Showing papers in "Human Brain Mapping in 1995"

Spatial registration and normalization of images

Abstract: This paper concerns the spatial and intensity transformations that map one image onto another. We present a general technique that facilitates nonlinear spatial (stereotactic) normalization and image realignment. This technique minimizes the sum of squares between two images following nonlinear spatial deformations and transformations of the voxel (intensity) values. The spatial and intensity transformations are obtained simultaneously, and explicitly, using a least squares solution and a series of linearising devices. The approach is completely noninteractive (automatic), nonlinear, and noniterative. It can be applied in any number of dimensions. Various applications are considered, including the realignment of functional magnetic resonance imaging (MRI) time-series, the linear (affine) and nonlinear spatial normalization of positron emission tomography (PET) and structural MRI images, the coregistration of PET to structural MRI, and, implicitly, the conjoining of PET and MRI to obtain high resolution functional images. © 1995 Wiley-Liss, Inc.

Automatic 3-D model-based neuroanatomical segmentation

Abstract: Explicit segmentation is required for many forms of quantitative neuroanatomic analysis However, manual methods are time-consuming and subject to errors in both accuracy and reproducibility (precision) A 3-D model-based segmentation method is presented in this paper for the completely automatic identification and delineation of gross anatomical structures of the human brain based on their appearance in magnetic resonance images (MRI) The approach depends on a general, iterative, hierarchical non-linear registration procedure and a 3-D digital model of human brain anatomy that contains both volumetric intensity-based data and a geometric atlas Here, the traditional segmentation strategy is inverted: instead of matching geometric contours from and idealized atlas directly to the MRI data, segmentation is achieved by identifying the non-linear spatial transformation that best maps corresponding intensity-based features between a model image and a new MRI brain volume When completed, atlas contours defined on the model image are mapped through the same transformation to segment and label individual structures in the new data set Using manually segmented sturcture boundaries for comparison, measures of volumetric difference and volumetric overlap were less than 2% and better than 97% for realistic brain phantom data, and less than 10% and better than 85%, respectively, for human MRI data This compares favorably to intra-observer variability estimates of 49% and 87%, respectively The procedure performs well, is objective and its implementation robust The procedure requires no manual intervention, and is thus applicable to studies of large numbers of subjects The general method for non-linear image matching is also useful for non-linear mapping of brain data sets into stereotaxic space if the target volume is already in stereotaxic space © 1995 Wiley-Liss, Inc

Regional cerebral blood flow during object naming and word reading.

Abstract: By several accounts, reading single words may be accomplished either by sequentially transcribing orthographic units into their corresponding sounds (an indirect route), or by directly associating a visual word form to the semantic or articulatory representation (a direct route) By contrast, the similar task of naming objects must rely only on a direct route, since objects cannot be “sounded out” To study the localization of cognitive processes specific to reading, we used positron emission tomography (PET) to measure regional cerebral blood flow while subjects named words and pictures of objects silently or aloud Group averages of blood flow changes were obtained for experimental vs control tasks Object and word presentations elicited similar blood flow increases in extra-striate visual cortices compared with a visual noise control Silent reading invoked a neural network very similar to that seen when subjects named objects silently, consistent with a “direct” route Naming objects aloud produced the addition of motor output regions to this network By contrast, oral reading produced a markedly different pattern of activated regions, suggesting reliance on a separate phonological pathway These results provide support for the dual coding hypothesis in reading and challenge the use of strict hierarchical models of cognitive operations in PET activation studies © 1995 Wiley-Liss, Inc

Clustered pixels analysis for functional MRI activation studies of the human brain

Abstract: Conventional t-statistics and cross-correlation coefficients are commonly used for analysis of functional magnetic resonance images. The sensitivity of these statistics is usually low because severe Bonferroni-type corrections are required for multiple statistical comparisons to minimize the false-positive error. In the human brain, most functional areas are larger in size than a single image pixel, and coactivation of numerous contiguous pixels is expected. The probability of occurrence of clusters due to random noise is small and can be modeled. Cluster size and intensity thresholding can be used to assess statistical significance. Previous cluster analysis strategies used Gaussian models, working best with low spatial resolution images (e.g., positron emission tomography). We present a new cluster analysis model applicable to data with little or even no covariance between adjacent pixels. Computer simulations and phantom experiments were used to verify this strategy. Our new method is substantially more sensitive than both the conventional intensity-only thresholding (IOT) method and the previous cluster method for signal change less than 6%, with maximum significant enhancement in sensitivity of 12.8 and 3.8 times, respectively. The results obtained from normal volunteers with visual stimulation further confirm the effectiveness of our new approach and show an average increase in detected activation area of 3.1 times over the IOT method and of 1.6 times over the previous cluster method using the new approach. ©1996 Wiley-Liss, Inc.

A modality-independent approach to spatial normalization of tomographic images of the human brain

Abstract: A modality-independent approch for interactive spatial normalization of tomographic images of the human brain is described and its performance evaluated. Spatial normalization is accomplished using a nine-parameter affine transformation to interactively align and adjust the shape of a subject brain to the reference brain detailed in the 1988 atlas of Talairach et al. A user-friendly software application was developed using the X-windows Motif environment to guide the user through this process. This software supports data types from a wide variety of tomographic imagers and produces output in spatially concise formats. The parameters used for spatial alignment and shape normalization are presented and methods to apply them discussed. Where normalization parameters cannot be obtained directly from the image, as with positron emission tomography (PET), methods for estimating them are given. Evaluation of a new four-landmark method to fit the AC-PC line in 16 magnetic resonance imaging (MRI) studies indicated an average difference assessed as the distance between the true and fitted AC-PC line at four locations of 0.82 mm when using a 2-D weighted fit. The same landmarks were evaluated using lower spatial resolution PET-like images simulated from the 16 MRI studies. The difference between the PET and MR image volumes following alignment was minimal, with mean rotational differences of less than 0.2 deg and mean translational differences of generally less than 2 mm. Spatial normalization is illustrated for single photon emission computed tomography (SPECT), X-ray computed tomography (CT), PET, and MR image volumes. Modality-independent spatial normalization can be consistently and reliably performed with the methods and software presented. © 1995 Wiley-Liss, Inc.

A PET Investigation of implicit and explicit sequence learning

Abstract: The purpose of this study was to determine the mediating neuroanatomy of implicit and explicit sequence learning using a modified version of the serial reaction time (SRT) paradigm. Subjects were seven healthy, right-handed adults (three male, four female, mean age 26.7, range 18–43 yr). PET data were acquired via the oxygen-15-labeled-carbon dioxide inhalation method while subjects performed the SRT. Subjects were scanned during two blocks each of (1) no sequence (Random), (2) single-blind, 12-item sequence (Implicit), and (3) unblinded, same sequence (Explicit). Whole-brain-normalized images reflecting relative regional cerebral blood flow (rCBF) were transformed to Talairach space, and statistical parametric maps (SPMs) of z-scores were generated for comparisons of interest. The threshold for significant activation was defined as z-score ≥ 3.00. Behavioral data demonstrated significant learning (P < .05) for Implicit and Explicit conditions. Tests of explicit knowledge reflected non-significant explicit contamination during the Implicit condition. Foci of significant activation in the Implicit condition were found in right ventral premotor cortex, right ventral caudate/nucleus accumbens, right thalamus, and bilateral area 19; activation in the Explicit condition included primary visual cortex, peri-sylvian cortex, and cerebellar vermis. Activations in visual and language areas during the Explicit condition may reflect conscious learning strategies including covert verbal rehearsal and visual imagery. Right-sided premotor, striatal, and thalamic activations support the notion that implicit sequence learning is mediated by cortico-striatal pathways, preferentially within the right hemisphere. © 1996 Wiley-Liss, Inc.

Hemispheric differences in neural systems for face working memory : a PET-rCBF study

Abstract: Neural systems that participate in working memory for faces were investigated in an experiment designed to distinguish face perception areas from working memory areas. Regional cerebral blood flow (rCBF) was measured using positron emission tomography (PET) while subjects performed a sensorimotor control task, a face perception control task, and five working memory tasks with parametrically varied retention intervals, ranging from 1 to 21 sec. Striate and ventral occipitotemporal extrastriate areas demonstrated a simple negative correlation between rCBF and retention delay, indicating that these areas participate principally in perceptual operations performed during visual stimulation. By contrast, right and left frontal areas demonstrated rCBF increases that were significantly more sustained across delays than were increases in ventral extrastriate areas, but the relation between rCBF and retention interval differed significantly by hemisphere. Whereas right frontal rCBF showed a nonsignificant tendency to diminish at longer delays, left inferior frontal, middle frontal, and anterior cingulate cortex, as well as left parietal and inferior temporal cortex, demonstrated their largest rCBF increases at the longest delays. These results indicate that right frontal and left frontal, parietal, and temporal areas all participate in face working memory, but that left hemisphere areas are associated with a more durable working memory representation or strategy that subjects rely on increasingly with longer retention intervals. One possible explanation for this hemispheric difference is that left hemisphere activity is associated with a face representation that embodies the result of more analysis and elaboration, whereas right frontal activity is associated with a simpler, icon-like image of a face that is harder to maintain in working memory. © 1995 Wiley-Liss, Inc.

Motion detection and correction in functional MR imaging

Abstract: Subject motion present during the time course of functional activation studies is a pervasive problem in mapping the spatial and temporal characteristics of brain activity. In functional MRI (fMRI) studies, the observed signal changes are small. Therefore, it is crucial to reduce the effect of subject motion during the acquisition of image data in order to differentiate true brain activation from artifactual signal changes due to subject motion. We have adapted a technique for automatic motion detection and correction which is based on the ratio-variance minimization algorithm to the fMRI subject motion problem. This method was used for retrospective correction of subject motion in the acquired data and resulted in improved functional maps. In this paper we have designed and applied a series of tests to evaluate the performance of this technique which span the classes of image characteristics common to fMRI. These areas include tests of the accuracy and range of motion as well as measurement of the effect of image signal to noise ratio, focal activation, image resolution, and image coverage on the motion detection system. Also, we have evaluated the amount of residual motion remaining after motion correction, and the ability of this technique to reduce the motion-induced artifacts and restore regions of activation lost due to subject motion. In summary, this method performed well in the range of image characteristics common for fMRI experiments, reducing motion to under 0.5 mm, and removed significant motion-induced artifacts while restoring true regions of activation. Motion correction is expected to become a routine requirement in the analysis of fMRI experiments. © 1995 Wiley-Liss, Inc.

A functional magnetic resonance imaging study of cortical regions associated with motor task execution and motor ideation in humans

Abstract: Although motor performance may be enhanced through mental practice, the neurophysiological substrate of mental stimulation (ideation) of a motor task is not well established. We used blood oxygen level-dependent contrast echo planar imaging at 1.5 T to identify regions of increased neural activity during the performance and ideation of a motor task. Five subjects performed a sequential finger-to-thumb opposition task and also imagined themselves performing the task in the absence of actual muscle movement. In all subjects, the left primary sensorimotor cortex showed more activation with actual movement than with motor ideation, but two subjects had significant activation with motor ideation. The left premotor area showed comparable activation with both actual and imagined performance in three subjects. These findings support the involvement of the primary motor area as well as the premotor area in motor ideation. © 1995 Wiley-Liss, Inc.

Simultaneous functional magnetic resonance imaging and electrophysiological recording

Abstract: The purpose of this study was to develop a method for obtaining simultaneous electrophysiological and functional magnetic resonance imaging data. Using phantom experiments and tests on several of the investigators, a method for obtaining simultaneous electrophysiological and fMRI data was developed and then tested in three volunteers including two task activation experiments. It was then applied in a sleep experiment (n = 12). Current limiting resistance and low-pass filtering were added to the electrophysiological circuit. Potential high frequency current loops were avoided in the electrical layout near the subject. MRI was performed at 1.5 T using conventional and echo planar imaging sequences. There was no evidence of subject injury. Expected correlations were observed between the electrophysiological and fMRI data in the task activation experiments. The fMRI data were not significantly degraded by the electrophysiological apparatus. Alpha waves were detected from within the magnet in seven of the 15 experimental sessions. There was degradation of the electrophysiological data due to ballistocardiographic artifacts (pulsatile whole body motion time-locked to cardiac activity) which varied between subjects from being minimal to becoming large enough to make detection of alpha waves difficult. We concluded that simultaneous fMRI and electrophysiological recording is possible with minor modifications of standard electrophysiological equipment. Our initial results suggest this can be done safely and without compromise of the fMRI data. The usefulness of this technique for studies of such things as sleep and epilepsy is promising. Applications requiring higher precision electrophysiological data, such as evoked response measurements, may require modifications based on ballistocardiographic effects. © 1995 Wiley-Liss, Inc.

Characterising the complexity of neuronal interactions

Abstract: This work addresses the complexity of neuronal interactions, the nature of this complexity and how it can be characterised in real neurophysiological processes. A measure of complexity has been introduced recently (Tononi et al. [1994]: Proc Natl Acad Sci USA 91:5033–5037) that is sensitive to the joint constraints imposed by two principles of brain organisation: functional segregation and functional integration. Functional segregation implies that the dynamics of a cortical area should reflect the multidimensional attributes for which that area is specialised (in other words, regional dynamics should show a relatively high entropy). Conversely, functional integration implies a distributed and divergent influence of every cortical area on the remaining areas (i.e., the overall dynamics should show a low entropy). Our measure is based on the profile of entropies of different sized regions of the brain. Complexity is high when smaller regions have (on average) a relatively high entropy with respect to the entropy of the whole system. This measure is equivalent to the (average) mutual information between all small regions and the rest of the system in question. We have applied this measure to nonlinear simulations and to neurophysiological data obtained with fMRI during photic stimulation. Because patterns of activity in the brain are intermediate between a state of incoherence, with regionally specific dynamics and a state of global coherence, we predicted that simulated nonlinear processes with similar characteristics would have a high complexity. In the language of nonlinear dynamics we hypothesised that the greatest complexity would be found somewhere between high-dimensional, chaotic behaviour and low-dimensional, orderly behaviour. Equivalently, using the metaphor of loosely coupled oscillators, we predicted that complexity would be highest in the domain between asynchronous oscillations and global synchrony. This hypothesis was confirmed using nonlinear neuronal simulations. In addition, we demonstrate that the complexity of neurophysiological data is easily measured and can show a significant complexity when compared to suitable control processes. © 1996 Wiley-Liss, Inc.

Topography of diprenorphine binding in human cingulate gyrus and adjacent cortex derived from coregistered PET and MR images

Abstract: Positron emission tomography (PET) studies of ligand binding lack sufficient anatomical detail to evaluate topographical variations in binding within each of the lobes of the human cerebral cortex. This study employed PET to localize [11C]diprenorphine binding to opioid receptors and magnetic resonance (MR) imaging for defining medial surface structures. Continuous arterial sampling for metabolite corrected [11C]diprenorphine levels and CNS blood flow were used to model the volume of distribution (VDtot) of binding for three subjects. The PET images of VDtot were coregistered to the MR images for each case and 37 regions of interest were used to calculate VDtot. The VDtot was averaged for the three cases and coregistered with an MR reconstruction of the medial surface and plotted onto a flat map of this region. The average VDtot showed that binding was highest in anterior cingulate, rostral cingulofrontal transition, and prefrontal cortices, while binding in caudal parts of anterior cingulate and superior frontal cortices, and posterior cingulate cortex varied from high to low. Three statistical levels of binding were defined in relation to the high binding in perigenual area 24: high and equal to area 24, moderate and significantly lower than area 24 (p <0.01), or low (p <0.001). These levels of binding were plotted onto an unfolded map of the medial cortex. The VDtot was high in rostral cortex, and a strip of high binding continued caudally on the dorsal lip of the cingulate gyrus. There were patches of high binding in cinguloparietal transition, posterior parietal, and supplementary motor cortices. Four regions had low binding: (1) areas 29 and 30 in the callosal sulcus, (2) fundus of the cingulate sulcus likely involving the cingulate motor areas, (3) fundus of the superior cingulate sulcus involving two divisions of supplementary motor cortex, and (4) sensorimotor cortex on the paracentral lobule. Variations in binding may reflect functional specializations such as low binding in the cingulate motor and visuospatial areas and high levels in areas involved in processing information with affective content. The higher sensitivity of three-dimensional scanning and coregistration of PET and MR images makes it feasible to analyze single individuals and, by performing pixel-by-pixel spectral analysis and generation of parametric maps, statistical analyses are possible. © 1995 Wiley-Liss, Inc.

Identifying objects at different levels of hierarchy: A positron emission tomography study

Abstract: Subjects decided whether names were appropriate for accompanying pictures while their local cerebral blood flow was monitored using positron emission tomography (PET); in one condition, the names were at the “entry” level (i.e., the level spontaneously named, as in “bird” for a robin), in another condition they were at a superordinate level (e.g., “animal”), and in another they were at a subordinate level (e.g., “robin”). The results indicated that different processes are used to evaluate terms at the different levels of analysis. Specifically, there was evidence that memory search is used to evaluate superordinates, but one must collect additional perceptual information to evaluate subordinates. In addition, in another condition the subjects saw written words that named the entry-level term and decided whether the object could be named by superordinate terms. Similar, but not identical, activation was observed as was found when subjects evaluated superordinate terms for pictures. © 1995 Wiley-Liss, Inc.

In vivo imaging of neuromodulation using positron emission tomography: Optimal ligand characteristics and task length for detection of activation

Abstract: Considerable evidence suggests that cognitive state affects local levels of neurotransmitter in the brain. We introduce a compartment model of neuroreceptor ligand kinetics to describe the effect of change in cognitive state on positron emission tomography (PET) signal dynamics. The model is used to establish optimal experimental conditions, timing of activation, and ligand characteristics, for detecting cognitive activation. The model, which follows free and bound endogenous neurotransmitter, describes the PET curve predicted for a single injection of radioligand in the presence or absence of activation. Activation was conceptualized as the performance of a task that raises the level of neurotransmitter that competes for receptor sites with the radioligand. Simulating the dopamine system, for example, required making assumptions regarding the kinetic rate constants for binding/dissociation of endogenous dopamine to/from the receptor and dopamine concentrations in the synapse. Simulations suggest that activation of dopamine should be detectable with PET and the D2 receptor ligand (llC)raclopride, although this ligand might not be optimal. Aspects of experimental design can be modified to optimize the likelihood of detecting neurotransmitter changes. The ideal radioligand for these studies should bind irreversibly to its receptor. Furthermore, the task should commence at injection time and last for at least 7 minutes. Optimal task duration depends on the dynamics of free radioligand in the tissue and can be determined via model simulations for any well-characterized receptor ligand. Flow effects were shown to be distinguishable from those of neurotransmitter activation. General principles regarding desirable ligand characteristics and activation timing held for both the D2 receptor and the dopamine transporter site. o 1995 Wiley-Liss, Inc.

In vivo imaging of neuromodulatory synaptic transmission using PET: A review of relevant neurophysiology

Abstract: Recent data from positron emission tomography (PET) imaging studies suggest the possibility of studying synaptic transmission in vivo in humans. The approach will require a synthesis of two established techniques: brain activation studies (conventionally performed by measuring regional cerebral blood flow or metabolism) and neurotransmitter receptor imaging (using radiolabelled ligands that bind to specific neuroreceptors). By comparing neuroreceptor binding in subjects at rest and while performing an activation task, it may be possible to determine whether a particular neurotransmitter is involved in performance of the task. The underlying principle is that endogenous neurotransmitter competes with the injected radioligand for the same receptors, thereby inhibiting ligand binding. This effect will be even more pronounced during activation, as the synaptic concentration of transmitter rises. Thus, activation of a specific neurotransmitter will be detected as a decrease in specific binding of the radioligand. In this paper we review neurophysiological and biochemical literature to estimate the endogenous neurotransmitter concentration changes that will be expected to occur during an activation task, using the dopamine system as an example. We calculate that the average synaptic dopamine concentration is =lo0 nM and that it approximately doubles during activation. This, along with consideration of the concentration of radioligand and affinities of the ligand and dopamine for dopamine receptors, suggests that physiological activation of a specific neurotransmitter system is likely to be detectable with PET. D 1995 wiley-Liss, inc.

Regional cerebral activity associated with the incidental processing of pseudo‐words

Abstract: In order to study brain activity associated with “incidental” cognitive processing, regional cerebral blood flow (rCBF) was measured in six volunteers while they monitored a sequence of pseudo-words (e.g., FLOPE) for the rare occasions when the letters were displayed in blue rather than white. In the control condition, the same pseudo-word was presented repeatedly. In one experimental condition all 60 pseudo-words were different, while in the other there were 18 repetitions. Although it was not necessary to “read” the pseudo-words to perform the monitoring task, subsequent forced choice recognition memory for these stimuli was significantly greater than chance. Furthermore, there were significant differences in blood flow between the three conditions. When different pseudo-words were presented there was significantly greater activity in brain areas concerned with shape and object identity (extrastriate cortex bilaterally), with visual word form (left inferior temporal gyrus), and with articulatory word form (Broca's area) even though none of this information about the pseudo-words was needed for performance of the monitoring task. In the condition in which some of the words were repeated, there was significantly reduced activity in the right lingual gyrus. This area may therefore be a possible anatomical locus for repetition priming with verbal stimuli. These results indicate the importance of taking into account incidental processing when designing tasks for functional imaging experiments. © 1995 Wiley-Liss, Inc.

High-resolution functional magnetic resonance imaging of cortical activation during tactile exploration

Abstract: Cortical activation during tactile exploration of macrogeometric objects was investigated in six healthy individuals with the use of magnetic resonance imaging (MRI) sensitized to changes in cerebral blood oxygenation. Dynamic measurement of task-related signal alterations were performed at 2.0 T using a rapid gradient-echo MRI sequence (TR/TE=63/30 ms, flip angle 10°, measuring time 6 s) at high spatial resolution (0.8 × 1.6 mm2). Four contiguous sections (thickness 4 mm) parallel to the bicommissural plane covered the hand area of the primary sensorimotor cortex (M1, S1), the supplementary motor area (SMA), premotor areas (PMA), and superior parts of the parietal cortex (PC). Task-related activation was determined by correlating signal intensity time courses with the stimulus protocol on a pixel-by-pixel basis. In contrast to predominantly contralateral M1 activation, effects in the hand area of S1 were not restricted to the contralateral side but were equally present in the posterior section of ispilateral S1. Furthermore, bilateral responses were encountered in SMA and PC, while observations within PMA remained inconsistent. These findings in single subjects readily demonstrate a highly resolved and interindividually reproducible pattern of cortical activation in relation to exploratory finger movements and associated intergration of somatosensory information. © 1995 Wiley-Liss, Inc.

Cortical surface modeling reveals gross morphometric correlates of individual differences

Abstract: Advances in human neurobiology are now made possible through methods which combine structural magnetic resonance imaging (MRI), three-dimensional reconstruction, and statistical analysis. MRI-based reconstruction enables the in vivo quantification of regional cortical surface area (rCSA) while inter-group comparisons uncover relationships of cortical morphometry with genotype, sex, and developmental abnormalities. In studies on normals we have found strong associations between the rCSA of monozygotic twins as compared to unrelated pairings. Further analysis of this data uncovered significant differences between the male and female twins in left hemisphere rCSA. When these methods were applied to brains of dyslexic subjects and controls, we identified a pattern of differences involving all major subdivisions of both hemispheres. Taken together, these techniques can illuminate structure- function issues in both normal and diseased brains. o 1996 Wiley-Liss, Inc.

Cortical fields participating in spatial frequency and orientation discrimination: Functinal anatomy by positron emission tomography

Abstract: With the purpose of localising those anatomical structures participating in the discrimination of spatial frequencies and orientations of gratings, we measured regional cerebral blood flow (rCBF) changes with positron emission tomography (PET) and 15O-butanol as tracer in ten healthy young male volunteers. The subjects performed two-alterative forced-choice discriminations of pairs of squarewave gratings regarding their spatial frequencies or orientations (spatial frequency and orientation tasks) or pairs of a grating and a two-dimensional random noise pattern regarding the presence or absence of grating pattern (reference task). In both the spatial frequency and orientation discrimination tasks a widely distributed network of functional fields is activated in the occipital, temporal, parietal, and frontal cortices and in the cerebellum. Spatial frequency discrimination required the activation of more cortical fields than orientation discrimination, and whereas the total volume of activated fields in temporal and frontal lobes were similar in the two tasks, the volumes of activated fields in the occipital lobes as well as in the parietal lobes were about two and a half times larger in spatial frequency discrimination than in orientation discrimination. The two networks of cortical fields were partially overlapping in the two tasks. The findings indicate that the discrimination of spatial frequency and orientation signals engages functional networks of cortical fields widely distributed in the human brain. Whereas both the occipito-temporal and occipito-parietal visual pathways are involved in both tasks, the processing and analysis of spatial frequency information activates occipital and parietal lobe regions more extensively than those of orientation information. © 1995 Wiley-Liss, Inc.

Regulation of rCBF by diffusible signals: An analysis of constraints on diffusion and elimination

Abstract: Local changes in cerebral hemodynamics are observed within a few hundred milliseconds of changes in neural activity. If hemodynamic responses are mediated by passive diffusion of a spatial signal (from the site of neural activity to the microvessels) then the dynamics of the response suggest a lower limit on the signal's apparent diffusion and elimination. The aim of this work was to estimate these limits and narrow the field of possible candidate substances. A simple biophysical simulation was used to examine how the time course of concentration changes in a spatial signal, at the site of action (microvessels), depends on key diffusion parameters (source geometry, apparent diffusion and elimination half-life). The simulations suggested 1) that the rise in signal concentration is mostly a function of source geometry and diffusion. Conversely falls in concentration depend on elimination and 2) even when sources are very sparsely distributed Nitric Oxide would have a sufficiently fast diffusion and elimination to signal the early components of activity-dependent hemody- namic response by passive diffusion.

Frontal component of the somatosensory evoked potential

Abstract: Electric stimulation of the median nerve at the wrist evokes a series of electric potentials that can be recorded from the scalp or directly from the cortex. These somatosensory evoked potentials (SEP) include a parietal negativity with a maximum 20 ms after the stimulus, which originates in the somatosensory cortex, probably area 3b (Allison et al. [1991a], Brain 114:2465–2503 and Desmedt et al. [1987], Electroenceph Clin Neurophysiol 68:1–19). Thirty milliseconds after the stimulus, a negative potential (N30) occurs at frontal recording sites. Recently it was observed that the amplitude of this potential is altered in patients with dystonia, Parkinson's disease, and Huntington's chorea. It has been argued that the N30 potential stems from cortical areas other than the somatosensory cortex, for example, the supplementary motor area. We used multichannel recordings to investigate the scalp distribution of the N20 and the N30 potentials in healthy subjects. We found that the N20 as well as the N30 potentials were accompanied by a corresponding positivity at frontal and parietal recording sites, respectively. The N20/P20 and the N30/P30 potential fields had a mirrorlike appearance, and both showed a polarity reversal near the central sulcus. This and the results of correlation analyses led us to the conclusion that the N30 generator is located near the central sulcus. © 1995 Wiley-Liss, Inc.