Electrophysiological hallmarks of location-based and object-based visual multiple objects tracking.

Converging evidence shows that our visual system can track multiple visual, independently moving items over time. This is accomplished location-based by maintaining the individual spatial information of each target item or object-based by constructing an abstract object-based representation out of the tracked items. Previous work showed specific behavioural, electrophysiological and haemodynamic markers for location-based or object-based representations of the relevant targets by probing the encoded information subsequently after tracking. However, domain-specific differences of representational correlates during visual tracking itself have not been reported yet. The current study aims to identify spectral properties of the electrophysiological signal during tracking that might indicate location-based versus object-based maintenance of visual information. Subjects had to covertly track four out of eight visually identical items for several seconds while electrophysiological signals were recorded. Subsequently, a probe consisting of four items appeared and the subjects had to indicate with a button press whether the probe matched all targets or not. Subjects employing an object-based strategy showed an enhanced gamma response during the presentation of the target items at the beginning of the trial. On the other hand, subjects using a location-based strategy showed enhanced gamma synchronization throughout the tracking itself. Both the object- and location-based gamma responses yielded identical spatial topographical field distributions. These results indicate that object-based tracking is supported by enhanced encoding during the initial presentation of the targets to be tracked. Location-based tracking is characterized by the sustained maintenance of the individual targets during the entire tracking period in that same processing network.

Brain activity is contingent on neuropsychological function in a functional magnetic resonance imaging study of verbal working memory in amyotrophic lateral sclerosis.

BACKGROUND AND PURPOSE: Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that causes progressive degeneration of neurons in motor and non-motor brain regions, affecting multiple cognitive domains such as memory. A functional magnetic resonance imaging (fMRI) study was performed to explore working memory function in ALS. METHODS: To contribute to the growing research field that employs structural and functional neuroimaging to investigate the effect of ALS on different working memory components, the localization and intensity of alterations in neural activity was explored using fMRI. Being the first study to specifically address verbal working memory via fMRI in the context of ALS, the verbal n-back task with 0-back and 2-back conditions was employed. RESULTS: Despite ALS patients showing unimpaired accuracies (p = 0.724) and reaction times (p = 0.0785), there was significantly increased brain activity of frontotemporal and parietal regions in the 2-back minus 0-back contrast in patients compared to controls (using nonparametric statistics with 5000 permutations and a T threshold of 2.5). DISCUSSION: Increased brain activity of the frontotemporal and parietal regions during working memory performance was largely associated with better neuropsychological function within the ALS group, suggesting a compensatory effect during working memory execution. This study therefore adds to the current knowledge on neural correlates of working memory in ALS and contributes to a more nuanced understanding of hyperactivity during cognitive processes in fMRI studies of ALS.

Modulating the global orientation bias of the visual system changes population receptive field elongations.

The topographical structure of the visual system in individual subjects can be visualized using fMRI. Recently, a radial bias for the long axis of population receptive fields (pRF) has been shown using fMRI. It has been theorized that the elongation of receptive fields pointing toward the fovea results from horizontal local connections bundling orientation selective units mostly parallel to their polar position within the visual field. In order to investigate whether there is a causal relationship between orientation selectivity and pRF elongation the current study employed a global orientation adapter to modulate the orientation bias for the visual system while measuring spatial pRF characteristics. The hypothesis was that the orientation tuning change of neural populations would alter pRF elongations toward the fovea particularly at axial positions parallel and orthogonal to the affected orientation. The results indeed show a different amount of elongation of pRF units and their orientation at parallel and orthogonal axial positions relative to the adapter orientation. Within the lower left hemifield, pRF radial bias and elongation showed an increase during adaptation to a 135° grating while both parameters decreased during the presentation of a 45° adapter stimulus. The lower right visual field showed the reverse pattern. No modulation of the pRF topographies were observed in the upper visual field probably due to a vertical visual field asymmetry of sensitivity toward the low contrast spatial frequency pattern of the adapter stimulus. These data suggest a direct relationship between orientation selectivity and elongation of population units within the visual cortex.

Cortical, subcortical and spinal neural correlates of slackline training-induced balance performance improvements.

Humans develop posture and balance control during childhood. Interestingly, adults can also learn to master new complex balance tasks, but the underlying neural mechanisms are not fully understood yet. Here, we combined broad scale brain connectivity fMRI at rest and spinal excitability measurements during movement. Six weeks of slackline training improved the capability to walk on a slackline which was paralleled by functional connectivity changes in brain regions associated with posture and balance control and by task-specific changes of spinal excitability. Importantly, the performance of trainees was not better than control participants in a different, untrained balance task. In conclusion, slackline training induced large-scale neuroplasticity which solely transferred into highly task specific performance improvements.

Neural correlates of effort-dependent and effort-independent cognitive fatigue components in patients with multiple sclerosis.

BACKGROUND: Among patients with multiple sclerosis (MS), fatigue is the most commonly reported symptom. It can be subdivided into an effort-dependent (fatigability) and an effort-independent component (trait-fatigue). OBJECTIVE: The objective was to disentangle activity changes associated with effort-independent "trait-fatigue" from those associated with effort-dependent fatigability in MS patients. METHODS: This study employed behavioral measures and functional magnetic imaging to investigate neural changes in MS patients associated with fatigue. A total of 40 MS patients and 22 age-matched healthy controls performed in a fatigue-inducing N-back task. Effort-independent fatigue was assessed using the Fatigue Scale of Motor and Cognition (FSMC) questionnaire. RESULTS: Effort-independent fatigue was observed to be reflected by activity increases in fronto-striatal-subcortical networks primarily involved in the maintenance of homeostatic processes and in motor and cognitive control. Effort-dependent fatigue (fatigability) leads to activity decreases in attention-related cortical and subcortical networks. CONCLUSION: These results indicate that effort-independent (fatigue) and effort-dependent fatigue (fatigability) in MS patients have functionally related but fundamentally different neural correlates. Fatigue in MS as a general phenomenon is reflected by complex interactions of activity increases in control networks (effort-independent component) and activity reductions in executive networks (effort-dependent component) of brain areas.

Spatial elongation of population receptive field profiles revealed by model-free fMRI back-projection.

Estimates of visual field topographies in human visual cortex obtained through fMRI traveling wave techniques usually provide the parameters of population receptive field (pRF) location (polar angle, eccentricity) and receptive field size. These parameters are obtained by fitting the recorded data to a standard model population receptive field. In this work, pRF profiles are measured directly by back-projecting preprocessed fMRI time-series to sweeps of a bar across the visual field in different angles. The current data suggest that the model-free pRF profiles contain information not only about receptive field location and size but also about the pRF shape characteristics. The elongation (ellipticity) of pRFs decreases along the early visual hierarchy to a different degree for the ventral and the dorsal stream. Furthermore, ellipticity changes as a function of eccentricity. pRF orientation shows a high degree of collinearity with its angular position within the visual field. Using model-free pRF measurements, the traveling wave technique provides additional characteristics of pRF topographies that are not restricted to size and provide robust measures within the single subject.

Spatio-temporal dynamics of attentional selection stages during multiple object tracking.

Subjects can visually track several moving items simultaneously, a fact that is difficult to explain by classical attention models. Previous work revealed that building a global shape based on the spatial position of the tracked items improves performance. Here we investigated the involved neural processes and the role of attention. A task-irrelevant probe stimulus was presented during multiple objects tracking at a fixed spatial location. Depending on the tracked item's trajectories the probe appeared either outside, inside, or on the edge of aforementioned global shape. Event-related potentials to the probe stimulus revealed two subsequent stages of attentional selection during multiple object tracking. After 100ms attention was deployed on the edge/boundary of the figure formed by the tracked items. In the following 80ms, attention spread from the outline to the full figure. These findings clarify the eminent contribution of attentional mechanisms in multiple objects tracking.

Perimovement decrease of alpha/beta oscillations in the human nucleus accumbens.

The human nucleus accumbens is thought to play an important role in guiding future action selection via an evaluation of current action outcomes. Here we provide electrophysiological evidence for a more direct, i.e., online, role during action preparation. We recorded local field potentials from the nucleus accumbens in patients with epilepsy undergoing surgery for deep brain stimulation. We found a consistent decrease in the power of alpha/beta oscillations (10-30 Hz) before and around the time of movements. This perimovement alpha/beta desynchronization was observed in seven of eight patients and was present both before instructed movements in a serial reaction time task as well as before self-paced, deliberate choices in a decision making task. A similar beta decrease over sensorimotor cortex and in the subthalamic nucleus has been directly related to movement preparation and execution. Our results support the idea of a direct role of the human nucleus accumbens in action preparation and execution.

Neural sources of visual working memory maintenance in human parietal and ventral extrastriate visual cortex.

Maintaining information in visual working memory is reliably indexed by the contralateral delay activity (CDA) - a sustained modulation of the event-related potential (ERP) with a topographical maximum over posterior scalp regions contralateral to the memorized input. Based on scalp topography, it is hypothesized that the CDA reflects neural activity in the parietal cortex, but the precise cortical origin of underlying electric activity was never determined. Here we combine ERP recordings with magnetoencephalography based source localization to characterize the cortical current sources generating the CDA. Observers performed a cued delayed match to sample task where either the color or the relative position of colored dots had to be maintained in memory. A detailed source-localization analysis of the magnetic activity in the retention interval revealed that the magnetic analog of the CDA (mCDA) is generated by current sources in the parietal cortex. Importantly, we find that the mCDA also receives contribution from current sources in the ventral extrastriate cortex that display a time-course similar to the parietal sources. On the basis of the magnetic responses, forward modeling of ERP data reveals that the ventral sources have non-optimal projections and that these sources are therefore concealed in the ERP by overlapping fields with parietal projections. The present observations indicate that visual working memory maintenance, as indexed by the CDA, involves the parietal cortical regions as well as the ventral extrastriate regions, which code the sensory representation of the memorized content.

Spatio-temporal patterns of brain activity distinguish strategies of multiple-object tracking.

Human observers can readily track up to four independently moving items simultaneously, even in the presence of moving distractors. Here we combined EEG and magnetoencephalography recordings to investigate the neural processes underlying this remarkable capability. Participants were instructed to track four of eight independently moving items for 3 sec. When the movement ceased a probe stimulus consisting of four items with a higher luminance was presented. The location of the probe items could correspond fully, partly, or not at all with the tracked items. Participants reported whether the probe items fully matched the tracked items or not. About half of the participants showed slower RTs and higher error rates with increasing correspondence between tracked items and the probe. The other half, however, showed faster RTs and lower error rates when the probe fully matched the tracked items. This latter behavioral pattern was associated with enhanced probe-evoked neural activity that was localized to the lateral occipital cortex in the time range 170-210 msec. This enhanced response in the object-selective lateral occipital cortex suggested that these participants performed the tracking task by visualizing the overall shape configuration defined by the vertices of the tracked items, thereby producing a behavioral advantage on full-match trials. In a later time range (270-310 msec) probe-evoked neural activity increased monotonically as a function of decreasing target-probe correspondence in all participants. This later modulation, localized to superior parietal cortex, was proposed to reflect the degree of mismatch between the probe and the automatically formed visual STM representation of the tracked items.

Distinct representations of attentional control during voluntary and stimulus-driven shifts across objects and locations.

Efficient interaction with the sensory environment requires the rapid reallocation of attentional resources between spatial locations, perceptual features, and objects. It is still a matter of debate whether one single domain-general network or multiple independent domain-specific networks mediate control during shifts of attention across features, locations, and objects. Here, we employed functional magnetic resonance imaging to directly compare the neural mechanisms controlling attention during voluntary and stimulus-driven shifts across objects and locations. Subjects either maintained or switched voluntarily and involuntarily their attention to objects located at the same or at a different visual location. Our data demonstrate shift-related activity in multiple frontoparietal, extrastriate visual, and default-mode network regions, several of which were commonly recruited by voluntary and stimulus-driven shifts between objects and locations. However, our results also revealed object- and location-selective activations, which, moreover, differed substantially between voluntary and stimulus-driven attention. These results suggest that voluntary and stimulus-driven shifts between objects and locations recruit partially overlapping, but also separable, cortical regions, implicating the parallel existence of domain-independent and domain-specific reconfiguration signals that initiate attention shifts in dependence of particular demands.

Location- and Object-Based Representational Mechanisms Account for Bilateral Field Advantage in Multiple-Object Tracking.

Keeping track of multiple visually identical and independently moving objects is a remarkable feature of the human visual system. Theoretical accounts for this ability focus on resource-based models that describe parametric decreases of performance with increasing demands during the task (i.e., more relevant items, closer distances, higher speed). Additionally, the presence of two central tracking resources, one within each hemisphere, has been proposed, allowing for an independent maintenance of moving targets within each visual hemifield. Behavioral evidence in favor of such a model shows that human subjects are able to track almost twice as many targets across both hemifields compared with within one hemifield. A number of recent publications argue for two separate and parallel tracking mechanisms during standard object tracking tasks that allow for the maintenance of the relevant information in a location-based and object-based manner. Unique electrophysiological correlates for each of those processes have been identified. The current study shows that these electrophysiological components are differentially present during tracking within either the left or right hemifield. The present results suggest that targets are mostly maintained as an object-based representation during left hemifield tracking, while location-based resources are preferentially engaged during right hemifield tracking. Interestingly, the manner of representation does not seem to have an impact on behavioral performance within the subjects, while the electrophysiological component indicating object-based tracking does correlate with performance between subjects. We propose that hemifield independence during multiple-object tracking may be an indication of the underlying hemispheric bias for parallel location-based and object-based tracking mechanisms.

Spatiotemporal dynamics of feature-based attention spread: evidence from combined electroencephalographic and magnetoencephalographic recordings.

Attentional selection on the basis of nonspatial stimulus features induces a sensory gain enhancement by increasing the firing-rate of individual neurons tuned to the attended feature, while responses of neurons tuned to opposite feature-values are suppressed. Here we recorded event-related potentials (ERPs) and magnetic fields (ERMFs) in human observers to investigate the underlying neural correlates of feature-based attention at the population level. During the task subjects attended to a moving transparent surface presented in the left visual field, while task-irrelevant probe stimuli executing brief movements into varying directions were presented in the opposite visual field. ERP and ERMF amplitudes elicited by the unattended task-irrelevant probes were modulated as a function of the similarity between their movement direction and the task-relevant movement direction in the attended visual field. These activity modulations reflecting globally enhanced processing of the attended feature were observed to start not before 200 ms poststimulus and were localized to the motion-sensitive area hMT. The current results indicate that feature-based attention operates in a global manner but needs time to spread and provide strong support for the feature-similarity gain model.

Neural basis of multisensory looming signals.

Approaching or looming signals are often related to extremely relevant environmental events (e.g. threats or collisions) making these signals critical for survival. However, the neural network underlying multisensory looming processing is not yet fully understood. Using functional magnetic resonance imaging (fMRI) we identified the neural correlates of audiovisual looming processing in humans: audiovisual looming (vs. receding) signals enhance fMRI-responses in low-level visual and auditory areas plus multisensory cortex (superior temporal sulcus; plus parietal and frontal structures). When characterizing the fMRI-response profiles for multisensory looming stimuli, we found significant enhancements relative to the mean and maximum of unisensory responses in looming-sensitive visual and auditory cortex plus STS. Superadditive enhancements were observed in visual cortex. Subject-specific region-of-interest analyses further revealed superadditive response profiles within all sensory-specific looming-sensitive structures plus bilateral STS for audiovisual looming vs. summed unisensory looming conditions. Finally, we observed enhanced connectivity of bilateral STS with low-level visual areas in the context of looming processing. This enhanced coupling of STS with unisensory regions might potentially serve to enhance the salience of unisensory stimulus features and is accompanied by superadditive fMRI-responses. We suggest that this preference in neural signaling for looming stimuli effectively informs animals to avoid potential threats or collisions.

Induction of cognitive fatigue in MS patients through cognitive and physical load.

The objective of the study was to investigate whether cognitive fatigue in patients with multiple sclerosis (MS) is a spontaneous phenomenon or whether it can be provoked or exacerbated through cognitive effort and motor exercise. Thirty two patients with definite MS and cognitive fatigue according to the Fatigue Scale for Motor and Cognitive Functions (FSMC ≥ 22) performed attention tests (alertness, selective, and divided attention subtests from the TAP test battery for attention performance) twice during rest (baseline), and before and after treadmill training and cognitive load (a standardised battery of neuropsychological tests lasting 2.5 hours). Subjective exhaustion was assessed with a 10-point rating scale. Tonic alertness turned out to be the most sensitive test and showed significantly increased reaction times after treadmill training and after cognitive load. Patients' subjective assessment of exhaustion (10-point rating scale) and the objective test results were discrepant. In contrast, healthy control subjects (N = 20) did not show any decline of performance in the subtest alertness after cognitive or physical load. Data favour the concept that fatigue is induced by physical and mental load. Discrepancies between subjective and objective assessment offer therapeutic options. The common notion of a purely "subjective" lack of physical and/or mental energy should be reconsidered.

The locations of stroke lesions next to the posterior internal capsule may predict the recovery of the related proprioceptive deficits.

Background: Somatosensory deficits after stroke correlate with functional disabilities and impact everyday-life. In particular, the interaction of proprioception and motor dysfunctions affects the recovery. While corticospinal tract (CST) damage is linked to poor motor outcome, much less is known on proprioceptive recovery. Identifying a predictor for such a recovery could help to gain insights in the complex functional recovery processes thereby reshaping rehabilitation strategies. Methods: 50 patients with subacute stroke were tested before and after neurological rehabilitation. Proprioceptive and motor impairments were quantified with three clinical assessments and four hand movement and proprioception measures using a robotic device. Somatosensory evoked potentials (SSEP) to median nerve stimulation and structural imaging data (MRI) were also collected. Voxel-based lesion-symptom mapping (VLSM) along with a region of interest (ROI) analysis were performed for the corticospinal tract (CST) and for cortical areas. Results: Before rehabilitation, the VLSM revealed lesion correlates for all clinical and three robotic measures. The identified voxels were located in the white matter within or near the CST. These regions associated with proprioception were located posterior compared to those associated with motor performance. After rehabilitation the patients showed an improvement of all clinical and three robotic assessments. Improvement in the box and block test was associated with an area in anterior CST. Poor recovery of proprioception was correlated with a high lesion load in fibers towards primary sensorymotor cortex (S1 and M1 tract). Patients with loss of SSEP showed higher lesion loads in these tracts and somewhat poorer recovery of proprioception. The VSLM analysis for SSEP loss revealed a region within and dorsal of internal capsule next to the posterior part of CST, the posterior part of insula and the rolandic operculum. Conclusion: Lesions dorsal to internal capsule next to the posterior CST were associated with proprioceptive deficits and may have predictive value. Higher lesion load was correlated with poorer restoration of proprioceptive function. Furthermore, patients with SSEP loss trended towards poor recovery of proprioception, the corresponding lesions were also located in the same location. These findings suggest that structural imaging of the internal capsule and CST could serve as a recovery predictor of proprioceptive function.

Serial attentional resource allocation during parallel feature value tracking.

The visual system has evolved the ability to track features like color and orientation in parallel. This property aligns with the specialization of processing these feature dimensions in the visual cortex. But what if we ask to track changing feature-values within the same feature dimension? Parallel tracking would then have to share the same cortical representation, which would set strong limitations on tracking performance. We address this question by measuring the precision of color representations when human observers track the color of two superimposed dot clouds that simultaneously change color along independent trajectories in color-space. We find that tracking precision is highly imbalanced between streams and that tracking precision changes over time by alternating between streams at a rate of ~1 Hz. These observations suggest that, while parallel color tracking is possible, it is highly limited, essentially allowing for only one color-stream to be tracked with precision at a given time.

Neural correlates of hysterical blindness.

The neural mechanisms underlying conversion disorders such as hysterical blindness are at present unknown. Typically, patients are diagnosed through exclusion of neurological disease and the absence of pathologic neurophysiological diagnostic findings. Here, we investigate the neural basis of this disorder by combining electrophysiological (event-related potentials) and hemodynamic measures (functional magnet resonance tomography) in a patient with hysterical blindness before and after successful treatment. Importantly, the blindness was limited to the left upper and right lower visual quadrant offering the possibility to use the other 2 sighted quadrants as controls. While the functional magnetic resonance imaging activations were normal for visual stimulation electrophysiological indices of visual processing were modulated in a specific manner. Before treatment, the amplitude of the N1 event-related potentials component had smaller amplitudes for stimuli presented in the blind quadrants of the visual field. Following successful treatment the N1 component elicited by stimuli presented in formerly blind quadrants had a normal distribution without any amplitude differences between the 4 quadrants. The current findings point out that dissociative disorders such as hysterical blindness may have neurophysiological correlates. Furthermore, the observed neurophysiological pattern suggests an involvement of attentional mechanisms in the neural basis hysterical blindness.

Action imagery combined with action observation activates more corticomotor regions than action observation alone.

BACKGROUND AND PURPOSE: Both action observation (AO) and action imagery have been proposed as therapeutic options for stroke rehabilitation. Currently, it is not clear to what extent their underlying neuronal mechanisms differ from each other and whether one of these therapeutic options might be preferable for this purpose. METHODS: Twenty-six neurologically healthy subjects were investigated using functional magnetic resonance imaging during AO alone and during AO with additional action imagery of video clips showing simple, object-related hand actions. RESULTS: The blood oxygenation level dependent (BOLD) signal induced by AO increased in a bihemispheric, symmetrical network of areas including the occipital, superior, and inferior parietal cortex, dorsal and ventral premotor regions, and the prefrontal cortex. The addition of imagery to the AO elicited additional activation in both cerebellar hemispheres, caudate nucleus, ventral and dorsal premotor cortex, inferior parietal cortex, and the supplementary motor area. DISCUSSION AND CONCLUSION: These data reveal more profound activations of the motor system during AO in conjunction with imagery than during AO alone. These results may have important implications for neurorehabilitation and motor learning.

Adaptive and Maladaptive Brain Functional Network Reorganization After Stroke in Hemianopia Patients: An Electroencephalogram-Tracking Study.

Objective: Hemianopia after occipital stroke is believed to be mainly due to local damage at or near the lesion site. However, magnetic resonance imaging studies suggest functional connectivity network (FCN) reorganization also in distant brain regions. Because it is unclear whether reorganization is adaptive or maladaptive, compensating for, or aggravating vision loss, we characterized FCNs electrophysiologically to explore local and global brain plasticity and correlated FCN reorganization with visual performance. Methods: Resting-state electroencephalography (EEG) was recorded in chronic, unilateral stroke patients and healthy age-matched controls (n = 24 each). This study was approved by the local ethics committee. The correlation of oscillating EEG activity was calculated with the imaginary part of coherence between pairs of regions of interest, and FCN graph theory metrics (degree, strength, clustering coefficient) were correlated with stimulus detection and reaction time. Results: Stroke brains showed altered FCNs in the alpha- and low beta-band in numerous occipital, temporal brain structures. On a global level, FCN had a less efficient network organization whereas on the local level node networks were reorganized especially in the intact hemisphere. Here, the occipital network was 58% more rigid (with a more "regular" network structure) whereas the temporal network was 32% more efficient (showing greater "small-worldness"), both of which correlated with worse or better visual processing, respectively. Conclusions: Occipital stroke is associated with both local and global FCN reorganization, but this can be both adaptive and maladaptive. We propose that the more "regular" FCN structure in the intact visual cortex indicates maladaptive plasticity, where less processing efficacy with reduced signal/noise ratio may cause the perceptual deficits in the intact visual field (VF). In contrast, reorganization in intact temporal brain regions is presumably adaptive, possibly supporting enhanced peripheral movement perception.