Real-time optical imaging of naturally evoked electrical activity in intact frog brain

In Vivo Whole-Cell Patch-Clamp Methods: Recent Technical Progress and Future Perspectives

Brain functions are fundamental for the survival of organisms, and they are supported by neural circuits consisting of a variety of neurons. To investigate the function of neurons at the single-cell level, researchers often use whole-cell patch-clamp recording techniques. These techniques enable us to record membrane potentials (including action potentials) of individual neurons of not only anesthetized but also actively behaving animals. This whole-cell recording method enables us to reveal how neuronal activities support brain function at the single-cell level. In this review, we introduce previous studies using in vivo patch-clamp recording techniques and recent findings primarily regarding neuronal activities in the hippocampus for behavioral function. We further discuss how we can bridge the gap between electrophysiology and biochemistry.

Dynamic Patterns of Spontaneous Ongoing Activity in the Visual Cortex of Anesthetized and Awake Monkeys are Different

Ongoing internal cortical activity plays a major role in perception and behavior both in animals and humans. Previously we have shown that spontaneous patterns resembling orientation-maps appear over large cortical areas in the primary visual-cortex of anesthetized cats. However, it remains unknown 1) whether spontaneous-activity in the primate also displays similar patterns and 2) whether a significant difference exists between cortical ongoing-activity in the anesthetized and awake primate. We explored these questions by combining voltage-sensitive-dye imaging with multiunit and local-field-potential recordings. Spontaneously emerging orientation and ocular-dominance maps, spanning up to 6 × 6 mm2, were readily observed in anesthetized but not in awake monkeys. Nevertheless, spontaneous correlated-activity involving orientation-domains was observed in awake monkeys. Under both anesthetized and awake conditions, spontaneous correlated-activity coincided with traveling waves. We found that spontaneous activity resembling orientation-maps in awake animals spans smaller cortical areas in each instance, but over time it appears across all of V1. Furthermore, in the awake monkey, our results suggest that the synaptic strength had been completely reorganized including connections between dissimilar elements of the functional architecture. These findings lend support to the notion that ongoing-activity has many more fast switching representations playing an important role in cortical function and behavior.

Next-generation probes, particles, and proteins for neural interfacing

Bidirectional interfacing with the nervous system enables neuroscience research, diagnosis, and therapy. This two-way communication allows us to monitor the state of the brain and its composite networks and cells as well as to influence them to treat disease or repair/restore sensory or motor function. To provide the most stable and effective interface, the tools of the trade must bridge the soft, ion-rich, and evolving nature of neural tissue with the largely rigid, static realm of microelectronics and medical instruments that allow for readout, analysis, and/or control. In this Review, we describe how the understanding of neural signaling and material-tissue interactions has fueled the expansion of the available tool set. New probe architectures and materials, nanoparticles, dyes, and designer genetically encoded proteins push the limits of recording and stimulation lifetime, localization, and specificity, blurring the boundary between living tissue and engineered tools. Understanding these approaches, their modality, and the role of cross-disciplinary development will support new neurotherapies and prostheses and provide neuroscientists and neurologists with unprecedented access to the brain.

The Possible Role of TASK Channels in Rank-Ordered Recruitment of Motoneurons in the Dorsolateral Part of the Trigeminal Motor Nucleus

Because a rank-ordered recruitment of motor units occurs during isometric contraction of jaw-closing muscles, jaw-closing motoneurons (MNs) may be recruited in a manner dependent on their soma sizes or input resistances (IRs). In the dorsolateral part of the trigeminal motor nucleus (dl-TMN) in rats, MNs abundantly express TWIK (two-pore domain weak inwardly rectifying K channel)-related acid-sensitive-K(+) channel (TASK)-1 and TASK3 channels, which determine the IR and resting membrane potential. Here we examined how TASK channels are involved in IR-dependent activation/recruitment of MNs in the rat dl-TMN by using multiple methods. The real-time PCR study revealed that single large MNs (>35 μm) expressed TASK1 and TASK3 mRNAs more abundantly compared with single small MNs (15-20 μm). The immunohistochemistry revealed that TASK1 and TASK3 channels were complementarily distributed in somata and dendrites of MNs, respectively. The density of TASK1 channels seemed to increase with a decrease in soma diameter while there were inverse relationships between the soma size of MNs and IR, resting membrane potential, or spike threshold. Dual whole-cell recordings obtained from smaller and larger MNs revealed that the recruitment of MNs depends on their IRs in response to repetitive stimulation of the presumed Ia afferents. 8-Bromoguanosine-cGMP decreased IRs in small MNs, while it hardly changed those in large MNs, and subsequently decreased the difference in spike-onset latency between the smaller and larger MNs, causing a synchronous activation of MNs. These results suggest that TASK channels play critical roles in rank-ordered recruitment of MNs in the dl-TMN.

Voltage-Sensitive Dye Imaging of Neocortical Activity

Neural computations underlying sensory perception, cognition, and motor control are performed by populations of neurons at different anatomical and temporal scales. Few techniques are currently available for exploring the dynamics of local and large range populations. Voltage-sensitive dye imaging (VSDI), based on organic voltage probes, reveals neural population activity in areas ranging from a few tens of micrometers to a couple of centimeters, or two areas up to ~10 cm apart. VSDI provides a submillisecond temporal resolution and a spatial resolution of ~50 µm. The dye signal emphasizes subthreshold synaptic potentials. VSDI has been applied in the mouse, rat, gerbil, ferret, tree shrew, cat, and monkey cortices to explore the lateral spread of retinotopic or somatotopic activation; the dynamic spatiotemporal pattern resulting from sensory activation, including the somatosensory, olfactory, auditory, and visual modalities; and motor preparation and the properties of spontaneously occurring population activity. In this introduction, we focus on VSDI in vivo and review results obtained mostly in the visual system in our laboratory.

Functional segregation of retinal ganglion cell projections to the optic tectum of rainbow trout

The interpretation of visual information relies on precise maps of retinal representation in the brain coupled with local circuitry that encodes specific features of the visual scenery. In nonmammalian vertebrates, the main target of ganglion cell projections is the optic tectum. Although the topography of retinotectal projections has been documented for several species, the spatiotemporal patterns of activity and how these depend on background adaptation have not been explored. In this study, we used a combination of electrical and optical recordings to reveal a retinotectal map of ganglion cell projections to the optic tectum of rainbow trout and characterized the spatial and chromatic distribution of ganglion cell fibers coding for increments (ON) and decrements (OFF) of light. Recordings of optic nerve activity under various adapting light backgrounds, which isolated the input of different cone mechanisms, yielded dynamic patterns of ON and OFF input characterized by segregation of these two fiber types. Chromatic adaptation decreased the sensitivity and response latency of affected cone mechanisms, revealing their variable contributions to the ON and OFF responses. Our experiments further demonstrated restricted input from a UV cone mechanism to the anterolateral optic tectum, in accordance with the limited presence of UV cones in the dorsotemporal retina of juvenile rainbow trout. Together, our findings show that retinal inputs to the optic tectum of this species are not homogeneous, exhibit highly dynamic activity patterns, and are likely determined by a combination of biased projections and specific retinal cell distributions and their activity states.

Imaging the Dynamics of Mammalian Neocortical Population Activity In-Vivo

Neural computations underlying sensory perception, cognition and motor control are performed by populations of neurons at different anatomical and temporal scales. Few techniques are currently available for exploring dynamics of local and large range populations. Voltage-sensitive dye imaging (VSDI) reveals neural population activity in areas ranging from a few tens of microns to a couple of centimeters, or two areas up to ~10 cm apart. VSDI provides a sub-millisecond temporal resolution, and a spatial resolution of about 50 μm. The dye signal emphasizes subthreshold synaptic potentials. VSDI has been applied in the mouse, rat, gerbil, ferret, tree shrew, cat and monkey cortices, in order to explore lateral spread of retinotopic or somatotopic activation, the dynamic spatiotemporal pattern resulting from sensory activation, including the somatosensory, olfactory, auditory, and visual modalities, as well as motor preparation and the properties of spontaneously-occurring population activity. In this chapter we focus on VSDI in-vivo and review results obtained mostly in the visual system in our laboratory.

Evaluation of voltage-sensitive fluorescence dyes for monitoring neuronal activity in the embryonic central nervous system

Using an optical imaging technique with voltage-sensitive dyes (VSDs), we investigated the functional organization and architecture of the central nervous system (CNS) during embryogenesis. In the embryonic nervous system, a merocyanine-rhodanine dye, NK2761, has proved to be the most useful absorption dye for detecting neuronal activity because of its high signal-to-noise ratio (S/N), low toxicity and small dye bleaching. In the present study, we evaluated the suitability of fluorescence VSDs for optical recording in the embryonic CNS. We screened eight styryl (hemicyanine) dyes in isolated brainstem-spinal cord preparations from 7-day-old chick embryos. Measurements of voltage-related optical signals were made using a multiple-site optical recording system. The signal size, S/N, photobleaching, effects of perfusion and recovery of neural responses after staining were compared. We also evaluated optical responses with various magnifications. Although the S/N was lower than with the absorption dye, clear optical responses were detected with several fluorescence dyes, including di-2-ANEPEQ, di-4-ANEPPS, di-3-ANEPPDHQ, di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA. Di-2-ANEPEQ showed the largest S/N, whereas its photobleaching was faster and the recovery of neural responses after staining was slower. Di-4-ANEPPS and di-3-ANEPPDHQ also exhibited a large S/N but required a relatively long time for recovery of neural activity. Di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA showed smaller S/Ns than di-2-ANEPEQ, di-4-ANEPPS and di-3-ANEPPDHQ; but the recovery of neural responses after staining was faster. This study demonstrates the potential utility of these styryl dyes in optical monitoring of voltage changes in the embryonic CNS.

Temporally-structured acquisition of multidimensional optical imaging data facilitates visualization of elusive cortical representations in the behaving monkey

Fundamental understanding of higher cognitive functions can greatly benefit from imaging of cortical activity with high spatiotemporal resolution in the behaving non-human primate. To achieve rapid imaging of high-resolution dynamics of cortical representations of spontaneous and evoked activity, we designed a novel data acquisition protocol for sensory stimulation by rapidly interleaving multiple stimuli in continuous sessions of optical imaging with voltage-sensitive dyes. We also tested a new algorithm for the "temporally structured component analysis" (TSCA) of a multidimensional time series that was developed for our new data acquisition protocol, but was tested only on simulated data (Blumenfeld, 2010). In addition to the raw data, the algorithm incorporates prior knowledge about the temporal structure of the data as well as input from other information. Here we showed that TSCA can successfully separate functional signal components from other signals referred to as noise. Imaging of responses to multiple visual stimuli, utilizing voltage-sensitive dyes, was performed on the visual cortex of awake monkeys. Multiple cortical representations, including orientation and ocular dominance maps as well as the hitherto elusive retinotopic representation of orientation stimuli, were extracted in only 10s of imaging, approximately two orders of magnitude faster than accomplished by conventional methods. Since the approach is rather general, other imaging techniques may also benefit from the same stimulation protocol. This methodology can thus facilitate rapid optical imaging explorations in monkeys, rodents and other species with a versatility and speed that were not feasible before.

An evaluation of in vivo voltage-sensitive dyes: pharmacological side effects and signal-to-noise ratios after effective removal of brain-pulsation artifacts

In the current study, we investigated pharmacological side effects and signal-to-noise ratios (SNRs) of two commonly used voltage-sensitive dyes (VSDs): the blue dye RH-1691 (1 mg/ml) and the red dye di-4-ANEPPS (0.1 mg/ml), applied in vivo to the rat barrel cortex. Blue dyes are often favored over red dyes in in vivo studies due to their apparent superior SNR, partly because their fluorescence spectrum is farther away from the hemoglobin absorption spectrum, making them less prone to heartbeat-associated brain-pulsation artifacts (BPA). We implemented a previously reported template-based BPA removal algorithm and evaluated its applicability to di-4-ANEPPS before comparing characteristics of the two dyes. Somatosensory-evoked potentials (SEPs) were also recorded. Whereas SEPs recorded before and after application of di-4-ANEPPS failed to exhibit demonstrable differences, RH-1691 caused a significant and prolonged increase in SEP amplitude for several hours. In contrast, neither dye influenced the spontaneous cortical activity as assessed by the spectral content of the EEG. Both dyes turned out to be strikingly similar with respect to changes in fractional fluorescence as a function of SEP response amplitude, as well as regarding shot noise characteristics after removal of the BPA. Thus there is strong evidence that the increased SNR for RH-1691 is a consequence of an artificially increased signal. When applying an appropriate BPA removal algorithm, di-4-ANEPPS has proven to be suitable for single-trial in vivo VSD imaging (VSDI) and produces no detectable neurophysiological changes in the system under investigation. Taken together, our data argue for a careful re-evaluation of pharmacological side effects of RH-1691 and support the applicability of di-4-ANEPPS for stable single-trial in vivo VSDI recordings.

Imaging neural circuit dynamics with a voltage-sensitive fluorescent protein

Population signals from neuronal ensembles in cortex during behavior are commonly measured with EEG, local field potential (LFP), and voltage-sensitive dyes. A genetically encoded voltage indicator would be useful for detection of such signals in specific cell types. Here we describe how this goal can be achieved with Butterfly, a voltage-sensitive fluorescent protein (VSFP) with a subthreshold detection range and enhancements designed for voltage imaging from single neurons to brain in vivo. VSFP-Butterfly showed reliable membrane targeting, maximum response gain around standard neuronal resting membrane potential, fast kinetics for single-cell synaptic responses, and a high signal-to-noise ratio. Butterfly reports excitatory postsynaptic potentials (EPSPs) in cortical neurons, whisker-evoked responses in barrel cortex, 25-Hz gamma oscillations in hippocampal slices, and 2- to 12-Hz slow waves during brain state modulation in vivo. Our findings demonstrate that cell class-specific voltage imaging is practical with VSFP-Butterfly, and expand the genetic toolbox for the detection of neuronal population dynamics.

Visualization and manipulation of neural activity in the developing vertebrate nervous system

Neural activity during vertebrate development has been unambiguously shown to play a critical role in sculpting circuit formation and function. Patterned neural activity in various parts of the developing nervous system is thought to modulate neurite outgrowth, axon targeting, and synapse refinement. The nature and role of patterned neural activity during development has been classically studied with in vitro preparations using pharmacological manipulations. In this review we discuss newly available and developing molecular-genetic tools for the visualization and manipulation of neural activity patterns specifically during development.

Flow detection of propagating waves with temporospatial correlation of activity

Voltage-sensitive dye imaging (VSDI) allows population patterns of cortical activity to be recorded with high temporal resolution, and recent findings ascribe potential significance to these spatial propagation patterns--both for normal cortical processing and in pathologies such as epilepsy. However, analysis of these spatiotemporal patterns has been mostly qualitative to date. In this report, we describe an algorithm to quantify fast local flow patterns of cortical population activation, as measured with VSDI. The algorithm uses correlation of temporal features across space, and therefore differs from conventional optical flow algorithms which use correlation of spatial features over time. This alternative approach allows us to take advantage of the characteristics of fast optical imaging data, which have very high temporal resolution but less spatial resolution. We verify the method both on artificial and biological data, and demonstrate its use.

Lateral Spread of Orientation Selectivity in V1 is Controlled by Intracortical Cooperativity

Neurons in the primary visual cortex receive subliminal information originating from the periphery of their receptive fields (RF) through a variety of cortical connections. In the cat primary visual cortex, long-range horizontal axons have been reported to preferentially bind to distant columns of similar orientation preferences, whereas feedback connections from higher visual areas provide a more diverse functional input. To understand the role of these lateral interactions, it is crucial to characterize their effective functional connectivity and tuning properties. However, the overall functional impact of cortical lateral connections, whatever their anatomical origin, is unknown since it has never been directly characterized. Using direct measurements of postsynaptic integration in cat areas 17 and 18, we performed multi-scale assessments of the functional impact of visually driven lateral networks. Voltage-sensitive dye imaging showed that local oriented stimuli evoke an orientation-selective activity that remains confined to the cortical feedforward imprint of the stimulus. Beyond a distance of one hypercolumn, the lateral spread of cortical activity gradually lost its orientation preference approximated as an exponential with a space constant of about 1 mm. Intracellular recordings showed that this loss of orientation selectivity arises from the diversity of converging synaptic input patterns originating from outside the classical RF. In contrast, when the stimulus size was increased, we observed orientation-selective spread of activation beyond the feedforward imprint. We conclude that stimulus-induced cooperativity enhances the long-range orientation-selective spread.

Voltage-sensitive dye imaging: Technique review and models

In this review, we present the voltage-sensitive dye imaging (VSDI) method. The possibility offered for in vivo (and in vitro) brain imaging is unprecedented in terms of spatial and temporal resolution. However, the unresolved multi-component origin of the optical signal encourages us to perform a detailed analysis of the method limitation and the existing models. We propose a biophysical model at a mesoscopic scale in order to understand and interpret this signal.

Propagating waves of activity in the neocortex: what they are, what they do

The development of voltage-sensitive dyes (VSD) and fast optical imaging techniques have brought us a new tool for examining spatiotemporal patterns of population neuronal activity in the neocortex. Propagating waves have been observed during almost every type of cortical processing examined by VSD imaging or electrode arrays. These waves provide subthreshold depolarization to individual neurons and increase their spiking probability. Therefore, the propagation of the waves sets up a spatiotemporal framework for increased excitability in neuronal populations, which can help to determine when and where the neurons are likely to fire. In this review, first discussed is propagating waves observed in various systems and possible mechanisms for generating and sustaining these waves. Then discussed are wave dynamics as an emergent behavior of the population activity that can, in turn, influence the activity of individual neurons. The functions of spontaneous and sensory-evoked waves remain to be explored. An important next step will be to examine the interaction between dynamics of propagating waves and functions in the cortex, and to verify if cortical processing can be modified when these waves are altered.

Normalization of voltage-sensitive dye signal with functional activity measures

In general, signal amplitude in optical imaging is normalized using the well-established DeltaF/F method, where functional activity is divided by the total fluorescent light flux. This measure is used both directly, as a measure of population activity, and indirectly, to quantify spatial and spatiotemporal activity patterns. Despite its ubiquitous use, the stability and accuracy of this measure has not been validated for voltage-sensitive dye imaging of mammalian neocortex in vivo. In this report, we find that this normalization can introduce dynamic biases. In particular, the DeltaF/F is influenced by dye staining quality, and the ratio is also unstable over the course of experiments. As methods to record and analyze optical imaging signals become more precise, such biases can have an increasingly pernicious impact on the accuracy of findings, especially in the comparison of cytoarchitechtonic areas, in area-of-activation measurements, and in plasticity or developmental experiments. These dynamic biases of the DeltaF/F method may, to an extent, be mitigated by a novel method of normalization, DeltaF/DeltaF(epileptiform). This normalization uses as a reference the measured activity of epileptiform spikes elicited by global disinhibition with bicuculline methiodide. Since this normalization is based on a functional measure, i.e. the signal amplitude of "hypersynchronized" bursts of activity in the cortical network, it is less influenced by staining of non-functional elements. We demonstrate that such a functional measure can better represent the amplitude of population mass action, and discuss alternative functional normalizations based on the amplitude of synchronized spontaneous sleep-like activity. These findings demonstrate that the traditional DeltaF/F normalization of voltage-sensitive dye signals can introduce pernicious inaccuracies in the quantification of neural population activity. They further suggest that normalization-independent metrics such as waveform propagation patterns, oscillations in single detectors, and phase relationships between detector pairs may better capture the biological information which is obtained by high-sensitivity imaging.

Removal of spatial biological artifacts in functional maps by local similarity minimization

Functional maps obtained by various technologies, including optical imaging techniques, f-MRI, PET, and others, may be contaminated with biological artifacts such as vascular patterns or large patches of parenchyma. These artifacts originate mostly from changes in the microcirculation that result from either activity-dependent changes in volume or from oximetric changes that do not co-localize with neuronal activity per se. Standard methods do not always suffice to reduce such artifacts, in which case conspicuous spatial artifacts mask details of the underlying activity patterns. Here we propose a simple algorithm that efficiently removes spatial biological artifacts contaminating high-resolution functional maps. We validated this procedure by applying it to cortical maps resulting from optical imaging, based either on voltage-sensitive dye signals or on intrinsic signals. To remove vascular spatial patterns we first constructed a template of typical artifacts (vascular/cardiac pulsation/vasomotion), using principle components derived from baseline information obtained in the absence of stimulation. Next, we modified this template by means of local similarity minimization (LSM), achieved by measuring neighborhood similarity between contaminated data and the artifact template and then abolishing the similarity. LSM thus removed spatial patterns originating from the cortical vasculature components, including large fields of capillary parenchyma, helping to unveil details of neuronal activity patterns that were otherwise masked by these vascular artifacts. Examples obtained from our imaging experiments with anaesthetized cats and behaving monkeys showed that the LSM method is both general and reproducible, and is often superior to other available procedures.

Imaging nervous system activity

Optical imaging methods rely upon visualization of three types of signals: (1) intrinsic optical signals, including light scattering and reflectance, birefringence, and spectroscopic changes of intrinsic molecules, such as NADH or oxyhemoglobin; (2) changes in fluorescence or absorbance of voltage-sensitive membrane dyes; and (3) changes in fluorescence or absorbance of calcium-sensitive indicator dyes. Of these, the most widely used approach is fluorescent microscopy of calcium-sensitive dyes. This unit describes protocols for the use of calcium-sensitive dyes and voltage-dependent dyes for studies of neuronal activity in culture, tissue slices, and en-bloc preparations of the central nervous system.

Imaging in vivo: watching the brain in action

The appeal of in vivo cellular imaging to any neuroscientist is not hard to understand: it is almost impossible to isolate individual neurons while keeping them and their complex interactions with surrounding tissue intact. These interactions lead to the complex network dynamics that underlie neural computation which, in turn, forms the basis of cognition, perception and consciousness. In vivo imaging allows the study of both form and function in reasonably intact preparations, often with subcellular spatial resolution, a time resolution of milliseconds and a purview of months. Recently, the limits of what can be achieved in vivo have been pushed into terrain that was previously only accessible in vitro, due to advances in both physical-imaging technology and the design of molecular contrast agents.

Optical and electrophysiological recordings of corticospinal synaptic activity and its developmental change in in vitro rat slice co-cultures

Electrophysiological recordings and optical imaging with a fast voltage-sensitive dye (di-4-ANNEPS) were used to directly examine the spatiotemporal properties of in vitro corticospinal synapses formed in co-cultures of cerebral cortex and spinal cord slices. Whole cell recordings from spinal cord cells showed both monosynaptic and polysynaptic excitatory postsynaptic currents (EPSCs) in response to stimulation of corticospinal axons. Monosynaptic EPSCs and excitatory postsynaptic potentials (EPSPs) were isolated in artificial cerebrospinal fluid containing high concentrations of divalent cations. Optical imaging and extracellular recordings were done simultaneously. Both EPSPs and optically recorded excitatory postsynaptic potentials (optEPSPs) lasted 300-500 ms and were almost always positive. The major component of these long-lasting potentials was blocked by ifenprodil, a specific antagonist of the NR2B subunit-containing N-methyl-d-aspartate receptor (NMDAR). The spatial distribution of corticospinal optEPSPs paralleled that of the corticospinal field excitatory postsynaptic potentials (fEPSPs), suggesting that positive fEPSP amplitude is a reliable indicator of the distribution of corticospinal synapses. Corticospinal optEPSPs spread into the ventrolateral region by 6-7 days in vitro (DIV), but were restricted to the dorsomedial area by 11-13 DIV, suggesting synapses were eliminated from the ventrolateral side of the spinal cord. After the recordings were complete, corticospinal fibers were often anterogradely labeled with biocytin to assess the relation between presynaptic fiber distribution and the optical signals (optically-recorded presynaptic fiber volley (opt-prevolley) and optEPSP). The distributions of the opt-prevolleys and optEPSPs correlated well with the distribution of presynaptic fibers, suggesting the opt-prevolley reflects corticospinal fiber activity and that the fibers made synapses relatively evenly along their axons. The NR2B-mediated component of the corticospinal synaptic response declined during the interval between 6 and 7 DIV and 11-13 DIV, suggesting that a shift in the NMDAR subtype from NR2B to something else (perhaps NR2A) may be involved in regulating developmental plasticity in the rat spinal cord and the process of corticospinal synapse elimination.

Differential dynamics of transient neuronal assemblies in visual compared to auditory cortex

Large-scale, coherent, but highly transient networks of neurons, 'neuronal assemblies', operate over a sub-second time frame. Such assemblies of brain cells need not necessarily respect well-defined anatomical compartmentalisation, but represent an intermediate level of brain organisation between identified brain regions and individual neurons dependent on the activity status of the synaptic connections and axonal projections. To study neuronal assemblies both in slices and in the living brain, optical imaging using voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution in real-time. Applying VSDI technique to compare assemblies in visual versus auditory cortices under standardised experimental protocols, we observed no significant variations in the basic parameters of fluorescence signal and assembly size: such results might be predicted from the canonical invariance of cortical structures across modalities. However, further analysis revealed less obvious yet significant differences in the assembly dynamics of the two regions. The neural assemblies spread widely across layers in the two cortices following paired-pulse stimulation of putative layer 4. The respective patterns of activity started to differentiate within a specific time frame (250-300 ms). The signal was predominant near the point of stimulation in the visual cortex, whereas in the auditory cortex the signal was stronger in the superficial layers. This modality-specific divergence in assembly dynamics highlights a previously under-appreciated level of neuronal processing. Additionally, these findings could prompt a new approach to the understanding of how information from different senses, transmitted as action potentials with identical electrochemical characteristics across different cortices, be it visual or auditory, can eventually yield, nonetheless, the qualitatively distinct experiences of seeing or hearing.

Combined Voltage and Calcium Epifluorescence Imaging In Vitro and In Vivo Reveals Subthreshold and Suprathreshold Dynamics of Mouse Barrel Cortex

Cortical dynamics can be imaged at high spatiotemporal resolution with voltage-sensitive dyes (VSDs) and calcium-sensitive dyes (CaSDs). We combined these two imaging techniques using epifluorescence optics together with whole cell recordings to measure the spatiotemporal dynamics of activity in the mouse somatosensory barrel cortex in vitro and in the supragranular layers in vivo. The two optical signals reported distinct aspects of cortical function. VSD fluorescence varied linearly with membrane potential and was dominated by subthreshold postsynaptic potentials, whereas the CaSD signal predominantly reflected local action potential firing. Combining VSDs and CaSDs allowed us to monitor the synaptic drive and the spiking activity of a given area at the same time in the same preparation. The spatial extent of the two dye signals was different, with VSD signals spreading further than CaSD signals, reflecting broad subthreshold and narrow suprathreshold receptive fields. Importantly, the signals from the dyes were differentially affected by pharmacological manipulations, stimulation strength, and depth of isoflurane anesthesia. Combined VSD and CaSD measurements can therefore be used to specify the temporal and spatial relationships between subthreshold and suprathreshold activity of the neocortex.

Cortical response field dynamics in cat visual cortex

Little is known about the "inverse" of the receptive field--the region of cortical space whose spatiotemporal pattern of electrical activity is influenced by a given sensory stimulus. We refer to this activated area as the cortical response field, the properties of which remain unexplored. Here, the dynamics of cortical response fields evoked in visual cortex by small, local drifting-oriented gratings were explored using voltage-sensitive dyes. We found that the cortical response field was often characterized by a plateau of activity, beyond the rim of which activity diminished quickly. Plateau rim location was largely independent of stimulus orientation. However, approximately 20 ms following plateau onset, 1-3 peaks emerged on it and were amplified for 25 ms. Spiking was limited to the peak zones, whose location strongly depended on stimulus orientation. Thus, alongside selective amplification of a spatially restricted suprathreshold response, wider activation to just below threshold encompasses all orientation domains within a well-defined retinotopic vicinity of the current stimulus, priming the cortex for processing of subsequent stimuli.

A K+-selective cGMP-gated ion channel controls chemosensation of sperm

Eggs attract sperm by chemical factors, a process called chemotaxis. Sperm from marine invertebrates use cGMP signalling to transduce incident chemoattractants into changes in the Ca2+ concentration in the flagellum, which control the swimming behaviour during chemotaxis. The signalling pathway downstream of the synthesis of cGMP by a guanylyl cyclase is ill-defined. In particular, the ion channels that are involved in Ca2+ influx and their mechanisms of gating are not known. Using rapid voltage-sensitive dyes and kinetic techniques, we record the voltage response that is evoked by the chemoattractant in sperm from the sea urchin Arbacia punctulata. We show that the chemoattractant evokes a brief hyperpolarization followed by a sustained depolarization. The hyperpolarization is caused by the opening of K+-selective cyclic-nucleotide-gated (CNG) channels in the flagellum. Ca2+ influx commences at the onset of recovery from hyperpolarization. The voltage threshold of Ca2+ entry indicates the involvement of low-voltage-activated Ca(v) channels. These results establish a model of chemosensory transduction in sperm whereby a cGMP-induced hyperpolarization opens Ca(v) channels by a 'recovery-from-inactivation' mechanism and unveil an evolutionary kinship between transduction mechanisms in sperm and photoreceptors.

Heterogeneous spatial patterns of long-term potentiation in rat hippocampal slices

Although LTP (long-term potentiation) of synaptic transmission has received much attention as a model for learning and memory, its function within a neural circuit context remains poorly understood. To monitor LTP over an extensive circuit, we imaged responses in hippocampal slices using a voltage-sensitive dye. Following theta-burst stimulation, evoked optical signals showed an increase that lasted 40 min or more. Weak stimuli only potentiated the local area around the stimulating electrode, but stronger stimuli induced LTP over a wide area with a complex and non-uniform spatial pattern. The expression of LTP showed distinct peaks and valleys that depended on which axons were activated. Interestingly, the spatial distribution of LTP bore no relation to the spatial distribution of single-shock responses, but closely resembled the distribution of postsynaptic spikes evoked by theta bursts. Thus, postsynaptic spikes during induction constitute a critical determinant for the expression of LTP in intact circuits.

Spatiotemporal patterns of an evoked network oscillation in neocortical slices: coupled local oscillators

We have discovered an evoked network oscillation in rat neocortical slices and have examined its spatiotemporal patterns with voltage-sensitive dye imaging. The slices (visual and auditory cortices) were prepared in a medium of low calcium, high magnesium and with sodium replaced by choline to reduce the excito-toxicity and sodium loading. After slicing, the choline was washed out while normal calcium, magnesium, and sodium concentrations were restored. The oscillation was evoked by a single electrical shock to slices bathed in normal artificial cerebral spinal fluid (ACSF). The oscillation was organized as an all-or-none epoch containing 4-13 cycles at a central frequency approximately 25 Hz. The activity can be reversibly blocked by 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX). 2-amino-5-phosphonopentanoic acid (APV), and atropine but not by bicuculline, indicating polysynaptic excitatory mechanisms. Voltage-sensitive dye imaging showed high-amplitude oscillation signals in superficial and middle cortical layers. Spatiotemporally, the oscillations were organized as waves, propagating horizontally along cortical laminar. Each oscillation cycle was associated with one wave propagating in space. The waveforms were often different at different locations (e.g., extra cycles), suggesting the co-existence of multiple local oscillators. For different cycles, the waves often initiated at different locations, suggesting that local oscillators are competing to initiate each oscillation cycle. Overall our results suggest that this cortical network oscillation is organized at two levels: locally, oscillating neurons are tightly coupled to form local oscillators, and globally the coupling between local oscillators is weak, allowing abrupt spatial phase lags and propagating waves with multiple initiation sites.

Roles of periventricular neurons in retinotectal transmission in the optic tectum

The midbrain roof is a retinorecipient region referred to as the optic tectum in lower vertebrates, and the superior colliculus in mammals. The retinal fibers projecting to the tectum transmit visual information to tectal retinorecipient neurons. Periventricular neurons are a subtype of these neurons that have their somata in the deepest layer of the teleostean tectum and apical dendrites ramifying at more superficial layers consisting of retinal fibers. The retinotectal synapses between the retinal fibers and periventricular neurons are glutamatergic, and ionotropic glutamate receptors mediate the transmission in these synapses. This transmission involves long-term potentiation, and is modulated by hormone action. Visual information processed in the periventricular neurons is transmitted to adjacent tectal cells and target nuclei of periventricular neuron axonal branches, some of which relay the visual information to other brain areas controlling behavior. We demonstrated that periventricular neurons play a principal role in visual information processing in the teleostean optic tectum; the effects of tectal output on behavior is discussed also in the present review.

Visualizing the Cortical Representation of Whisker Touch: Voltage-Sensitive Dye Imaging in Freely Moving Mice

Voltage-sensitive dye imaging resolves the spatiotemporal dynamics of supragranular subthreshold cortical activity with millisecond temporal resolution and subcolumnar spatial resolution. We used a flexible fiber optic image bundle to visualize voltage-sensitive dye dynamics in the barrel cortex of freely moving mice while simultaneously filming whisker-related behavior to generate two movies matched frame-by-frame with a temporal resolution of up to 2 ms. Sensory responses evoked by passive whisker stimulation lasted longer and spread further across the barrel cortex in awake mice compared to anesthetized mice. Passively evoked sensory responses were large during behaviorally quiet periods and small during active whisking. However, as an exploring mouse approached an object while whisking, large-amplitude, propagating cortical sensory activity was evoked by active whisker-touch. These experiments demonstrate that fiber optics can be used to image cortical sensory activity with high resolution in freely moving animals. The results demonstrate differential processing of sensory input depending upon behavior.

Apolipoprotein L-I promotes trypanosome lysis by forming pores in lysosomal membranes

Apolipoprotein L-I is the trypanolytic factor of human serum. Here we show that this protein contains a membrane pore-forming domain functionally similar to that of bacterial colicins, flanked by a membrane-addressing domain. In lipid bilayer membranes, apolipoprotein L-I formed anion channels. In Trypanosoma brucei, apolipoprotein L-I was targeted to the lysosomal membrane and triggered depolarization of this membrane, continuous influx of chloride, and subsequent osmotic swelling of the lysosome until the trypanosome lysed.

Noninvasive detection of functional brain activity with near-infrared diffusing-wave spectroscopy

We use near-infrared dynamic multiple scattering of light [diffusing-wave spectroscopy (DWS)] to detect the activation of the somato-motor cortex in 11 right-handed volunteers performing a finger opposition task separately with their right and left hands. Temporal autocorrelation functions g(1)(r,tau) of the scattered light field are measured during 100-s periods of motor task alternating with 100-s resting baseline periods. From an analysis of the experimental data with an analytical theory for g(1)(r,tau) from a three-layer geometry with optical and dynamical heterogeneity representing scalp, skull, and cortex, we obtain quantitative estimates of the diffusion coefficient in cortical regions. Consistent with earlier results, the measured cortical diffusion coefficient is found to be increased during the motor task, with a strong contralateral and a weaker ipsilateral increase consistent with the known brain hemispheric asymmetry for right-handed subjects. Our results support the interpretation of the increase of the cortical diffusion coefficient during finger opposition being due to the functional increase in cortical blood flow rate related to vasodilation.

Functional imaging with cellular resolution reveals precise micro-architecture in visual cortex

Neurons in the cerebral cortex are organized into anatomical columns, with ensembles of cells arranged from the surface to the white matter. Within a column, neurons often share functional properties, such as selectivity for stimulus orientation; columns with distinct properties, such as different preferred orientations, tile the cortical surface in orderly patterns. This functional architecture was discovered with the relatively sparse sampling of microelectrode recordings. Optical imaging of membrane voltage or metabolic activity elucidated the overall geometry of functional maps, but is averaged over many cells (resolution >100 microm). Consequently, the purity of functional domains and the precision of the borders between them could not be resolved. Here, we labelled thousands of neurons of the visual cortex with a calcium-sensitive indicator in vivo. We then imaged the activity of neuronal populations at single-cell resolution with two-photon microscopy up to a depth of 400 microm. In rat primary visual cortex, neurons had robust orientation selectivity but there was no discernible local structure; neighbouring neurons often responded to different orientations. In area 18 of cat visual cortex, functional maps were organized at a fine scale. Neurons with opposite preferences for stimulus direction were segregated with extraordinary spatial precision in three dimensions, with columnar borders one to two cells wide. These results indicate that cortical maps can be built with single-cell precision.

VSDI: a new era in functional imaging of cortical dynamics

During the last few decades, neuroscientists have benefited from the emergence of many powerful functional imaging techniques that cover broad spatial and temporal scales. We can now image single molecules controlling cell differentiation, growth and death; single cells and their neurites processing electrical inputs and sending outputs; neuronal circuits performing neural computations in vitro; and the intact brain. At present, imaging based on voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution for imaging neocortical functions in the living brain, and has paved the way for a new era in the functional imaging of cortical dynamics. It has facilitated the exploration of fundamental mechanisms that underlie neocortical development, function and plasticity at the fundamental level of the cortical column.

Imaging cortical correlates of illusion in early visual cortex

Exploring visual illusions reveals fundamental principles of cortical processing. Illusory motion perception of non-moving stimuli was described almost a century ago by Gestalt psychologists. However, the underlying neuronal mechanisms remain unknown. To explore cortical mechanisms underlying the 'line-motion' illusion, we used real-time optical imaging, which is highly sensitive to subthreshold activity. We examined, in the visual cortex of the anaesthetized cat, responses to five stimuli: a stationary small square and a long bar; a moving square; a drawn-out bar; and the well-known line-motion illusion, a stationary square briefly preceding a long stationary bar presentation. Whereas flashing the bar alone evoked the expected localized, short latency and high amplitude activity patterns, presenting a square 60-100 ms before a bar induced the dynamic activity patterns resembling that of fast movement. The preceding square, even though physically non-moving, created gradually propagating subthreshold cortical activity that must contribute to illusory motion, because it was indistinguishable from cortical representations of real motion in this area. These findings demonstrate the effect of spatio-temporal patterns of subthreshold synaptic potentials on cortical processing and the shaping of perception.

Imaging spatiotemporal dynamics of surround inhibition in the barrels somatosensory cortex

Sensory processing and its perception require that local information would also be available globally. Indeed, in the mammalian neocortex, local excitation spreads over large distances via the long-range horizontal connections in layer 2/3 and may spread over an entire cortical area if excitatory polysynaptic pathways are also activated. Therefore, a balance between local excitation and surround inhibition is required. Here we explore the spatiotemporal aspects of cortical depolarization and hyperpolarization of rats anesthetized with urethane. New voltage-sensitive dyes (VSDs) were used for high-resolution real-time visualization of the cortical responses to whisker deflections and cutaneous stimulations of the whisker pad. These advances facilitated imaging of ongoing activity and evoked responses even without signal averaging. We found that the motion of a single whisker evoked a cortical response exhibiting either one or three phases. During a triphasic response, there was first a cortical depolarization in a small cortical region the size of a single cortical barrel. Subsequently, this depolarization increased and spread laterally in an oval manner, preferentially along rows of the barrel field. During the second phase, the amplitude of the evoked response declined rapidly, presumably because of recurrent inhibition. Subsequently, the third phase exhibiting a depolarization rebound was observed and clear, and approximately 16 Hz oscillations were detected. Stimulus conditions revealing a net surround hyperpolarization during the second phase were also found. By using new, improved VSD, the present findings shed new light on the spatial parameters of the intricate spatiotemporal cortical interplay of inhibition and excitation.

Spatiotemporal dynamics of sensory responses in layer 2/3 of rat barrel cortex measured in vivo by voltage-sensitive dye imaging combined with whole-cell voltage recordings and neuron reconstructions

The spatiotemporal dynamics of the sensory response in layer 2/3 of primary somatosensory cortex evoked by a single brief whisker deflection was investigated by simultaneous voltage-sensitive dye (VSD) imaging and whole-cell (WC) voltage recordings in the anesthetized rat combined with reconstructions of dendritic and axonal arbors of L2/3 pyramids. Single and dual WC recordings from pyramidal cells indicated a strong correlation between the local VSD population response and the simultaneously measured subthreshold postsynaptic potential changes in both amplitude and time course. The earliest VSD response was detected 10-12 msec after whisker deflection centered above the barrel isomorphic to the stimulated principal whisker. It was restricted horizontally to the size of a single barrel-column coextensive with the dendritic arbor of barrel-column-related pyramids in L2/3. The horizontal spread of excitation remained confined to a single barrel-column with weak whisker deflection. With intermediate deflections, excitation spread into adjacent barrel-columns, propagating twofold more rapidly along the rows of the barrel field than across the arcs, consistent with the preferred axonal arborizations in L2/3 of reconstructed pyramidal neurons. Finally, larger whisker deflections evoked excitation spreading over the entire barrel field within approximately 50 msec before subsiding over the next approximately 250 msec. Thus the subthreshold cortical map representing a whisker deflection is dynamic on the millisecond time scale and strongly depends on stimulus strength. The sequential spatiotemporal activation of the excitatory neuronal network in L2/3 by a simple sensory stimulus can thus be accounted for primarily by the columnar restriction of L4 to L2/3 excitatory connections and the axonal field of barrel-related pyramids.

Beyond the visible--imaging the human brain with light

Optical approaches to investigate cerebral function and metabolism have long been applied in invasive studies. From the neuron cultured to the exposed cortex in the human during neurosurgical procedures, high spatial resolution can be reached and several processes such as membrane potential, cell swelling, metabolism of mitochondrial chromophores, and vascular response can be monitored, depending on the respective preparation. The authors focus on an extension of optical methods to the noninvasive application in the human. Starting with the pioneering work of Jöbsis 25 years ago, near-infrared spectroscopy (NIRS) has been used to investigate functional activation of the human cerebral cortex. Recently, several groups have started to use imaging systems that allow the generation of images of a larger area of the subject's head and, thereby, the production of maps of cortical oxygenation changes. Such images have a much lower spatial resolution compared with the invasively obtained optical images. The noninvasive NIRS images, however, can be obtained in undemanding set-ups that can be easily combined with other functional methods, in particular EEG. Moreover, NIRS is applicable to bedside use. The authors briefly review some of the abundant literature on intrinsic optical signals and the NIRS imaging studies of the past few years. The weaknesses and strengths of the approach are critically discussed. The authors conclude that NIRS imaging has two major advantages: it can address issues concerning neurovascular coupling in the human adult and can extend functional imaging approaches to the investigation of the diseased brain.

Long-term voltage-sensitive dye imaging reveals cortical dynamics in behaving monkeys

A novel method of chronic optical imaging based on new voltage-sensitive dyes (VSDs) was developed to facilitate the explorations of the spatial and temporal patterns underlying higher cognitive functions in the neocortex of behaving monkeys. Using this system, we were able to explore cortical dynamics, with high spatial and temporal resolution, over period of <or=1 yr from the same patch of cortex. The visual cortices of trained macaques were stained one to three times a week, and immediately after each staining session, the monkey started to perform the behavioral task, while the primary and secondary visual areas (V1 and V2) were imaged with a fast optical imaging system. Long-term repeated VSD imaging (VSDI) from the same cortical area did not disrupt the normal cortical architecture as confirmed repeatedly by optical imaging based on intrinsic signals. The spatial patterns of functional maps obtained by VSDI were essentially identical to those obtained from the same patch of cortex by imaging based on intrinsic signals. On comparing the relative amplitudes of the evoked signals and differential map obtained using these two different imaging methodologies, we found that VSDI emphasizes subthreshold activity more than imaging based on intrinsic signals, that emphasized more spiking activity. The latency of the VSD-evoked response in V1 ranged from 46 to 68 ms in the different monkeys. The amplitude of the V2 response was only 20-60% of that in V1. As expected from the anatomy, the retinotopic responses to local visual stimuli spread laterally across the cortical surface at a spreading velocity of 0.15-0.19 m/s over a larger area than that expected by the classical magnification factor, reaching its maximal anisotropic spatial extent within approximately 40 ms. We correlated the observed dynamics of cortical activation patterns with the monkey's saccadic eye movements and found that due to the slow offset of the cortical response relative to its onset, there was a short period of simultaneous activation of two distinct patches of cortex following a saccade to the visual stimulus. We also found that a saccade to a small stimulus was followed by direct transient activation of a cortical region in areas of V1 and V2, located retinotopically within the saccadic trajectory.

Multiple-site optical recording for characterization of functional synaptic organization of the optic tectum of rainbow trout

To map the functional synaptic organization over a wide area in the optic tectum, we directly monitored two-dimensional propagation of postsynaptic depolarization evoked by firing of retinotectal afferents in optic tectum slices prepared from rainbow trout (Oncorhynchus mykiss), using a voltage-sensitive dye and a photodiode array system. The postsynaptic responses to afferent stimulation first propagated in the stratum opticum and stratum fibrosum et griseum superficiale in an anterograde fashion in the afferents and then expanded vertically into the deep layers. This vertical propagation appeared to occur along a bundle-like structure that corresponded well with a cluster of neurons whose somata are located in the stratum periventriculare. Pharmacological studies showed that these postsynaptic responses were mediated by ionotropic glutamate receptors. On the other hand, the optical signals appeared to consist of at least two components (a transient signal and a slow signal). The second transient signal summated with the first slow signal by paired stimulation, suggesting that the transient and slow signals originated from different cell types. Taken together, these results showed that the functional synaptic organization of the teleost optic tectum comprises of two depolarization-signal propagating paths along a horizontal layer structure and a vertical bundle-like structure and that these synaptic responses occur via glutamatergic transmission.

Optical imaging combined with targeted electrical recordings, microstimulation, or tracer injections

To facilitate the combination of optical imaging with various electrode-based techniques, we have designed and produced a skull-mounting "sliding-top cranial window" and a removable "electrode positioner microdrive". These new devices were used to study sensory processing in chronic and acute experiments in the cerebral cortices of cats and monkeys. This assembly allows simultaneous optical imaging of intrinsic signals or voltage-sensitive dyes combined with extracellular recording (single and multiple unit recording and local field potential), intracellular recording, microstimulation, or targeted injection of tracers. After the functional architecture is determined by optical imaging, electrodes are targeted into a selected cortical site under full visual control, at a variety of penetration angles (30-90 degrees ), accessing a large cortical area. The device consists of three parts: (1) a skull-mounting chamber, (2) a sliding cap, and (3) a microdrive. The microdrive can easily be removed and the cranial window is then sealed and covered with a flat protective cover. For chronic experiments, this arrangement allows the animal to be handled over a long period while fitted with a sealed cranial window of minimal volume and weight, and with negligible risk of accidental damage or infection.

Spatio-temporal dynamics of odor representations in the mammalian olfactory bulb

We explored the spatio-temporal dynamics of odor-evoked activity in the rat and mouse main olfactory bulb (MOB) using voltage-sensitive dye imaging (VSDI) with a new probe. The high temporal resolution of VSDI revealed odor-specific sequences of glomerular activation. Increasing odor concentrations reduced response latencies, increased response amplitudes, and recruited new glomerular units. However, the sequence of glomerular activation was maintained. Furthermore, we found distributed MOB activity locked to the nasal respiration cycle. The spatial distribution of its amplitude and phase was heterogeneous and changed by sensory input in an odor-specific manner. Our data show that in the mammalian olfactory bulb, odor identity and concentration are represented by spatio-temporal patterns, rather than spatial patterns alone.

Dynamics and constancy in cortical spatiotemporal patterns of orientation processing

How does the high selectivity to stimulus orientation emerge in the visual cortex? Thalamic feedforward-dominated models of orientation selectivity predict constant selectivity during the visual response, whereas intracortical recurrent models predict dynamic improvement in selectivity. We imaged the cat visual cortex with voltage-sensitive dyes to measure orientation-tuning dynamics of a large neuronal population. Tuning-curve width did not narrow after response onset, whereas the difference between preferred and orthogonal responses (modulation depth) first increased, then declined. We identified a suppression of the evoked responses, referred to as the evoked deceleration-acceleration (DA) notch, which was larger for the orthogonal response. Furthermore, peak selectivity of the tuning curves was contemporaneous with the evoked DA notch. These findings suggest that in the cat brain, sustained visual cortical processing does not narrow orientation tuning; rather, intracortical interactions may amplify modulation depth and suppress the orthogonal response relatively more than the preferred. Thus, feedforward models and recurrent models of orientation selectivity must be combined.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Linking spontaneous activity of single cortical neurons and the underlying functional architecture

The relation between the activity of a single neocortical neuron and the dynamics of the network in which it is embedded was explored by single-unit recordings and real-time optical imaging. The firing rate of a spontaneously active single neuron strongly depends on the instantaneous spatial pattern of ongoing population activity in a large cortical area. Very similar spatial patterns of population activity were observed both when the neuron fired spontaneously and when it was driven by its optimal stimulus. The evoked patterns could be used to reconstruct the spontaneous activity of single neurons.

Imaging cortical dynamics at high spatial and temporal resolution with novel blue voltage-sensitive dyes

Conventional imaging techniques have provided high-resolution imaging either in the spatial domain or in the temporal domain. Optical imaging utilizing voltage-sensitive dyes has long had the unrealized potential to achieve high resolution in both domains simultaneously, providing subcolumnar spatial detail with millisecond precision. Here, we present a series of developments in voltage-sensitive dyes and instrumentation that make functional imaging of cortical dynamics practical, in both anesthetized and awake behaving preparations, greatly facilitating exploration of the cortex. We illustrate this advance by analyzing the millisecond-by-millisecond emergence of orientation maps in cat visual cortex.

Acetylcholine suppresses the spread of excitation in the visual cortex revealed by optical recording: possible differential effect depending on the source of input

Optical recording with a voltage-sensitive dye was performed in visual cortical slices of the rat to determine the effect of acetylcholine (ACh) on the spread of excitation. In the presence of ACh, the spread of excitation initiated by stimulation at the white matter/layer VI (WM/VI) was greatly suppressed throughout the cortex, with less suppression in the middle layers. By comparing the effect of ACh with that of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the fraction of the synaptic component that was sensitive to ACh was evaluated. ACh suppressed approximately 40-50% (maximum 55.8%, n = 11) of the initial synaptic component in the superficial and deep layers. In the middle, however, the effect was weakest and only approximately 20-30% (minimum 20.9%, n = 11) of the initial synaptic component was suppressed. On the basis of histological analysis, the region with the weakest ACh effect extended from upper V to lower II/III. To identify the site of ACh action in terms of pre- versus postsynaptic localization, exogenous glutamate was applied. Because ACh did not suppress the excitation induced by glutamate, the site of the ACh action was indicated to be presynaptic. When layer II/III was stimulated instead of WM/VI, the suppression was uniform throughout the cortex. A muscarinic receptor antagonist, atropine, blocked the suppression by ACh. In conclusion, our results indicate the following two points. First, ACh strongly suppresses intracortical connectivity through presynaptic muscarinic receptors. Secondly, in contrast to the intracortical connection, some group(s) of fibres, possibly thalamocortical afferents that arise from white matter and terminate in the middle cortical layers are suppressed much less by ACh. While ACh has been reported to have confusingly diverse effects, e.g. direct depolarization and hyperpolarization as well as synaptic facilitation and suppression, its effect on the propagation of excitation is very clear; suppression on intracortical connection, leaving thalamocortical inputs rather intact. We postulate that cholinergic innervation enables the afferent input to have a relatively dominant effect in the cortex.