Imaging cortical electrical stimulation in vivo: fast intrinsic optical signal versus voltage-sensitive dyes

Assessing the Impact of Electrosuit Therapy on Cerebral Palsy: A Study on the Users' Satisfaction and Potential Efficacy

The aim of this study is to evaluate the effectiveness of electrosuit therapy in the clinical treatment of children with Cerebral Palsy, focusing on the effect of the therapy on spasticity and trunk control. Moreover, the compliance of caregivers with respect to the use of the tool was investigated. During the period ranging from 2019 to 2022, a total of 26 children (18 M and 8 F), clinically stable and affected by CP and attending the Neurorehabilitation Unit of the "Padre Pio Foundation and Rehabilitation Centers", were enrolled in this study. A subset of 12 patients bought or rented the device; thus, they received the administration of the EMS-based therapy for one month, whereas the others received only one-hour training to evaluate the feasibility (by the caregivers) and short-term effects. The Gross Motor Function Classification System was utilized to evaluate gross motor functions and to classify the study sample, while the MAS and the LSS were employed to assess the outcomes of the EMS-based therapy. Moreover, between 80% and 90% of the study sample were satisfied with the safety, ease of use, comfort, adjustment, and after-sales service. Following a single session of electrical stimulation with EMS, patients exhibited a statistically significant enhancement in trunk control. For those who continued this study, the subscale of the QUEST with the best score was adaptability (0.74 ± 0.85), followed by competence (0.67 ± 0.70) and self-esteem (0.59 ± 0.60). This study investigates the impact of the employment of the EMS on CP children's ability to maintain trunk control. Specifically, after undergoing a single EMS session, LSS showed a discernible improvement in children's trunk control. In addition, the QUEST and the PIADS questionnaires demonstrated a good acceptability and satisfaction of the garment by the patients and the caregivers.

Electric Fields Induced in the Brain by Transcranial Electric Stimulation: A Review of In Vivo Recordings

Transcranial electrical stimulation (tES) techniques, such as direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), cause neurophysiological and behavioral modifications as responses to the electric field are induced in the brain. Estimations of such electric fields are based mainly on computational studies, and in vivo measurements have been used to expand the current knowledge. Here, we review the current tDCS- and tACS-induced electric fields estimations as they are recorded in humans and non-human primates using intracerebral electrodes. Direct currents and alternating currents were applied with heterogeneous protocols, and the recording procedures were characterized by a tentative methodology. However, for the clinical stimulation protocols, an injected current seems to reach the brain, even at deep structures. The stimulation parameters (e.g., intensity, frequency and phase), the electrodes’ positions and personal anatomy determine whether the intensities might be high enough to affect both neuronal and non-neuronal cell activity, also deep brain structures.

Non-Invasive Transcutaneous Spinal DC Stimulation as a Neurorehabilitation ALS Therapy in Awake G93A Mice: The First Step to Clinical Translation

Spinal direct current stimulation (sDCS) modulates motoneuron (MN) excitability beyond the stimulation period, making it a potential neurorehabilitation therapy for amyotrophic lateral sclerosis (ALS), a MN degenerative disease in which MN excitability dysfunction plays a critical and complex role. Recent evidence confirms induced changes in MN excitability via measured MN electrophysiological properties in the SOD1 ALS mouse during and following invasive subcutaneous sDCS (ssDCS). The first aim of our pilot study was to determine the clinical potential of these excitability changes at symptom onset (P90-P105) in ALS via a novel non-invasive transcutaneous sDCS (tsDCS) treatment paradigm on un-anesthetized SOD1-G93A mice. The primary outcomes were motor function and survival. Unfortunately, skin damage avoidance limited the strength of applied stimulation intensity, likewise limiting measurable primary effects. The second aim of this study was to determine which orientation of stimulation (anodal vs cathodal, which are expected to have opposing effects) is beneficial vs harmful in ALS. Despite the lack of measured primary effects, strong trends in survival of the anodal stimulation group, combined with an analysis of survival variance and correlations among symptoms, suggest anodal stimulation is harmful at symptom onset. Therefore, cathodal stimulation may be beneficial at symptom onset if a higher stimulation intensity can be safely achieved via subcutaneously implanted electrodes or alternative methods. Importantly, the many logistical, physical, and stimulation parameters explored in developing this novel non-invasive treatment paradigm on unanesthetized mice provide insight into an appropriate and feasible methodology for future tsDCS study designs and potential clinical translation.

Voltage-Sensitive Dye versus Intrinsic Signal Optical Imaging: Comparison of Tactile Responses in Primary and Secondary Somatosensory Cortices of Rats

Studies using functional magnetic resonance imaging assume that hemodynamic responses have roughly linear relationships with underlying neural activity. However, to accurately investigate the neurovascular transfer function and compare its variability across brain regions, it is necessary to obtain full-field imaging of both electrophysiological and hemodynamic responses under various stimulus conditions with superior spatiotemporal resolution. Optical imaging combined with voltage-sensitive dye (VSD) and intrinsic signals (IS) is a powerful tool to address this issue. We performed VSD and IS imaging in the primary (S1) and secondary (S2) somatosensory cortices of rats to obtain optical maps of whisker-evoked responses. There were characteristic differences in sensory responses between the S1 and S2 cortices: VSD imaging revealed more localized excitatory and stronger inhibitory neural activity in S1 than in S2. IS imaging revealed stronger metabolic responses in S1 than in S2. We calculated the degree of response to compare the sensory responses between cortical regions and found that the ratio of the degree of response of S2 to S1 was similar, irrespective of whether the ratio was determined by VSD or IS imaging. These results suggest that neurovascular coupling does not vary between the S1 and S2 cortices.

Spatial extent of cortical motor hotspot in navigated transcranial magnetic stimulation

BACKGROUND

Motor mapping with navigated transcranial magnetic stimulation (nTMS) requires defining a "hotspot", a stimulation site consistently producing the highest-amplitude motor-evoked potentials (MEPs). The exact location of the hotspot is difficult to determine, and the spatial extent of high-amplitude MEPs usually remains undefined due to MEP variability and the spread of the TMS-induced electric field (E-field). Therefore, here we aim to define the hotspot as a sub-region of a motor map.

NEW METHOD

We analyzed MEP amplitude distributions in motor mappings of 30 healthy subjects in two orthogonal directions on the motor cortex. Based on the widths of these distributions, the hotspot extent was estimated as an elliptic area. In addition, E-field distributions induced by motor map edge stimulations were simulated for ten subjects, and the E-field attenuation was analyzed to obtain another estimate for hotspot extent.

RESULTS

The median MEP-based hotspot area was 13 mm² (95% confidence interval (CI) = [10, 18] mm²). The mean E-field-based hotspot area was 26 mm² (95% CI = [13, 38] mm²).

COMPARISON WITH EXISTING METHODS

In contrast to the conventional hotspot, the new definition considers its spatial extent, indicating the most easily excited area where subsequent nTMS stimuli should be targeted for maximal response. The E-field-based hotspot provides an estimate for the extent of cortical structures where the E-field is close to its maximum.

CONCLUSIONS

The nTMS hotspot should be considered as an area rather than a single qualitatively defined spot due to MEP variability and E-field spread.

Anionic Conjugated Polyelectrolytes for FRET-based Imaging of Cellular Membrane Potential

We report a Förster resonance energy transfer (FRET)-based imaging ensemble for the visualization of membrane potential in living cells. A water-soluble poly(fluorene-cophenylene) conjugated polyelectrolyte (FsPFc10) serves as a FRET donor to a voltage-sensitive dye acceptor (FluoVolt^™ ). We observe FRET between FsPFc10 and FluoVolt^™ , where the enhancement in FRET-sensitized emission from FluoVolt^™ is measured at various donor/acceptor ratios. At a donor/acceptor ratio of 1, the excitation of FluoVolt^™ in a FRET configuration results in a three-fold enhancement in its fluorescence emission (compared to when it is excited directly). FsPFc10 efficiently labels the plasma membrane of HEK 293T/17 cells and remains resident with minimal cellular internalization for ~ 1.5 h. The successful plasma membrane-associated colabeling of the cells with the FsPFc10-FluoVolt^™ donor-acceptor pair is confirmed by dual-channel confocal imaging. Importantly, cells labeled with FsPFc10 show excellent cellular viability with no adverse effect on cell membrane depolarization. During depolarization of membrane potential, HEK 293T/17 cells labeled with the donor-acceptor FRET pair exhibit a greater fluorescence response in FluoVolt^™ emission relative to when FluoVolt^™ is used as the sole imaging probe. These results demonstrate the conjugated polyelectrolyte to be a new class of membrane labeling fluorophore for use in voltage sensing schemes.

The effect of coil placement and orientation on the assessment of focal excitability in motor mapping with navigated transcranial magnetic stimulation

BACKGROUND

Navigated transcranial magnetic stimulation (nTMS) is used for mapping muscle representations in the primary motor cortex. We used sulcus-aligned mapping and electric field (E-field) modeling to investigate the excitability of the motor hand area for further understanding the methodological limitations of nTMS.

NEW METHOD

We studied 10 healthy volunteers to locate the cortical target eliciting the largest responses (the hotspot) in the first dorsal interosseous (FDI) muscle. Six additional targets were placed along the central sulcus at 5-mm distances. Resting motor thresholds (rMTs) and optimal coil orientations were determined at all targets, and a conventional motor mapping was conducted. The cortical E-fields, induced by stimulating the targets with rMT intensities and optimal coil orientations, were modeled in a realistic head geometry to estimate the activated cortical sites.

RESULTS

The rMTs increased with increasing distance from the hotspot (p < 0.001). The greatest motor-evoked potential (MEP) amplitudes occurred with the coil perpendicular to the sulcus and with the coil pointing towards the hotspot or the center of gravity of the motor map. The E-field strengths at the hotspot (99±26 V/m) remained above previously estimated thresholds for activation.

COMPARISON WITH EXISTING METHODS

Depending on the target location, optimal coil orientations may deviate significantly from the conventional perpendicular-to-sulcus angle, which is often assumed optimal. These orientations seem to maintain the E-field stable in the hand knob, regardless of the sulcal shape near the stimulated target.

CONCLUSIONS

The coil orientation is crucial for the accuracy of motor mapping, and the apparent motor map may extend due to remote hotspot activation.

Sound- and current-driven laminar profiles and their application method mimicking acoustic responses in the mouse auditory cortex in vivo

The local application of electrical currents to the cortex is one of the most commonly used techniques to activate neurons, and this intracortical stimulation (ICS) could potentially lead to new types of neuroprosthetic devices that can be directly applied to the cortex. To identify whether ICS-activated circuits are physiological vs. profoundly artificial, it is necessary to record in vivo the responses of the same neuronal population to both natural sensory stimuli and artificial electric stimuli. However, few studies have extensively reported simultaneous electrophysiological recordings combined with ICS. Here, we evaluated the similarity between sound- and ICS-driven cortical response patterns in different cortical layers. In the mouse auditory cortex, we performed laminar recordings using 16-channel silicon electrodes and ICS using sharp glass-pipette electrodes containing biocytin for layer identification. In different cortical depths, short current pulses were delivered in vivo to mice under urethane anesthesia. For the recorded data, we mainly analyzed properties of local field potentials and current source densities (CSDs). We demonstrated that electrical stimulation evoked different excitation patterns according to the stimulated cortical layer; responses to electric stimuli in layer 4 were most likely to mimic acoustic responses. Next, we proposed a CSD-based stimulation method to artificially synthesize sound-driven responses, using an approximation method associated with a linear combination of CSD patterns electrically stimulated in the different cortical layers. The result indicates that synthesized responses were consistent with the canonical model of sound processing. Using these approaches, we provide a new technique in which natural sound-driven responses can be mimicked by well-designed computational stimulation pattern sequences in a layer-dependent manner. These findings may aid in the future development of an electrical stimulation methodology for a cortical prosthesis.

Evaluating the potential of using quantum dots for monitoring electrical signals in neurons

Success in the projects aimed at providing an advanced understanding of the brain is directly predicated on making critical advances in nanotechnology. This Perspective addresses the unique interface of neuroscience and nanomaterials by considering the foundational problem of sensing neuron membrane voltage and offers a potential solution that may be facilitated by a prototypical nanomaterial. Despite substantial improvements, the visualization of instantaneous voltage changes within individual neurons, whether in cell culture or in vivo, at both the single-cell and network level at high speed remains complex and problematic. The unique properties of semiconductor quantum dots (QDs) have made them powerful fluorophores for bioimaging. What is not widely appreciated, however, is that QD photoluminescence is exquisitely sensitive to proximal electric fields. This property should be suitable for sensing voltage changes that occur in the active neuronal membrane. Here, we examine the potential role of QDs in addressing the important challenge of real-time optical voltage imaging.

RGB camera-based imaging of cerebral tissue oxygen saturation, hemoglobin concentration, and hemodynamic spontaneous low-frequency oscillations in rat brain following induction of cortical spreading depression

To evaluate cerebral hemodynamics and spontaneous low-frequency oscillations (SLFOs) of cerebral blood flow in rat brain, we investigated an imaging method using a digital RGB camera. In this method, the RGB values were converted into tristimulus values in the CIE (Commission Internationale de l'Eclairage) XYZ color space, which is compatible with the common RGB working spaces. Monte Carlo simulation for light transport in tissue was then used to specify the relationship among the tristimulus XYZ values and the concentrations of oxygenated hemoglobin (CHbO), deoxygenated hemoglobin (CHbR), and total hemoglobin (CHbT) and cerebral tissue oxygen saturation (StO2). Applying the fast Fourier transform to each pixel of the sequential images of CHbT along the timeline, SLFOs of cerebral blood volume were visualized as a spatial map of power spectral density (PSD) at specific frequencies related to vasomotion. To confirm the feasibility of this method, we performed in vivo experiments using exposed rat brain during a cortical spreading depression (CSD) evoked by topical application of KCl. Cerebral hemodynamic responses to CSD such as initial hypoperfusion, profound hyperemia, and post-CSD oligemia and hypoxemia were successfully visualized with this method. At the transition to the hyperemia phase from hypoperfusion, CHbO and StO2 were significantly increased, which implied vasodilatation in arterioles and increased cerebral blood volume in response to CSD. In the wake of the hyperemic phase, CHbO and CHbT were significantly reduced to 25 ± 12% and 3.5 ± 1% of baseline, respectively, suggesting long-lasting vasoconstriction after CSD. In this persistent oligemia, StO2 significantly dropped to at most 23 ± 12% of the level before CSD, indicating long-lasting hypoxemia. The PSD value of SLFOs in CHbT for arteriole regions during CSD was significantly reduced to 28 ± 20% of baseline with respect to the pre-CSD level, which was correlated with the reduction in StO2. The results showed the possibility of RGB camera-based diffuse reflectance spectroscopy imaging for evaluating cerebral hemodynamics and SLFOs under normal and pathologic conditions.

Review of functional and clinical relevance of intrinsic signal optical imaging in human brain mapping

Intrinsic signal optical imaging (ISOI) within the first decade of its use in humans showed its capacity as a precise functional mapping tool. It is a powerful tool that can be used intraoperatively to help a surgeon to directly identify functional areas of the cerebral cortex. Its use is limited to the intraoperative setting as it requires a craniotomy and durotomy for direct visualization of the brain. It has been applied in humans to study language, somatosensory and visual cortices, cortical hemodynamics, epileptiform activity, and lesion delineation. Despite studies showing clear evidence of its usefulness in clinical care, its clinical use in humans has not grown. Impediments imposed by imaging in a human operating room setting have hindered such work. However, recent studies have been aimed at overcoming obstacles in clinical studies establishing the benefits of its use to patients. This review provides a description of ISOI and its use in human studies with an emphasis on the challenges that have hindered its widespread use and the recent studies that aim to overcome these hurdles. Clinical studies establishing the benefits of its use to patients would serve as the impetus for continued development and use in humans.

Quantum Dot-Peptide-Fullerene Bioconjugates for Visualization of in Vitro and in Vivo Cellular Membrane Potential

We report the development of a quantum dot (QD)-peptide-fullerene (C60) electron transfer (ET)-based nanobioconjugate for the visualization of membrane potential in living cells. The bioconjugate is composed of (1) a central QD electron donor, (2) a membrane-inserting peptidyl linker, and (3) a C60 electron acceptor. The photoexcited QD donor engages in ET with the C60 acceptor, resulting in quenching of QD photoluminescence (PL) that tracks positively with the number of C60 moieties arrayed around the QD. The nature of the QD-capping ligand also modulates the quenching efficiency; a neutral ligand coating facilitates greater QD quenching than a negatively charged carboxylated ligand. Steady-state photophysical characterization confirms an ET-driven process between the donor-acceptor pair. When introduced to cells, the amphiphilic QD-peptide-C60 bioconjugate labels the plasma membrane by insertion of the peptide-C60 portion into the hydrophobic bilayer, while the hydrophilic QD sits on the exofacial side of the membrane. Depolarization of cellular membrane potential augments the ET process, which is manifested as further quenching of QD PL. We demonstrate in HeLa cells, PC12 cells, and primary cortical neurons significant QD PL quenching (ΔF/F0 of 2-20% depending on the QD-C60 separation distance) in response to membrane depolarization with KCl. Further, we show the ability to use the QD-peptide-C60 probe in combination with conventional voltage-sensitive dyes (VSDs) for simultaneous two-channel imaging of membrane potential. In in vivo imaging of cortical electrical stimulation, the optical response of the optimal QD-peptide-C60 configuration exhibits temporal responsivity to electrical stimulation similar to that of VSDs. Notably, however, the QD-peptide-C60 construct displays 20- to 40-fold greater ΔF/F0 than VSDs. The tractable nature of the QD-peptide-C60 system offers the advantages of ease of assembly, large ΔF/F0, enhanced photostability, and high throughput without the need for complicated organic synthesis or genetic engineering, respectively, that is required of traditional VSDs and fluorescent protein constructs.

Simultaneous Evaluation of Cerebral Hemodynamics and Light Scattering Properties of the In Vivo Rat Brain Using Multispectral Diffuse Reflectance Imaging

The simultaneous evaluation of cerebral hemodynamics and the light scattering properties of in vivo rat brain tissue is demonstrated using a conventional multispectral diffuse reflectance imaging system. This system is constructed from a broadband white light source, a motorized filter wheel with a set of narrowband interference filters, a light guide, a collecting lens, a video zoom lens, and a monochromatic charged-coupled device (CCD) camera. An ellipsoidal cranial window is made in the skull bone of a rat under isoflurane anesthesia to capture in vivo multispectral diffuse reflectance images of the cortical surface. Regulation of the fraction of inspired oxygen using a gas mixture device enables the induction of different respiratory states such as normoxia, hyperoxia, and anoxia. A Monte Carlo simulation-based multiple regression analysis for the measured multispectral diffuse reflectance images at nine wavelengths (500, 520, 540, 560, 570, 580, 600, 730, and 760 nm) is then performed to visualize the two-dimensional maps of hemodynamics and the light scattering properties of the in vivo rat brain.

Label-Free Vibrational Spectroscopic Imaging of Neuronal Membrane Potential

Detecting membrane potentials is critical for understanding how neuronal networks process information. We report a vibrational spectroscopic signature of neuronal membrane potentials identified through hyperspectral stimulated Raman scattering (SRS) imaging of patched primary neurons. High-speed SRS imaging allowed direct visualization of puff-induced depolarization of multiple neurons in mouse brain slices, confirmed by simultaneous calcium imaging. The observed signature, partially dependent on sodium ion influx, is interpreted as ion interactions on the CH₃ Fermi resonance peak in proteins. By implementing a dual-SRS balanced detection scheme, we detected single action potentials in electrically stimulated neurons. These results collectively demonstrate the potential of sensing neuronal activities at multiple sites with a label-free vibrational microscope.

Evaluation of Cerebral Hemodynamics and Tissue Morphology of In Vivo Rat Brain Using Spectral Diffuse Reflectance Imaging

We investigated a quantitative imaging of reduced scattering coefficients μ_s'( λ) and the absorption coefficients μ_a( λ) of in vivo cortical tissues in the range from visible to near-infrared (NIR) wavelengths based on diffuse reflectance spectral imaging technique. In this method, diffuse reflectance images of in vivo cortical tissue are acquired at nine wavelengths (500, 520, 540, 560, 570, 580, 600, 730, and 760 nm). A multiple regression analysis aided by the Monte Carlo simulation for the absorbance spectra is then utilized to estimate the optical coefficients of cortical tissue. This analysis calculates the concentration of oxygenated hemoglobin and that of deoxygenated hemoglobin, the scattering amplitude a and the scattering power b. The spectrum of absorption coefficient is deduced from the estimated concentrations of oxygenated hemoglobin and deoxygenated hemoglobin. The spectrum of reduced scattering coefficient is determined by the estimated scattering amplitude and scattering power. The particle size distribution of microstructure is calculated from the estimated scattering power b for evaluating the morphological change in brain tissue quantitatively. Animal experiments with in vivo exposed brain of rats demonstrated that the responses of the absorption properties to hyperoxic and anoxic conditions are in agreement with the expected well-known cortical hemodynamics. The average particle size was significantly reduced immediately after the onset of anoxia and then it was changed into an increase, which implied the swelling and shrinkage of the cellular and subcellular structures induced by loss of tissue viability in brain tissue.

High-dynamic-range fluorescence laminar optical tomography (HDR-FLOT)

Three-dimensional fluorescence laminar optical tomography (FLOT) can achieve resolutions of 100-200 µm and penetration depths of 2-3 mm. FLOT has been used in tissue engineering, neuroscience, as well as oncology. The limited dynamic range of the charge-coupled device-based system makes it difficult to image fluorescent samples with a large concentration difference, limits its penetration depth, and diminishes the quantitative accuracy of 3D reconstruction data. Here, incorporating the high-dynamic-range (HDR) method widely used in digital cameras, we present HDR-FLOT, increasing penetration depth and improving the ability to image fluorescent samples with a large concentration difference. The method was tested using an agar phantom and a B6 mouse for brain imaging in vivo.

In Vivo Mesoscopic Voltage-Sensitive Dye Imaging of Brain Activation

Functional mapping of brain activity is important in elucidating how neural networks operate in the living brain. The whisker sensory system of rodents is an excellent model to study peripherally evoked neural activity in the central nervous system. Each facial whisker is represented by discrete modules of neurons all along the pathway leading to the neocortex. These modules are called "barrels" in layer 4 of the primary somatosensory cortex. Their location (approximately 300-500 μm below cortical surface) allows for convenient imaging of whisker-evoked neural activity in vivo. Fluorescence laminar optical tomography (FLOT) provides depth-resolved fluorescence molecular information with an imaging depth of a few millimeters. Angled illumination and detection configurations can improve both resolution and penetration depth. We applied angled FLOT (aFLOT) to record 3D neural activities evoked in the whisker system of mice by deflection of a single whisker in vivo. A 100 μm capillary and a pair of microelectrodes were inserted to the mouse brain to test the capability of the imaging system. The results show that it is possible to obtain 3D functional maps of the sensory periphery in the brain. This approach can be broadly applicable to functional imaging of other brain structures.

Short-term dynamics of causal information transfer in thalamocortical networks during natural inputs and microstimulation for somatosensory neuroprosthesis

Recording the activity of large populations of neurons requires new methods to analyze and use the large volumes of time series data thus created. Fast and clear methods for finding functional connectivity are an important step toward the goal of understanding neural processing. This problem presents itself readily in somatosensory neuroprosthesis (SSNP) research, which uses microstimulation (MiSt) to activate neural tissue to mimic natural stimuli, and has the capacity to potentiate, depotentiate, or even destroy functional connections. As the aim of SSNP engineering is artificially creating neural responses that resemble those observed during natural inputs, a central goal is describing the influence of MiSt on activity structure among groups of neurons, and how this structure may be altered to affect perception or behavior. In this paper, we demonstrate the concept of Granger causality, combined with maximum likelihood methods, applied to neural signals recorded before, during, and after natural and electrical stimulation. We show how these analyses can be used to evaluate the changing interactions in the thalamocortical somatosensory system in response to repeated perturbation. Using LFPs recorded from the ventral posterolateral thalamus (VPL) and somatosensory cortex (S1) in anesthetized rats, we estimated pair-wise functional interactions between functional microdomains. The preliminary results demonstrate input-dependent modulations in the direction and strength of information flow during and after application of MiSt. Cortico-cortical interactions during cortical MiSt and baseline conditions showed the largest causal influence differences, while there was no statistically significant difference between pre- and post-stimulation baseline causal activities. These functional connectivity changes agree with physiologically accepted communication patterns through the network, and their particular parameters have implications for both rehabilitation and brain—machine interface SSNP applications.

Neurovascular coupling: in vivo optical techniques for functional brain imaging

Optical imaging techniques reflect different biochemical processes in the brain, which is closely related with neural activity. Scientists and clinicians employ a variety of optical imaging technologies to visualize and study the relationship between neurons, glial cells and blood vessels. In this paper, we present an overview of the current optical approaches used for the in vivo imaging of neurovascular coupling events in small animal models. These techniques include 2-photon microscopy, laser speckle contrast imaging (LSCI), voltage-sensitive dye imaging (VSDi), functional photoacoustic microscopy (fPAM), functional near-infrared spectroscopy imaging (fNIRS) and multimodal imaging techniques. The basic principles of each technique are described in detail, followed by examples of current applications from cutting-edge studies of cerebral neurovascular coupling functions and metabolic. Moreover, we provide a glimpse of the possible ways in which these techniques might be translated to human studies for clinical investigations of pathophysiology and disease. In vivo optical imaging techniques continue to expand and evolve, allowing us to discover fundamental basis of neurovascular coupling roles in cerebral physiology and pathophysiology.

In vivo imaging of brain metabolism activity using a phosphorescent oxygen-sensitive probe

Several approaches have been adopted for real-time imaging of neural activity in vivo. We tested a new cell-penetrating phosphorescent oxygen-sensitive probe, NanO2-IR, to monitor temporal and spatial dynamics of oxygen metabolism in the neocortex following peripheral sensory stimulation. Probe solution was applied to the surface of anesthetized mouse brain; optical imaging was performed using a MiCAM-02 system. Trains of whisker stimuli were delivered and associated changes in phosphorescent signal were recorded in the contralateral somatosensory ("barrel") cortex. Sensory stimulation led to changes in oxygenation of activated areas of the barrel cortex. The oxygen imaging results were compared to those produced by the voltage-sensitive dye RH-1691. While the signals emitted by the two probes differed in shape and amplitude, they both faithfully indicated specific whisker evoked cortical activity. Thus, NanO2-IR probe can be used as a tool in visualization and real-time analysis of sensory-evoked neural activity in vivo.

Photoacoustic and optical coherence tomography of epilepsy with high temporal and spatial resolution and dual optical contrasts

Epilepsy mapping with high spatial and temporal resolution has great significance for both fundamental research on epileptic neurons and the clinical management of epilepsy. In this communication, we demonstrate for the first time in vivo epilepsy mapping with high spatial and temporal resolution and dual optical contrasts in an animal model. Through the variations of a depthresolved optical coherence tomography signal with optical scattering contrast, we observed that epileptic neuron activities modulated the optical refractive index of epileptic neurons and their surrounding tissue. Simultaneously, through neurovasculature coupling mechanisms and optical absorption contrast, we used photoacoustic signals to document the hemodynamic changes of the microvasculature surrounding the epileptic neurons. The epilepsy mapping results were confirmed by a simultaneously recorded electroencephalogram signal during epileptic seizure. Our new epilepsy mapping tool, with high temporal and spatial resolution and dual optical contrasts, may find many applications, such as drug development and epilepsy surgery.

Label-free imaging of membrane potential using membrane electromotility

Electrical activity may cause observable changes in a cell's structure in the absence of exogenous reporter molecules. In this work, we report a low-coherence interferometric microscopy technique that can detect an optical signal correlated with the membrane potential changes in individual mammalian cells without exogenous labels. By measuring milliradian-scale phase shifts in the transmitted light, we can detect changes in the cells' membrane potential. We find that the observed optical signals are due to membrane electromotility, which causes the cells to deform in response to the membrane potential changes. We demonstrate wide-field imaging of the propagation of electrical stimuli in gap-junction-coupled cell networks. Membrane electromotility-induced cell deformation may be useful as a reporter of electrical activity.

Living Brain Optical Imaging: Technology, Methods and Applications

Within the last few decades, optical imaging methods have yielded revolutionary results when applied to all parts of the central nervous system. The purpose of this review is to analyze research possibilities and limitations of several novel imaging techniques and show some of the most interesting achievements obtained by these methods. Here we covered intrinsic optical imaging, voltage-sensitive dye, photoacoustic, optical coherence tomography, near-infrared spectroscopy and some other techniques. All of them are mainly applicable for experimental neuroscience but some of them also suitable for the clinical studies.

Advantages and limitations of brain imaging methods in the research of absence epilepsy in humans and animal models

The purpose of this review is to analyze research possibilities and limitations of several methods, technical tools and their combinations for elucidation of absence epilepsy mechanisms, particularly the childhood absences. Despite the notable collection of simultaneous recording of clinical electroencephalography (EEG) and behavioral changes in relation to absence seizures, shortcomings of scalp EEG in both spatial resolution and precise detection of subcortical centers have limited the understanding of the fundamental mechanisms of altered brain function during and after recurrent epileptic paroxysms. Therefore, in the past decade, EEG recordings have often been combined with simultaneous imaging methods in epilepsy studies. Among imaging methods, the following ones are used regularly: functional magnetic resonance imaging (fMRI), positron-emission tomography (PET), low-resolution electromagnetic tomography (LORETA), single photon emission spectroscopy (SPECT), near-infrared spectroscopy (NIRS), and optical imaging of intrinsic signals (IOS). In addition, voltage-sensitive dye optical imaging method and even photoacoustic microscopy can be applied to animal models of epilepsy. Samplings of some of the most relevant data obtained by the above methods are presented. It appears that the elaboration of more adequate animal models of the patterns of absence seizures during the early postnatal period is necessary for better correspondence of human and animal absence phenomena.

In vivo imaging of epileptic activity using 2-NBDG, a fluorescent deoxyglucose analog

Accurately locating epileptic foci has great importance in advancing the treatment of epilepsy. In this study, epileptic seizures were first induced by intracortical injection of 4-aminopyridine in rats. A fluorescent deoxyglucose substitute, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG), was then continuously injected via the tail vein. Brain glucose metabolism was subsequently monitored by fluorescence imaging of 2-NBDG. The initial uptake rate of 2-NBDG at the injection site of 4-aminopyridine significantly exceeded that of the control injection site, which indicated local hypermetabolism induced by seizures. Our results show that 2-NBDG can be used for localizing epileptic foci.

Photoacoustic microscopy of microvascular responses to cortical electrical stimulation

Advances in the functional imaging of cortical hemodynamics have greatly facilitated the understanding of neurovascular coupling. In this study, label-free optical-resolution photoacoustic microscopy (OR-PAM) was used to monitor microvascular responses to direct electrical stimulations of the mouse somatosensory cortex through a cranial opening. The responses appeared in two forms: vasoconstriction and vasodilatation. The transition between these two forms of response was observed in single vessels by varying the stimulation intensity. Marked correlation was found between the current-dependent responses of two daughter vessels bifurcating from the same parent vessel. Statistical analysis of twenty-seven vessels from three different animals further characterized the spatial-temporal features and the current dependence of the microvascular response. Our results demonstrate that OR-PAM is a valuable tool to study neurovascular coupling at the microscopic level.

Study of the cortical representation of whisker directional deflection using voltage-sensitive dye optical imaging

Using voltage-sensitive dye optical imaging methods, we visualized neural activity in the rat barrel cortex in response to the deflection of a single whisker in different directions. Obtained data indicates that fast movements of single whiskers in varying directions correlate with different patterns of activation in the somatosensory cortex. A functional map was created based on the voltage-sensitive dye optical signal. This supports prior research that vibrissae deflections cause responses in different cortical neurons within the barrel field according to the direction of the deflection. By analogy with the orientation columns in the visual cortex, directionally biased single-whisker responses to different directions of deflection could be a possible mechanism for the directional selectivity of this important sensory response.

Frequency analysis of the visual steady-state response measured with the fast optical signal in younger and older adults

Relatively high frequency activity (>4Hz) carries important information about the state of the brain or its response to high frequency events. The electroencephalogram (EEG) is commonly used to study these changes because it possesses high temporal resolution and a good signal-to-noise ratio. However, it provides limited spatial information. Non-invasive fast optical signals (FOS) have been proposed as a neuroimaging tool combining spatial and temporal resolution. Yet, this technique has not been applied to study high frequency brain oscillations because of its relatively low signal-to-noise ratio. Here we investigate the sensitivity of FOS to relatively high-frequency brain oscillations. We measured the steady-state optical response elicited in medial and lateral occipital cortex by checkerboard reversals occurring at 4, 6, and 8Hz in younger and older adults. Stimulus-dependent oscillations were observed at the predicted stimulation frequency. In addition, in the younger adults the FOS steady-state response was smaller in lateral than medial areas, whereas in the older adults it was reversed in these two cortical regions. This may reflect diminished top-down inhibitory control in the older adults. The results indicate that FOS can be used to study the modulation of relatively high-frequency brain oscillations in adjacent cortical regions.

Fast propagating waves within the rodent auditory cortex

Central processing of acoustic signals is assumed to take place in a stereotypical spatial and temporal pattern involving different fields of auditory cortex. So far, cortical propagating waves representing such patterns have mainly been demonstrated by optical imaging, repeatedly in the visual and somatosensory cortex. In this study, the surface of rat auditory cortex was mapped by recording local field potentials (LFPs) in response to a broadband acoustic stimulus. From the peak amplitudes of LFPs, cortical activation maps were constructed over 4 cortical auditory fields. Whereas response onset had same latencies across primary auditory field (A1), anterior auditory field (AAF), and ventral auditory field and longer latencies in posterior auditory field, activation maps revealed a reproducible wavelike pattern of activity propagating for ∼45 ms poststimulus through all cortical fields. The movement observed started with 2 waves within the primary auditory fields A1 and AAF moving from ventral to dorsal followed by a motion from rostral to caudal, passing continuously through higher-order fields. The pattern of propagating waves was well reproducible and showed only minor changes if different anesthetics were used. The results question the classical "hierarchical" model of cortical areas and demonstrate that the different fields process incoming information as a functional unit.