Mesoscale functional connectivity in macaque visual areas.

Studies of resting-state functional connectivity (rsFC) have provided rich insights into the structures and functions of the human brain. However, most rsFC studies have focused on large-scale brain connectivity. To explore rsFC at a finer scale, we used intrinsic signal optical imaging to image the ongoing activity of the anesthetized macaque visual cortex. Differential signals from functional domains were used to quantify network-specific fluctuations. In 30-60 min resting-state imaging, a series of coherent activation patterns were observed in all three visual areas we examined (V1, V2, and V4). These patterns matched the known functional maps (ocular dominance, orientation, color) obtained in visual stimulation conditions. These functional connectivity (FC) networks fluctuated independently over time and exhibited similar temporal characteristics. Coherent fluctuations, however, were observed from orientation FC networks in different areas and even across two hemispheres. Thus, FC in the macaque visual cortex was fully mapped both on a fine scale and over a long range. Hemodynamic signals can be used to explore mesoscale rsFC in a submillimeter resolution.

Comparing BOLD and VASO-CBV population receptive field estimates in human visual cortex.

Vascular Space Occupancy (VASO) is an alternative fMRI approach based on changes in Cerebral Blood Volume (CBV). VASO-CBV fMRI can provide higher spatial specificity than the blood oxygenation level-dependent (BOLD) method because the CBV response is thought to be limited to smaller vessels. To investigate how this technique compares to BOLD fMRI for cognitive neuroscience applications, we compared population receptive field (pRF) mapping estimates between BOLD and VASO-CBV. We hypothesized that VASO-CBV would elicit distinct pRF properties compared to BOLD. Specifically, since pRF size estimates also depend on vascular sources, we hypothesized that reduced vascular blurring might yield narrower pRFs for VASO-CBV measurements. We used a VASO sequence with a double readout 3D EPI sequence at 7T to simultaneously measure VASO-CBV and BOLD responses in the visual cortex while participants viewed conventional pRF mapping stimuli. Both VASO-CBV and BOLD images show similar eccentricity and polar angle maps across all participants. Compared to BOLD-based measurements, VASO-CBV yielded lower tSNR and variance explained. The pRF size changed with eccentricity similarly for VASO-CBV and BOLD, and the pRF size estimates were similar for VASO-CBV and BOLD, even when we equate variance explained between VASO-CBV and BOLD. This result suggests that the vascular component of the pRF size is not dominating in either VASO-CBV or BOLD.

Neurovascular coupling and oxygenation are decreased in hippocampus compared to neocortex because of microvascular differences.

The hippocampus is essential for spatial and episodic memory but is damaged early in Alzheimer's disease and is very sensitive to hypoxia. Understanding how it regulates its oxygen supply is therefore key for designing interventions to preserve its function. However, studies of neurovascular function in the hippocampus in vivo have been limited by its relative inaccessibility. Here we compared hippocampal and visual cortical neurovascular function in awake mice, using two photon imaging of individual neurons and vessels and measures of regional blood flow and haemoglobin oxygenation. We show that blood flow, blood oxygenation and neurovascular coupling were decreased in the hippocampus compared to neocortex, because of differences in both the vascular network and pericyte and endothelial cell function. Modelling oxygen diffusion indicates that these features of the hippocampal vasculature may restrict oxygen availability and could explain its sensitivity to damage during neurological conditions, including Alzheimer's disease, where the brain's energy supply is decreased.

Developmental patterns of CBF and BOLD responses to visual stimulus.

To investigate the developmental changes of cerebral blood flow (CBF) and hemodynamic responses to changing neural activity, we used the arterial spin label (ASL) technique to measure resting CBF and simultaneous CBF / blood-oxygen-level dependent (BOLD) signal changes during visual stimulation in 97 typically developing children and young adults (age 13.35 [6.02, 25.25] (median [min, max]) years old at the first time point). The longitudinal study protocol included three MRIs (2.7 ± 0.06 obtained), one year apart, for each participant. Mixed-effect linear and non-linear statistical models were used to analyze age effects on CBF and BOLD signals. Resting CBF decreased exponentially with age (p = 0.0001) throughout the brain, and developmental trajectories differed across brain lobes. The absolute CBF increase in visual cortex during stimulation was constant over the age range, but the fractional CBF change increased with age (p = 0.0001) and the fractional BOLD signal increased with age (p = 0.0001) correspondingly. These findings suggest that the apparent neural hemodynamic coupling in visual cortex does not change after age six years, but age-related BOLD signal changes continue through adolescence primarily due to the changes with age in resting CBF.

Dopaminergic neuromodulation has no detectable effect on visual-cue induced haemodynamic response function in the visual cortex: A double-blind, placebo-controlled functional magnetic resonance imaging study.

The aim of this study was to investigate the effect of acute dopamine agonistic and antagonistic manipulation on the visual-cue induced blood oxygen level-dependent signal response in healthy volunteers. Seventeen healthy volunteers in a double-blind placebo-controlled cross-over design received either a dopamine antagonist, agonist or placebo and underwent functional magnetic resonance imaging. Using classical inference and Bayesian statistics, we found no effect of dopaminergic modulation on properties of visual-cue induced blood oxygen level-dependent signals in the visual cortex, particularly on distinct properties of the haemodynamic response function (amplitude, time-to-peak and width). Dopamine-related effects modulating the neurovascular coupling in the visual cortex might be negligible when measured via functional magnetic resonance imaging.

Task-related activity in human visual cortex.

The brain exhibits widespread endogenous responses in the absence of visual stimuli, even at the earliest stages of visual cortical processing. Such responses have been studied in monkeys using optical imaging with a limited field of view over visual cortex. Here, we used functional MRI (fMRI) in human participants to study the link between arousal and endogenous responses in visual cortex. The response that we observed was tightly entrained to task timing, was spatially extensive, and was independent of visual stimulation. We found that this response follows dynamics similar to that of pupil size and heart rate, suggesting that task-related activity is related to arousal. Finally, we found that higher reward increased response amplitude while decreasing its trial-to-trial variability (i.e., the noise). Computational simulations suggest that increased temporal precision underlies both of these observations. Our findings are consistent with optical imaging studies in monkeys and support the notion that arousal increases precision of neural activity.

Vasomotion as a Driving Force for Paravascular Clearance in the Awake Mouse Brain.

Paravascular drainage of solutes, including ß-amyloid (Aß), appears to be an important process in brain health and diseases such as Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). However, the major driving force for clearance remains largely unknown. Here we used in vivo two-photon microscopy in awake head-fixed mice to assess the role of spontaneous vasomotion in paravascular clearance. Vasomotion correlated with paravascular clearance of fluorescent dextran from the interstitial fluid. Increasing the amplitude of vasomotion by means of visually evoked vascular responses resulted in increased clearance rates in the visual cortex of awake mice. Evoked vascular reactivity was impaired in mice with CAA, which corresponded to slower clearance rates. Our findings suggest that low-frequency arteriolar oscillations drive drainage of solutes. Targeting naturally occurring vasomotion in patients with CAA or AD may be a promising early therapeutic option for prevention of Aß accumulation in the brain.

Visual hallucinations in Alzheimer's disease do not seem to be associated with chronic hypoperfusion of to visual processing areas V2 and V3 but may be associated with reduced cholinergic input to these areas.

BACKGROUND: Up to 20% of patients with AD experience hallucinations. The pathological substrate is not known. Visual hallucinations (VH) are more common in dementia with Lewy bodies (DLB). In autopsy studies, up to 60% of patients with AD have concomitant Lewy body pathology. Decreased perfusion of the occipital lobe has been implicated in DLB patients with VH, and post-mortem studies point to both decreased cholinergic activity and reduced oxygenation of the occipital cortex in DLB. METHODS: We used biochemical methods to assess microvessel density (level of von Willebrand factor, a marker of endothelial cell content), ante-mortem oxygenation (vascular endothelial growth factor, a marker of tissue hypoxia; myelin-associated glycoprotein to proteolipid protein-1 ratio, a measure of tissue oxygenation relative to metabolic demand), cholinergic innervation (acetylcholinesterase and choline acetyltransferase), butyrylcholinesterase and insoluble α-synuclein content in the BA18 and BA19 occipital cortex obtained post-mortem from 23 AD patients who had experienced visual hallucinations, 19 AD patients without hallucinations, 19 DLB patients, and 36 controls. The cohorts were matched for age, gender and post-mortem interval. RESULTS: There was no evidence of reduced microvessel density, hypoperfusion or reduction in ChAT activity in AD with visual hallucinations. Acetylcholinesterase activity was reduced in both BA18 and BA19, in all 3 dementia groups, and the concentration was also reduced in BA19 in the DLB and AD without visual hallucinations groups. Insoluble α-synuclein was raised in the DLB group in both areas but not in AD either with or without visual hallucinations. CONCLUSIONS: Our results suggest that visual hallucinations in AD are associated with cholinergic denervation rather than chronic hypoperfusion or α-synuclein accumulation in visual processing areas of the occipital cortex.

A critical assessment of data quality and venous effects in sub-millimeter fMRI.

Advances in hardware, pulse sequences, and reconstruction techniques have made it possible to perform functional magnetic resonance imaging (fMRI) at sub-millimeter resolution while maintaining high spatial coverage and acceptable signal-to-noise ratio. Here, we examine whether sub-millimeter fMRI can be used as a routine method for obtaining accurate measurements of fine-scale local neural activity. We conducted fMRI in human visual cortex during a simple event-related visual experiment (7 T, gradient-echo EPI, 0.8-mm isotropic voxels, 2.2-s sampling rate, 84 slices), and developed analysis and visualization tools to assess the quality of the data. Our results fall along three lines of inquiry. First, we find that the acquired fMRI images, combined with appropriate surface-based processing, provide reliable and accurate measurements of fine-scale blood oxygenation level dependent (BOLD) activity patterns. Second, we show that the highly folded structure of cortex causes substantial biases on spatial resolution and data visualization. Third, we examine the well-recognized issue of venous contributions to fMRI signals. In a systematic assessment of large sections of cortex measured at a fine scale, we show that time-averaged T2*-weighted EPI intensity is a simple, robust marker of venous effects. These venous effects are unevenly distributed across cortex, are more pronounced in gyri and outer cortical depths, and are, to a certain degree, in consistent locations across subjects relative to cortical folding. Furthermore, we show that these venous effects are strongly correlated with BOLD responses evoked by the experiment. We conclude that sub-millimeter fMRI can provide robust information about fine-scale BOLD activity patterns, but special care must be exercised in visualizing and interpreting these patterns, especially with regards to the confounding influence of the brain's vasculature. To help translate these methodological findings to neuroscience research, we provide practical suggestions for both high-resolution and standard-resolution fMRI studies.

Low-Intensity Pulsed Ultrasound Stimulation Induces Coupling Between Ripple Neural Activity and Hemodynamics in the Mouse Visual Cortex.

Several studies have separately investigated neural activities and hemodynamic responses induced by low-intensity pulsed ultrasound stimulation (LIPUS), less is known about their coupling under LIPUS. This study aims to investigate the neurovascular coupling with LIPUS by measuring neural activity and hemodynamics. We found that the relative power and sample entropy of local field potential at the ripple band have a significant correlation to relative cerebral blood flow over time (correlation coefficients: 0.66 ± 0.13 [P < 0.01] and -0.58 ± 0.11 [P < 0.05]). These results demonstrate that LIPUS can induce neurovascular coupling in the mouse visual cortex.

Two distinct profiles of fMRI and neurophysiological activity elicited by acetylcholine in visual cortex.

Cholinergic neuromodulation is involved in all aspects of sensory processing and is crucial for processes such as attention, learning and memory, etc. However, despite the known roles of acetylcholine (ACh), we still do not how to disentangle ACh contributions from sensory or task-evoked changes in functional magnetic resonance imaging (fMRI). Here, we investigated the effects of local injection of ACh on fMRI and neural signals in the primary visual cortex (V1) of anesthetized macaques by combining pharmaco-based MRI (phMRI) with electrophysiological recordings, using single electrodes and electrode arrays. We found that local injection of ACh elicited two distinct profiles of fMRI and neurophysiological activity, depending on the distance from the injector. Near the injection site, we observed an increase in the baseline blood oxygen-level-dependent (BOLD) and cerebral blood flow (CBF) responses, while their visual modulation decreased. In contrast, further from the injection site, we observed an increase in the visually induced BOLD and CBF modulation without changes in baseline. Neurophysiological recordings suggest that the spatial correspondence between fMRI responses and neural activity does not change in the gamma, high-gamma, and multiunit activity (MUA) bands. The results near the injection site suggest increased inhibitory drive and decreased metabolism, contrasting to the far region. These changes are thought to reflect the kinetics of ACh and its metabolism to choline.

Patterns of Cortical Visual Field Defects From Embolic Stroke Explained by the Anastomotic Organization of Vascular Microlobules.

The cerebral cortex is supplied by vascular microlobules, each comprised of a half dozen penetrating arterioles that surround a central draining venule. The surface arterioles that feed the penetrating arterioles are interconnected via an extensively anastomotic plexus. Embolic occlusion of a small surface arteriole rarely produces a local infarct, because collateral blood flow is available through the vascular reticulum. Collateral flow also protects against infarct after occlusion of a single penetrating arteriole. Cortical infarction requires blockage of a major arterial trunk, with arrest of blood flow to a relatively large vascular territory. For striate cortex, the major vessels compromised by emboli are the inferior calcarine and superior calcarine arteries, as well as the distal branches of the middle cerebral artery. Their vascular territories have a fairly consistent relationship with the retinotopic map. Consequently, occlusion by emboli results in stereotypical visual field defects. The organization of the arterial supply to the occipital lobe provides an anatomical explanation for a phenomenon that has long puzzled neuro-ophthalmologists, namely, that of the myriad potential patterns of cortical visual field loss, only a few are encountered commonly from embolic cortical stroke.

Measuring visual cortical oxygenation in diabetes using functional near-infrared spectroscopy.

AIMS: Diabetes mellitus affects about 6% of the world's population, and the chronic complications of the disease may result in macro- and micro-vascular changes. The purpose of the current study was to shed light on visual cortical oxygenation in diabetic individuals. We then aimed to compare the haemodynamic response (HDR) to visual stimulation with glycaemic control, given the likelihood of diabetic individuals suffering from such macro- and micro-vascular insult. METHODOLOGY: Thirty participants took part in this explorative study, fifteen of whom had diabetes and fifteen of whom were non-diabetic controls. The HDR, measured as concentrations of oxyhaemoglobin [HbO] and deoxyhaemoglobin [HbR], to visual stimulation was recorded over the primary visual cortex (V1) using a dual-channel oximeter. The stimulus comprised a pattern-reversal checkerboard presented in a block design. Participants' mean glycated haemoglobin (HbA1c) level (± SD) was 7.2 ± 0.6% in the diabetic group and 5.5 ± 0.4% in the non-diabetic group. Raw haemodynamic data were normalised to baseline, and the last 15 s of data from each 'stimulus on' and 'stimulus off' condition were averaged over seven duty cycles for each participant. RESULTS: There were statistically significant differences in ∆[HbO] and ∆[HbR] to visual stimulation between diabetic and non-diabetic groups (p < 0.05). In the diabetic group, individuals with type 1 diabetes displayed an increased [HbO] (p < 0.01) and decreased [HbR] (p < 0.05) compared to their type 2 counterparts. There was also a linear relationship between both ∆[HbO] and ∆[HbR] as a function of HbA1c level (p < 0.0005). CONCLUSIONS: Our findings suggest that fNIRS can be used as a quantitative measure of cortical oxygenation in diabetes. Diabetic individuals have a larger HDR to visual stimulation compared to non-diabetic individuals. This increase in ∆[HbO] and decrease in ∆[HbR] appears to be correlated with HbA1c level.

The pial vasculature of the mouse develops according to a sensory-independent program.

The cerebral vasculature is organized to supply the brain's metabolic needs. Sensory deprivation during the early postnatal period causes altered neural activity and lower metabolic demand. Neural activity is instructional for some aspects of vascular development, and deprivation causes changes in capillary density in the deprived brain region. However, it is not known if the pial arteriole network, which contains many leptomeningeal anastomoses (LMAs) that endow the network with redundancy against occlusions, is also affected by sensory deprivation. We quantified the effects of early-life sensory deprivation via whisker plucking on the densities of LMAs and penetrating arterioles (PAs) in anatomically-identified primary sensory regions (vibrissae cortex, forelimb/hindlimb cortex, visual cortex and auditory cortex) in mice. We found that the densities of penetrating arterioles were the same across cortical regions, though the hindlimb representation had a higher density of LMAs than other sensory regions. We found that the densities of PAs and LMAs, as well as quantitative measures of network topology, were not affected by sensory deprivation. Our results show that the postnatal development of the pial arterial network is robust to sensory deprivation.

Reduced Cerebral Blood Flow in the Visual Cortex and Its Correlation With Glaucomatous Structural Damage to the Retina in Patients With Mild to Moderate Primary Open-angle Glaucoma.

PURPOSE: Altered ocular and cerebral vascular autoregulation and vasoreactivity have been demonstrated in patients with primary open-angle glaucoma (POAG). In the present study, we investigated the correlations between reduced cerebral blood flow (CBF) in early and higher-tier visual cortical areas and glaucomatous changes in the retinas of patients with mild to moderate POAG. PATIENTS AND METHODS: 3-dimensional pseudocontinuous arterial spin labelling magnetic resonance imaging at 3 T was performed in 20 normal controls and 15 mild to moderate POAG patients. Regions of interest were selected based on the Population-Average, Landmark- and Surface-based (PALS) atlas of the human cerebral cortex. Arterial spin labelling-measured CBF values were extracted in the early and higher-tier visual cortical areas and were compared between patients and controls using a 2-sample t test. Pearson correlation analyses were used to assess the correlations between reduced CBF and cup-to-disc ratio, retinal nerve fiber layer thickness, and ganglion cell complex thickness. RESULTS: Reduced CBF in early visual cortical areas (V1, V2, and ventral posterior area) and in the higher-tier visual left lateral occipital cortex was presented in mild to moderate POAG patients compared with controls. Furthermore, reduced CBF of the right areas V2 and ventral posterior area was correlated with cup-to-disc ratio, total ganglion cell complex thickness, and average retinal nerve fiber layer thickness. CONCLUSIONS: In conclusion, the complex pathologic progress of POAG includes abnormal cerebral perfusion within the visual cortex since the mild to moderate disease stages. The association of cerebral perfusion changes with alterations of the optic disc and the retina may contribute to the early diagnosis of POAG.

Anton-Babinski syndrome, case report. / Ceguera de Anton-Babinski, a propósito de un caso.

CLINICAL CASE: A 22 year-old woman complained about blurred vision after an episode of recovered cardiorespiratory arrest. She had bilateral low visual acuity («count fingers¼) and no ophthalmological or visual pathways changes. She also had an apparent lack of awareness of the deficit. The Magnetic Resonance Imaging (MRI) showed ischaemic changes in both occipital lobes. As a result, she was diagnosed with Anton-Babinski syndrome. DISCUSSION: This is a rare disease that should be suspected in strange or poorly congruent visual loss. It is usually due to an ischaemic injury in this region of brain, manifesting itself with low vision not perceived by the patient (visual confabulation). It can simulate a non-organic visual loss or psychiatric disease.

Size-Dependent Axonal Bouton Dynamics following Visual Deprivation In Vivo.

Persistent synapses are thought to underpin the storage of sensory experience, yet little is known about their structural plasticity in vivo. We investigated how persistent presynaptic structures respond to the loss of primary sensory input. Using in vivo two-photon (2P) imaging, we measured fluctuations in the size of excitatory axonal boutons in L2/3 of adult mouse visual cortex after monocular enucleation. The average size of boutons did not change after deprivation, but the range of bouton sizes was reduced. Large boutons decreased, and small boutons increased. Reduced bouton variance was accompanied by a reduced range of correlated calcium-mediated neural activity in L2/3 of awake animals. Network simulations predicted that size-dependent plasticity may promote conditions of greater bidirectional plasticity. These predictions were supported by electrophysiological measures of short- and long-term plasticity. We propose size-dependent dynamics facilitate cortical reorganization by maximizing the potential for bidirectional plasticity.

Stimulus-evoked changes in cerebral vessel diameter: A study in healthy humans.

The high metabolic demand of neuronal tissue, coupled with its relatively low energy storage capacity, requires that increases in neuronal activation are quickly matched with increased blood flow to ensure efficient supply of oxygen and nutrients to the tissue. For this to occur, dilation of nearby arterioles must be coordinated with the dilation of larger upstream feeding arteries. As it stands, the exact spatial extent of such dilation in humans is unknown. Using non-invasive time-of-flight magnetic resonance angiography in healthy participants, we developed an automatic methodology for reconstructing cerebral arterial vessels and quantifying their diameter on a voxel-by-voxel basis. Specifically, we isolated the posterior cerebral artery (PCA) supplying each occipital lobe and quantified its vasodilation induced by visual stimulation. Stimulus-induced changes were strongest (∼30%) near primary visual cortex and progressively decreased in a non-linear fashion as a function of distance. Surprisingly, weak - albeit significant - changes (∼2%) were observed ∼70 mm from the visual cortex. This demonstrates that visual stimulation modulates vascular tone along the bulk of the PCA segment, and thus may have important implications for our understanding of functional hyperemia in healthy and diseased states.

Effects of pentoxifylline on hemodynamic, hemorheological, and microcirculatory parameters in young SHRs during arterial hypertension development.

The most common form of hypertension in young adults is isolated diastolic hypertension. Diastolic arterial pressure is determined by the total peripheral resistance and depends on both vascular hindrance and blood viscosity. The aim of our work was to study the efficiency of pentoxifylline (PTX) in young spontaneously hypertensive rats (SHRs) during the development of arterial hypertension. The effects of a treatment course with PTX (100 mg/kg/day p.o. for 6 weeks, from 5 to 11 weeks old) on the mean, systolic, and diastolic blood pressure (BP); stroke volume; cardiac output; total peripheral resistance (TPR); whole blood viscosity (BV); plasma viscosity; hematocrit; RBC aggregation and deformability; local cerebral blood flow (lCBF); and microvascularization of the visual cortex were studied in SHRs in comparison with control SHRs and Wistar Kyoto rats. PTX-treated SHRs had significantly lower systolic, diastolic, and mean BP (by 24%, 26%, and 15%, respectively) and BV (by 5-9%) and a higher erythrocyte deformability index (by 1.5-2%), lCBF (by 42%), average diameter of capillaries (by 11%), density of the capillary network (by 23%), and percentage of capillaries with a diameter of 3-7 µm in comparison with control SHRs. In conclusion, PTX exerted positive effects on the hemodynamic, hemorheological, and microcirculatory parameters in SHRs during the development of arterial hypertension.

Neuronal synchrony and the relation between the blood-oxygen-level dependent response and the local field potential.

The most widespread measures of human brain activity are the blood-oxygen-level dependent (BOLD) signal and surface field potential. Prior studies report a variety of relationships between these signals. To develop an understanding of how to interpret these signals and the relationship between them, we developed a model of (a) neuronal population responses and (b) transformations from neuronal responses into the functional magnetic resonance imaging (fMRI) BOLD signal and electrocorticographic (ECoG) field potential. Rather than seeking a transformation between the two measures directly, this approach interprets each measure with respect to the underlying neuronal population responses. This model accounts for the relationship between BOLD and ECoG data from human visual cortex in V1, V2, and V3, with the model predictions and data matching in three ways: across stimuli, the BOLD amplitude and ECoG broadband power were positively correlated, the BOLD amplitude and alpha power (8-13 Hz) were negatively correlated, and the BOLD amplitude and narrowband gamma power (30-80 Hz) were uncorrelated. The two measures provide complementary information about human brain activity, and we infer that features of the field potential that are uncorrelated with BOLD arise largely from changes in synchrony, rather than level, of neuronal activity.