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

Optical imaging of intrinsic signals in ferret auditory cortex: responses to narrowband sound stimuli

This paper describes optical imaging of the auditory cortex in the anesthetized ferret, particularly addressing optimization of narrowband stimuli. The types of sound stimuli used were tone-pip trains and sinusoidal frequency and amplitude modulated (SFM and SAM) tones. By employing short illumination wavelengths (546 nm), we have successfully characterized the tonotopic arrangement, in agreement with the well-established electrophysiological tonotopic maps of the ferret auditory primary field (AI). The magnitude of the optical signal increased with sound level, was maximal for a modulation frequency (MF) of 2-4 Hz, and was larger for tone-pip trains and SFM sounds than for SAM sounds. Accordingly, an optimal narrowband stimulus was defined. Thus optical imaging can be used successfully to obtain frequency maps in auditory cortex by an appropriate choice of stimulus parameters. In addition, background noise consisting of 0.1-Hz oscillations could be reduced by introduction of blood pressure enhancing drugs. The optical maps were largely independent of 1) the type of narrowband stimulus, 2) the sound level, and 3) the MF. This stability of the optical maps was not predicted from the electrophysiological literature.

Inhibition of vasomotion in hippocampal cerebral arterioles during increases in neuronal activity

The activity of small arterioles, internal diameter 9.9 +/- 0.8 microm (SEM), was investigated in the CA1 region of hippocampal slices maintained in vitro at 34 degrees C. Under resting conditions, the vessels were quiescent. However, in the presence of the thromboxane A2 agonist U46619 (75-100 nM), rhythmic contractile activity (vasomotion, 1.1-9.9 min(-1), mean 4.1 +/- 0.7 min(-1) SEM) developed in the smooth muscle cells of the vessel walls. Electrical stimulation of the Schaffer collateral fibre pathway was used to evoke increases in neuronal activity in CA1 in the vicinity of the vessels under investigation. A 3-min period of electrical stimulation of the Schaffer collateral fibre pathway produced a significant reduction in vasomotion in 8/8 vessels. During stimulation, vasomotion either ceased completely (n = 5) or the frequency decreased from 7.1, 3.3 and 3.2 min(-1) to 1.2, 0.4 and 0.6 min(-1), respectively (n = 3). In addition, the amplitude of the residual contractions was reduced by 66%, 12% and 52%. In the presence of 1 microM tetrodotoxin (TTX) (n = 4) to block the generation of action potentials, vasomotion was still present. However, the inhibition of vasomotion evoked by increased neuronal activity was blocked concomitant with the abolition of the field potentials recorded in CA1 in response to the stimulation of the Schaffer collaterals. These findings suggest that a reduction in vasomotion may contribute to the local hyperaemia, which accompanies increases in synaptic activity in the brain.

Fine-scale organization of SI (area 3b) in the squirrel monkey revealed with intrinsic optical imaging

Optical imaging of intrinsic cortical activity was used to study the somatotopic map and the representation of pressure, flutter, and vibration in area 3b of the squirrel monkey (Saimiri sciureus) cortex under pentothal or isoflurane anesthesia. The representation of the fingerpads in primary somatosensory cortex was investigated by stimulating the glabrous skin of distal fingerpads (D1-D5) with Teflon probes (3-mm diam) attached through an armature to force feedback-controlled torque motors. Under pentothal anesthesia, intrinsic signal maps in area 3b obtained in response to stimulation (trapezoidal indentation) of individual fingerpads showed focal activations. These activations (ranging from 0.5 to 1.0 mm) were discrete and exhibited minimal overlap between adjacent fingerpad representations. Consistent with previously published maps, a somatotopic representation of the fingerpads was observed with an orderly medial to lateral progression from the D5 to D1 fingerpads. Under isoflurane anesthesia, general topography was still maintained, but the representation of fingerpads on adjacent fingers had higher degrees of overlap than with pentothal anesthesia. Multi- and single-unit recordings in the activation zones confirmed the somatotopic maps. To examine preferential inputs from slowly adapting type I (SA) and rapidly adapting type I (RA) and type II (PC) mechanoreceptors, we applied stimuli consisting of sinusoidal indentations that produce sensations of pressure (1 Hz), flutter (30 Hz), and vibration (200 Hz). Under pentothal anesthesia, activation patterns to these different stimuli were focal and coincided on the cortex. Under isoflurane, activation zones from pressure, flutter, and vibratory stimuli differed in size and shape and often contained multiple foci, although overall topography was maintained. Subtraction and vector maps revealed cortical areas (approximate 250-microm diam) that were preferentially activated by the sensations of pressure, flutter, and vibration. Multi- and single-unit recordings aided in the interpretation of the imaging maps. In conclusion, the cortical signals observed with intrinsic signal optical imaging delineated a somatotopic organization of area 3b and revealed different topographical cortical activation patterns for pressure, flutter, and vibratory stimuli. These patterns were dependent on anesthesia type. Possible relationships of these anesthesia effects to somatosensory cortical plasticity are discussed.

Optical tomographic imaging of dynamic features of dense-scattering media

Methods used in optical tomography have thus far proven to produce images of complex target media (e.g., tissue) having, at best, relatively modest spatial resolution. This presents a challenge in differentiating artifact from true features. Further complicating such efforts is the expectation that the optical properties of tissue for any individual are largely unknown and are likely to be quite variable due to the occurrence of natural vascular rhythms whose amplitudes are sensitive to a host of autonomic stimuli that are easily induced. We recognize, however, that rather than frustrating efforts to validate the accuracy of image features, the time-varying properties of the vasculature can be exploited to aid in such efforts, owing to the known structure-dependent frequency response of the vasculature and to the fact that hemoglobin is a principal contrast feature of the vasculature at near-infrared wavelengths. To accomplish this, it is necessary to generate a time series of image data. In this report we have tested the hypothesis that through analysis of time-series data, independent contrast features can be derived that serve to validate, at least qualitatively, the accuracy of imaging data, in effect establishing a self-referencing scheme. A significant finding is the observation that analysis of such data can produce high-contrast images that reveal features that are mainly obscured in individual image frames or in time-averaged image data. Given the central role of hemoglobin in tissue function, this finding suggests that a wealth of new features associated with vascular dynamics can be identified from the analysis of time-series image data.

Scattered-light imaging in vivo tracks fast and slow processes of neurophysiological activation

We imaged fast optical changes associated with evoked neural activation in the dorsal brainstem of anesthetized rats, using a novel imaging device. The imager consisted of a gradient-index (GRIN) lens, a microscope objective, and a miniature charged-coupled device (CCD) video camera. We placed the probe in contact with tissue above cardiorespiratory areas of the nucleus of the solitary tract and illuminated the tissue with 780-nm light through flexible fibers around the probe perimeter. The focus depth was adjusted by moving the camera and microscope objective relative to the fixed GRIN lens. Back-scattered light images were relayed through the GRIN lens to the CCD camera. Video frames were digitized at 100 frames per second, along with tracheal pressure, arterial blood pressure, and electrocardiogram signals recorded at 1 kHz per channel. A macroelectrode placed under the GRIN lens recorded field potentials from the imaged area. Aortic, vagal, and superior laryngeal nerves were dissected free of surrounding tissue within the neck. Separate shocks to each dissected nerve elicited evoked electrical responses and caused localized optical activity patterns. The optical response was modeled by four distinct temporal components corresponding to putative physical mechanisms underlying scattered light changes. Region-of-interest analysis revealed image areas which were dominated by one or more of the different time-course components, some of which were also optimally recorded at different tissue depths. Two slow optical components appear to correspond to hemodynamic responses to metabolic demand associated with activation. Two fast optical components paralleled electrical evoked responses.

The cortical representation of the hand in macaque and human area S-I: high resolution optical imaging

High-resolution images of the somatotopic hand representation in macaque monkey primary somatosensory cortex (area S-I) were obtained by optical imaging based on intrinsic signals. To visualize somatotopic maps, we imaged optical responses to mild tactile stimulation of each individual fingertip. The activation evoked by stimulation of a single finger was strongest in a narrow transverse band ( approximately 1 x 4 mm) across the postcentral gyrus. As expected, a sequential organization of these bands was found. However, a significant overlap, especially for the activated areas of fingers 3-5, was found. Surprisingly, in addition to the finger-specific domains, we found that for each of the fingers, weak stimulation activated also a second "common patch" of cortex, located just medially to the representation of the finger. These results were confirmed by targeted multiunit and single-unit recordings guided by the optical maps. The maps remained very stable over many hours of recording. By optimizing the imaging procedures, we were able to obtain the functional maps extremely rapidly (e.g., the map of five fingers in the macaque monkey could be obtained in as little as 5 min). Furthermore, we describe the intraoperative optical imaging of the hand representation in the human brain during neurosurgery and then discuss the implications of the present results for the spatial resolution accomplishable by other neuroimaging techniques, relying on responses of the microcirculation to sensory-evoked electrical activity. This study demonstrates the feasibility of using high-resolution optical imaging to explore reliably short- and long-term plasticity of cortical representations, as well as for applications in the clinical setting.

Building surfaces from borders in Areas 17 and 18 of the cat

Several brightness illusions indicate that borders can dramatically affect the perception of adjoining surfaces. In the Craik-O'Brien-Cornsweet illusion, in particular, two equiluminant surfaces can appear different in brightness due to the contrast border between them. Although the psychophysical nature of this phenomenon has been well characterized, the neural circuitry underlying this effect is unexplored. Here, we have asked whether there are cells in visual cortex which respond to edge-induced illusory brightness percepts such as the Cornsweet. Using optical imaging and single unit recordings methods, we have studied responses of the primary (Area 17) and second (Area 18) visual cortical areas of the anesthetized cat to both real luminance change and Cornsweet brightness change. We find that there are indeed cells whose responses are modulated in phase with the modulation of the Cornsweet stimulus. These cells are present in both Area 17 and Area 18, but are more prevalent in Area 18. These responses are generally weak and are found even when receptive fields are distant from the contrast border. Consistent with perception, cells which respond to the Cornsweet border are modulated in antiphase to the Narrow Real (another border-induced illusory brightness stimulus). Remarkably, we also find evidence of edge-induced responses to illusory brightness change using intrinsic signal optical imaging. Both real luminance change and edge-induced brightness change produces a greater imaged response in Area 18 than in Area 17. Thus, in the absence of direct luminance stimulation, cells in visual cortex can respond to modulation of distant border contrasts. We suggest that the perception of surface brightness was encoded in the early visual cortical pathway by both surface luminance contrast signals in Area 17 (Rossi, A. F., Rittenhouse, C. D., & Paradiso, M. A. (1996). The representation of brightness in primary visual cortex. Science, 273, 1104-7) and border-induced contrast signals that predominate in Area 18.

Tuning and topography in an odor map on the rat olfactory bulb

The sense of smell originates in a diverse array of receptor neurons, comprising up to 1000 different types. To understand how these parallel channels encode chemical stimuli, we recorded the responses of glomeruli in the olfactory bulbs of the anesthetized rat, by optical imaging of intrinsic signals. Odor stimulation produced two kinds of optical responses at the surface of the bulb: a broad diffuse component superposed by discrete small spots. Histology showed that the spots correspond to individual glomeruli, and that approximately 400 of them can be monitored in this way. Based on its wavelength-dependence, this optical signal appears to derive from changes in light scattering during neural activity. Pure odorants generally activated several glomeruli in a bilaterally symmetric pattern, whose extent varied greatly with concentration. A simple formalism for ligand binding accounts quantitatively for this concentration dependence and yields the effective affinity with which a glomerulus responds to an odorant. When tested with aliphatic molecules of increasing carbon chain length, many glomeruli were sharply tuned for one or two adjacent chain lengths. Glomeruli with similar tuning properties were located near each other, producing a systematic map of molecular chain length on the surface of the olfactory bulb. Given local inhibitory circuits within the olfactory bulb, this can account for the observed functional inhibition between related odors. We explore several parallels to the function and architecture of the visual system that help interpret the neural representation of odors.

Comparison of neuronal and hemodynamic measures of the brain response to visual stimulation: an optical imaging study

The noninvasive mapping of hemodynamic brain activity has led to significant advances in neuroimaging. This approach is based in part on the assumption that hemodynamic changes are proportional to (and therefore constitute a linear measure of) neuronal activity. We report a study investigating the quantitative relationship between neuronal and hemodynamic measures. This study exploited the fact that optical imaging methods can simultaneously provide noninvasive measures of neuronal and hemodynamic activity from the same region of the brain. We manipulated visual stimulation frequency and measured responses from the medial occipital area of 8 young adults. The results were consistent with a model postulating a linear relationship between the neuronal activity integrated over time and the amplitude of the hemodynamic response. The hemodynamic response colocalized with the neuronal response. These data support the use of quantitative neuroimaging methods to infer the intensity and localization of neuronal activity in occipital areas.

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Signal source separation in the analysis of neural activity in brain

This research examines the spatial and temporal characteristics of the responses to stimulation of the barrel cortex in anesthetized rats using optical imaging with particular emphasis on methods of analysis which reduce the effects of low-frequency oscillations on localization of the activated cortical region. Image sequences were captured using a light source with a narrow bandwidth of wavelength (590 +/- 2 nm). On each trial image data were collected at 15 Hz and stored over a 12-s period starting 8 s before stimulation onset. Stimulation was for 1 s using an oscillating whisker vibrator (approximately 1-mm deflection at 5 Hz). For each subject a total of 30 experimental trials were collected and averaged. There was an interstimulus interval of 26 s. The trial-averaged data were analyzed using two related signal source separation algorithms. Both algorithms use a weak model of the expected temporal response as a filter to exclude contributions from low-frequency baseline oscillations which we call the V-signal. We found that both algorithms successfully separated most of the effects of the V-signal from the response to the stimulation. The performance of the algorithms compared favorably with the performance of related algorithms without weak constraints and the "ratio of means" strategy used by C. H. Chen-Bee et al. (1996b, J. Neurosci. Methods 68:28-37; C. H. Chen-Bee et al., 2000, J. Neurosci. Methods 97:157-173).

Spontaneous low frequency oscillations of cerebral hemodynamics and metabolism in human adults

We investigated slow spontaneous oscillations in cerebral oxygenation in the human adult's visual cortex. The rationale was (1) to demonstrate their detectability by near infrared spectroscopy (NIRS); (2) to analyze the spectral power of as well as the phase relationship between the different NIRS parameters (oxygenated and deoxygenated hemoglobin and cytochrome-oxidase; oxy-Hb/deoxy-Hb/Cyt-ox). Also (3) influences of functional stimulation and hypercapnia on power and phase shifts were investigated. The results show that-in line with the literature-low frequency oscillations (LFO) centred around 0.1 s(-1) and even slower oscillations at about 0.04 s(-1) (very low frequency, VLFO) can be distinguished. Their respective power differs between oxy-Hb, deoxy-Hb, and Cyt-ox. Either frequency (LFO and VLFO) is altered in magnitude by functional stimulation of the cortical area examined. Also we find a change of the phase shift between the vascular parameters (oxy-Hb, tot-Hb) and the metabolic parameter (Cyt-ox) evoked by the stimulation. It is shown that hypercapnia attenuates the LFO in oxy-Hb and deoxy-Hb.

CONCLUSIONS

(1) spontaneous vascular and metabolic LFO and VLFO can be reproducibly detected by NIRS in the human adult. (2) Their spectral characteristics and their response to hypercapnia are in line with those described in exposed cortex (for review see (Hudetz et al., 1998)) and correspond to findings with transcranial doppler sonography (TCD) (Diehl et al., 1995) and fMRI (Biswal et al., 1997). (3) The magnitude of and phase relation between NIRS-parameters at the LFO may allow for a local noninvasive assessment of autoregulatory mechanisms in the adult brain.

Spectroscopic analysis of neural activity in brain: increased oxygen consumption following activation of barrel cortex

This research investigates the hemodynamic response to stimulation of the barrel cortex in anaesthetized rats using optical imaging and spectroscopy (Bonhoeffer and Grinvald, 1996; Malonek and Grinvald, 1996; Mayhew et al., 1999). A slit spectrograph was used to collect spectral image data sequences. These were analyzed using an algorithm that corrects for the wavelength dependency in the optical path lengths produced by the light scattering properties of tissue. The analysis produced the changes in the oxy- and deoxygenation of hemoglobin following stimulation. Two methods of stimulation were used. One method mechanically vibrated a single whisker, the other electrically stimulated the whisker pad. The electrical stimulation intensity varied from 0.4 to 1.6 mA. The hemodynamic responses to stimulation increased as a function of intensity. At 0.4 mA they were commensurate with those from the mechanical stimulation; however, the responses at the higher levels were greater by a factor of approximately 10. For both methods of data collection, the results of the spectroscopic analysis showed an early increase in deoxygenated hemoglobin (Hbr) with no evidence for a corresponding decrease in oxygenated hemoglobin (HbO(2)). Evidence for increased oxygen consumption (CMRO(2)) was obtained by converting the fractional changes in blood volume (Hbt) into estimates of changes in blood flow (Grubb et al., 1974) and using the resulting time course to scale the fractional changes in Hbr. The results show an early increase CMRO(2) peaking approximately 2 s after stimulation onset. Using these methods, we find evidence for increased oxygen consumption following increased neural activity even at low levels of stimulation intensity.

Malformation of the functional organization of somatosensory cortex in adult ephrin-A5 knock-out mice revealed by in vivo functional imaging

The molecular mechanisms that coordinate the functional organization of the mammalian neocortex are largely unknown. We tested the involvement of a putative guidance label, ephrin-A5, in the functional organization of the somatosensory cortex by quantifying the functional representations of individual whiskers in vivo in adult ephrin-A5 knock-out mice, using intrinsic signal optical imaging. In wild-type mice ephrin-A5 is expressed in a gradient in the somatosensory cortex during development. In adult ephrin-A5 knock-out mice, we found a spatial gradient of change in the amount of cortical territory shared by individual whisker functional representations across the somatosensory cortex, as well as a gradient of change in the distance between the functional representations. Both gradients of change were in correspondence with the developmental expression gradient of ephrin-A5 in wild-type mice. These changes involved malformations of the cortical spacing of the thalamocortical components, without concurrent malformations of the intracortical components of individual whisker functional representations. Overall, these results suggest that a developmental guidance label, such as ephrin-A5, is involved in establishing certain spatial relationships of the functional organization of the adult neocortex, and they underscore the advantage of investigating gene manipulation using in vivo functional imaging.

Temporal and topographical characterization of language cortices using intraoperative optical intrinsic signals

We used intraoperative optical imaging of intrinsic signals (iOIS) and electrocortical stimulation mapping (ESM) to compare functionally active brain regions in 10 awake patients undergoing neurosurgical resection. Patients performed two to four tasks, including visual and auditory naming, word discrimination, and/or orofacial movements. All iOIS maps included areas identified by ESM mapping. However, iOIS also revealed topographical specificity dependent on language task. In Broca's area, naming paradigms activated both anterior and posterior inferior frontal gyrus (IFG), while the word discrimination paradigm activated only posterior IFG. In Wernicke's area, object naming produced activations localizing over the inferior and anterior/posterior regions, while the word discrimination task activated superior and anterior cortices. These results may suggest more posterior phonological activation and more anterior semantic activations in Broca's area, and more anterior/superior phonological activation and more posterior/inferior semantic activations in Wernicke's area. Although similar response onset was observed in Broca's and Wernicke's areas, temporal differences were revealed during block paradigm (20-s) activations. In Broca's area, block paradigms yielded a boxcar temporal activation profile (in all tasks) that resembled response profiles observed in motor cortex (with orofacial movements). In contrast, activations in Wernicke's area responded with a more dynamic profile (including early and late peaks) which varied with paradigm performance. Wernicke's area profiles were very similar to response profiles observed in sensory and visual cortex. The differing temporal patterns may therefore reflect unique processing performed by receptive (Wernicke's) and productive (Broca's) language centers. This study is consistent with task-specific semantic and phonologic regions within Broca's and Wernicke's areas and also is the first report of response profile differences dependent on cortical region and language task.

Principal component analysis and blind separation of sources for optical imaging of intrinsic signals

The analysis of data sets from optical imaging of intrinsic signals requires the separation of signals, which accurately reflect stimulated neuronal activity (mapping signal), from signals related to background activity. Here we show that blind separation of sources by extended spatial decorrelation (ESD) is a powerful method for the extraction of the mapping signal from the total recorded signal. ESD is based on the assumptions (i) that each signal component varies smoothly across space and (ii) that every component has zero cross-correlation functions with the other components. In contrast to the standard analysis of optical imaging data, the proposed method (i) is applicable to nonorthogonal stimulus-conditions, (ii) can remove the global signal, blood-vessel patterns, and movement artifacts, (iii) works without ad hoc assumptions about the data structure in the frequency domain, and (iv) provides a confidence measure for the signals (Z score). We first demonstrate on orientation maps from cat and ferret visual cortex, that principal component analysis, which acts as a preprocessing step to ESD, can already remove global signals from image stacks, as long as data stacks for at least two-not necessarily orthogonal-stimulus conditions are available. We then show that the full ESD analysis can further reduce global signal components and-finally-concentrate the mapping signal within a single component both for differential image stacks and for image stacks recorded during presentation of a single stimulus.

Selective measurement of endothelial or smooth muscle [Ca(2+)](i) in pressurized/perfused cerebral arteries with fura-2

Despite a critical role for calcium in endothelial regulation of cerebrovascular tone, endothelial intracellular calcium ([Ca(2+)](i)) has never been measured in the context of an intact pressurized cerebral vessel. The purpose of the present study was to selectively measure endothelial or smooth muscle [Ca(2+)](i) and diameter in a pressurized/perfused cerebral vessel. In a pressurized rat middle cerebral artery, fura-2 AM was administered selectively to either the luminal (endothelium) or abluminal (smooth muscle) side of the vessel. Selectivity of loading was determined by measuring fura-2 fluorescence before and after removal of the endothelium. Removal of the endothelium virtually eliminated fura-2 fluorescence. In addition, 2-methylthioadenosine triphosphate (2MeS-ATP, a selective endothelial P2 receptor agonist) was used to infer the selectivity of fura-2 loading. It was reasoned that 2MeS-ATP should produce a decrease in smooth muscle [Ca(2+)](i) and an increase in endothelial [Ca(2+)](i) in selectively loaded vessels, consistent with its role as an NO-dependent dilator. In smooth muscle loaded vessels, [Ca(2+)](i) went from 252+/-8 to 82+/-9 nM following luminal administration of 2MeS-ATP, whereas in endothelial loaded vessels, [Ca(2+)](i) went from 137+/-11 to 271+/-20 nM. Thus, a method is provided which allows for selective measurement of endothelial or smooth muscle [Ca(2+)](i) with simultaneous measurement of diameter in a pressurized cerebral vessel.

Near-infrared study of fluctuations in cerebral hemodynamics during rest and motor stimulation: temporal analysis and spatial mapping

We have noninvasively studied the motor cortex hemodynamics in human subjects under rest and motor stimulation conditions using a multichannel near-infrared tissue spectrometer. Our instrument measures optical maps of the cerebral cortex at two wavelengths (758 and 830 nm), with an acquisition time of 160 ms per map. We obtained optical maps of oxy- and deoxy-hemoglobin concentration changes in terms of amplitudes of folding average, power spectrum and coherence at the stimulation repetition frequency, and the phase synchronization index. Under periodic motor stimulation conditions, we observed coherence and frequency or phase synchronization of the local hemodynamic changes with stimulation. Our main findings are the following: (1) The amplitude of the hemodynamic response to the motor stimulation is comparable to the amplitude of the fluctuations at rest. (2) The spatial patterns of the oxy- and deoxy-hemoglobin responses to the stimulation are different. (3) The hemodynamic response to stimulation shows a spatial localization and a level of phase synchronization with the motor stimulation that depends on the stimulation period.

Quantitative multi-modal functional MRI with blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR)

A magnetic resonance imaging (MRI) method is described that allows interleaved measurements of transverse (R(2)(*) and R(2)) and longitudinal (R(1)) relaxation rates of tissue water in conjunction with spin labeling. The image-contrasts are intrinsically blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) weighted, but each contrast is made quantitative by two echo time (TE) and inversion recovery time (TIR) acquisitions with gradient echo (GE) and spin echo (SE) weighted echo-planar imaging (EPI). The EPI data were acquired at 7 Tesla with nominal spatial resolution of 430 x 430 x 1000 microm(3) in rat brain in vivo. The method is termed as blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR) and allows acquisition of R(2)(*), R(2), and CBF maps in an interleaved manner within approximately 12 minute. The basic theory of the method, associated experimental/systematic errors, and temporal restrictions are discussed. The method is validated by comparison of multi-modal maps obtained by BOLDED AFFAIR (i.e., two TE and TIR values with GE and SE sequences) and conventional approach (i.e., multiple TE and TIR values with GE and SE sequences) during varied levels of whole brain activity. Preliminary functional data from a rat forepaw stimulation model demonstrate the feasibility of this method for functional MRI (fMRI) studies. It is expected that with appropriate precautions this method in conjunction with contrast agent-based MRI has great potential for quantitative fMRI studies of mammalian cortex.

Three distinct auditory areas of cortex (AI, AII, and AAF) defined by optical imaging of intrinsic signals

Using pure-tone sound stimulation, three separate auditory areas are revealed by optical imaging of intrinsic signals in the temporal cortex of the chinchilla (Chinchilla laniger). These areas correlate with primary auditory cortex (AI) and two secondary areas, AII and the anterior auditory field (AAF). We have distinguished AI on the basis of concurrent single-unit electrophysiological recording; neurons within the AI intrinsic signal region have short (<15 ms) onset-response latencies compared with neurons recorded in AII and the AAF. Within AI, AII, and AAF we have been able to define cochleotopic or tonotopic organization from the differences in intrinsic signal areas evoked by pure tones at octave-spaced frequencies from 500 Hz to 16 kHz. The maps in AI and AII are arranged orthogonal to each other.

Spectroscopic analysis of changes in remitted illumination: the response to increased neural activity in brain

Imaging of neural activation has been used to produce maps of functional architecture and metabolic activity. There is some uncertainty associated with the sources underlying the intrinsic signals. It has been reported that following increased neural activity there was little increased oxygen consumption ( approximately 5%), although glucose consumption increased by approximately 50%. The research we describe uses a modification of the Beer-Lambert Law called path-length scaling analysis (PLSA) to analyze the spectra of the hemodynamic and metabolic responses to vibrissal stimulation in rat somatosensory cortex. The results of the PLSA algorithm were compared with those obtained using a linear spectrographic analysis method (we refer to this as LMCA). There are differences in the results of the analysis depending on which of the two algorithms (PLSA or LMCA) is used. Using the LMCA algorithm, we obtain results showing an increase in the volume of Hbr at approximately 2 s, following onset of stimulation but no complementary decrease in oxygenated haemoglobin (HbO(2)). These results are similar to a previous report. In contrast, after using the PLSA algorithm, the time series of the chromophore changes shows no evidence for an increase in the volume of deoxygenated haemoglobin (Hbr). However, after further analysis of the time series from the PLSA using general linear models (GLM) to remove contributions from low frequency baseline oscillations, both the HbO(2) and Hbr times series of the response to stimulation were found to be biphasic with an early decrease in saturation peaking approximately 1 s after onset of stimulation followed by a larger increase in saturation peaking at approximately 3 s. Finally, following the PLSA-then-GLM analysis procedure, we do not find convincing evidence for an increase in cytochrome oxidation following stimulation, though we demonstrate the PLSA algorithm to be capable of disassociating changes in cytochrome oxidation state from changes in hemoglobin oxygenation.

fMRI and EEG responses to periodic visual stimulation

EEG/VEP and fMRI responses to periodic visual stimulation are reported. The purpose of these experiments was to look for similar patterns in the time series produced by each method to help understand the relationship between the two. The stimulation protocol was the same for both sets of experiments and consisted of five complete cycles of checkerboard pattern reversal at 1.87 Hz for 30 s followed by 30 s of a stationary checkerboard. The fMRI data was analyzed using standard methods, while the EEG was analyzed with a new measurement of activation-the VEPEG. Both VEPEG and fMRI time series contain the fundamental frequency of the stimulus and quasi harmonic components-an unexplained double frequency commonly found in fMRI data. These results have prompted a reappraisal of the methods for analyzing fMRI data and have suggested a connection between our findings and much older published invasive electrophysiological measurements of blood flow and the partial pressures of oxygen and carbon dioxide. Overall our new analysis suggests that fMRI signals are strongly dependant on hydraulic blood flow effects. We distinguish three categories of fMRI signal corresponding to: focal activated regions of brain tissue; diffuse nonspecific regions of steal; and major cerebral vessels of arterial supply or venous drainage. Each category of signal has its own finger print in frequency, amplitude, and phase. Finally, we put forward the hypothesis that modulations in blood flow are not only the consequence but are also the cause of modulations in functional activity.

Analysis of dynamic brain imaging data

Modern imaging techniques for probing brain function, including functional magnetic resonance imaging, intrinsic and extrinsic contrast optical imaging, and magnetoencephalography, generate large data sets with complex content. In this paper we develop appropriate techniques for analysis and visualization of such imaging data to separate the signal from the noise and characterize the signal. The techniques developed fall into the general category of multivariate time series analysis, and in particular we extensively use the multitaper framework of spectral analysis. We develop specific protocols for the analysis of fMRI, optical imaging, and MEG data, and illustrate the techniques by applications to real data sets generated by these imaging modalities. In general, the analysis protocols involve two distinct stages: "noise" characterization and suppression, and "signal" characterization and visualization. An important general conclusion of our study is the utility of a frequency-based representation, with short, moving analysis windows to account for nonstationarity in the data. Of particular note are 1) the development of a decomposition technique (space-frequency singular value decomposition) that is shown to be a useful means of characterizing the image data, and 2) the development of an algorithm, based on multitaper methods, for the removal of approximately periodic physiological artifacts arising from cardiac and respiratory sources.

Spontaneous fluctuations in cerebral oxygen supply. An introduction

Spontaneous, low frequency (4-12 cpm) fluctuations, independent of the cardiac and respiratory cycles, in human and animal brains were first recorded with the O2 polarographic technique in the late 1950s. They were seen in NADH and cytochrome oxidase and associated with spontaneous vasomotion pial and large cerebral arteries. Renewed interest in spontaneous fluctuations was generated by studies with laser-Doppler flowmetry (LDF), reflectance oximetry and functional MRI. Spontaneous fluctuations were consistently produced when cerebral perfusion was challenged by systemic or local manipulations; the fluctuation amplitude reached 30-40% of the mean. The most potent stimuli are hypotension, hyperventilation, cerebral artery occlusion and cerebral vasoconstriction elicited, for example, by a nitric oxide synthase inhibitor but not by indomethacin. The fluctuations are suspended by CO2 and halothane at concentrations that produce hyperemia. Recently, spontaneous fluctuations were recorded by LDF microprobes in areas as small as 130 microns and by video-microscopy in single capillaries. The fluctuations were absent in severe, focally ischemic brain territories. The dependence of spontaneous fluctuations on intravascular pressure argues for the importance of a myogenic mechanism, however, neuronal modulation may also play a role. Coherence of small vessel vasomotion may be required for the emergence of regional flow fluctuations. There is a need to elucidate the spatial and frequency domains in which fluctuations are present under normal physiological conditions and those in which they may reflect brain injury and pathologies of diagnostic or prognostic value.

Fluctuations and stimulus-induced changes in blood flow observed in individual capillaries in layers 2 through 4 of rat neocortex

Cortical blood flow at the level of individual capillaries and the coupling of neuronal activity to flow in capillaries are fundamental aspects of homeostasis in the normal and the diseased brain. To probe the dynamics of blood flow at this level, we used two-photon laser scanning microscopy to image the motion of red blood cells (RBCs) in individual capillaries that lie as far as 600 micrometers below the pia mater of primary somatosensory cortex in rat; this depth encompassed the cortical layers with the highest density of neurons and capillaries. We observed that the flow was quite variable and exhibited temporal fluctuations around 0.1 Hz, as well as prolonged stalls and occasional reversals of direction. On average, the speed and flux (cells per unit time) of RBCs covaried linearly at low values of flux, with a linear density of approximately 70 cells per mm, followed by a tendency for the speed to plateau at high values of flux. Thus, both the average velocity and density of RBCs are greater at high values of flux than at low values. Time-locked changes in flow, localized to the appropriate anatomical region of somatosensory cortex, were observed in response to stimulation of either multiple vibrissae or the hindlimb. Although we were able to detect stimulus-induced changes in the flux and speed of RBCs in some single trials, the amplitude of the stimulus-evoked changes in flow were largely masked by basal fluctuations. On average, the flux and the speed of RBCs increased transiently on stimulation, although the linear density of RBCs decreased slightly. These findings are consistent with a stimulus-induced decrease in capillary resistance to flow.

Single-shot half k-space high-resolution gradient-recalled EPI for fMRI at 3 Tesla

Half k-space gradient-recalled echo-planar imaging (GR-EPI) is discussed in detail. T2* decay during full k-space GR-EPI gives rise to unequal weighting of the lines of k-space, loss of signal intensity at the center of k-space, and a point-spread function that limits resolution. In addition, the long readout time for high-resolution full k-space acquisition gives rise to severe susceptibility effects. These problems are substantially reduced by acquiring only half of k-space and filling the empty side by Hermitian conjugate formation. Details of the pulse sequence and image reconstruction are presented. The point-spread function is 3(1/2) times narrower for half than full k-space acquisition. Experiments as well as theoretical considerations were carried out in a context of fMRI using a whole-brain local gradient and an RF coil at 3 Tesla. Using a bandwidth of +/-83 kHz, well-resolved single-shot images of the human brain, as well as good quality fMRI data sets were obtained with a matrix of 192 x 192 over 16 x 16 cm2 FOV using half k-space techniques. The combination of high spatial resolution using the methods presented in this article and the high temporal resolution of EPI opens opportunities for research into fMRI contrast mechanisms. Increase of percent signal change as the resolution increases is attributed to reduction of partial volume effects of activated voxels. Histograms of fMRI pixel responses are progressively weighted to higher percent signal changes as the resolution increases. The conclusion has been reached that half k-space GR-EPI is generally superior to full k-space GR-EPI and should be used even for low-resolution (64 x 64) EPI.

Refractory periods observed by intrinsic signal and fluorescent dye imaging

All perfusion-based imaging modalities depend on the relationship between neuronal and vascular activity. However, the relationship between stimulus and response was never fully characterized. With the use of optical imaging (intrinsic signals and intravascular fluorescent dyes) during repetitive stimulation paradigms, we observed reduced responses with temporally close stimuli. Cortical evoked potentials, however, did not produce the same reduced responsiveness. We therefore termed these intervals of reduced responsiveness "refractory periods." During these refractory periods an ability to respond was retained, but at a near 60% reduction in the initial magnitude. Although increasing the initial stimulus duration lengthened the observed refractory periods, significantly novel or temporally spaced stimuli overcame them. We observed this phenomenon in both rodent and human subjects in somatosensory and auditory cortices. These results have significant implications for understanding the capacities, mechanisms, and distributions of neurovascular coupling and thereby possess relevance to all perfusion-dependent functional imaging techniques.

Wavelength-dependent differences between optically determined functional maps from macaque striate cortex

This study investigates the role of wavelength in determining the source and dynamic range of activity-driven reflectance changes in macaque striate cortex. By using short (600 nm) and long (720 nm) wavelengths to map ocular dominance, orientation, and position from the same region of cortex on alternate trials, we isolated wavelength-dependent differences in the contributions of different tissue compartments. In agreement with previous reports, 600-nm illumination was found to produce optical signals that were more than twice the size of those obtained with 720-nm illumination. In addition, 600- and 720-nm images were found to correlate everywhere except in regions occluded by blood vessels, where the images obtained at 600 nm correlated with the overlying vasculature. Since the 720-nm images do not correlate with the vasculature, this difference suggests that differential images obtained under 600-nm illumination are disproportionately sensitive to vascular events (e.g., changes in blood flow, volume, etc.). This finding is supported by the absorption spectra of hemoglobin and its derivatives, which absorb 600-nm light 4-1000 times more strongly than 720-nm light. Hence, for the 40% of cortex covered by blood vessels larger than 50 microns, images obtained at 600 nm are dominated by the vascular compartment to the exclusion of signals from the neural compartment below.

An evaluation of linear model analysis techniques for processing images of microcirculation activity

Sequences of images of the cortical surface can be processed to reveal information about the cortical microcirculation, regional cerebral blood flow (rCBF), and changes induced by neuronal activity. This study examined the use of different analysis methodologies on intrinsic optical images taken from rat sensory motor cortex and testes. Generalized linear model (GLM) analysis was used and compared with standard signal processing methods including principal component analysis. The GLM method has been used by Friston et al. (1994, Hum. Brain Map., 1: 214-220) in the analysis of functional magnetic resonance imagery to identify regions of focal activity. We investigated the use of this method to analyze video image data of the modulation of rCBF from rat cortex. The results revealed spatiotemporal variations in rCBF in response to stimulation within local regions of cortex. The advantage of the GLM method is that it augments ordinary signal processing methods with an estimate of statistical reliability. The use of different wavelengths of illumination reveals spatial structures with different temporal relationships. In image time series data collected under green and red illumination a phase difference was found in the low frequency approximately 0.1 Hz vasomotion oscillation. This phase difference occurred in data from both cortex and testes. A possible explanation of these differences is that the spectral absorption characteristics of the tissue reflect changes in the volume proportions of the different hemoglobin derivatives in interacting with the modulation of the volume of blood. It is suggested that the combination of these effects produces the phase differences we detect.

A miniature CCD video camera for high-sensitivity light measurements in freely behaving animals

We developed a miniaturized, high-sensitivity camera that can be placed in areas of difficult access in freely behaving animals for neural tissue imaging. The device consists of a charged coupled device (CCD) chip, a coherent image conduit and miniature light emitting diodes (LEDs). An amplifier circuit is constructed on the camera chip and nine wires are attached for external connections. Placement of LEDs around the image conduit perimeter provides dark-field illumination, which increases detection of cellular-related light scattering changes and doubles the depth-of-view over conventional reflectance imaging procedures. The device has been successfully used to record from several deep brain structures, including the ventral medullary surface of sleeping and waking cats. The procedure allows assessment of light scattering changes that result from neural activity or detection of vital dyes to metabolic or voltage-induced activation.