adaptation and perceptual learning can change how we perceive the visual environment, reflecting the plasticity of the visual system. Our previous work has investigated the interaction between the two aspects of visual plasticity. One of the main findings is that multiple days of repeated motion adaptation attenuates motion aftereffect, which is explained by habituation of motion adaptation. Interestingly, there was almost no transfer of the effect to the untrained adapter, which differed from the trained adapter in the features including retinotopic location, spatiotopic location, and motion direction. Given that the reference frame of motion aftereffect is proposed to be retinotopic, it remains unclear whether the effect we refer to as habituation effect of motion adaptation is more like a special type of motion adaptation or not. Therefore, in three experiments, we examined the role of retinotopic location, spatiotopic location, and motion direction on the transfer of habituation, respectively. In each experiment, only one of the features was kept the same for the trained and untrained conditions. We found that the habituation effect transferred across both the retinotopic and spatiotopic locations as long as the adapting direction remained the same. The findings indicate that the effect we refer to as habituation of motion adaptation is anchored neither in eye-centered (retinotopic) nor world-centered (spatiotopic) coordinates. Rather, it is specific to the direction of the adapter. Therefore, the habituation effect of motion adaptation cannot be ascribed to a variant of motion adaptation.