Cerebral vasomotion: a 0.1-Hz oscillation in reflected light imaging of neural activity

Astrocytic calcium signaling: the information currency coupling neuronal activity to the cerebral microcirculation

In the brain, increased neuronal synaptic activity is accompanied by an increase in local cerebral blood flow that serves to satisfy neuronal metabolic demands. This linkage between neuronal activity and local blood flow has been appreciated for more than 100 years. Although this process has been exploited clinically in the form of functional imaging techniques to map brain function, the mechanisms by which increased synaptic activity is communicated to the cerebral microcirculation to generate a vasodilatory response are poorly understood. Recent studies, however, have illuminated a central role for astrocytic calcium (Ca(2+)) signals as mediators of this process of neurovascular coupling. This review highlights recent evidence implicating astrocytes in the regulation of intracerebral arteriolar diameter, with particular emphasis on the putative signaling molecules and pathways proposed to exert changes on arteriolar physiology.

Coupled oxygenation oscillation measured by NIRS and intermittent cerebral activation on EEG in premature infants

Electroencephalography of premature neonates shows a physiological discontinuity of electrical activity during quiet sleep. Near infrared spectroscopy (NIRS) shows spontaneous oscillations of hemoglobin oxygenation and volume. Similar oscillations are visible in term neonates and adults, with NIRS and other functional imaging techniques (fMRI, Doppler, etc.), but are generally thought to result from vasomotion and to be a physiological artifact of limited interest. The origin and possible relationship to neuronal activity of the baseline changes in the NIRS signal have not been established. We carried out simultaneous EEG-NIRS recordings on six healthy premature neonates and four premature neonates presenting neurological distress, to determine whether changes in the concentration of cerebral oxy- and deoxy- and total hemoglobin were related to the occurrence of spontaneous bursts of cerebral electric activity. Bursts of electroencephalographic activity in neonates during quiet sleep were found to be coupled to a transient stereotyped hemodynamic response involving a decrease in oxy-hemoglobin concentration, sometimes beginning a few seconds before the onset of electroencephalographic activity, followed by an increase, and then a return to baseline. This pattern could be either part of the baseline oscillations or superimposed changes to this baseline, influencing its shape and phase. The temporal patterns of NIRS parameters present an unique configuration, and tend to be different between our healthy and pathological subjects. Studies of physiological activities and of the effects of intrinsic regulation on the NIRS signal should increase our understanding of these patterns and EEG-NIRS studies should facilitate the integration of NIRS into the set of clinical tools used in neurology.

The triphasic intrinsic signal: implications for functional imaging

Intrinsic signal optical imaging with red illumination (ISOI) is used extensively to provide high spatial resolution maps of stimulus-evoked hemodynamic-related signals as an indirect means to map evoked neuronal activity. This evoked signal is generally described as beginning with an undershoot or "dip" in signal that is faster, more transient, and weaker compared with the subsequent signal overshoot. In contrast, the evoked signal detected with blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is generally described as containing an undershoot after the initial dip and overshoot, even though it, too, detects hemodynamic-related signals and its first two phases appear complementary to those of ISOI. Here, we used ISOI with 635 nm illumination to image over 13.5 s after a 1 s stimulus delivery to detect and successfully use the ISOI undershoot phase for functional mapping. Eight spatiotemporal attributes were assessed per signal phase including maximum areal extent and peak magnitude, both of which were largest for the ISOI overshoot, followed by the undershoot and then the initial dip. Peak activity location did not colocalize well between the three phases; furthermore, we found mostly modest correlations between attributes within each phase and sparse correlations between phases. Extended (13.5 s) electrophysiology recordings did not exhibit a reoccurrence of evoked suprathreshold or subthreshold neuronal responses that could be associated with the undershoot. Beyond the undershoot, additional overshoot/undershoot fluctuations were also mapped, but were typically less spatiotemporally specific to stimulus delivery. Implications for ISOI and BOLD fMRI are discussed.

Simultaneous laser Doppler flowmetry and arterial spin labeling MRI for measurement of functional perfusion changes in the cortex

This study compares laser Doppler flowmetry (LDF) and arterial spin labeling (ASL) for the measurement of functional changes in cerebral blood flow (CBF). The two methods were applied concurrently in a paradigm of electrical whisker stimulation in the anaesthetised rat. Multi-channel LDF was used, with each channel corresponding to different fiber separation (and thus measurement depth). Continuous ASL was applied using separate imaging and labeling coils at 3 T. Careful experimental set up ensured that both techniques recorded from spatially concordant regions of the barrel cortex, where functional responses were maximal. Strong correlations were demonstrated between CBF changes measured by each LDF channel and ASL in terms of maximum response magnitude and response time-course within a 6-s-long temporal resolution imposed by ASL. Quantitatively, the measurements of the most superficial LDF channels agreed strongly with those of ASL, whereas the deeper LDF channels underestimated consistently the ASL measurement. It was thus confirmed that LDF quantifies CBF changes consistently at a superficial level, and for this case the two methods provided concordant measures of functional CBF changes, despite their essentially different physical principles and spatiotemporal characteristics.

Efficiency and cost of economical brain functional networks

Brain anatomical networks are sparse, complex, and have economical small-world properties. We investigated the efficiency and cost of human brain functional networks measured using functional magnetic resonance imaging (fMRI) in a factorial design: two groups of healthy old (N = 11; mean age = 66.5 years) and healthy young (N = 15; mean age = 24.7 years) volunteers were each scanned twice in a no-task or "resting" state following placebo or a single dose of a dopamine receptor antagonist (sulpiride 400 mg). Functional connectivity between 90 cortical and subcortical regions was estimated by wavelet correlation analysis, in the frequency interval 0.06-0.11 Hz, and thresholded to construct undirected graphs. These brain functional networks were small-world and economical in the sense of providing high global and local efficiency of parallel information processing for low connection cost. Efficiency was reduced disproportionately to cost in older people, and the detrimental effects of age on efficiency were localised to frontal and temporal cortical and subcortical regions. Dopamine antagonism also impaired global and local efficiency of the network, but this effect was differentially localised and did not interact with the effect of age. Brain functional networks have economical small-world properties-supporting efficient parallel information transfer at relatively low cost-which are differently impaired by normal aging and pharmacological blockade of dopamine transmission.

Sensory-evoked intrinsic optical signals in the olfactory bulb are coupled to glutamate release and uptake

Functional imaging signals arise from metabolic and hemodynamic activity, but how these processes are related to the synaptic and electrical activity of neurons is not well understood. To provide insight into this issue, we used in vivo imaging and simultaneous local pharmacology to study how sensory-evoked neural activity leads to intrinsic optical signals (IOS) in the well-defined circuitry of the olfactory glomerulus. Odor-evoked IOS were tightly coupled to release of glutamate and were strongly modulated by activation of presynaptic dopamine and GABA-B receptors. Surprisingly, IOS were independent of postsynaptic transmission through ionotropic or metabotropic glutamate receptors, but instead were inhibited when uptake by astrocytic glutamate transporters was blocked. These data suggest that presynaptic glutamate release and uptake by astrocytes form a critical pathway through which neural activity is linked to metabolic processing and hence to functional imaging signals.

MRI of spontaneous fluctuations after acute cerebral ischemia in nonhuman primates

PURPOSE

To study the spontaneous low-frequency blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signal fluctuations during hyperacute focal cerebral ischemia.

MATERIALS AND METHODS

A stroke model in nonhuman primates (macaques) was used in this study. Spontaneous fluctuations were recorded using a series of gradient-recalled echo (GRE) echo-planar imaging (EPI) images. Fast Fourier transformation (FFT) was performed on the serial EPI data to calculate the frequency and magnitude of the spontaneous fluctuations. Diffusion tensor imaging (DTI) and perfusion-weighted imaging (PWI) were preformed to detect the ischemic lesion.

RESULTS

The frequency of these fluctuations decreased in the periinfarct tissue in the ipsilateral hemisphere, while their magnitude increased. This area of abnormal signal fluctuations often extended beyond the hyperacute diffusion/perfusion abnormality.

CONCLUSION

This study suggests that measurement of the spontaneous fMRI signal fluctuations provides different information than is available from diffusion/perfusion or T2-weighted MRI.

Mapping iso-orientation columns by contrast agent-enhanced functional magnetic resonance imaging: reproducibility, specificity, and evaluation by optical imaging of intrinsic signal

Activation resembling ocular dominance or orientation columns has been mapped with high-resolution functional magnetic resonance imaging (fMRI). However, the neuronal interpretation of these functional maps is unclear because of the poor sensitivity of fMRI, unknown point spread function (PSF), and lack of comparison with independent techniques. Here we show that cerebral blood volume (CBV)-weighted fMRI with a blood plasma contrast agent (monocrystalline iron oxide nanoparticles), in combination with continuous temporally encoded stimulation, can map columnar neuronal activity in the cat primary visual cortex with high sensitivity, selectivity, and reproducibility. We examined hemodynamic response PSF by comparing these CBV-based signals with oxygen metabolism-based negative blood oxygenation level-dependent signals. A significant positive correlation exists between CBV- and metabolism-based iso-orientation maps, suggesting that the hemodynamic PSF is narrower than intercolumn distances. We also compared CBV-based fMRI with optical intrinsic signal (OIS) imaging, a technique that identifies sites of increased neuronal activity, to investigate neuronal correlation. Iso-orientation maps obtained by fMRI and OIS were well matched, indicating that areas of the highest orientation-selective CBV signals correspond to sites of increased neural activity. Using CBV-based fMRI, we successfully mapped orientation-selective functional architecture in the medial bank of the visual cortex, an area inaccessible to OIS imaging. Thus, we conclude that contrast agent-based fMRI, in combination with continuous temporally encoded stimulation, is a highly sensitive technique capable of mapping neural activity at the resolution of functional columns without depth limitation.

Local potassium signaling couples neuronal activity to vasodilation in the brain

The mechanisms by which active neurons, via astrocytes, rapidly signal intracerebral arterioles to dilate remain obscure. Here we show that modest elevation of extracellular potassium (K+) activated inward rectifier K+ (Kir) channels and caused membrane potential hyperpolarization in smooth muscle cells (SMCs) of intracerebral arterioles and, in cortical brain slices, induced Kir-dependent vasodilation and suppression of SMC intracellular calcium (Ca2+) oscillations. Neuronal activation induced a rapid (<2 s latency) vasodilation that was greatly reduced by Kir channel blockade and completely abrogated by concurrent cyclooxygenase inhibition. Astrocytic endfeet exhibited large-conductance, Ca2+-sensitive K+ (BK) channel currents that could be activated by neuronal stimulation. Blocking BK channels or ablating the gene encoding these channels prevented neuronally induced vasodilation and suppression of arteriolar SMC Ca2+, without affecting the astrocytic Ca2+ elevation. These results support the concept of intercellular K+ channel-to-K+ channel signaling, through which neuronal activity in the form of an astrocytic Ca2+ signal is decoded by astrocytic BK channels, which locally release K+ into the perivascular space to activate SMC Kir channels and cause vasodilation.

Synchronization between arterial blood pressure and cerebral oxyhaemoglobin concentration investigated by wavelet cross-correlation

Wavelet cross-correlation (WCC) is used to analyse the relationship between low-frequency oscillations in near-infrared spectroscopy (NIRS) measured cerebral oxyhaemoglobin (O(2)Hb) and mean arterial blood pressure (MAP) in patients suffering from autonomic failure and age-matched controls. Statistically significant differences are found in the wavelet scale of maximum cross-correlation upon posture change in patients, but not in controls. We propose that WCC analysis of the relationship between O(2)Hb and MAP provides a useful method of investigating the dynamics of cerebral autoregulation using the spontaneous low-frequency oscillations that are typically observed in both variables without having to make the assumption of stationarity of the time series. It is suggested that for a short-duration clinical test previous transfer-function-based approaches to analyse this relationship may suffer due to the inherent nonstationarity of low-frequency oscillations that are observed in the resting brain.

Quantitative evaluation of interrelations between spontaneous low-frequency oscillations in cerebral hemodynamics and systemic cardiovascular dynamics

A common issue in blood-related brain-function measurements, such as optical topography, is that the observed signals are usually corrupted with strong noise that is primarily spontaneous low-frequency oscillations (LFOs) in cerebral hemodynamics, which are difficult to separate from the signals due to functional brain activity because of their common spectral range. We discuss the analysis of information transfer between LFOs around 0.1 Hz in the hemoglobin concentration change (HbCC) in the cerebral cortex, the heart rate (HR), and the mean arterial blood pressure (ABP) to understand the origin of spontaneous LFOs in cerebral hemodynamics. As measures of information transfer, we used transfer entropy (TE) for two-variable system analysis and introduced intrinsic transfer entropy for further analysis of three-variable systems by extending the original TE. Data for analysis were obtained from simultaneous measurements with optical topography and infrared finger plethysmography under rest conditions. The analysis revealed that the LFOs in oxy HbCC, a parameter of cerebral hemodynamics, mainly stem from HR, but its contribution is only about 20%. In addition, the intrinsic contribution of ABP is about 5% and the common contribution of HR and ABP is about 10%. From these, HR and ABP cannot account for more than the half the information carried with variable oxy HbCC, which suggests the origin of LFOs in cerebral hemodynamics may lie in the regulation of regional cerebral blood flow change and energetic metabolism rather than due to the systemic regulation of the cardiovascular system.

Blood volume and hemoglobin oxygenation response following electrical stimulation of human cortex

Our understanding of perfusion-based human brain mapping techniques relies on a detailed knowledge of the relationship between neuronal activity and cerebrovascular hemodynamics. We performed optical imaging of intrinsic signals at wavelengths sensitive to total hemoglobin (Hbt; which correlate with cerebral blood volume (CBV)) and deoxygenated hemoglobin (Hbr) directly in humans during neurosurgical operations and investigated the optical signals associated with bipolar cortical stimulation at a range of amplitudes. Cortical stimulation elicited a rapid focal increase in Hbr (initial dip) in all subjects. An equally rapid increase in Hbt (<200 ms), with a slightly higher signal-to-noise ratio, was also highly localized for <2 s in spite of the non-columnar nature of the stimulus, after which the signal spread to adjacent gyri. A later decrease in Hbr (>3 s), which is relevant to the blood oxygen level dependent (BOLD) signal, was poorly localized. Increasing the stimulus amplitude elicited a linear increase in the area of the optical signal for Hbt and the initial dip but not the late decrease in Hbr, and a nonlinear increase in optical signal amplitude with a plateau effect for initial dip, Hbt and late decrease in Hbr.

Dynamic physiological modeling for functional diffuse optical tomography

Diffuse optical tomography (DOT) is a noninvasive imaging technology that is sensitive to local concentration changes in oxy- and deoxyhemoglobin. When applied to functional neuroimaging, DOT measures hemodynamics in the scalp and brain that reflect competing metabolic demands and cardiovascular dynamics. The diffuse nature of near-infrared photon migration in tissue and the multitude of physiological systems that affect hemodynamics motivate the use of anatomical and physiological models to improve estimates of the functional hemodynamic response. In this paper, we present a linear state-space model for DOT analysis that models the physiological fluctuations present in the data with either static or dynamic estimation. We demonstrate the approach by using auxiliary measurements of blood pressure variability and heart rate variability as inputs to model the background physiology in DOT data. We evaluate the improvements accorded by modeling this physiology on ten human subjects with simulated functional hemodynamic responses added to the baseline physiology. Adding physiological modeling with a static estimator significantly improved estimates of the simulated functional response, and further significant improvements were achieved with a dynamic Kalman filter estimator (paired t tests, n=10, P<0.05). These results suggest that physiological modeling can improve DOT analysis. The further improvement with the Kalman filter encourages continued research into dynamic linear modeling of the physiology present in DOT. Cardiovascular dynamics also affect the blood-oxygen-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI). This state-space approach to DOT analysis could be extended to BOLD fMRI analysis, multimodal studies and real-time analysis.

Spatial specificity of the enhanced dip inherently induced by prolonged oxygen consumption in cat visual cortex: Implication for columnar resolution functional MRI

Since changes in oxygen consumption induced by active neurons are specific to cortical columns, the small and transient "dip" of deoxyhemoglobin signal, which indicates an increase in oxygen consumption, has been of great interest. In this study, we succeeded in enhancing and sustaining the dip in the deoxyhemoglobin-weighted 620-nm intrinsic optical imaging signals from a 10-s orientation-selective stimulation in cat visual cortex by reducing arterial blood pressure with sodium nitroprusside (a vasodilator) to mitigate the contribution of stimulus-induced blood supply. During this condition, intact spiking activity and a significant reduction of stimulus-induced blood volume changes (570-nm intrinsic signals) were confirmed. The deoxyhemoglobin signal from the prolonged dip was highly localized to iso-orientation domains only during the initial approximately 2 s; the signal specificity weakened over time although the domains were still resolvable after 2 s. The most plausible explanation for this time-dependent spatial specificity is that deoxyhemoglobin induced by oxygen consumption drains from active sites, where spiking activity occurs, to spatially non-specific downstream vessels over time. Our results suggest that the draining effect of pial and intracortical veins in dHb-based imaging techniques, such as blood oxygenation-level dependent (BOLD) functional MRI, is intrinsically unavoidable and reduces its spatial specificity of dHb signal regardless of whether the stimulus-induced blood supply is spatially specific.

Separate spatial scales determine neural activity-dependent changes in tissue oxygen within central visual pathways

The relationship between oxygen levels and neural activity in the brain is fundamental to functional neuroimaging techniques. We have examined this relationship on a fine spatial scale in the lateral geniculate nucleus (LGN) and visual cortex of the cat using a microelectrode sensor that provides simultaneous colocalized measurements of oxygen partial pressure in tissue (tissue oxygen) and multiunit neural activity. In previous work with this sensor, we found that changes in tissue oxygen depend strongly on the location and spatial extent of neural activation. Specifically, focal neural activity near the microelectrode elicited decreases in tissue oxygen, whereas spatially extended activation, outside the field of view of our sensor, yielded mainly increases. In the current study, we report an expanded set of measurements to quantify the spatiotemporal relationship between neural responses and changes in tissue oxygen. For the purpose of data analysis, we develop a quantitative model that assumes that changes in tissue oxygen are composed of two response components (one positive and one negative) with magnitudes determined by neural activity on separate spatial scales. Our measurements from visual cortex and the LGN are consistent with this model and suggest that the positive response spreads over a distance of 1-2 mm, whereas the negative component is confined to a few hundred micrometers. These results are directly relevant to the mechanisms that generate functional brain imaging signals and place limits on their spatial properties.

Development of contralateral and ipsilateral frequency representations in ferret primary auditory cortex

Little is known about the maturation of functional maps in the primary auditory cortex (A1) after the onset of sensory experience. We used intrinsic signal imaging to examine the development of the tonotopic organization of ferret A1 with respect to contralateral and ipsilateral tone stimulation. Sound-evoked responses were recorded as early as postnatal day (P) 33, a few days after hearing onset. From P36 onwards, pure tone stimuli evoked restricted, tonotopically organized patches of activity. There was an age-dependent increase in the cortical area representing each octave, with a disproportionate expansion of cortical territory representing frequencies > 4 kHz after P60. Similar tonotopic maps were observed following stimulation of the contralateral and ipsilateral ears. During the first few weeks following hearing onset, no differences were found in the area of cortical activation or in the magnitude of the optical responses evoked by stimulation of each ear. In older animals, however, contralateral stimuli evoked stronger responses and activated a larger A1 area than ipsilateral stimuli. Our findings indicate that neither the tonotopic organization nor the representation of inputs from each ear reach maturity until approximately 1 month after hearing onset. These results have important implications for cortical signal processing in juvenile animals.

A resilient, low-frequency, small-world human brain functional network with highly connected association cortical hubs

Small-world properties have been demonstrated for many complex networks. Here, we applied the discrete wavelet transform to functional magnetic resonance imaging (fMRI) time series, acquired from healthy volunteers in the resting state, to estimate frequency-dependent correlation matrices characterizing functional connectivity between 90 cortical and subcortical regions. After thresholding the wavelet correlation matrices to create undirected graphs of brain functional networks, we found a small-world topology of sparse connections most salient in the low-frequency interval 0.03-0.06 Hz. Global mean path length (2.49) was approximately equivalent to a comparable random network, whereas clustering (0.53) was two times greater; similar parameters have been reported for the network of anatomical connections in the macaque cortex. The human functional network was dominated by a neocortical core of highly connected hubs and had an exponentially truncated power law degree distribution. Hubs included recently evolved regions of the heteromodal association cortex, with long-distance connections to other regions, and more cliquishly connected regions of the unimodal association and primary cortices; paralimbic and limbic regions were topologically more peripheral. The network was more resilient to targeted attack on its hubs than a comparable scale-free network, but about equally resilient to random error. We conclude that correlated, low-frequency oscillations in human fMRI data have a small-world architecture that probably reflects underlying anatomical connectivity of the cortex. Because the major hubs of this network are critical for cognition, its slow dynamics could provide a physiological substrate for segregated and distributed information processing.

Spontaneous slow hemodynamic oscillations are impaired in cerebral microangiopathy

Small-vessel disease or cerebral microangiopathy (CMA) is a common finding in elderly people. It is related to a variety of vascular risk factors and may finally lead to subcortical ischemic vascular dementia. Because vessel stiffness is increased, we hypothesized that slow spontaneous oscillations are reduced in cerebral hemodynamics. Accordingly, we examined spontaneous oscillations in the visual cortex of 13 patients suffering from CMA, and compared them with 14 age-matched controls. As an imaging method we applied functional near-infrared spectroscopy, because it is particularly sensitive to the microvasculature. Spontaneous low-frequency oscillations (LFOs) (0.07 to 0.12 Hz) were specifically impaired in CMA in contrast to spontaneous very-low-frequency oscillations (0.01 to 0.05 Hz), which remained unaltered. Vascular reagibility was reduced during visual stimulation. Interestingly, changes were tightly related to neuropsychological deficits, namely executive dysfunction. Vascular alterations had to be attributed mainly to the vascular risk factor arterial hypertension. Further, results suggest that the impairments might be, at least partly, reversed by medical treatment such as angiotensin-converting enzyme inhibitors/angiotensin II receptor blockers. Results indicate that functional near-infrared spectroscopy may detect changes in the microvasculature due to CMA, namely an impairment of spontaneous LFOs, and of vascular reagibility. Hence, CMA accelerates microvascular changes due to aging, leading to impairments of autoregulation.

Fine functional organization of auditory cortex revealed by Fourier optical imaging

We provide an overall view of the functional tonotopic organization of the auditory cortex in the rat. We apply a recently developed technique for acquiring intrinsic signal optical maps, Fourier imaging, in the rat auditory cortex. These highly detailed maps, derived in a several-minute-long recording procedure, delineate multiple auditory cortical areas and demonstrate their shapes, sizes, and tonotopic order. Beyond the primary auditory cortex, there are at least three distinct areas with fine-scale tonotopic organization, as well as at least one additional high-frequency field. The arrangement of all of these cortical areas is consistent across subjects. The accuracy of these optical maps was confirmed by microelectrode mapping in the same subjects. This imaging method allows fast mapping of the auditory cortex at high spatial resolution comparable to that provided by conventional microelectrode technique. Although spiking activity is largely responsible for the evoked intrinsic signals, certain features of the optical signal cannot be explained by spiking activity only, and should probably be attributed to other mechanisms inducing metabolic activity, such as subthreshold membrane phenomena.

The effect of hypercapnia on the neural and hemodynamic responses to somatosensory stimulation

Modern non-invasive imaging techniques utilize the coupling between neural activity and changes in blood flow, volume and oxygenation to map the functional architecture of the human brain. An understanding of how the hemodynamic response is influenced by pre-stimulus baseline perfusion is important for the interpretation of imaging data. To address this issue, the present study measured hemodynamics with optical imaging spectroscopy and laser Doppler flowmetry, while multi-channel electrophysiology was used to record local field potentials (LFP) and multi-unit activity (MUA). The response to whisker stimulation in rodent barrel cortex was recorded during baseline (normocapnia) and elevated perfusion rates produced by two levels of hypercapnia (5 and 10%). With the exception of the 'washout' of deoxyhemoglobin, which was attenuated, all aspects of the neural and hemodynamic response to whisker stimulation were similar during 5% hypercapnia to those evoked during normocapnia. In contrast, 10% hypercapnia produced cortical arousal and a reduction in both the current sink and MUA elicited by stimulation. Blood flow and volume responses were reduced by a similar magnitude to that observed in the current sink. The deoxyhemoglobin 'washout', however, was attenuated to a greater degree than could be expected from the neural activity. These data suggest that imaging techniques based on perfusion or blood volume changes may be more robust to shifts in baseline than those based on the dilution of deoxyhemoglobin, such as conventional BOLD fMRI.

An arteriolar compliance model of the cerebral blood flow response to neural stimulus

Although functional magnetic resonance imaging (fMRI) is a widely used and powerful tool for studying brain function, the quantitative interpretation of fMRI measurements for basic neuroscience and clinical studies can be complicated by inter-subject and inter-session variability arising from modulation of the baseline vascular state by disease, aging, diet, and pharmacological agents. In particular, recent studies have shown that the temporal dynamics of the cerebral blood flow (CBF) and the blood oxygenation level dependent (BOLD) responses to stimulus are modulated by changes in baseline CBF induced by various vasoactive agents and by decreases in vascular compliance associated with aging. These effects are not readily explained using current models of the CBF and BOLD responses. We present here a second-order nonlinear feedback model of the evoked CBF response in which neural activity modulates the compliance of arteriolar smooth muscle. Within this model framework, the baseline vascular state affects the dynamic response by changing the relative contributions of an active smooth muscle component and a passive connective tissue component to the overall vessel compliance. Baseline dependencies of the BOLD signal are studied by coupling the arteriolar compliance model with a previously described balloon model of the venous compartment. Numerical simulations show that the proposed model describes to first order the observed dependence of CBF and BOLD responses on the baseline vascular state.

Spatiotemporal evolution of functional hemodynamic changes and their relationship to neuronal activity

Brain imaging techniques such as functional magnetic resonance imaging (fMRI) have provided a wealth of information about brain organization, but their ability to investigate fine-scale functional architecture is limited by the spatial specificity of the hemodynamic responses upon which they are based. We investigated the spatiotemporal evolution of hemodynamic responses in rat somatosensory cortex to electrical hindpaw stimulation. We combined the advantages of optical intrinsic signal imaging and spectroscopy to produce high-resolution two-dimensional maps of functional changes in tissue oxygenation and blood volume. Cerebral blood flow changes were measured with laser-Doppler flowmetry, and simultaneously recorded field potentials allowed comparison between hemodynamic changes and underlying neuronal activity. For the first 2 to 3 secs of activation, hemodynamic responses overlapped in a central parenchymal focus. Over the next several seconds, cerebral blood volume changes propagated retrograde into feeding arterioles, and oxygenation changes anterograde into draining veins. By 5 to 6 secs, responses localized primarily in vascular structures distant from the central focus. The peak spatial extent of the hemodynamic response increased linearly with synaptic activity. This spatial spread might be because of lateral subthreshold activation or passive vascular overspill. These results imply early microvascular changes in volume and oxygenation localize to activated neural columns, and that spatial specificity will be optimal within a 2- to 3-sec window after neuronal activation.

Varieties of attention-deficit/hyperactivity disorder-related intra-individual variability

Intra-individual variability in behavior and functioning is ubiquitous among children with attention-deficit/hyperactivity disorder (ADHD), but it has not been systematically examined or integrated within causal models. This article seeks to provide a conceptual, methodologic, and analytic framework as a foundation for future research. We first identify five key research questions and methodologic issues. For illustration, we examine the periodic structure of Eriksen Flanker task reaction time (RT) data obtained from 24 boys with ADHD and 18 age-matched comparison boys. Reaction time variability in ADHD differed quantitatively from control subjects, particularly at a modal frequency around .05 Hz (cycle length approximately 20 sec). These oscillations in RT were unaffected by double-blind placebo and were suppressed by double-blind methylphenidate. Together with converging lines of basic and clinical evidence, these secondary data analyses support the speculative hypothesis that the increased power of multisecond oscillations in ADHD RT data, and by inference, in attentional performance, represents a catecholaminergic deficit in the ability to appropriately modulate such oscillations in neuronal activity. These results highlight the importance of retaining time-series data and quantitatively examining intra-subject measures of variability as a putative endophenotype for ADHD.

Extraction of the average and differential dynamical response in stimulus-locked experimental data

In optical imaging experiments of primary visual cortex, visual stimuli evoke a complicated dynamics. Typically, any stimulus with sufficient contrast evokes a response. Much of the response is the same regardless of which stimulus is presented. For instance, when oriented drifting gratings are presented to the visual system, over 90% of the response is the same from orientation to orientation. Small differences may be seen, however, between the responses to different orientations. A problem in the analysis of optical measurements of the response to stimulus in cortical tissue is the distinction of the 'global' or 'non-specific' response from the 'differential' or 'stimulus-specific' response. This problem arises whenever the signal of interest is the difference in response to various stimuli and is evident in many kinds of uni- and multivariate data. To this end, we present enhancements to a frequency-based method that we previously introduced called the periodic stacking method. These enhancements allow us to separately estimate the dynamics of both the average signal across all stimuli (the 'global' response) and deviations from the average amongst the various stimuli (the 'stimulus-specific' response) evoked in response to a set of stimuli. We also discuss improvements in the signal-to-noise ratio, relative to standard trial averaging methods, that result from the data-adaptive smoothing in our method.

Temporal dependence in uncoupling of blood volume and oxygenation during interictal epileptiform events in rat neocortex

We investigated the dynamic spatiotemporal relationships of cerebral blood volume (CBV), deoxygenated hemoglobin (Hbr), and light scatter (LS) associated with interictal epileptiform events with multiwavelength optical recording of intrinsic signals and simultaneous field potential recording. Interictal spikes (IISs) were induced with iontophoresis of bicuculline methiodide in rat neocortex. Intrinsic signal changes appeared as early as 100 msec after the IIS at all wavelengths and could be appreciated after only a single IIS. Initially, the largest signal arose from a focal increase in deoxygenation, which lasted for approximately 2 sec, consistent with an "initial dip." An equally early focal increase in CBV had a smaller amplitude than the Hbr signal until >2 sec after the IIS, when its amplitude surpassed that of the Hbr signal but also spread to a larger, less focal area. The most spatially restricted and smallest amplitude signal was produced by LS. A later hyperoxygenation, or increase in blood oxygenation level-dependent signal, was often seen in the draining veins but inconsistently seen in the IIS focus. An inverted optical signal was recorded at all wavelengths from multiple regions in the surrounding cortex within 100 msec of the IIS. We therefore conclude that the IIS induces a rapid increase in metabolic demand, which cannot be met by a rapid, initially focal but small increase in CBV that results in a prolonged increase in Hbr (epileptic dip in oxygenated hemoglobin). The inverted optical signal in the surround arises from a decrease in CBV and a decrease in Hbr, likely resulting from a combination of shunting of CBV to the focus and decreased metabolic demand resulting from decreased neuronal activity, consistent with "surround inhibition."

Cortical spreading depression produces long-term disruption of activity-related changes in cerebral blood volume and neurovascular coupling

Cortical spreading depression (CSD) is a pronounced depolarization of neurons and glia that spreads slowly across the cortex followed by a period of depressed electrophysiological activity. The vascular changes associated with CSD are a large transient increase in blood flow followed by a prolonged decrease lasting greater than 1 h. Currently, the profile of functional vascular activity during this hypovolemic period has not been well characterized. Perfusion-based imaging techniques such as functional magnetic resonance imaging (fMRI) assume a tight coupling between changes in neuronal and vascular activity. Under normal conditions, these variables are well correlated. Characterizing the effect of CSD on this relationship is an important step to understand the impact acute pathophysiological events may have on neurovascular coupling. We examine the effect of CSD on functional changes in cerebral blood volume (CBV) evoked by cortical electrophysiological activity for 1 h following CSD induction. CBV signal amplitude, duration, and time to peak show little recovery at 60 min post-induction. Analysis of spontaneous vasomotor activity suggests a decrease in vascular reactivity may play a significant role in the disruption of normal functional CBV responses. Electrophysiological activity is also attenuated but to a lesser degree. CBV and evoked potentials are not well correlated following CSD, suggesting a breakdown of the neurovascular coupling relationship.

Neuronal activity-related coupling in cortical arterioles: involvement of astrocyte-derived factors

Neuronal activity-evoked dilatation was investigated in cortical arterioles in brain slices from mature rats maintained in vitro at 31-33 degrees C. In the presence of the thromboxane A2 agonist U46619 (75 nM) to preconstrict vessels, internal diameter decreased by 14.2% and rhythmic contractile activity (vasomotion) developed. Addition of the epoxygenase inhibitor miconazole (20 microm) produced a further decrease in diameter and increase in the frequency of vasomotion, suggesting that tonic release of epoxygenase products maintains a level of cerebrovascular dilator tone. Addition of 1 mum AMPA for 5 min evoked a 15.4 +/- 3.7% increase in diameter and the frequency of vasomotion decreased by -6.7 +/- 1.4 contractions min(-1). The response persisted in the presence of 1 mum TTX, indicating that it was independent of neuronal activity and thus likely to have been evoked by activation of AMPA receptors on astrocytes rather than neurones. The response to the brief (5 min) application of AMPA remained unchanged in the presence of miconazole (20 microm). Prolonged (30 min) application of AMPA produced a +12.1 +/- 1.5% increase in internal diameter and reduction in vasomotion (-8.4 +/- 1.7 contractions min(-1)) that were sustained throughout the stimulation period. However, when AMPA was applied in the presence of miconazole (20 microm) it evoked only a transient increase in diameter (+9.8 +/- 3.1%) and decrease in vasomotion (-6.6 +/- 1.5 contractions min(-1)) that lasted for less than 10 min despite continued application of AMPA. The results suggest that products of epoxygenase activity, probably epoxyeicosatrienoic acids (EETs) are involved in activity-related dilatation in cortical arterioles. Whilst epoxygenase activity is not required to initiate dilatation, it appears to be involved in sustaining the response. Thus EETs released from membrane stores could contribute to the initial stages, but once these have been depleted de novo synthesis of EETs is required to maintain the effect.

Caffeine alters the temporal dynamics of the visual BOLD response

The blood oxygenation level-dependent (BOLD) responses to visual stimuli, using both a 1-s long single trial stimulus and a 20-s long block stimulus, were measured in a 4-T magnetic field both before and immediately after a 200-mg caffeine dose. In addition, resting levels of cerebral blood flow (CBF) were measured using arterial spin labeling. For the single trial stimulus, the caffeine dose significantly (p<0.05) reduced the time to peak (TTP), the time after the peak at which the response returned to 50% of the peak amplitude (TA50), and the amplitude of the poststimulus undershoot in all subjects (N=5). Other parameters, such as the full-width half-maximum (FWHM) and the peak amplitude, also showed significant changes in the majority of subjects. For the block stimulus, the TTP, TA50, and the time for the response to reach 50% of the peak amplitude (T50) were significantly reduced. In some subjects, oscillations were observed in the poststimulus portion of the response with median peak periods of 9.1 and 9.5 s for the single trial and block responses, respectively. Resting CBF was reduced by an average of 24%. The reproducibility of the results was verified in one subject who was scanned on 3 different days. The dynamic changes are similar to those previously reported for baseline CBF reductions induced by hypocapnia and hyperoxia.

Calcium Dynamics in Cortical Astrocytes and Arterioles During Neurovascular Coupling

Neuronal activity in the brain is thought to be coupled to cerebral arterioles (functional hyperemia) through Ca 2+ signals in astrocytes. Although functional hyperemia occurs rapidly, within seconds, such rapid signaling has not been demonstrated in situ, and Ca 2+ measurements in parenchymal arterioles are still lacking. Using a laser scanning confocal microscope and fluorescence Ca 2+ indicators, we provide the first evidence that in a brain slice preparation, increased neuronal activity by electrical stimulation (ES) is rapidly signaled, within seconds, to cerebral arterioles and is associated with astrocytic Ca 2+ waves. Smooth muscle cells in parenchymal arterioles exhibited Ca 2+ and diameter oscillations (“vasomotion”) that were rapidly suppressed by ES. The neuronal-mediated Ca 2+ rise in cortical astrocytes was dependent on intracellular (inositol trisphosphate [IP 3 ]) and extracellular voltage-dependent Ca 2+ channel sources. The Na + channel blocker tetrodotoxin prevented the rise in astrocytic [Ca 2+ ] i and the suppression of Ca 2+ oscillations in parenchymal arterioles to ES, indicating that neuronal activity was necessary for both events. Activation of metabotropic glutamate receptors in astrocytes significantly decreased the frequency of Ca 2+ oscillations in parenchymal arterioles. This study supports the concept that astrocytic Ca 2+ changes signal the cerebral microvasculature and indicate the novel concept that this communication occurs through the suppression of arteriolar [Ca 2+ ] i oscillations and corresponding vasomotion. The full text of this article is available online at http://circres.ahajournals.org.

Haemodynamic responses to sensory stimulation are enhanced following acute cocaine administration

Cocaine enhances neural activity in response to sensory stimulation, an effect that may play a role in the development of drug craving. However, cocaine-induced sensory enhancement may be difficult to study in humans using neuroimaging if the global increases in baseline haemodynamic parameters, which cocaine produces, interfere with the ability of enhanced sensory-related neural activity to lead to enhanced haemodynamic responses. To investigate the effect of cocaine-induced baseline haemodynamic changes on sensory-related haemodynamic (and electrophysiological) responses, field potential (FP) and haemodynamic responses (obtained using optical imaging spectroscopy and laser-Doppler flowmetry) in the barrel cortex of the anaesthetised rat were measured during mechanical whisker stimulation following cocaine (0.5 mg/kg) or saline administration. During cocaine infusion, the relationship between blood flow and volume transiently decoupled. Following this, cocaine caused large baseline increases in blood flow (133%) and volume (33%), which peaked after approximately 6 min and approached normal levels again after 25 min. During the peak baseline increases, FP responses to whisker stimulation were similar to saline whereas several haemodynamic response parameters were slightly reduced. After the peak, significant increases in FP responses were observed, accompanied by significantly enhanced haemodynamic responses, even though the haemodynamic baselines remained elevated. Hence, the haemodynamic response to sensory stimulation is transiently reduced in the presence of large increases in baseline but, after the baseline peak, enhanced neural responses are faithfully accompanied by enhanced haemodynamic responses. The findings suggest that any cocaine-induced enhancement of sensory-related neural activity in humans is likely to be detectable by neuroimaging.

Spontaneous low-frequency oscillations decline in the aging brain

It is well known that aging leads to a degeneration of the vascular system. Hence, one may hypothesize that spontaneous oscillations decrease in the cerebral microvasculature with aging. Accordingly, the authors investigated the age dependency of spontaneous oscillations in the visual cortex during rest and functional activation. Functional near-infrared spectroscopy was used because it is particularly sensitive to the microvasculature. Visual stimulation led to an increase of oxyhemoglobin, total hemoglobin, and a decrease of deoxyhemoglobin, without any influence of age. Peaks of normalized power spectral density were detected for spontaneous low-frequency (0.07 to 0.11 Hz) and very-low-frequency (0.01 to 0.05 Hz) oscillations, with a higher amplitude for oxyhemoglobin than for deoxyhemoglobin. Spontaneous low-frequency oscillations of oxyhemoglobin and deoxyhemoglobin declined strongly with aging during both rest and visual stimulation. Reduction of spontaneous low-frequency oscillations might indicate a declining spontaneous activity in microvascular smooth muscle cells, in conjunction with an increased vessel stiffness with aging.

Functional signal- and paradigm-dependent linear relationships between synaptic activity and hemodynamic responses in rat somatosensory cortex

Linear relationships between synaptic activity and hemodynamic responses are critically dependent on functional signal etiology and paradigm. To investigate these relationships, we simultaneously measured local field potentials (FPs) and optical intrinsic signals in rat somatosensory cortex while delivering a small number of electrical pulses to the hindpaw with varied stimulus intensity, number, and interstimulus interval. We used 570 and 610 nm optical signals to estimate cerebral blood volume (CBV) and oxygenation, respectively. The spatiotemporal evolution patterns and trial-by-trial correlation analyses revealed that CBV-related optical signals have higher fidelity to summed evoked FPs (SigmaFPs) than oxygenation-derived signals. CBV-related signals even correlated with minute SigmaFP fluctuations within trials of the same stimulus condition. Furthermore, hemodynamic signals (CBV and late oxygenation signals) increased linearly with SigmaFP while varying stimulus number, but they exhibited a threshold and steeper gradient while varying stimulus intensity, suggesting insufficiency of the homogeneity property of linear systems and the importance of spatiotemporal coherence of neuronal population activity in hemodynamic response formation. These stimulus paradigm-dependent linear and nonlinear relationships demonstrate that simple subtraction-based analyses of hemodynamic signals produced by complex stimulus paradigms may not reflect a difference in SigmaFPs between paradigms. Functional signal- and paradigm-dependent linearity have potentially profound implications for the interpretation of perfusion-based functional signals.

Optical imaging of intrinsic signals: recent developments in the methodology and its applications

Since optical imaging (OI) of intrinsic signals was first developed in the 1980s, significant advances have been made regarding our understanding of the origins of the recorded signals. The technique has been refined and the range of its applications has been broadened considerably. Here we review recent developments in methodology and data analysis as well as the latest findings on how intrinsic signals are related to metabolic cost and electrophysiological activity in the brain. We give an overview of what optical imaging has contributed to our knowledge of the functional architecture of sensory cortices, their development and plasticity. Finally, we discuss the utility of OI for functional studies of the human brain as well as in animal models of neuropathology.

Large-scale organization of ferret auditory cortex revealed using continuous acquisition of intrinsic optical signals

We have adapted a new approach for intrinsic optical imaging, in which images were acquired continuously while stimuli were delivered in a series of continually repeated sequences, to provide the first demonstration of the large-scale tonotopic organization of both primary and nonprimary areas of the ferret auditory cortex. Optical responses were collected during continuous stimulation by repeated sequences of sounds with varying frequency. The optical signal was averaged as a function of time during the sequence, to produce reflectance modulation functions (RMFs). We examined the stability and properties of the RMFs and show that their zero-crossing points provide the best temporal reference points for quantifying the relationship between the stimulus parameter values and optical responses. Sequences of different duration and direction of frequency change gave rise to comparable results, although in some cases discrepancies were observed, mostly between upward- and downward-frequency sequences. We demonstrated frequency maps, consistent with previous data, in primary auditory cortex and in the anterior auditory field, which were verified with electrophysiological recordings. In addition to these tonotopic gradients, we demonstrated at least 2 new acoustically responsive areas on the anterior and posterior ectosylvian gyri, which have not previously been described. Although responsive to pure tones, these areas exhibit less tonotopic order than the primary fields.

Cytochrome-c-oxidase redox changes during visual stimulation measured by near-infrared spectroscopy cannot be explained by a mere cross talk artefact

The detection of redox changes in cytochrome-c-oxidase ([Cyt-ox]) in response to cerebral activation by non-invasive NIRS is hampered by methodological spectroscopic issues related to the modification of the Beer-Lambert law. Also, the question whether a change in the enzyme's redox-state is elicited by functional stimulation is unresolved. In a previous study, we found physiological evidence in favour of an activation-induced increase in oxidation of the enzyme [J. Cereb. Blood Flow Metab. 19 (1999) 592], while in a second study on spectroscopic cross talk, we found that the [Cyt-ox] changes to potentially be an artefact of the spectroscopic approach [J. Biomed. Opt. 7 (2002) 51]. Here, we use two different stimuli which differentially activate areas either rich or poor in [Cyt-ox] content (blob/interblob in visual cortex V1 and pale/thin stripes in V2) to further clarify this apparent discrepancy. In a first experiment, two stimuli were presented in an alternating fashion for 20 s and all stimulation periods were separated by resting periods of 40 s. We observed similar changes in [Cyt-ox] for both stimuli. To become more sensitive to the potentially very small optical changes related to changes in [Cyt-ox], we tried to minimise global haemodynamic and metabolic effects in a second experiment by omitting the resting periods. Our hypothesis was that [Cyt-ox] changes could be fully explained by cross talk as it is predicted from our last study [J. Biomed. Opt. 7 (2002) 51]. However, in more than half of the experiments, we were not able to model the changes in Cyt-ox calculated from measured attenuation spectra as a cross talk artefact. We interpret this finding as an argument in favour of the existence of [Cyt-ox] changes in response to functional stimulation. This finding, however, does not lessen the liability of the [Cyt-ox] changes to cross talk and calls for great caution when [Cyt-ox] changes are derived from NIRS measurements based on the modified Beer-Lambert approach. Further (invasive) validation studies are required.

Linear and Nonlinear Relationships between Neuronal Activity, Oxygen Metabolism, and Hemodynamic Responses

We investigated the relationship between neuronal activity, oxygen metabolism, and hemodynamic responses in rat somatosensory cortex with simultaneous optical intrinsic signal imaging and spectroscopy, laser Doppler flowmetry, and local field potential recordings. Changes in cerebral oxygen consumption increased linearly with synaptic activity but with a threshold effect consistent with the existence of a tissue oxygen buffer. Modeling analysis demonstrated that the coupling between neuronal activity and hemodynamic response magnitude may appear linear over a narrow range but incorporates nonlinear effects that are better described by a threshold or power law relationship. These results indicate that caution is required in the interpretation of perfusion-based indicators of brain activity, such as functional magnetic resonance imaging (fMRI), and may help to refine quantitative models of neurovascular coupling.

Columnar specificity of microvascular oxygenation and volume responses: implications for functional brain mapping

Cortical neurons with similar properties are grouped in columnar structures and supplied by matching vascular networks. The hemodynamic response to neuronal activation, however, is not well described on a fine spatial scale. We investigated the spatiotemporal characteristics of microvascular responses to neuronal activation in rat barrel cortex using optical intrinsic signal imaging and spectroscopy. Imaging was performed at 570 nm to provide functional maps of cerebral blood volume (CBV) changes and at 610 nm to estimate oxygenation changes. To emphasize parenchymal rather than large vessel contributions to the functional hemodynamic responses, we developed an ANOVA-based statistical analysis technique. Perfusion-based maps were compared with underlying neuroanatomy with cytochrome oxidase staining. Statistically determined CBV responses localized accurately to individually stimulated barrel columns and could resolve neighboring columns with a resolution better than 400 microm. Both CBV and early oxygenation responses extended beyond anatomical boundaries of single columns, but this vascular point spread did not preclude spatial specificity. These results indicate that microvascular flow control structures providing targeted flow increases to metabolically active neuronal columns also produce finely localized changes in CBV. This spatial specificity, along with the high contrast/noise ratio, makes the CBV response an attractive mapping signal. We also found that functional oxygenation changes can achieve submillimeter specificity not only during the transient deoxygenation ("initial dip") but also during the early part of the hyperoxygenation. We, therefore, suggest that to optimize hemodynamic spatial specificity, appropriate response timing (using < or =2-3 sec changes) is more important than etiology (oxygenation or volume).

Two-photon imaging of capillary blood flow in olfactory bulb glomeruli

Analysis of the spatiotemporal coupling between neuronal activity and cerebral blood flow requires the precise measurement of the dynamics of RBC flow in individual capillaries that irrigate activated neurons. Here, we use two-photon microscopy in vivo to image individual RBCs in glomerular capillaries in the rat dorsal olfactory bulb. We find that odor stimulation evokes capillary vascular responses that are odorant- and glomerulus-specific. These responses consist of increases as well as decreases in RBC flow, both resulting from independent changes in RBC velocity or linear density. Finally, measuring RBC flow with micrometer spatial resolution and millisecond temporal resolution, we demonstrate that, in olfactory bulb superficial layers, capillary vascular responses precisely outline regions of synaptic activation.

Functional assessment of Broca’s area using near infrared spectroscopy in humans

We used near-infrared spectroscopy (NIRS) to compare functional hemoglobin concentration changes (delta[oxy-Hb] and delta[deoxy-Hb]) over human language and motor cortices. Eight subjects performed finger opposition, tongue movement, and covert visual object naming in an interleaved block paradigm design. NIRS revealed paradigm specific patterns of delta[oxy-Hb] and delta[deoxy-Hb] providing cortical localization of each function. During each task, significant response overlap was observed when comparing the [oxy-Hb] signals, whereas delta[deoxy-Hb] seemed more localized. Furthermore, by applying magnitude and time to significance measures to the delta[deoxy-Hb] response profile, Broca's area was easily distinguished from neighboring tongue (and hand) motor representation. Delta[oxy-Hb] did not provide this level of specificity. These findings suggest delta[deoxy-Hb] as the preferential NIRS parameter to map language cortices.

Spatiotemporal characteristics of cerebral blood volume changes in rat somatosensory cortex evoked by sciatic nerve stimulation and obtained by optical imaging

The spatiotemporal characteristics of changes in cerebral blood volume associated with neuronal activity were investigated in the hindlimb somatosensory cortex of alpha-chloralose-urethane anesthetized rats (n=10) with optical imaging at 570 nm through a thinned skull. Activation of the cortex was carried out by electrical stimulation of the contralateral sciatic nerve with 5-Hz, 0.3-V pulses (0.5 ms) for 2 s. The stimulation evoked a monophasic decrease in optical reflectance at the cortical parenchyma and arterial sites soon after the onset of stimulation, whereas no similar response was observed at vein compartments. The optical signal changes reached 10% of the peak response 0.70 +/- 0.32 s after the start of stimulation, and no significant time lag in this 10% start latency time was observed between the response at the cortical parenchyma and artery compartments. The decrease in optical reflectance reached a peak (0.25 +/- 0.047%) 2.66 +/- 0.61 s after stimulus onset at parenchymal sites, which is 0.40 +/- 0.20 s earlier (P<0.05) than that at arterial sites (0.50 +/- 0.068% 3.06 +/- 0.70 s). Varying the locations within the cortical parenchyma and arterial compartments did not significantly affect the temporal characteristics of the evoked signal. These results suggest that stimulation of the sciatic nerve evokes an increase in local blood volume in both capillaries (cortical parenchyma) and arterioles soon after the onset of a stimulus, but the blood volume increase evoked in capillaries could not be entirely accounted for by the dilation of arterioles.

New paradigm for optical imaging: temporally encoded maps of intrinsic signal

We present a new technique for acquiring and analyzing intrinsic signal optical images of brain activity, using continuous stimulus presentation and data acquisition. The main idea is to present a temporally periodic stimulus and to analyze the component of the response at the stimulus frequency. Advantages of the new technique include the removal of heart, respiration, and vasomotor artifacts, a dramatic increase in spatial resolution, and a 30-fold or greater reduction in acquisition time. We also present a novel approach to localizing instantaneous neuronal responses using time-reversed stimuli that is widely applicable to brain imaging. To demonstrate the power of the technique, we present high-resolution retinotopic maps of five visual areas in mouse cortex and orientation maps in cat visual cortex.

Correlated multisecond oscillations in firing rate in the basal ganglia: modulation by dopamine and the subthalamic nucleus

Previous studies from this laboratory have shown that many neurons in the basal ganglia have multisecond (<0.5 Hz) periodicities in firing rate in awake rats. The frequency and regularity of these oscillations are significantly increased by systemically injected dopamine (DA) agonists. Because oscillatory activity should have greater functional impact if shared by many neurons, the level of correlation of multisecond oscillations was assessed by recording pairs of neurons in the globus pallidus and substantia nigra pars reticulata in the same hemisphere, or pairs of globus pallidus neurons in opposite hemispheres in awake, immobilized rats. Cross-correlation (90-180 s lags) and spectral analysis were used to characterize correlated oscillations. Thirty-eight percent of pairs recorded in baseline (n=50) demonstrated correlated multisecond oscillations. Phase relationships were near 0 or 180 degrees. DA agonist injection significantly increased the incidence of correlation (intra- and interhemispheric) to 94% (n=17). After DA agonist injection, phase relationships of globus pallidus/substantia nigra neuron pairs were exclusively concentrated near 180 degrees, and phases of interhemispheric pairs of globus pallidus neurons were concentrated near 0 degrees. After subthalamic nucleus lesion (n=8), the incidence of correlated multisecond oscillations (or of multisecond oscillations per se) was not changed, although the consistent phase relationship between the globus pallidus and substantia nigra pars reticulata was disrupted. Subthalamic lesion also blocked apomorphine-induced decreases in oscillatory period and increases in oscillation amplitude, and significantly attenuated apomorphine-induced changes in mean firing rate. The data demonstrate that multisecond oscillations in the basal ganglia can be correlated between nuclei, and that DA receptor activation increases the level of correlation and organizes internuclear phase relationships at these multisecond time scales. While the subthalamic nucleus is not necessary for generating or transmitting these slow oscillations, it is involved in DA agonist-induced modulation of mean firing rate, oscillatory period, and internuclear phase relationship. These data further support a role for DA in modulating coherent oscillatory activity in the basal ganglia, and for the subthalamic nucleus in shaping the effects of DA receptor stimulation on basal ganglia output.

Shedding light on brain mapping: advances in human optical imaging

Several functional brain imaging techniques have been used to study human cortical organization. Optical imaging of intrinsic signals (OIS) offers perhaps the best combination of spatial coverage, resolution and speed for mapping the functional topography of human cortex. In this review, we discuss recent advances in optical imaging technology and methodology that have made human OIS easier to implement and more accessible, including improvements in detector characteristics and the development of sophisticated algorithms for reducing motion artifact. Moreover, we discuss how these advances have helped enhance our understanding of the functional organization of the human brain. We also review newly developed analyses for interpreting and validating optical signals, including refined signal analysis techniques and multimodality comparisons. Combined, these advances have enabled the study of not only primary sensory and motor cortices, but also higher cognitive processes such as language production and comprehension. Continued improvement and implementation of this technique promises to shed new light on the functional organization of human cortex.

Spatiotemporal analysis of optical imaging data

Previous methods for analyzing optical imaging data have relied heavily on temporal averaging. However, response dynamics are rich sources of information. Here, we develop and present a method that combines principal component analysis and multitaper harmonic analysis to extract the statistically significant spatial and temporal response from optical imaging data. We apply the method to both simulated data and experimental optical imaging data from the cat primary visual cortex.

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.

Intraoperative optical intrinsic signal imaging: a clinical tool for functional brain mapping

Optical imaging of intrinsic signals (OIS) is a well-established neuroimaging modality by which functional cortical activity is mapped by detecting activity-related changes in cortical light reflectance. Light reflectance changes are detected by a charged-coupled device camera that captures images of the exposed cortex both at rest and during activity. Although to date OIS has only been used for research purposes, intraoperative OIS (iOIS) holds promise as a clinical mapping tool. In general, iOIS demonstrates good spatial correlation with electrocortical stimulation mapping (ECSM) and other electrophysiological modalities. Additionally, iOIS offers high spatial resolution (in microns), does not make contact with the surface of the brain, and introduces no potentially harmful compounds. Moreover, mapping is relatively rapid. The authors review the potential contribution of iOIS to the intraoperative environment. Specifically, they review iOIS methodology, discuss signal origin, compare OIS with other functional mapping modalities, and explain its potential benefits and limitations. They propose that iOIS may, in the future, be used in conjunction with ECSM to improve the resolution and accuracy of intraoperative mapping, decrease total time of intraoperative mapping, and possibly improve neurological outcomes. Additional studies will be required to quantify the sensitivity and specificity of optical maps relative to ECSM before it can be implemented clinically.