Department of Radiology, Center for Functional Brain Imaging and Clinical MRI, Temple University School of Medicine, Philadelphia, PA, USA. Department of Radiology, Center for Functional Brain Imaging, Temple University School of Medicine, Philadelphia, PA, USA.
Introduction
Principles of Functional MRI
Thus, it is difficult to compare BOLD signal changes in different brain regions, from different imaging laboratories and/or from different magnetic fields. In this introductory chapter, we discuss the source of the BOLD signal and the improvement of BOLD fMRI techniques.
Physiological Changes
The relationship between CBF and CBV obtained in monkeys during CO2 modulation can be described as. From equation (1.2), the relative change of CMRO2 can be obtained from information on the relative changes of CBF and U.
Functional Imaging Contrasts
The change in total blood volume can be measured using contrast agents because the contrast agents are distributed throughout the vascular system. Since venous blood volume represents 75% of total blood volume,15 it is possible that the change in venous blood volume predominates.
T 1 Weighted fMRI
However, based on separate measurements of arterial and venous blood volume changes during hypercapnia by a new 19F NMR technique and video microscopy, a relative change in venous blood volume is approximately half of the relative total CBV change (see Figure 1.2B).15. Functional brain mapping has been successfully obtained during motor, visual and cognitive tasks (see Figure 1.4 for finger movements).
T 2 and T 2 Based fMRI
Intravascular Component
When spin-echo data acquisition is used, spins in large vessels will not be refocused, removing very large arterial contributions. When spin-echo data acquisition is used, the EV effect of large vessels can be minimized.
Extravascular Component
A power term g will be 1.5 for a 3 mm container and 1.1 for a 30 mm container, showing that a smaller size container is more sensitive to the frequency shift (such as induced by magnetic field). Spin-echo and gradient-echo BOLD signal changes as a function of vessel size can be seen in figure 1.10.46 In capillaries, a change in R2 is similar to that of R2∗.
Spin Echo versus Gradient Echo BOLD
In spin-echo BOLD fMRI, the EV effect around small vessels is supra-linearly dependent on the magnetic field, while that around large vessels is reduced. However, because the dephasing effect around vessels is refocused, the sensitivity of spin-echo BOLD signal is greatly reduced.
Contrast-to-Noise Ratio
Thus, the spin-echo technique is more specific for the parenchyma than gradient-echo BOLD fMRI. DS for spin-echo BOLD fMRI will be maximized by setting the T2 TE of the tissue.
Spatial and Temporal Resolution of fMRI Spatial Resolution
Temporal Resolution
Later, temporal characteristics of fMRI responses can be correlated with behavioral data such as response time. Differences in the underlying temporal behavior of neuronal activity can be distinguished from variations in hemodynamic response time between subjects and brain regions (see review article68).
Conclusions
Thus, the spatial resolution of fMRI can be achieved down to an order of one column. By using a multiple-experiment approach with different stimulus intervals or durations, the temporal resolution can be improved to 100 milliseconds.
In vivo validation of the BOLD mechanism: A review of signal changes in gradient echo functional MRI in the presence of flow. Multi-slice perfusion-based functional MRI using the FAIR technique: Comparison of CBF and BOLD effects.
General Overview
The described phenomenon forms the basis of the blood oxygenation level-dependent (BOLD) contrast used in fMRI. Higher contrast is achieved by sensitizing the MR signal to a specific target and, in some cases, suppressing that of the target's surroundings.
Spin-Echo and Gradient-Echo Imaging Methods
It has been shown that the dynamics of the MR signal, especially the rate at which the signal decays (relaxation), is dependent on the oxygen level in the blood.12,13 In particular, increased magnetic sensitivity caused by deoxygenation has an influence on the signal. Such sequences are designed to be sensitive to small variations in magnetic fields (microscopic field gradients) caused by changing blood oxygen levels.
Spin-Echo Formation Mechanism
A 2-radian RF pulse is applied to the sample along the y¢ axis, forcing magnetization of the mass in the transverse plane. This is due to the loss of phase coherence encountered by random field fluctuations that cannot be recovered by refocusing the RF pulse.
Spin-Echo Imaging Pulse Sequence
The process of MR signal acquisition and construction of the raw signal matrix is often called the k-space coverage. Further, the detection circuit is turned on in the presence of the frequency encoding gradient and the MR signal is sampled along that k-space line.
Contrast Characteristics of Spin-Echo Sequences
In this case, the spin-echo is completely refocused and the MR signal does not retain the trace of the inhomogeneity. Those excited spins that experienced the refocusing pulse before exiting the imaging volume during the acquisition period contribute to the MR signal.
Gradient-Echo Formation Mechanism
Immediately at the end of the excitation pulse, all spin isochromats precess at the same rate. At time t3, another gradient field of the same magnitude and direction, but opposite polarity, is applied to the sample.
Gradient-Echo Imaging Pulse Sequence
The main effect of the refocusing pulse used in the spin echo forming mechanism is achieved by destroying phase angles of spin isochromats and reversing the direction of the dephasing. Reversing gradients only in the direction of the phase encoding does not fully rephase transverse magnetization.
Contrast Characteristics of Gradient-Echo Sequences
In the case of the spin-echo sequence, in contrast to the reduction of the MR signal, it caused Figure 2.9. In the case of relatively short TR or rapid blood flow, the MR signal increases due to partial desaturation with fresh spins.
Echo Planar Imaging Methods
MR signal enhancement occurs when a sufficient number of unsaturated spins enter the image portion during several TR periods. In general, the increased sensitivity of gradient-echo methods to such perturbations originates from the uncompensated rotational phase contributions developed during the formation of an MR signal echo.
Echo Planar Imaging Pulse Sequences
Thus, the effective TE determined at the time of the primary echo determines the overall MR signal loss. To this end, the significant decrease in MR signal amplitude over the acquisition window constitutes image blurring.
Spiral-Echo Planar Imaging Methods
There are apparently two important ways to follow the spiral – namely from the center to the periphery of k-space and in the opposite direction. The spiral-out pulse sequence traverses the center of k-space at the beginning of the recording period and therefore achieves the contrast-defining echo immediately after the excitation pulse.
Basic Types of Neuroimaging Inference
Usually, experiments from these three categories concern changes in bulk neural activity measured by neuroimaging methods. However, other types of endings are also possible; for example, methods exist to measure the degree of effective connectivity between different cortical regions—the degree to which one cortical region influences neural activity in another region.1.
Manipulation of the Cognitive Process
Compared to cognitive subtraction methods, failure of the pure insertion assumption is less likely for parametric designs since the cognitive process is present during all conditions. Reduced responses of this cortical area to the second presentation of the same face viewed from a different angle would support this claim.
Properties of the BOLD fMRI System That Impact Experimental Design
Specifically, the smooth shape of the HRF makes it difficult to distinguish closely spaced neural events. Interestingly, the implications for experimental design of the shape of HRF and the noise properties of BOLD fMRI are contradictory.
Different Temporal Structures of BOLD fMRI Experiments
These fast event-related designs are quite sensitive to the exact specification of the HRF for their success (unless a basis set is used for analysis; see below). Here, a sparse event-related design is used together with an exquisite coupling of the timing of stimulus presentation to image acquisition.
Data Preprocessing
A difference in the time of onset of the smooth BOLD hemodynamic response evoked by two different stimuli within a cortical region is sought. Traveling wave stimuli are used to define topographical maps of cortical responses, the best known of which is the retinotopic organization of early visual areas.14 These designs use stimuli that vary continuously over some sensory space (eg retinal eccentricity), and identify for any point within a cortical area that was the optimal position of the stimulus within the sensory space for evoking neural activity.
Distortion Correction
These designs are often combined with cortical planar mapping techniques to display the results.15.
Slice Acquisition Correction
Motion Correction
Spatial Normalization
Subsequently, hypotheses regarding the response of this functionally defined area to other types of stimuli can be tested independently across subjects within this area. This powerful approach allows drawing cross-subject conclusions regarding the responses of a particular functional area (e.g., the fusiform facial area) at the expense of making statements about a particular position in a standardized anatomical space.
Spatial Smoothing
For example, we can identify an area that responds more to images of faces than to general objects.
Statistical Analysis
In some circumstances, the sign of the relationship between the variable of interest and the data may be reversed. The product is a statistical map in which each voxel in the brain contains a corresponding statistical value for the contrast of the variables of interest.
Challenges in fMRI and Its Limitations
The challenges in achieving reduced acquisition times with high spatial and temporal resolution will be discussed in detail in the sections that follow. Most cognitive and many sensory/motor fMRI experiments are limited in terms of spatial and temporal resolution by a range of factors.
MR Physics-Based Limitations in fMRI
Much research is underway examining brain function at all these levels, and as these systems are better understood, the required spatial and temporal resolution for specific fMRI investigations will become clearer.
Physics-Based Limitations on Spatial Resolution
If all other imaging parameters are held constant, the signal varies in direct proportion to the voxel volume. At the extremes of spatial resolution in MR, water diffusion is the ultimate limiting factor (see Callaghan PT, Principles of Nuclear Magnetic Resonance Microscopy, Oxford Science Publications, 1993).
SNR and Field Strength
As will be seen below, there are a number of other considerations—such as the number of slices acquired, temporal resolution, image distortion, and brain coverage—. Most studies to date that have used very high spatial resolution tend to focus on a specific cortical area, and studies involving whole-brain coverage are usually performed at much more modest spatial resolutions.
Static Field Inhomogeneities
Static field effects are more pronounced along the largest dimension of a voxel, and the source of signal loss is decreasing along this dimension. Sensitivity increases with field strength, and using an asymmetric echo planar (EPI) acquisition, it is possible to tune the sensitivity to field inhomogeneities at different scales.16–18.
Effect of Acquisition TR
Moving to higher field strengths, such as 3T or greater, provides sufficient BOLD contrast using spin echo techniques or asymmetric spin echo methods, where microscopic field effects are not refocused by the spin echo, but macroscopic effects are refocused. Spin echo-based BOLD imaging has been demonstrated at 1.5T15, but the sensitivity at this relatively low field strength is insufficient for most fMRI applications.
Physiological Factors Influencing Spatial Resolution
Physiological-Based Limitations/Constraints in fMRI
In the case of the cardiac cycle, the entire brain pulsates with each heartbeat, introducing both very small motions and variable vessel flow conditions that must be averaged out of the data. Respiration changes the sensitivity in the chest, as deoxygenated air in the chest is replaced by oxygenated air, and while the chest is obviously some distance from the brain, these changes in receptivity can be traced in the brain.19.
Blood Oxygenation Changes and Localization
Functional Spatial Limitations
Brain System Dependent Limitations
Draining Vein Problem
The fact that the BOLD response in draining veins can be large allows them to be identified by the large percentage signal change measured. The sensitivity of gradient echo and spin echo sequences to microscopic susceptibility changes is essentially the same; Therefore, the choice of gradient-echo imaging over spin-echo imaging is made to allow for the contribution of larger vessels to the BOLD signal change to get an additional boost from the larger signal changes associated with these vessels.
Initial Dip
Cheng and colleagues,29 for example, produced maps of cortical columns in the visual cortex by adapting this approach to stable stimulus presentations. Theoretically, the initial immersion can provide extremely good localization of the activated regions because there is no problem of drainage veins with this phenomenon, and this directly reflects the increased oxygen consumption.40 Kim and colleagues41 exploited this phenomenon to map iso orientation columns on the mouse. the visual cortex.
Subject Movement
This initial dip is followed by an increase in signal intensity as the blood flow response kicks in and overcompensates for the increased oxygen consumption, leading to an increase in the ratio of oxyhemoglobin to deoxyhemoglobin in the blood and the increase in BOLD contrast that most fMRI experiments typically rely on . Second, the BOLD signal change associated with the initial dip is approximately one-tenth of what is observed with the later BOLD signal increases;.
Other Physiological Changes Associated with Brain Activation
Moving to higher field strengths may allow for a reduction in overall study time and may therefore also allow for the use of more restraints, especially if the subject is aware of the fact that the discomfort is only of short duration. In most cases, however, the gain in SNR when moving to higher field strengths is used to increase spatial resolution or add more tasks, both of which are good choices, but as a result the study time remains fixed.
Threshold Effects and Localization
Temporal Resolution of the BOLD Response
The TR should be of the order of two seconds or less, with shorter TR again being better, or a technique whereby the time slot between stimulus presentation and image acquisition is varied so that the blood flow response is sampled over the course of many events with high temporal. However, it is very difficult to compare the blood flow responses across different brain regions, as local differences in the structure of the microvasculature may account for the differences in the observed time course.
Pulse Sequences for fMRI: Spatial/Temporal Resolution
However, this is not possible without first characterizing local variations in the structure of the microvasculature in different cortical areas. Spiral scanning, as the name implies, moves spirals, in or out, from the center of the data space and can be less demanding on the gradient hardware.
Imaging Approaches to Other Physiological Measurements
Collecting fewer phase-encoded steps with this approach can be used to reduce imaging time or can allow more slices to be collected within a TR window, thereby increasing spatial coverage without sacrificing temporal resolution. Doubling the capture bandwidth to 128 kilohertz will reduce the image distortion by a factor of two and reduce the SNR by a factor of 2, allowing a few more slices to be squeezed into the TR interval.
Arterial Spin Labeling
Magnetization transfer (MT) effects can also cause changes in signal intensity in an image slice, leading to spurious CBF measures. The MT effect causes direct signal changes in the imaged slice even in the absence of perfusion.
Sensitivity
Summary
Spatiotemporal dynamics of BOLD fMRI signals: toward mapping submillimeter cortical columns using the early negative response. Investigating the human hippocampal formation using a random event-related paradigm and z-simulated functional MRI.
Clinical Challenges of fMRI
Cognitive impairment can also alter the patient's ability to perform tasks, both with regard to motivation and the difficulty of the task. In addition, the altered anatomy resulting from intracerebral lesions can hinder spatial registration and normalization tools commonly used in group statistics, making it difficult to directly compare patient results with those from a normative sample.3 This chapter discusses in more detail the challenges of fMRI in clinical populations. , including issues of field strength and sequence selection, study and task design, and data analysis.
A Brief History of Clinical Brain Mapping
Hemodynamic Basis of fMRI Maps
The uncertainty of this neurovascular coupling function introduces one of the greatest challenges and one of the major sources of error in the interpretation of clinical fMRI studies. In a study of 98 patients, Krings and colleagues showed that the distance of a central mass from the motor region significantly affected the magnitude of activation, even within patients without paresis.10 Other studies found similar suppression of the hemodynamic response. adjacent pathology.11,12 Conversely, in a study of 14 patients, Schlosser and colleagues suggested that fMRI activation patterns within patients with frontal lobe tumors, when mapped using a verbal fluency paradigm, were comparable to signals in normal controls.13 Similarly, Righini and colleagues studied 17 patients with frontoparietal masses and found little difference in motor activations between the affected and unaffected hemispheres.2 The discrepancy in these studies highlights the need to be aware of the possibility that adjacent pathologies cerebral hemodynamics can change, but that this change is most probably pathology and location dependent and possibly task dependent.
Technical Considerations
After all, if the voxel sizes are too large, the lack of differences between subjects or groups may not actually mean that there is no difference. This is particularly important for language mapping, in which investigators expect to find multiple language regions of the temporal lobe.18 A lack of signal in these regions does not indicate a lack of activity, but may be due to a lack of sensitivity to recognize relevant signals.
Scan Sequence and Susceptibility
One alternative to the commonly used gradient echo EPI scan is asymmetric spin echo. As a result, the spin-echo sequence reduces susceptibility artifacts at air-tissue boundaries, but will also result in a loss of CNR due to reduced BOLD contrast.
Mapping the Oxy/Deoxyhemoglobin Signal
Study and Task Design
Task Selection
Syllable repetition was assumed to be a combination of phonation and lip closure for the purpose of this study. Not surprisingly, the authors showed that identification of activity in the medial temporal lobe (including hippocampal and parahippocampal structures) varied depending on the control condition used.
Practice Effects
Therefore, it is critical to plan experiments efficiently and to constantly provide new stimuli and tasks to ensure that practice-related changes do not contaminate the results (unless, of course, practice-related effects are being investigated.).
Task Difficulty
Kim and colleagues40 found that the pattern of reorganization in patients with focal lesions varied across individuals and appeared to be related to whether the lesions were cortical or subcortical. Together, the existing data suggest that patients with deficits tend to use compensatory strategies that engage additional brain regions to perform the task.
Analysis
For example, Sonty and colleagues52 showed that patients with primary progressive aphasia had activation in primary language areas like normal patients, but also had additional language activation in regions outside the language cortices, suggesting the use of compensatory strategies. The pattern of fMRI activation during compensation may give a false impression about the localization of the function; For example, with increased compensatory RH, activation may falsely suggest that the patient has right hemisphere speech dominance.
Within Subject Versus Group Analysis
Attention should also be paid to the technique used to quantify fMRI activations and techniques used to minimize false-positive and false-negative results.
Dependent Measures
Due to the low baseline speech rate of the right hemisphere, it is very difficult to accurately compare populations. Functional MRI asymmetry in the temporal lobes (regardless of language task) did not correlate with Wada asymmetries.
Conjunction Analysis
In this image, fMRI activations of language expression (generated by connectivity analysis) are seen adjacent to a frontal AVM, identifying Broca's area. 2002 (a three expressive language task: visual object naming, word generation and auditory response naming) and receptive language (linking visual responsive naming and sentence comprehension) and compared these fMRI activations with intraoperative ESM (Figure 5.3 A, B).
Reproducibility
Consequently, areas negative for language by ESM may still show fMRI activations, producing false positives. However, the use of the conjunction analysis minimizes this false-positive rate by identifying only those areas that are consistently activated across language tasks.
Applying fMRI to Clinical Planning
These studies strongly argue for the unreliability of activation magnitude as a measure of response magnitude. The latter method of measuring percentage signal change is also preferable because it can detect differences in response magnitude across tasks in voxels that were already activated in the original task.
Reliability of Signal Adjacent to Pathology
Relationship to Outcomes
Functional MRI of somatosensory activation in rats: effect of hypercapnic upregulation on perfusion and BOLD imaging. Three-dimensional tailored RF pulses for reducing susceptibility artifacts in T(*)(2)-weighted functional MRI.
Historical Milestones That Enable Imaging of Cortical Processes That Underlie Mental Events Using MRI
Brain Mapping for Neurosurgery and Cognitive Neuroscience
These early PET studies established proof-of-principle that activity associated with cognitive events was observable in the living human brain via hemodynamic variations within locally active neural areas. Static visual fields indicate a homophonic quadrant field defect in the left superior quadrant associated with damage to the visual projection fibers in the right occipital lobe after resection of a lesion in the occipital region of the right hemisphere.
Development of Magnetic Resonance Imaging (MRI) to Visualize Living Brain Structure
The Development of fMRI
The BOLD Response
This technique, which exploited the fundamental link between the MR signal, blood flow, and neural events, was called functional magnetic resonance imaging (fMRI) (see review by John Gore12 for more information). The timeline marks milestones in neuroscience and neuroscience. medicine parallels physics and engineering that led to the development of functional magnetic resonance imaging (fMRI) and its ability to image active human cortical tissue corresponding to a specific cognitive function. (detailed description of these relationships).
Hypothesis of Functional Specialization
In the case of the well-studied visual system, more than 30 separate and distinct visual cortical areas are organized into well-defined processing pathways. Visual signals from the retina are transmitted to the lateral geniculate nucleus of the thalamus, and then to primary visual cortex where visual information fan outs to the outer visual cortical areas.
Identification and Preservation of Cortical Areas Specialized for Essential Tasks
Within a coherent task battery, it is desirable to maximize reliability by using multiple tasks to target related functions. All functions are repeated using both active (willing) and passive (receptive) modes to ensure that it is applicable to patients with a range of symptoms and performance.
A Multifunction Task Battery
Each of the target functions and structures associated with each task is illustrated in Figure 6.6 for a healthy volunteer. An integrated fMRI procedure for preoperative mapping of cortical areas associated with tactile, motor, language, and visual functions.
Healthy Volunteers and Patients
Sensitivity of Task Battery: Healthy Volunteers
Sensitivity of Task Battery: Surgical Population
Explanations for the three failed Wernicke's area patients included motion artifact (n = 1) and likely lack of coherence (n = 2). Explanations for the five failed Broca's Area patients included neurologic deficits (n = 3), likely borderline compliance (n = 1), and an uncorrectable head motion artifact (n = 1), although a false-negative finding cannot be excluded .
Comparison of Task Sensitivity for Patients and Healthy Volunteers
Using the combined performance of the picture naming task and the passive listening task, the fMRI signal was observed in Wernicke's area in 31 of 34 (91%) patients with pathology in the superior temporal gyrus and Broca's area in 17 of 22 patients (77%) with pathology in the inferior frontal gyrus . An fMRI signal was observed in the visual cortex (calcarine sulcus and inferior occipital gyrus) in all six patients with lesions in these cortical areas.
Accuracy of Task Battery: Comparison with Intraoperative Electrophysiology
Comparison of fMRI, Wada, and Intraoperative Language Mapping
Comparison of fMRI and Visual Fields
Case Example 1: Motor and Language Mapping
Integrative Mapping of Sensory and Motor Functions
Interoperative Mapping of Language Functions
Postsurgical Status
Case Example 2: Language Mapping—Late Bilingual Patient
These displacements were confirmed during surgery and the tumor was resected without impairing any of the language functions. A follow-up fMRI scan showed that the language areas associated with each language have moved back to the expected locations within the brain (Figure 6.10B).
Case Example 3: Language Mapping—Early Bilingual Patient
Functional MRI of her language areas for both languages revealed overlapping clusters in the putative Broca's area (left inferior frontal gyrus), as illustrated in Figure 6.11A, which was unchanged postoperatively (Figure 6.11B), consistent with the absence of postoperative morbidity . Kim and colleagues41 showed that the average distance between the centroid in putative Broca's area during L1 and L2 production in late bilingual subjects was approximately seven millimeters, while in early bilingual subjects the language activations were indistinguishable. were (Figure 6.12).
Case Example 4: Motor Mapping
On the preoperative side, fMRI maps have contributed to our risk-benefit assessments and to the decision whether or not to offer a patient surgery, although these decisions are based on the overall medical condition combined rather than on a single factor. On the intraoperative side, as shown above, fMRI results also served to guide intraoperative electrophysiology, thereby contributing to the efficiency of intraoperative procedures.
Determination of the Anatomy and Topography of Cortical Areas Specialized for Cognitive Tasks
Conservation of Effects versus Individual Differences
Generalizing the Results
It is assumed that the neural correlates of the cognitive task of interest are revealed by subtracting the baseline activity from the activity observed during the experimental task. On the other hand, the conjunctive approach used for intramodal studies may serve to increase confidence in the result by isolating the activity that is repeated in multiple executions of the same task.
Integration of Functionally Specialized Areas Associated with Cognitive Tasks: The Network Approach
Functional Neuroanatomy of Language Processes
A Large-Scale Network
Functional Neuroanatomy of Attention Processes
However, a rightward bias observed for the spatial preparation task suggested that activation within the system showed specific variations for attributes of the attention task (Figure 6.16). Although the same areas are involved in both tasks, a right-side bias in the intraparietal sulcus (IPS) was observed for the spatial priming task and suggests that, within this network, task-related variations are present.
Tests of Cognitive Theory Based on Mapping of Neural Correlates
Functional Neuroanatomy of Working Memory
A Fixed- or Variable-Area Network
The N Back Task and a Test of a Cognitive Theory
Functional Neuroanatomy of Selective Attention
A Neurological Model of Cognitive Interference
Left and right lateral views and top views of averaged PET images are shown. Different colors reflect the importance of activation, with red areas being the most important.
Functional Neuroanatomy of Executive Processes
Separate or Combined Systems
In Experiment 1, a classic Stroop task was used, that is, incongruent words appear infrequently while congruent words appear frequently (A). Experiment 2 (B) used an inverse Stroop task, that is, congruent words are rare events and incongruent words occur frequently.
The Go-No Go Task
However, all areas activated during dual-task performance were also activated during component tasks; that is, there was no evidence for a separate executive system. An increase in activity in a given area during a dual task is associated with an additional load on the second task.
Integration of Temporal and Spatial Information to Map Executive Processes
The results showed that activated areas varied with the sensory modality of the component tasks as expected based on these domain-specific functional specializations. These results can be interpreted as generally consistent with the hypothesis that these executive processes may be implemented by interactions between distinct anatomical and functional systems involved in the performance of component tasks rather than by a specific area or areas dedicated to a modular executive system and separable.
Integration of ERP and fMRI
Event-Related fMRI
This is an active area of research, and new evidence from studies using techniques such as electrophysiological recordings and event-related fMRI to investigate within-system effects may be needed to elucidate the modulation of processes both within and across these systems. related fMRI offers an additional class of task designs that extend and extend investigations of cognitive processes. The event-related approach thus allows trials to be categorized post hoc based on the subject's behavior.
The Oddball Task
Event-related fMRI provides an additional class of task designs that extend and extend investigations of cognitive processes. Results replicated previous fMRI findings, indicating activation of extrastriate visual cortex in appreciating the illusory contours (perceived between the corner elements of Kanizsa squares) and the lack of this activation when viewing stimuli in which the corner elements were rotated from alignment.
The Functional Neuroanatomy of Very High Level Cognitive Processes
Clinical application of functional magnetic resonance imaging in presurgical identification of the central sulcus. Functional evaluation using magnetic resonance imaging of the visual cortex in patients with retrochiasmatic lesions.
Episodic Memory
There are a number of episodic memory fMRI probes, many of which are designed specifically to address or manipulate certain aspects of episodic memory processing; a sample problem is shown in table 7.1. In general, episodic memory is thought to be subserved by a broad network of brain regions, primarily involving prefrontal and medial temporal circuits, including the hippocampal formation (dentate gyrus; CA1, CA2 and CA3 fields; and subiculum), entorhinal cortex, perirhinal cortex, parahippocampal complex and amygdala .7-10 Several models of the neural basis of specific episodic memory processes have been proposed.
Age Related Changes in Episodic Memory
Young adults showed greater activation than older adults in the left anterior inferior temporal cortex (BA 20) (A). However, the older adults showed under-recruitment of the left anterior hippocampus compared to the young adults.
Episodic Memory in Alzheimer’s Disease and Related Conditions
Patients with greater hippocampal preservation showed greater activity in bilateral prefrontal regions. Overall, those participants who were diagnosed with AD showed reduced hippocampal activation during episodic memory processing compared to either of the other groups.
Semantic Memory
During successful encoding, the younger group showed significantly more activation of the left anterior medial temporal lobe than older adults with impaired memory, but did not differ significantly from older adults with normal memory. Grön et al.83 studied fMRI brain activity patterns in older adults presenting for medical evaluation for subjective memory problems for the first time.
Semantic Memory in Aging and Dementia
There were only small group differences in brain activity, with slightly but not significantly greater precentral activation in younger compared to older adults. Note the expanded spatial extent of activation in the patient group in the left frontal region.
Working Memory
Furthermore, the extensive spatial extent of frontal activation within the patient group was directly correlated with the degree of atrophy in that frontal region.72 This finding provided preliminary fMRI-based support for the compensatory recruitment hypothesis in semantic memory in AD, suggesting that increased brain activation may disease-related structural changes in the brain help compensate, although other reasons for the changes in brain activation are also possible. Smith and colleagues reported bilaterally reduced brain activation in inferotemporal areas during language tasks in individuals at risk for AD due to their family history and ApoE status.
Working Memory in Aging and Dementia
Rombouts and colleagues84 recently investigated the effect of cholinergic enhancement on brain activity during working memory in a preliminary sample of patients with mild AD. Overall, the results point to age-related changes in the activity of working memory circuits that are largely characterized by declines in prefrontal and hippocampal regions.
Conclusion
Some Proposed Clinical Applications of fMRI Language Mapping
Language research has not simply been an additional, trivial extension of the classical Wernicke-Broca neuroanatomical model of language. It is easy to see, given the difficulty of assimilating this knowledge base, how attempts at language design based on nineteenth-century brain models could easily go wrong, producing uninteresting and incomprehensible results.
Second, functional data can be easily registered with very high-resolution standard MRI images acquired at the same brain location, improving the ability to associate functional foci with specific anatomical structures. Third, activation procedures can be performed repeatedly in the same subject within and across scanning sessions, providing improved statistical power, measures of test-retest reliability, the ability to monitor changes in activation serially over time, and the potential to explore a range. of cognitive processes.
Presurgical Applications
Fourth, fMRI can be applied to MRI scanners already in place in many medical facilities, with the addition of relatively inexpensive fast-acquisition pulse sequences, auxiliary coil hardware, and specialized stimulus delivery and response recording systems.
Prediction of Outcome in Aphasia
Diagnosis
Monitoring Treatment Effects
Some Theoretical Principles
A basic assumption of language mapping is that activation tasks can be designed to make different demands on these processing subsystems. The use of such stimuli in the baseline condition could result in an unwanted subtraction (or partial subtraction) of language-related activation.
Survey of Language Activation Protocols
The main language zones in the brain are Broca's Area (left posterior inferior frontal gyrus) and Wernicke's Area (left posterior superior temporal gyrus). Decision-making tasks that require a decision based on the phonological structure of the stimulus (e.g., rhyme detection, judgment of the number of syllables), and orthographic decision tasks that require a decision based on the letters in the stimulus (e.g., matching uppercase and lowercase letters, letter identification).
Language Task: Passively Listening to Words or Sentences
Language Task: Passively Listening to Words
Language Task: Word Generation
Language Task: Visual Object Naming
Language Task: Semantic Decision
Language Task: Sentence or Word Reading
