Neuro Imaging Laboratory, Division of Brain Mapping, Department of Neurology, UCLA School of Medicine, Los Angeles, California 90095. Department of Neurology and Brain Mapping Center, UCLA School of Medicine, Los Angeles, California 90095 Thomas A.
Introduction to Cartography
The purpose of this chapter is to provide an introduction and guidance through a complex series of intersecting fields, reflecting the depth and breadth of the disciplines required to bring contemporary brain mapping into the future. In this second edition of this text we have attempted to remain faithful to the full history of this very modern enterprise, while at the same time updating the remarkable progress that has occurred in the few short years since its first publication.
History of Cartography
The Role of Instrumentation
Computers
Visualization
The Dimensions of a Brain Map
The current meaning of the representation of different visually related areas, where each is indicated by a different color-coded scheme. Note the chronological evolution of the map theme as well as the improvement in sophistication.
Spatial Resolution
Attribute Data
Time Series
Derivatives and Probabilities
The Full Scope of Brain Mapping
Coordinates to Nomenclature
Templates
Electronic versions with point and vector lists describing anatomical structures were developed by Bloom et al. Numerous other approaches combined carefully delineated individuals or populations with spatial normalization to effectively label the anatomy of each individual placed in the atlas (Lancaster et al ., 1997; Schmahmann et al., 1999; Toga et al., 2001a; Thompson et al., 2001b).
Anatomic Collections
Numerous other approaches combined carefully delineated individuals or populations with spatial normalization to effectively label the anatomy of any individual located in the atlas (Lancaster et al., 1997; Schmahmann et al., 1999; Thompson et al., 2001b). things that excel at some aspect of neuro and related anatomies.
Visualizations
Thus, the contribution of anatomical assemblies is best seen when used in combination with other brain mapping methods or if the assemblies were acquired using new digital, 3D-capable techniques. 2000) have developed several collections that provide detailed maps of the cyto- and chemoarchitectures juxtaposed with other maps of the same subject. 1994a,b, 1995a,b) have also made 3D studies of the entire human brain at 100 µm resolution from postmortem material compared to MRI and PET antemortem of the same subject.
Transformations
Any successful curvilinear transformation to register brain data between subjects must be high-dimensional to accommodate fine anatomical variations (Christensen et al., 1996; Thompson and Toga, 1998). This is difficult or simply impossible to achieve with traditional image warping manipulations (Christensen et al., 1995).
Databases
Relationships to Other Biological Maps
Such deformable atlases (Seitz et al., 1990; Evans et al., 1991; Miller et al., 1993; Gee et al., 1993; Christensen et al., 1993; Sandor and Leahy Rizzo et al., 1995) can be used to transfer 3D maps of functional and vascular territories to the coordinate system of different subjects, as well as information about different tissue types and the boundaries of cytoarchitectonic fields and their neurochemical composition. Physical continuum models of the deformation address these problems by considering the deforming atlas image to be embedded in a three-dimensional deformable medium, which can be an elastic material or a viscous fluid.
The Human Genome
Cooperation
Stereotaxy
History
The Human Genome Project is an excellent example that offers lessons for brain mapping. Variations on this original approach produced several atlases of the human brain (Talairach et al., 1957; . Schaltenbrand and Bailey, 1959; Andrew and Watkins, 1969; Van Buren and Maccubin, 1962), some of which are still in common use today. .
A Uniform Descriptive and Measurement System
Atlases
Nomenclature
The history of neuroanatomy and its underlying language is often marked by the names of anatomists and other neuroscientists. When these investigators made their discoveries, they had to create a lexicon to communicate their findings.
Scale
The importance of naming conventions for modern brain mapping lies not only in the need for researchers to communicate efficiently and specifically; due to the computerized nature of brain maps, a standardized nomenclature is critical. The development of a universal ontology that creates an acceptable nomenclature and its relationships is essential for the creation of usable databases or metadata bases across laboratories and institutions.
Aliases
Nomenclature Maps
Detection Devices
Brain mapping methods are numerous and varied, often complementing each other with varying degrees of invasiveness, accuracy and object of measurement.
Structure
Additionally, variations in pulse sequences can result in image features that emphasize one or more aspects of the tissue, and by combining multiple scans, more complete views of brain anatomy can be achieved.
Diffusion Tensor Imaging
Computed Tomographic Angiograph (CTA)
Function
Brain Maps: Content and Format
The variety of brain maps has resulted in a diverse range of products. For example, maps devoted to anatomy must include the boundaries that describe each structure and, therefore, usually include contour images (sometimes superimposed on section images) or vector (digital) lists.
Volumes
In many cases, the focus of the map determines the format of the map and therefore its construction. The need for comprehensive maps that include many of these requirements requires a computerized approach that is adaptable to a variety of applications (see Fig. 5).
Explicit Geometry
Statistical Representations
Furthermore, numerical forms are most efficient for electronic communication and can then be converted into a visual representation. Mapping variance, biases and other population-based statistics is possible using probabilistic approaches where confidence limits can be set depending on the question being asked.
Multimodal Maps
Dynamic Maps
Temporal Maps of Brain Structure
Mapping Growth Patterns in Four Dimensions
While difference maps help determine whether structural changes have occurred in dementia (Freeborough et al., 1996), these. In the near future, these brain mapping techniques will allow mapping growth and degeneration in their full spatial and temporal complexity.
The Atlas Is the Database
- Summary
- Introduction
- Critical Variables in Brain Mapping Techniques
- The Concept of Resolution
- Intrinsic Resolution
- Signal-to-Noise Factors
- Image Resolution
- Axial Resolution
- Modulation Transfer Function
- Partial Volume Effects
- Resolution Uniformity
- Object Movement
In “Coplanar stereotactic atlas of the human brain – three-dimensional proportional system: a cerebral imaging approach”. That is, only 42% of the object's signal would be included in the final image.
Temporal Resolution
Sampling
Sampling Frequency
If one has a fast method of measurement, relative to the change in blood flow, the kinetics of the process can be observed. By rapidly sampling a single behavioral event, the kinetics of the biological process can be observed (Huang and Phelps, 1986).
Sampling Volume
Sites Accessed
Digital images are made of the entire block plane and currently have a spatial resolution in the range of 100–200 µm. In both, an identical compound can be used to trace a specific process in the brain.
Invasiveness
For valid animal models, invasive or more detailed experiments can be performed using approaches that would be logistically impossible or unethical in humans. MRI techniques are useful both in the macroscopic world of human subjects (Prichard and Brass, 1992) and microscopically (Damasio et al., 1991; Jacobs and Frasier, 1994).
Conclusions
The ideal brain mapping technique would have extremely high spatial and temporal resolution with the ability to continuously sample a large volume of the brain. In "Fifty-ninth James Arthur Lecture on the Evolution of the Human Brain." American Museum of Natural History, New York.
Mapping Neural Organization at the Cellular and Synaptic Levels
A major goal of neurobiology is to provide a detailed map of the anatomical connections between individual neurons and groups of neurons. Considering that neural organization is in a dynamic state, it is important not only to create a functional map of the brain, but also to clarify the principles and mechanisms that govern the development and plasticity of neural organization.
The Problem: Cellular Diversity and Complexity of Neural Tissues
Thus, neural organization can be considered to be in a dynamic state, especially at the subcellular level, and we are inherently challenged with mapping an actively changing terrain. It is hoped that the principles of neural organization will be elucidated by studying the process of building network connections during ontogenesis.
One Solution: Vital Fluorescent Labeling and 3D Confocal Imaging in Brain Slices
Live Brain Slice Preparation and Culture
Indeed, if brain tissues are isolated from developing animals, they can be maintained in vitro for long periods (weeks to months), extending far beyond the corresponding time from which tissues can be maintained ex vivo when taken from older (>P7 ) animals . Another disc culture method (Stoppini et al., 1991), where discs are grown on porous filter membranes, has also been shown to be very suitable for long-term culture.
Labeling Neuronal and Glial Cells in Brain Tissue Slices
The rolling tube cultures are especially useful for high-resolution microscopy because the slices stick together – and can. Tissue sections are prepared as described above and then placed on cell culture inserts (Falcon 3090 or 3102) containing polyethylene terephthalate, track-etched porous membranes (1 µm pore size).
Visualizing Neural Structure with Fluorescent Membrane Dyes
Small, localized injections can be performed by applying a series of short pressure pulses (1 ms duration, 80 psi) to the back of the pipette. Although a potential disadvantage of longer wavelength red light (relative to blue or green light) is that spatial resolution is somewhat reduced, optical imaging in tissues is expected to be improved with red dyes because: (1) there is less background autofluorescence of tissues at the longer wavelengths; (2) biological tissues should deflect red light less, improving the collection of light for imaging; and (3) the red light has lower energy than UV, blue, and green light and should therefore cause less photodynamic damage (see later).
Immunofluorescent Labeling
A new and potentially efficient approach to obtain Golgi labeling of neuronal and glial cells with fluorescent lipids has been described (Gan et al., 2000). For example, using combinations of three different lipophilic dyes, Gan et al. 2000) can distinguish single cells labeled in one of seven different spectral patterns.
Gene Transfection and Expression of Fluorescent Proteins
For example, O'Rourke et al. 1992) followed DiI-labeled migrating neuroblasts in slices of developing cerebral cortex by time-lapse confocal microscopy and then fixed the slices and stained the tissues immunohistochemically with an antiserum to reveal radial glial fibers. They first photoconverted the DiI to permanently label the labeled cells with a stable, electron-dense DAB reaction product and then permeabilized the tissues for antibody staining.
Ca 2+ -Sensitive Fluorescent Probes for Studying Neuronal and Glial Physiology
Imaging Methodology
Confocal Microscopy
Three-Dimensional Imaging
Dynamic Imaging in Live Brain Slices
- Maintaining Brain Slices on the Microscope Stage Factors that seem to be critical for maintaining healthy
- Depth of View
- Signal-to-Noise and Spatiotemporal Resolution When imaging any dynamic biological events, there is
- Focus Drift
- Photodynamic Damage
- Data Management
- Application: Mapping Neural Structure and Physiology in Developing
The first problem (poor staining) can be overcome if the dye is injected into the tissue with minimal disruption. Unfortunately, even when z-axis image stacks are stacked, tissue motions can be so severe that they require constant focus "adjustment" to keep cells within the stack.
Organization and Growth of Axonal Fibers
From an anatomical point of view, this has included three-dimensional mapping of axonal and dendritic branches as well as synaptic structures. These examples serve to demonstrate the spatial and temporal resolution available for mapping neural structure and physiology in semi-intact mammalian brain tissue.
Structure and Development of Neuronal Dendrites
Dynamic structural changes associated with axonal and dendritic growth and synapse formation are also investigated to elucidate the principles of developmental plasticity. From a physiological standpoint, we examined patterns of neuronal and glial activity in both acutely isolated and organotypically cultured brain slices from developing rodents.
Organization of Neural Synapses
We examined the three-dimensional organization of mossy fiber terminals in brain slices using the antibody against syn-I. The giant en passant varicosities (arrows), corresponding to synaptic terminals, can be seen along the length of the mossy fiber axons.
Intracellular Ca2+ Transients in Neurons and Glia
- Conclusions and Future Prospects
- Why (and Why Not) Voltage and Calcium Imaging
- Signal Type
- Dyes
Expression of the fusion protein, which is normally targeted to synapses, labels all synaptic sites in transfected neurons. Some of the work described here was carried out by the author in the laboratory of Dr.
Voltage-Sensitive Dyes
In the second part of the chapter, there is a more detailed description of voltage-sensitive dye measurements in turtle and calcium dye measurements in mice. Different types of staining were used in the two preparations described in this chapter.
Calcium Dyes
Amplitude of the Voltage or Calcium Change
Both signals discussed in this chapter are presented as a fractional intensity change (∆ F/F). These signals provide information about the time course of the potential or calcium concentration change, but no direct information about the absolute magnitude.
Noise in the Optical Measurements
For example, the magnitude of the optical signal in response to a sensory stimulus can be compared to the magnitude of the signal in response to an epileptic event (Orbach et al., 1985). However, to determine the amplitude of the voltage or calcium change from a ratio measurement, one needs to know the fraction of the fluorescence resulting from dye at the expected location, i.e. bound to active vs inactive membranes for voltage sensitive dyes or dye free in the axoplasm vs dye bound to protein or in intracellular compartments for calcium dyes.
Shot Noise
For example, at 1010 photons/ms, a fractional intensity change of 0.1% can be measured with a signal-to-noise ratio of 100 in one experiment. The discussion that follows will indicate aspects of the measurements and characteristics of both camera systems that cause them to deviate from the shot noise ideal.
Extraneous Noise
One type of extraneous noise is caused by fluctuations in the output power of the light source (see below). For this reason, the performance of a photodiode array system that achieves the upper limit is shown in Figure 1.
Dark Noise
Light Sources
A number of precautions have been described to reduce vibration noise (Salzberg et al., 1977; London et al., 1987). The pneumatic isolation mounts on many vibration isolation tables are more efficient at reducing vertical vibration than at reducing horizontal movement.
Optics
Numerical Aperture and Depth of Focus
Light Scattering and Out-of-Focus Light
Confocal Microscopes
Cameras
Parallel Readout Arrays
Serial Readout Arrays
Comparison of Local Field Potential and Voltage-Sensitive Dye Recording
Voltage-Sensitive Dye Recording in the Turtle Olfactory Bulb
This type of local field potential signal implies that a population of neurons is somehow synchronously active. We measured the voltage-sensitive dye signal associated with these oscillations in the box turtle.
Initial Dye Screening
Since the first discovery in the hedgehog (Adrian, 1942), odor-induced oscillations have been observed in phylogenetically distant species including grasshoppers, frogs, turtles, rabbits, monkeys and humans. Because the optical measurements have a spatial resolution approximately 25 times better than local field potential measurements (Fig. 8), a more detailed visualization of the spatiotemporal features of the oscillations was obtained.
Methods
Optical imaging was performed with optics ( Fig. 9 , right) optimized for light collection efficiency at low magnification. Fluorescence was measured using a 464-element photodiode array camera (NeuroPlex) placed on the true inverted image formed by the Macroscope.
Example Result
- Calcium Dye Recording in the Mouse Olfactory Bulb
- Intrinsic Imaging and Fluorescence Signals from In Vivo Mammalian Brain
- Summary and Future Directions
- Sources of Intrinsic Signals and Wavelength Dependency
We used calcium-sensitive dyes to image receptor neuron input to glomeruli in the dorsal olfactory bulb of the mouse. First, scattering will be minimal because the Calcium Green-1 dextran is only in the outer layer of the olfactory bulb.
The Blood Volume Component
This phenomenon of "scattering" has mostly been observed in the rodent somatosensory cortex in response to whisker stimulation (Godde et al., 1995; Chen-Bee and Frostig, 1996). Three main components of optical signals have been identified: blood volume changes, hemoglobin oximetry changes and light scattering (Frostig et al., 1990; Malonek and Grinvald, 1996; Mayhew et al., 1999; Nemoto et al., 1999).
The Oxymetry Component
1995) reported that internal optical and intravascular fluorescent dye maps overflow regions of electrophysiological activity (using single-unit recordings) by about 20%. This may be due to low-level neuronal activity occurring in adjacent barrels in response to stimulation of adjacent, non-primary whiskers.
Light Scattering Component
In the near-infrared range (>750 nm) the light-scattering component dominates the intrinsic signal. However, the difficulty of attributing a specific signal etiology to the light scattering signal still remains.
How Can the Different Components of the Intrinsic Signal Be Resolved?
Single-Wavelength Imaging
Recently, Nomura and colleagues introduced a new protocol for mapping light scattering changes in vivo without any contribution from hemoglobin absorption (Nomura et al., 2000). Similarly, the light scattering signal has emerged as an extremely useful mapping signal in slices (Stepnoski et al., 1991) and the isolated brain.
Spectroscopic Analysis of Intrinsic Signals
Studies comparing intrinsic and intravascular dye signals suggest that these light scattering changes correlate well with changes in cerebral blood volume and may not be directly influenced by hemoglobin levels and oxygenation (Fig. 2) (Narayan et al., 1995; different wavelengths of light penetrate into the cortex to different degrees (longer wavelengths penetrate deeper into the cortex), and therefore one should be cautious in interpreting results across wavelengths, as different cortical volumes may be sampled at different wavelengths (Mayhew et al., 1999).
The Time Course of Intrinsic Signals
Preparation of an Animal for Optical Imaging
Once spectral data are obtained, a modified Beer-Lambert law is used to extract the contribution of the various chromophores to the intrinsic signal. The first such study used a linear component analysis to extract the etiology of the intrinsic signals (Malonek and Grinvald, 1996), but this approach has been questioned recently.
Species
- Rodents
- Cats
- Nonhuman Primates
- Humans
The higher complexity of the cat brain makes it more likely that the results can be extrapolated to humans. D and E) Activations observed at 610 nm corresponding to stimulation of rodent whiskers and hindlimbs, respectively, using the cranial window preparation shown in (B).
Anesthesia
- Ventilation and End-Tidal CO2
- Blood gases
- Pulse Oxymetry
- Blood Pressure Monitoring
- Core Body Temperature
- The Cranial Window
- Thin Skull Preparation
- Glass Plate
- Synchronization with Heartbeat and Respiration Synchronization of image acquisition with respiration and
- Postacquisition Image Registration
- The Apparatus
In addition, if the chamber window is large, it is important that it has a metal cover that can be screwed into the chamber instead of a fragile cover glass. To increase the transparency of the skull during imaging, silicone oil should be applied to the skull.
The Camera
- Photodiode Arrays
- Video Cameras
- Slow-Scan CCD Cameras
- Differential Video Imaging
The number of photons that can be attributed to statistical fluctuations is equal to the square root of the total number of photons emitted. Due to the considerations previously mentioned (see Shot Noise above), it is important that well capacities are as large as possible.
Illumination
- Lamps
- Filters and Filter Wheels
- Light Guides
- Shutter
It is important to confirm even illumination of the brain by looking at an online image from the imaging device. Adjustable lenses can be used on the front of the light guides to focus the incident light on the cortex.
The Macroscope (Adapted from Bonhoeffer and Grinvald, 1996)
Lenses
In the tandem lens combination, commercial home video CCD lenses can also be used as the lens next to the camera. Using it as the lens next to the cortex can be problematic whenever the working distance is important.
Camera Mount
The advantage of using the home video lenses is that the numerical aperture of home video CCD lenses is often larger than that of 35mm camera lenses. The final working distance, or distance from the first lens to the sample, must be at least 3 cm to manipulate the sample, including correct alignment of light guides and electrode placement.
The Spectrophotometer
Data Acquisition
The camera can also be mounted on a conventional microscope or on an operative microscope, preferably one that offers a high numerical aperture and thus a short working distance (5-7 cm). Numerical aperture, lighting, working distance and precise mechanical stability should all be considered in the final design.
Basic Experimental Setup
Timing and Duration of a Single Data Acquisition
Therefore, to capture the entire temporal profile of the intrinsic signal response, one must image for approximately 15 s after stimulus onset. For experiments aimed at investigating physiology, the “return to baseline” phase of the intrinsic signal may be an important component to be imaged.
Interstimulus Interval
However, the intrinsic signal response generally peaks within 5 s of stimulus onset, so many groups take images for only 5 s after stimulus onset. These later changes would not have been identified if imaging did not last longer than 5 s.
The Amount of Data
For example, Berwick et al. 2000) found that inhibition of neuronal nitric oxide synthesis did not affect the early increase in deoxyhemoglobin but significantly attenuated the late functional increase in total hemoglobin and oxyhemoglobin.
Testing the Apparatus (Adapted from Bonhoeffer and Grinvald, 1996)
Data Analysis for Mapping Functional Architecture
The analysis of these different types of data is very different and is therefore discussed separately in this section. Therefore, it is critical to understand the assumptions and limitations of each approach, both when selecting a particular approach and when interpreting the results.
Analysis of Single-Wavelength Data
- Ratio Analysis
- Principal Component Analyses
- Correlation Maps
- The Generalized Linear Model
- Analysis Using Weak Models
At high SNR, the first principal component can be used as an accurate measure of the time course of the intrinsic signal response (Cannestra et al., 1996). At high SNR (as is often the case in intrinsic signal imaging in rodents), the first principal component can be used as an accurate measure of the time course of the intrinsic signal response (Cannestra et al., 1996) (Fig. 7).
Analysis of Spectral Data
Linear Spectroscopic Analysis
Recently, Zheng and colleagues (2001) proposed using “weak model” constraints or adjustments to two of the above analyzes (BSS and GLM). Zheng and colleagues (2001) have shown that the method is still applicable with shorter prestimulus baselines (1 s), although not as effective.
Path-Length Scaling Spectroscopic Analysis Nemoto and colleagues (1999) were the first group to
As should be clear from this discussion, the approach taken to the analysis of the spectral data can significantly alter the conclusions of the investigation. Therefore, investigators must critically assess any analytical or mathematical approach used to evaluate spectroscopic data.
Baseline Vasomotion
It is proposed that the true path lengths for different relative concentrations of oxyhemoglobin and deoxyhemoglobin can be calculated for each wavelength to better estimate true changes in chromophore concentrations.
Color Coding of Functional Maps
Chronic Optical Imaging
Consequently, subjects can easily tolerate long-term optical imaging, both within the span of a day and over a period of several weeks. To make optical imaging successful over a period of weeks or months, several methodological adjustments may be necessary.
Imaging through the Intact Skull and/or Dura
In addition to its high spatial and temporal resolution, one of the strengths of optical imaging of intrinsic signals is that it is relatively noninvasive, in that the cortex is not affected or damaged by the imaging procedures.
Chronic Optical Imaging in the Awake Monkey
Reproducibility of Optical Maps
Optical Imaging of the Human Neocortex
Haglund and colleagues were the first to observe optical signals in humans, reporting activity-related changes in cortical light reflectance during cortical stimulation, epileptiform afterdischarges, and cognitive tasks ( Haglund et al., 1992 ). Since then, there have been reports describing the evolution of optical signals in the human cortex (Toga et al., 1995a), the mapping of primary sensory and motor cortex (Cannestra et al., 1998a), and the delineation of language cortex within (Cannestra et al. al., 2000) and across languages (Pouratian et al., 2000a) and compared intraoperative intrinsic signals with preoperative BOLD f MRI signals (Cannestra et al., 2001; Pouratian et al., 2001).
Imaging during Neurosurgery
Strategies for Reducing Movement Artifacts Strategies for reducing cortical movement have previously
Several strategies have emerged to minimize the effect of cortical motion during imaging, including imaging through a sterile glass plate overlying the cortex (Section III.C.3) (Haglund et al., 1992), which allows image acquisition synchronized with respiration and heart. rate (Section III.C.4) (Toga et al., 1995a), and use post-acquisition image registration (Section III.C.5) (Haglund et al., 1992; Cannestra et al., 2000; Pouratian et al. al. ., 2000a). The first report of intrinsic signals in humans used a glass slide to immobilize the cortex.
Clinical Utility of Intrinsic Signal Maps
However, it is unclear how this physical confinement of the cortex affects the physiology underlying the intrinsic signaling etiology. This phenomenon of "spreading" has also been observed in the rodent somatosensory cortex in response to whisker stimulation (Godde et al., 1995; Chen-Bee and Frostig, 1996; . Masino and Frostig, 1996).
Optical Imaging through the Intact Human Skull—NIRS
Alternatively, this spread may represent an inaccurate colocalization of optical signals and electrophysiological activity due to an inaccurate physiological coupling of neuronal activity, metabolism, and perfusion. The significance of optical signal propagation needs to be elucidated before intrinsic signal maps can be used for clinical decision making.
Applications
Combining Optical Imaging with Other Techniques
Stimulation results in a movement over the motor cortex, a perception (over sensory or limbic areas) or task disruption when testing waking language. Furthermore, approximately 25% of the regions considered inactive by ESM were also optically active, indicating that intrinsic signal maps extend beyond the regions indicated by ESM. In humans, the spread may be partly due to the fact that optical imaging detects both essential and secondary (i.e. active but not necessary for task completion) cortex, while ESM detects only essential areas.
Targeting Tracer Injection into Selected Functional Domains
Alternatively, fiducial lesions can be recorded in the optical image and later identified and matched to the same lesions in the histological specimens. Optical imaging has been combined with histological examination in the macaque visual cortex as well as in area 18 in the cat (Malach et al., 1993; Kisvarday et al., 1994).
Combining Optical Imaging with Electrical Recordings or Stimulation
Using optic-map-directed injections, the authors were able to demonstrate that intrinsic anatomical connections tended to connect monocular regions with the same eye's ocular dominance columns (skip columns corresponding to the other eye), binocular regions projected to other binocular regions but not to monocular regions and cortical areas that serve a specific orientation preference associated with other areas with a similar orientation preference (Malach et al., 1993). It is becoming clear that studies of this kind can lead to a better understanding of the relationship between anatomy and function in the cerebral cortex at both the single-cell and macroscopic levels.
Combining Optical Imaging and fMRI
A 3D active surface algorithm is used to generate an external cortical surface mesh for each subject (MacDonald et al., 1994). Recent technological advances have led to the development of an inexpensive combined optical imaging and functional MRI (3 T) system for animals (Paley et al., 2001).
Combining Optical Imaging and Micro-PET
Applications
The advantage of having simultaneous intraoperative optical imaging and MRI is that both modalities can be used to image the brain after craniotomy and dural reflection, so that the brain is imaged in the same space and (possibly) time. Nevertheless, Toga and colleagues have confirmed the feasibility of intraoperative optical imaging in the intraoperative MR environment (unpublished data).
Characterization of Visual and Somatosensory Cortices
These include characterizing the functional architecture of sensory cortices, investigating patterns of functional perfusion and neurovascular physiology, imaging development and plasticity, and characterizing the dynamic profile of disease processes.
Characterization of Functional Perfusion and Neurovascular Physiology
Therefore, it was hypothesized that different language tasks (eg, naming visual objects versus discriminating words) would activate Broca's and Wernicke's area differently. In the study by Cannestra and colleagues (2000), investigators were able to use optical imaging of intrinsic signals to define subdivisions of Broca's and Wernicke's area and describe different patterns of temporal activations within each region.
Imaging Development and Plasticity
Pouratian and colleagues described developmental changes in intrinsic signals in the rodent barrel cortex, showing that while some intrinsic signals change throughout development (ie, 850 nm), intrinsic signals at other wavelengths remain relatively stable and unchanged throughout development. (ie, 550 and 610 nm) (Pouratian et al., 2000b). For example, Polley and colleagues have shown that long-term whisker removal and differential housing in rodents can alter intrinsic signaling responses in the barrel cortex differently in two distinct ways (Polley et al., 1999a).
Imaging Disease
Comparison of Intrinsic Optical Imaging with Other Imaging Techniques
NIRS
Laser Doppler Flowmetry (LDF)
However, LDF still offers the opportunity to monitor flow changes occurring at greater depths than is available through intrinsic signal imaging. Although traditionally used for retinal imaging, this technology has recently been used to monitor flow changes on the surface of the rodent brain following functional activation ( Nielsen et al., 2000 ).
Dye Imaging
Conclusions and Outlook
One of the most exciting and challenging applications of optical imaging of intrinsic signals is studying the human cortex. Optical imaging of the functional architecture in the cat visual cortex: the layout of direction and orientation domains.
Optical Window for Noninvasive Studies
Optical methods provide higher biochemical specificity of the signal, and approaches to quantification of substance concentrations are more straightforward (see considerations of a modified Beer-Lambert law below). 3) Optical methods are sensitive to very low substance concentration when using fluorescence methods. In principle, optical methods allow monitoring vascular, metabolic-cellular and neuronal responses.
Transmission/Reflection Mode for Near-Infrared Studies
Although the combination of respective advantages has been explored with combined approaches, the technical requirements are complex and the signal-to-noise ratio is lowered with such approaches. Moreover, combined approaches with fMRI, PET, and MEG/EEG do not interfere with the biophysics of other techniques.
Absorption and Scattering and a Modified Beer–Lambert Law
Other Optical Parameters Relevant for Near-Infrared Studies
When the delay is shorter than 10–8s, the phenomenon is called fluorescence, when it lies between 10–8 and 10–6s, it is called delayed fluorescence, when it is longer than 10–6s, it is called phosphorescence. Near-infrared applications using a correlation spectroscopy approach allow for non-invasive investigations in the animal (Cheung et al., 2001).
Technical Approaches for Near-Infrared Spectroscopy
The phase delay of the modulation is proportional to the mean time of flight [phase modulated spectroscopy (PMS) or NIRS frequency domain system]. All of the approaches mentioned above can be implemented in just one place (one channel) or multiple places.
Physiological Parameters of NIRS Measurements
Changes in the scattering properties of the tissue were described on two time scales. Others will not reach the detector as they are scattered out of the sample volume (photon 4).
Assessment of Physiological Parameters and Applications
Oxygenated Hemoglobin (Oxy-Hb) and Deoxygenated Hemoglobin (Deoxy-Hb)
While so far most fluorescent dyes have been designed for excitation with visible light, recently an increasing number of dyes have been developed for near-infrared wavelengths, which are potentially useful for non-invasive near-infrared studies (Bremer et al., 2001). In animals, the phenomenon of phosphorescence has been used to measure oxygen concentration (Rumsey et al., 1988; Lindauer et al., 2001).
Cytochrome c Oxidase Redox State
The black line in the upper right corner is the result of a two-component fitting process with “oxy-Hb” and “deoxy-Hb” components. Note that the fit is not "perfect". The residuals of this fit are given below (red dots). Changes, i.e. spectrum difference between resting state and activated visual cortex, could not be explained by hemoglobin chromophores alone (Fig. 5a).
Fast and Slow Light Scattering Signals
Near-Infrared Spectroscopy and Imaging: Applications
So far we have provided an overview of the general potential of optical techniques in neurophysiological research. There is no doubt that the potential of optical methods can be limited by signal-to-noise ratios when a slab of ~1 cm of highly scattering and absorbing tissue (i.e., the skull) is introduced, obscuring and distorting the image of the cerebral cortex. .
Functional Imaging of the Human Brain
Functional Imaging Based on Hemoglobin Oxygenation
Our incentive to advance further non-invasive optical methods is based on the enormous wealth of physiological information gathered from the invasive applications and the perspective of obtaining some of this information at the patient's bedside. 1997) and the BOLD signal in fMRI studies (Kleinschmidt et al., 1996; The bedside ability was illustrated by studies in which hemoglobin oxygenation changes were monitored during epileptic seizures (Steinhoff et al., 1996; Villringer et al., 1994; von Pannwitz et al., 1998).
Functional Imaging Based on Cytochrome c Oxidase Redox State
Based on these findings, we are confident that NIRS and NIRS imaging will require a bedside position for clinical and neonatal studies.
Functional Imaging of Fast Light Scattering Events Associated with Brain Activity
Studies on Neurovascular Coupling in Health and Disease
Gratton's group provided data that could allow noninvasive measurement of neurovascular coupling using an optical approach alone (Gratton et al., 2001). After global ischemia, there may be a dissociation between evoked neuronal activity, metabolism, and blood flow response (Schmitz et al., 1998).
Other Applications of Near-Infrared Spectroscopy and Imaging
Practical Aspects of NIRS Measurements
A number of studies indicate that neurovascular coupling can be altered in various types of disorders of the nervous system. Likewise, studies by Jens Dreier have suggested that a disorder of neurovascular coupling the.
Study Environment
Subject movement can be tolerated as long as it does not critically interfere with the experimental procedure. We conclude that a complete fixation of the probes on the subject's head is a prerequisite for a good NIRS measurement.
Probe Localization
In addition to the rather general origin of artefacts, the movement of the probes will introduce changes in reflectance that cannot be easily rejected by the data analysis. Unfortunately, the optimum of a fixed fixation of the probes may not coincide with the optimum comfort of the subject.
Interoptode Distance (IOD)
Sampling Rate
Coregistration of Basic Physiological Parameters
Experimental Protocols
NIRS Has an Excellent Temporal Resolution This strength of the method is obvious compared to PET
NIRS Has a Poor Spatial Resolution
This limits its ability to answer questions about functional anatomy of the cerebral cortex in the adult. When there is some a priori knowledge about the distribution of the areas under investigation, activation can be differentiated when the distance between the foci is on the order of a few centimeters.
NIRS Has a High Parameter Specificity
With a typical IOD of 3–4 cm, the one but following source will not significantly contribute photons to a sampling probe at a distance of 6–8 cm. By applying time-resolved techniques for imaging and optimization of detector sensitivity, the spatial resolution may well be increased by future systems based on such image reconstruction algorithms.
Problems and Perspectives
Functional brain imaging in sedated neonates using near-infrared topography during passive knee motion. Regional changes in cerebral hemodynamics as a consequence of a visual stimulus measured by near-infrared spectroscopy.
Why Measure Local Cerebral Blood Flow?
In general, the volume of the system and the time in the system vary together so that neither can fully account for the rate at which the fluxes vary. Fluorescent compounds, microscopes, video cameras, and computers now make it possible to follow changes in blood vessels in fine detail by quantification.
Function and Structural Contexts
This chapter reviews the techniques used to study dynamic changes in LCBF in real time by direct observation of the brain surface in experimental animals. Pathways for lateral and collateral flow exist in the cortical parenchyma, but to a first approximation, flow in the superficial vessels indicates flow in the underlying parenchyma.
Global Tracers
Within the brain parenchyma, capillary density directly correlates with metabolic capacity (Boero et al., 1999; Borowsky and Collins, 1989; Klein et al., 1986). These images illustrate the main features of this method; results are quantifiable, highly localized and comprehensive, regardless of location in the nervous system.
Volatile Tracers
Localization of the reactions in space can be improved with the configuration of the electrodes (smaller electrode surfaces and narrower spacing between electrodes). A good demonstration of the possible resolution is the localization of clearance near single retinal vessels and across the retina (Yu et al., 1991).
Doppler Flowmetry
Flow responses monitored over time were used to measure latencies to onset of flow changes, direction of flow changes (increase or decrease), changes in different stimulus parameters, changes with stimulation of different parts of the periphery around different vessels and intracortical circuits , and changes in flow from different layers of cortex (Moskalenko et al. Woolsey et al., 1996). The basis of this method is the transfer of thermal energy from circulating blood to cortical tissue, depending on local perfusion.
Video Microscopy
The full extent of the exposed cortex can be mapped with respect to the image of the surface (eg, Erinjeri and Woolsey, 2002). It also supports averaging, subtracting, and producing ratios between the images or pixels to detect intrinsic signals or to delineate vessels filled with contrast.
Localization of Activity Changes
For the flow techniques described in this chapter, the hardware and software configuration is unchanged. At 540–560 nm, the main component of the internal signal recorded is a change in red cell mass, a parameter that varies with flow.
Diameter
The fluorescent filter on the microscope transmits light from 520 to 560 nm within the absorption range for total Hb and therefore RBC volume (Frostig et al., 1990). Alternatively, filters in the 605 to 620 range can be used to detect light absorbed primarily by deoxyhemoglobin associated with capillary oxygen delivery to tissues (Erinjeri and Woolsey, 2002; Malonek et al., 1997).
Intravascular Dyes
In this and similar images obtained with sensory stimulation (Cox et al., 1993; Liang et al., 1995; Liu et al., 1994) or with pharmacological stimulation (Woolsey and Rovainen, 1991) the superficial arteries change, while the veins superficial no. Flows were slowed by a 1.6- and 7-fold increase in AVTT in two veins draining different parts of the cortex around an experimentally created area of ischemia (Wei et al.
Intravascular Particles
In capillaries where red blood cells move in single file, the speed and flux of individual blood cells can be measured automatically (Hudetz et al., 1995). Using CCD cameras with ultrafast frame rates (up to 1000 frames/s), fluorescently labeled RBCs can be tracked to measure velocity in arteries (Ishikawa et al., 1998).
Localization of Flow Changes
Donor rat RBCs labeled with fluorescein isothiocyanate (Sarelius and Duling, 1982) and other commercially available labels (PHK; Sigma) can be stored for one week before being resuspended in serum for injection (Liang et al., 1995). . Software has been developed to count these events automatically (Knuese et al., 1994) and, as with other video methods, analysis can be done post facto from the videotapes.
Conclusions and Prospects
Localized dynamic changes in cerebral blood flow in the rat cortex with whisker stimulation. Regional cerebral blood flow in the rat as determined by particle distribution and by diffusible tracer.
The Electroencephalogram and Averaged Event-Related Potentials
This chapter briefly reviews methodological issues in using the EEG and stimulus-related brain electrical activity such as evoked or "event-related" potentials (ERPs) to image cognitive brain functions. We begin by briefly considering the neural basis of the EEG and its measurement at the scalp.
Neuronal Generation and Transmission to the Scalp
Many of the fundamental issues addressed here are also directly extendable to magnetoencephalographic recordings; a summary of the differences between EEG and MEG approaches is presented in Wikswo et al. Another view focuses on the local spatial distribution of the magnitude of higher frequency macropotentials in specific structures.
Acquisition and Application of EEG Signals
This view is supported by reports that similar macropotential waveforms from widespread cortical and subcortical structures are associated with similar memory acquisition processes. For example, Freeman (Freeman, 1978; Freeman and Baird, 1987) describes 40- to 80-Hz EEG bursts of the rabbit olfactory bulb, which could represent a type of carrier wave whose spatial modulation encodes factors related to the stimulus and the animal's internal state. and other researchers have described similar spatial modulation of high-frequency activity in the visual (Gray et al., 1989) and sensorimotor (Murphy and Fetz, 1992) cortex of other species.
Event-Related Potentials
Improving the Spatial Resolution of the Electroencephalogram
High-Density Recordings
Spatial Sharpening and Anatomical Integration
These studies showed that blunted somatosensory responses isolated activity to the region of the central sulcus. The increased θ-band strength may be a marker of the continuous focused attention required to perform the task and may reflect activity in the anterior cingulate gyrus (Gevins et al., 1997) (Fig. 4).
Identifying the Generators of EEG
Analysis of Functional Networks
Some of the above methods can be used to characterize the spatiotemporal relationships of subsecond ERP components. Meanwhile, ERCs have provided fascinating glimpses into the complex, rapidly changing distributed neuronal processes underlying simple cognitive tasks.
500—875 ms Post—Cue
- The EEG as a Monitoring (vs Imaging) Modality
- Summary and Conclusions
- Introduction and Historical Perspective
- Structural versus Functional Brain Maps
- Strengths of Electrophysiological Mapping Methods Compared to Other
- Contrasts between Sensory versus Motor System Mapping
- Output Measures for Mapping Motor System Organization
What one intelligence test measures: A theoretical account of processing in the Raven Progressive Matrices Test. 1937 Penfield and Boldrey report data showing the first experimentally determined motor map of the human brain.