Checkerboard Stimuli Research Articles

Impaired Potentiation of Theta Oscillations During a Visual Cortical Plasticity Paradigm in Individuals With Schizophrenia.

Long-term potentiation (LTP) is a form of experience-dependent synaptic plasticity mediated by glutamatergic transmission at N-methyl-D-aspartate receptors (NMDARs). Impaired neuroplasticity has been implicated in the pathophysiology of schizophrenia, possibly due to underlying NMDAR hypofunction. Analogous to the high frequency electrical stimulation used to induce LTP in vitro and in vivo in animal models, repeated high frequency presentation of a visual stimulus in humans in vivo has been shown to induce enduring LTP-like neuroplastic changes in electroencephalography (EEG)-based visual evoked potentials (VEPs) elicited by the stimulus. Using this LTP-like visual plasticity paradigm, we previously showed that visual high-frequency stimulation (VHFS) induced sustained changes in VEP amplitudes in healthy controls, but not in patients with schizophrenia. Here, we extend this prior work by re-analyzing the EEG data underlying the VEPs, focusing on neuroplastic changes in stimulus-evoked EEG oscillatory activity following VHFS. EEG data were recorded from 19 patients with schizophrenia and 21 healthy controls during the visual plasticity paradigm. Event-related EEG oscillations (total power, intertrial phase coherence; ITC) elicited by a standard black and white checkerboard stimulus (~0.83 Hz, several 2-min blocks) were assessed before and after exposure to VHFS with the same stimulus (~8.9 Hz, 2 min). A cluster-based permutation testing approach was applied to time-frequency data to examine LTP-like plasticity effects following VHFS. VHFS enhanced theta band total power and ITC in healthy controls but not in patients with schizophrenia. The magnitude and phase synchrony of theta oscillations in response to a visual stimulus were enhanced for at least 22 min following VHFS, a frequency domain manifestation of LTP-like visual cortical plasticity. These theta oscillation changes are deficient in patients with schizophrenia, consistent with hypothesized NMDA receptor dysfunction.

Decoding visual information from high-density diffuse optical tomography neuroimaging data

BackgroundNeural decoding could be useful in many ways, from serving as a neuroscience research tool to providing a means of augmented communication for patients with neurological conditions. However, applications of decoding are currently constrained by the limitations of traditional neuroimaging modalities. Electrocorticography requires invasive neurosurgery, magnetic resonance imaging (MRI) is too cumbersome for uses like daily communication, and alternatives like functional near-infrared spectroscopy (fNIRS) offer poor image quality. High-density diffuse optical tomography (HD-DOT) is an emerging modality that uses denser optode arrays than fNIRS to combine logistical advantages of optical neuroimaging with enhanced image quality. Despite the resulting promise of HD-DOT for facilitating field applications of neuroimaging, decoding of brain activity as measured by HD-DOT has yet to be evaluated. ObjectiveTo assess the feasibility and performance of decoding with HD-DOT in visual cortex. Methods and ResultsTo establish the feasibility of decoding at the single-trial level with HD-DOT, a template matching strategy was used to decode visual stimulus position. A receiver operating characteristic (ROC) analysis was used to quantify the sensitivity, specificity, and reproducibility of binary visual decoding. Mean areas under the curve (AUCs) greater than 0.97 across 10 imaging sessions in a highly sampled participant were observed. ROC analyses of decoding across 5 participants established both reproducibility in multiple individuals and the feasibility of inter-individual decoding (mean AUCs > 0.7), although decoding performance varied between individuals. Phase-encoded checkerboard stimuli were used to assess more complex, non-binary decoding with HD-DOT. Across 3 highly sampled participants, the phase of a 60° wide checkerboard wedge rotating 10° per second through 360° was decoded with a within-participant error of 25.8±24.7°. Decoding between participants was also feasible based on permutation-based significance testing. ConclusionsVisual stimulus information can be decoded accurately, reproducibly, and across a range of detail (for both binary and non-binary outcomes) at the single-trial level (without needing to block-average test data) using HD-DOT data. These results lay the foundation for future studies of more complex decoding with HD-DOT and applications in clinical populations.

Cerebrovascular reactivity in cerebral amyloid angiopathy, Alzheimer disease, and mild cognitive impairment.

To assess cerebrovascular reactivity in response to a visual task in participants with cerebral amyloid angiopathy (CAA), Alzheimer disease (AD), and mild cognitive impairment (MCI) using fMRI. This prospective cohort study included 40 patients with CAA, 22 with AD, 27 with MCI, and 25 healthy controls. Each participant underwent a visual fMRI task using a contrast-reversing checkerboard stimulus. Visual evoked potentials (VEPs) were used to compare visual cortex neuronal activity in 83 participants. General linear models using least-squares means, adjusted for multiple comparisons with the Tukey test, were used to estimate mean blood oxygen level-dependent (BOLD) signal change during the task and VEP differences between groups. After adjustment for age and hypertension, estimated mean BOLD response amplitude was as follows: CAA 1.88% (95% confidence interval [CI] 1.60%-2.15%), AD 2.26% (1.91%-2.61%), MCI 2.15% (1.84%-2.46%), and control 2.65% (2.29%-3.00%). Only patients with CAA differed from controls (p = 0.01). In the subset with VEPs, group was not associated with prolonged latencies or lower amplitudes. Lower BOLD amplitude response was associated with higher white matter hyperintensity (WMH) volumes in CAA (for each 0.1% lower BOLD response amplitude, the WMH volume was 9.2% higher, 95% CI 6.0%-12.4%) but not other groups (p = 0.002 for interaction) when controlling for age and hypertension. Mean visual BOLD response amplitude was lowest in participants with CAA compared to controls, without differences in VEP latencies and amplitudes. This suggests that the impaired visual BOLD response is due to reduced vascular reactivity in CAA. In contrast to participants with CAA, the visual BOLD response amplitude did not differ between those with AD or MCI and controls.

Hemodynamic latency is associated with reduced intelligence across the lifespan: an fMRI DCM study of aging, cerebrovascular integrity, and cognitive ability.

Changes in neurovascular coupling are associated with both Alzheimer's disease and vascular dementia in later life, but this may be confounded by cerebrovascular risk. We hypothesized that hemodynamic latency would be associated with reduced cognitive functioning across the lifespan, holding constant demographic and cerebrovascular risk. In 387 adults aged 18-85 (mean = 48.82), dynamic causal modeling was used to estimate the hemodynamic response function in the left and right V1 and V3-ventral regions of the visual cortex in response to a simple checkerboard block design stimulus with minimal cognitive demands. The hemodynamic latency (transit time) in the visual cortex was used to predict general cognitive ability (Full-Scale IQ), controlling for demographic variables (age, race, education, socioeconomic status) and cerebrovascular risk factors (hypertension, alcohol use, smoking, high cholesterol, BMI, type 2 diabetes, cardiac disorders). Increased hemodynamic latency in the visual cortex predicted reduced cognitive function (p < 0.05), holding constant demographic and cerebrovascular risk. Increased alcohol use was associated with reduced overall cognitive function (Full Scale IQ 2.8 pts, p < 0.05), while cardiac disorders (Full Scale IQ 3.3 IQ pts; p < 0.05), high cholesterol (Full Scale IQ 3.9 pts; p < 0.05), and years of education (2 IQ pts/year; p < 0.001) were associated with higher general cognitive ability. Increased hemodynamic latency was associated with reduced executive functioning (p < 0.05) as well as reductions in verbal concept formation (p < 0.05) and the ability to synthesize and analyze abstract visual information (p < 0.01). Hemodynamic latency is associated with reduced cognitive ability across the lifespan, independently of other demographic and cerebrovascular risk factors. Vascular health may predict cognitive ability long before the onset of dementias.

Nonspecific Visuospatial Imagery as a Novel Mental Task for Online EEG-Based BCI Control.

Brain-computer interfaces (BCIs) can provide a means of communication to individuals with severe motor disorders, such as those presenting as locked-in. Many BCI paradigms rely on motor neural pathways, which are often impaired in these individuals. However, recent findings suggest that visuospatial function may remain intact. This study aimed to determine whether visuospatial imagery, a previously unexplored task, could be used to signify intent in an online electroencephalography (EEG)-based BCI. Eighteen typically developed participants imagined checkerboard arrow stimuli in four quadrants of the visual field in 5-s trials, while signals were collected using 16 dry electrodes over the visual cortex. In online blocks, participants received graded visual feedback based on their performance. An initial BCI pipeline (visuospatial imagery classifier I) attained a mean accuracy of [Formula: see text]% classifying rest against visuospatial imagery in online trials. This BCI pipeline was further improved using restriction to alpha band features (visuospatial imagery classifier II), resulting in a mean pseudo-online accuracy of [Formula: see text]%. Accuracies exceeded the threshold for practical BCIs in 12 participants. This study supports the use of visuospatial imagery as a real-time, binary EEG-BCI control paradigm.

Predicting Neural Response Latency of the Human Early Visual Cortex from MRI-Based Tissue Measurements of the Optic Radiation.

Although the non-invasive measurement of visually evoked responses has been extensively studied, the structural basis of variabilities in latency in healthy humans is not well understood. We investigated how tissue properties of optic radiation could predict interindividual variability in the latency of the initial visually evoked component (C1), which may originate from the primary visual cortex (V1). We collected C1 peak latency data using magnetoencephalography (MEG) and checkerboard stimuli, and multiple structural magnetic resonance imaging (MRI) data from 20 healthy subjects. While we varied the contrast and position of the stimuli, the C1 measurement was most reliable when high-contrast stimuli were presented to the lower visual field (LVF). We then attempted to predict interindividual variability in C1 peak latency in this stimulus condition with a multiple regression model using MRI parameters along the optic radiation. We found that this model could predict >20% of variance in C1 latency, when the data were averaged across the hemispheres. The model using the corticospinal tract did not predict variability in C1 latency, suggesting that there is no evidence for generalization to a non-visual tract. In conclusion, our results suggest that the variability in neural latencies in the early visual cortex in healthy subjects can be partly explained by tissue properties along the optic radiation. We discuss the challenges of predicting neural latency using current structural neuroimaging methods and other factors that may explain interindividual variance in neural latency.

Vascular physiology drives functional brain networks

We present the first evidence for vascular regulation driving fMRI signals in specific functional brain networks. Using concurrent neuronal and vascular stimuli, we collected 30 BOLD fMRI datasets in 10 healthy individuals: a working memory task, flashing checkerboard stimulus, and CO2 inhalation challenge were delivered in concurrent but orthogonal paradigms. The resulting imaging data were averaged together and decomposed using independent component analysis, and three “neuronal networks” were identified as demonstrating maximum temporal correlation with the neuronal stimulus paradigms: Default Mode Network, Task Positive Network, and Visual Network. For each of these, we observed a second network component with high spatial overlap. Using dual regression in the original 30 datasets, we extracted the time-series associated with these network pairs and calculated the percent of variance explained by the neuronal or vascular stimuli using a normalized R2 parameter. In each pairing, one network was dominated by the appropriate neuronal stimulus, and the other was dominated by the vascular stimulus as represented by the end-tidal CO2 time-series recorded in each scan. We acquired a second dataset in 8 of the original participants, where no CO2 challenge was delivered and CO2 levels fluctuated naturally with breathing variations. Although splitting of functional networks was not robust in these data, performing dual regression with the network maps from the original analysis in this new dataset successfully replicated our observations. Thus, in addition to responding to localized metabolic changes, the brain’s vasculature may be regulated in a coordinated manner that mimics (and potentially supports) specific functional brain networks. Multi-modal imaging and advances in fMRI acquisition and analysis could facilitate further study of the dual nature of functional brain networks. It will be critical to understand network-specific vascular function, and the behavior of a coupled vascular-neural network, in future studies of brain pathology.

Fixation durations in natural scene viewing are guided by peripheral scene content.

Fixation durations provide insights into processing demands. We investigated factors controlling fixation durations during scene viewing in two experiments. In Experiment 1, we tested the degree to which fixation durations adapt to global scene processing difficulty by manipulating the contrast (from original contrast to isoluminant) and saturation (original vs. grayscale) of the entire scene. We observed longer fixation durations for lower levels of contrast, and longer fixation durations for grayscale than for color scenes. Thus fixation durations were globally slowed as visual information became more and more degraded, making scene processing increasingly more difficult. In Experiment 2, we investigated two possible sources for this slow-down. We used “checkerboard” stimuli in which unmodified patches alternated with patches from which luminance information had been removed (isoluminant patches). Fixation durations showed an inverted immediacy effect (longer, rather than shorter, fixation durations on unmodified patches) along with a parafoveal-on-foveal effect (shorter fixation durations, when an unmodified patch was fixated next). This effect was stronger when the currently fixated patch was isoluminant as opposed to unmodified. Our results suggest that peripheral scene information substantially affects fixation durations and are consistent with the notion of competition among the current and potential future fixation locations.

Glycinergic and GABAergic interneurons shift the location and differentially alter the size of ganglion cell receptive field centers in the mammalian retina

By using the multi-electrode array (MEA) recording technique in conjunction with white-noise checkerboard stimuli and reverse correlation methods, we studied modulatory actions of glycinergic and GABAergic interneurons on spatiotemporal profiles of ganglion cells (GCs) in dark-adapted mouse retinas. We found that application of 2 µM strychnine decreased receptive field center radii of GCs by a mean value of 11%, and shifted the GC receptive field (RF) centers by a mean distance of 28.3 µm. On the other hand, 200 µM picrotoxin + 100 µM bicuculline + 50 µM TPMPA increased GC receptive field center radii by a mean value of 19%, and shifted the GC RF centers by a mean distance of 53.7 µm. Glycinergic neurons in the mouse retina are narrow-field amacrine cells that have been shown to mediate ON-OFF crossover inhibitory synapses within the RGs’ RF center, therefore they may increase the size and shift the location of GC RF center by synergistic addition to bipolar cell inputs to GCs. GABAergic neurons are wide-field amacrine cells and horizontal cells that are known to mediate antagonistic surround responses of GCs, and thus they decrease the GCs’ RF center size. Our results suggest that a major global function of glycinergic and GABAergic interneurons in the mammalian retina is to provide the flexibility for adjusting the size and location of GCs’ RF centers. The apparent shifts of GC RF centers suggest that the synergistic addition by GlyACs and the surround inhibition by GABAergic interneurons are not spatially symmetrical within GC RFs.

Signal change in BA17, 18 and 19: A comparative study of normal, glaucoma suspects and POAG patients

Magnetic resonance signal change is expected to be different between normal, glaucoma suspect (GS) and Primary Open Angle Glaucoma (POAG) subjects. Functional magnetic resonance imaging (fMRI) technique was employed to investigate the differences. This multiple subject studies were carried out to characterize the percentage of signal change (PSC) in Brodmann Area (BA) 17, 18 and 19 of the visual cortex. The block designed fMRI was implemented. The subjects were requested to focus on the black and white checkerboard stimulus of the visual world (A, B and CD), visual field (EF) and visual area (G). The data were analysed using Statistical Parametric Mapping (SPM8) via Matlab platform. results showed that GS subjects have the highest PSC, followed by normal and POAG subjects. However, no significant difference was observed in PSC between BA17, 18 and 19 for normal, GS and POAG groups with p = 0.0869, p = 0.0688 and p = 0.2690, respectively. In conclusion, none of the BA can be suggested to be the most dominant area in responding to the visual stimuli used in this study.

Imaging brain function with simultaneous BOLD and viscoelasticity contrast: fMRI/fMRE

Magnetic resonance elastography (MRE) is emerging as a new tool for studying viscoelastic changes in the brain resulting from functional processes. Here, we demonstrate a novel time series method to generate robust functional magnetic resonance elastography (fMRE) activation maps in response to a visual task with a flashing checkerboard stimulus. Using a single-shot spin-echo (SS-SE) pulse sequence, the underlying raw images inherently contain blood-oxygen-level dependent (BOLD) contrast, allowing simultaneous generation of functional magnetic resonance imaging (fMRI) activation maps from the magnitude and functional magnetic resonance elastography (fMRE) maps from the phase. This allows an accurate comparison of the spatially localized stiffness (fMRE) and BOLD (fMRI) changes within a single scan, eliminating confounds inherent in separately acquired scans. Results indicate that tissue stiffness within the visual cortex increases 6–11% with visual stimuli, whereas the BOLD signal change was 1–2%. Furthermore, the fMRE and fMRI activation maps have strong spatial overlap within the visual cortex, providing convincing evidence that fMRE is possible in the brain. However, the fMRE temporal SNR (tSNRfMRE) maps are heterogeneous across the brain. Using a dictionary matching approach to characterize the time series, the viscoelastic changes are consistent with a viscoelastic response function (VRF) time constant of 12.1 ​s ± 3.0 ​s for a first-order exponential decay, or a shape parameter of 8.1 ​s ± 1.4 ​s for a gamma-variate.

Reduced pattern electroretinogram signal in affected patients with Leber Hereditary Optic Neuropathy

AbstractPurposeThe pattern electroretinogram (PERG) signal has been reported to be a sensitive tool for the detection of retinal ganglion cell (RGC) dysfunction in both animal models and patients suffering from optic neuropathies. This study investigated whether functional PERG can detect different levels of RGC dysfunction in both carriers and affected patients with Leber Hereditary Optic Neuropathy (LHON).MethodsPERG signals were recorded using both checkerboard (0.8° check size, 2 reversals/sec) and bars (0.5 cycles/deg) stimuli with mean luminance of 300 cd/m2 and 100% contrast. PERG signals were measured against functional full field Photopic Negative Response (PhNR) recordings (red flashes at 5 cd s/m2 stimulus intensity on blue rod saturating background) and subjective measures of retinal sensitivity (mean deviation [MD] via 30‐2 automated visual fields) and structural metrics (both ganglion cell complex [GCC] and retinal nerve fiber layer [RNFL] thickness from optical coherence tomography [OCT] scans).ResultsPERG P50 amplitudes were similar across controls, carriers, and LHON affected patients. P50 implicit times were significantly reduced in LHON affected patients (CHECKS: 45.8 ± 1.1 ms; BARS: 43.5 ± 0.8 ms; n = 32) compared to controls (CHECKS: 49.3 ± 0.5 ms; BARS: 48.1 ± 0.4 ms; n = 72). N95 amplitudes elicited by the checkerboard stimulus were significantly reduced and implicit times significantly increased in LHON affected patients (−3.4 ± 0.4 µV; 93.6 ± 2.2 ms; n = 32) compared to controls (−5.9 ± 0.5 µV; 85.3 ± 0.8 ms; n = 72). No differences were seen in PERG signals between control or LHON affected patients and carriers. Although PERG parameters were significantly different in LHON affected patients, the measurements were not correlated with PhNR signals, MD and RNFL thickness in all four quadrants around the optic disc. On the other hand, PhNR amplitudes, which were significantly reduced in LHON affected (−16.3 ± 3.4 µV; n = 32) and in carriers (−21.8 ± 3.0 µV; n = 30) compared to controls (−34.2 ± 2.5 µV; n = 28), were correlated with MD (r = −0.611, p &lt; 0.0005; n = 28), average GCC (r = −0.618, p &lt; 0.0001; n = 32) and average RNFL thickness (r = −0.553, p &lt; 0.001; n = 31) in affected patients.ConclusionsSignificant differences in PERG recordings were measured between control and LHON affected patients. However, no significant differences were seen in carriers and PERG metrics were not correlated with functional PhNR parameters and other subjective and objective measurements tested. Our results suggest that PERG may not be as sensitive as PhNR to discriminate different levels of RGC dysfunction between LHON affected patients and carriers.

The effect of stimulus contrast on the harmonic components of steady state visual evoked potentials

AbstractPurposeSteady state visual evoked potentials (ssVEP) are widely used for objective functional diagnostics. Due to stimulation with intermittent stimuli, the evoked response contains frequency components related to the stimulation frequency and its harmonics. In general, the evoked amplitude at stimulation frequency is used for functional diagnostics. However, clinicals trails also reported main components at harmonic frequencies. The origin of the harmonic frequency components is not fully understood. This work aims to analyze the influence of the stimulation contrast on harmonic components of ssVEP.MethodsWe studied 10 healthy subjects (mean age 27.2 ± 5.8 years) using checkerboard stimuli (d = 7.2°) with a check size of 0.35°. Stimuli were presented in pattern‐reversal mode (7.5 changes per second) for 5 contrast levels (98%, 50%, 25%, 12.5%, 10%) at mean luminance of 50 cd/m2 and 20 cd/m2 in a random order. The active electrode was placed at Oz, reference electrode at FCz and ground electrode at FPz. The stimulations were performed monocularly in a dimmed room. The recorded ssVEP were transformed using Fourier Analysis. The normalized ratio of the amplitudes at stimulation frequency (7.5 Hz) and first harmonic (15 Hz) were calculated. For statistical analysis Wilcoxon test for paired samples were used.ResultsIn our investigation, all 10 subjects exhibited the main component at stimulation frequency (7.5 Hz). The normalized ratio of the amplitudes showed no significant differences between the 5 contrast levels for the mean luminance of 50 cd/m2 and 20 cd/m2.ConclusionThe presented work revealed, that main components at harmonic frequencies are not dependent on the stimulation contrast. The stimulation contrast has no significant effect on the harmonic responses of ssVEP.

Performance Evaluation of Visual Noise Imposed Stochastic Resonance Effect on Brain-Computer Interface Application: A Comparison Between Motion-Reversing Simple Ring and Complex Checkerboard Patterns.

Adding noise to a weak input signal can enhance the response of a non-linear system, a phenomenon known as stochastic resonance (SR). SR has been demonstrated in a variety of diverse sensory systems including the visual system, where visual noise enhances human motion perception and detection performance. The SR effect has not been extensively studied in brain-computer interface (BCI) applications. This study compares the performance of BCIs based on SR-influenced steady-state motion visual evoked potentials. Stimulation paradigms were used between a periodically monochromatic motion-reversing simple ring and complex alternating checkerboard stimuli. To induce the SR effect, dynamic visual noise was masked on both the periodic simple and complex stimuli. Offline results showed that the recognition accuracy of different stimulation targets followed an inverted U-shaped function of noise level, which is a hallmark of SR. With the optimal visual noise level, the proposed visual noise masked checkerboard BCI paradigm achieved faster and more stable detection performance due to the noise-enhanced brain responses. This work demonstrates that the SR effect can be employed in BCI applications and can achieve considerable performance improvements.

Older Adults Exhibit Greater Visual Cortex Inhibition and Reduced Visual Cortex Plasticity Compared to Younger Adults.

Recent evidence indicates that inhibition within the visual cortex is greater in older than young adults. Increased inhibition has been associated with reduced visual cortex plasticity in animal models. We investigated whether age-related increases in human visual cortex inhibition occur in conjunction with reduced visual cortex plasticity. Visual cortex inhibition was measured psychophysically using binocular rivalry alternation rates (AR) for dichoptic gratings. Slower ARs are associated with a greater concentration of the inhibitory neurotransmitter GABA within the human visual cortex. Visual cortex plasticity was measured using an established paradigm for induction of long-term potentiation (LTP) -like increases in visually evoked potential (VEP) amplitude. Following rapid visual stimulation, greater increases in VEP amplitude indicate greater visual cortex plasticity. The study involved two groups; young (18–40 years, n = 29) and older adults (60–80 years, n = 18). VEPs were recorded for a 1 Hz onset/offset checkerboard stimulus before and after 9 Hz visual stimulation with the same stimulus. ARs were slower in older than young adults. In contrast to most previous studies, VEP amplitudes were significantly reduced following the rapid visual stimulation for young adults; older adult VEP amplitudes were unaffected. Our AR results replicate previous observations of increased visual cortex inhibition in the older adults. Rapid visual stimulation significantly altered VEP amplitude in young adults, albeit in the opposite direction than predicted. VEP amplitudes did not change in older adults suggesting an association between increased inhibition and reduced plasticity within the human visual cortex.

Portable System for Neuro-Optical Diagnostics Using Virtual Reality Display.

A new product prototype system for diagnosing vision and neurological disorders, called NeuroDotVR, is described herein: this system utilizes a novel wireless NeuroDot brain sensor [Versek C et al. J Neural Eng. 2018 Aug; 15(4):046027] that quantitatively measures visual evoked potentials and fields resulting from custom visual stimuli displayed on a smartphone housed in a virtual reality headset. The NeuroDot brain sensor is unique in that it can be operated both in regular electroencephalography mode, as well as a new electric field encephalography mode, which yields improvements in signal sensitivity and provides new diagnostic information. Steady state and transient visual evoked potentials and fields using reversing checkerboard stimuli are presented with case studies in amblyopia, glaucoma, and dark adaptation. These preliminary data sets highlight potential clinical applications that may be pursued in further product development and scientific studies.

Steady-State Visually Evoked Potentials Elicited by Multifrequency Pattern-Reversal Stimulation.

PurposeIt has been shown that multifrequency stimulation with multifocal electroretinography can reduce recording time without a loss in signal-to-noise ratio. Here, we studied the applicability of multifrequency stimulations for steady-state visually evoked potential (VEP) recordings.MethodsMultifrequency VEPs were recorded monocularly from 10 healthy subjects using pattern-reversal stimuli. The reversal frequency varied between 5 and 30 Hz. Pattern-reversal checkerboard stimuli were generated using four square arrays, each containing 100 light-emitting diodes (LEDs), positioned in four quadrants. Each array had a temporal frequency that differed slightly from the nominal frequency. The long duration of the data acquisition ensured that the slightly different stimulus frequencies in the four LED arrays can be resolved and that the responses to the stimulus in each array can be distinguished (e.g., with a frequency resolution: 0.011 Hz at 12 Hz). The best response from the four recording electrode configuration, defined as the recording with the maximal signal-to-noise ratio, was used for further analysis. Algorithmic latencies were calculated from the ratio of phase data and frequencies in a range of 4 and 20 Hz.ResultsQuadrant-VEPs with simultaneous pattern-reversal stimulation yielded a significant dependency on temporal frequency and stimulus location. The frequency range leading to the maximal response amplitude was between 10 and 12 Hz. Response phases decreased approximately linearly, with increasing temporal frequency suggesting a mean algorithmic latency between 112 and 126 ms.ConclusionsMultifrequency stimulation using LED arrays is an efficient method for recording pattern-reversal VEPs while all stimuli are presented at the same time.Translational RelevanceSimultaneously recorded VEPs as performed by the multi-frequency method can be used for objective measurements of visual field defects.

Cerebral blood volume changes during the BOLD post-stimulus undershoot measured with a combined normoxia/hyperoxia method

Cerebral blood flow (CBF) and blood oxygenation level dependent (BOLD) signal measurements make it possible to estimate steady-state changes in the cerebral metabolic rate of oxygen (CMRO2) with a calibrated BOLD method. However, extending this approach to measure the dynamics of CMRO2 requires an additional assumption: that deoxygenated cerebral blood volume (CBVdHb) follows CBF in a predictable way. A test-case for this assumption is the BOLD post-stimulus undershoot, for which one proposed explanation is a strong uncoupling of flow and blood volume with an elevated level of CBVdHb during the post-stimulus period compared to baseline due to slow blood volume recovery (Balloon Model). A challenge in testing this model is that CBVdHb differs from total blood volume, which can be measured with other techniques. In this study, the basic hypothesis of elevated CBVdHb during the undershoot was tested, based on the idea that the BOLD signal change when a subject switches from breathing a normoxic gas to breathing a hyperoxic gas is proportional to the absolute CBVdHb. In 19 subjects (8F), dual-echo BOLD responses were measured in primary visual cortex during a flickering radial checkerboard stimulus in normoxia, and the identical experiment was repeated in hyperoxia (50% O2/balance N2). The BOLD signal differences between normoxia and hyperoxia for the pre-stimulus baseline, stimulus, and post-stimulus periods were compared using an equivalent BOLD signal calculated from measured R2* changes to eliminate signal drifts. Relative to the pre-stimulus baseline, the average BOLD signal change from normoxia to hyperoxia was negative during the undershoot period (p = 0.0251), consistent with a reduction of CBVdHb and contrary to the prediction of the Balloon Model. Based on these results, the BOLD post-stimulus undershoot does not represent a case of strong uncoupling of CBVdHb and CBF, supporting the extension of current calibrated BOLD methods to estimate the dynamics of CMRO2.

BOLD signal modulated with perception in the superficial layer of human V1 during binocular rivalry

During binocular rivalry, the alternation of the two eyes' percept correlates with the fluctuation of neural activities throughout the visual pathway, from as early as LGN and V1 to high-level occipital temporal visual cortices. However, it is not clear how feedforward and feedback processes interact to resolve the rivalrous inputs, and in particular, whether the observed neural fluctuations reflect feedforward, local or feedback modulations of resolved rivalry competition. To answer this question, we used ultra-high field fMRI at 7T with submillimeter resolution to measure rivalry-related signals from different layers of the human primary visual cortex, with the assumption that the middle layer is dominated by feedforward signals while feedback signals modulate the superficial and deep layers. BOLD signals in early visual cortex were acquired with T2*-weighted gradient echo EPI or T2-weighted balanced-SSFP pulse sequences. In the rivalry condition, a pair of orthogonal red and green gratings were dichoptically presented, and subjects reported their percept with button presses (red, green or mixed). In the replay condition, the same red/green gratings were monocularly presented in physical alternations to simulate the rivalry percept. The ocular dominance bias of V1 voxels was determined in separate scans using monocular checkerboard stimulus. Results show that in the replay condition, eye-specific modulation of BOLD signal was strongest in the middle (input) layer of V1; while in the rivalry condition, the rivalrous modulation of BOLD signal was strongest in the superficial layer. A transient signal was also observed, mainly from the superficial layer of V1, at the time points of perceptual transitions in the rivalry but not in the replay condition. These layer-specific fMRI findings support the idea that rivalry-related activity fluctuation in human primary visual cortex reflects feedback modulation from higher cortical areas. Meeting abstract presented at VSS 2018

Evoked potentials in diagnosis of visual dysfunction in amateur boxers

ABSTRACTObjective: Injuries with varying degrees of temporary or permanent visual dysfunction are common in boxing. This study presents clinical diagnostic information regarding the functional integrity of the visual system in elite amateur boxers. The objective of this study was to assess the presence of normal or abnormal pattern visual evoked potentials (VEP). VEP in boxers were analyzed in relation to the nonathletic group and years of boxing activity.Methods: Clinical examination involved 31 boxers (21 male and 10 female) and 31 controls homogeneous in terms of age and gender. Pattern-reversal VEP elicited by checkerboard stimuli with large (LC) and small checks (SC) under monocular condition were applied. The latency and amplitude of N75, P100 and N135 components of the VEP waveform were analyzed. Absolute values and interocular differences of P100 latency and N75-P100 amplitude were used in determining VEP abnormalities.Results: Individual analysis showed prolonged P100 latency in both eyes in one male boxer. Interocular P100 latency differences beyond 8 ms were observed in three male boxers. The N75-P100 amplitude of four boxers exceeded the normal range in both eyes for the LC stimulation and one boxer for the SC stimulation. Interocular N75-P100 amplitude differences beyond the normal range in two cases for both the LC and the SC stimulation were confirmed. There was a positive correlation between years of boxing activity and N75 latency in SC (R = 0.480, p < 0.05) and N75-P100 amplitude in LC (R = -0.370, p < 0.05).Conclusion: Long-term boxing training may cause impairments in neural conductivity in the visual pathway. VEP seem to be a valuable tool in the neurophysiological diagnosis of visual function in contact sports. They can be recommended as a systematical examination for boxers during training processes for the indication and reduction in the incidence of vision-threatening injuries.