Biomedical subjects

Ravi S Menon

Publications and source records attributed to Ravi S Menon.

49 records · Page 3Linked to original sources

PubMed2002

Brain activation during script-driven imagery induced dissociative responses in PTSD: a functional magnetic resonance imaging investigation.

BACKGROUND: The goal of this study was to examine the neuronal circuitry underlying dissociative responses to traumatic script-driven imagery in sexual-abuse-related posttraumatic stress disorder (PTSD). Pilot studies in our laboratory have shown that PTSD patients had very different responses to traumatic script-driven imagery. Approximately 70% of patients relived their traumatic experience and showed an increase in heart rate while recalling the traumatic memory. The other 30% of patients had a dissociative response with no concomitant increase in heart rate. This article focuses on the latter group. METHODS: The neuronal circuitry underlying dissociative responses in PTSD was studied using the traumatic script-driven symptom provocation paradigm adapted to functional magnetic resonance imaging (fMRI) at a 4 Tesla field strength in 7 subjects with sexual-abuse-related PTSD and 10 control subjects. RESULTS: Compared with control subjects, PTSD patients in a dissociative state showed more activation in the superior and middle temporal gyri (BA 38), the inferior frontal gyrus (BA 47), the occipital lobe (BA 19), the parietal lobe (BA 7), the medial frontal gyrus (BA 10), the medial cortex (BA 9), and the anterior cingulate gyrus (BA 24 and 32). CONCLUSIONS: These findings suggest that prefrontal and limbic structures underlie dissociative responses in PTSD. Differences observed clinically, psychophysiologically, and neurobiologically between patients who respond to traumatic script-driven imagery with dissociative versus nondissociative responses may suggest different neuronal mechanisms underlying these two distinct reactions.

Adult↗

PubMed2002

Differential effects of viewpoint on object-driven activation in dorsal and ventral streams.

Using fMRI, we showed that an area in the ventral temporo-occipital cortex (area vTO), which is part of the human homolog of the ventral stream of visual processing, exhibited priming for both identical and depth-rotated images of objects. This pattern of activation in area vTO corresponded to performance in a behavioral matching task. An area in the caudal part of the intraparietal sulcus (area cIPS) also showed priming, but only with identical images of objects. This dorsal-stream area treated rotated images as new objects. The difference in the pattern of priming-related activation in the two areas may reflect the respective roles of the ventral and dorsal streams in object recognition and object-directed action.

Adult↗

PubMed2002

Influence of hypoxia on wavelength dependence of differential pathlength and near-infrared quantification.

Continuous wave near-infrared spectroscopy (NIRS) measurements of cardiac correlated changes in attenuation in the adult human head were computed using a Fourier analysis technique that eliminates the positive error bias associated with the magnitude of the Fourier coefficient. These attenuation changes were used to determine wavelength dependence of differential pathlength, DP(lambda), at four stages during progressive hypoxia (21, 17, 13 and 9% FIO2) in normal volunteers. The effects of incorporating DP(lambda) into NIRS algorithms to compute relative concentration changes and absolute concentration of oxyhaemoglobin and deoxyhaemoglobin are discussed. Because variations in DP(lambda) are restricted to wavelengths below 780 nm, absolute concentration calculations are influenced by hypoxia-induced changes while relative concentrations are unaffected. However, even accounting for changes in DP(lambda) did not allow computation of physiologically reasonable absolute concentrations of the haemoglobin species.

Adult↗

PubMed2002

Imaging attentional modulation of pain in the periaqueductal gray in humans.

Pain is an unpleasant sensory and emotional experience usually triggered by stimulation of peripheral nerves and often associated with actual or potential tissue damage. It is well known that pain perception for patients and normal subjects can be modulated by psychological factors, such as attention, stress, and arousal. Our understanding of how this modulation occurs at a neuroanatomical level is poor. Here we neuroanatomically defined a key area in the network of brain regions active in response to pain that is modulated by attention to the painful stimulus. High-resolution functional magnetic resonance imaging was used to define brain activation to painful heat stimulation applied to the hand of nine normal subjects within the periaqueductal gray region. Subjects were asked to either focus on or distract themselves from the painful stimuli, which were cued using colored lights. During the distraction condition, subjects rated the pain intensity as significantly lower compared with when they attended to the stimulus. Activation in the periaqueductal gray was significantly increased during the distraction condition, and the total increase in activation was predictive of changes in perceived intensity. This provides direct evidence supporting the notion that the periaqueductal gray is a site for higher cortical control of pain modulation in humans.

Adult↗

PubMed2002

Postacquisition suppression of large-vessel BOLD signals in high-resolution fMRI.

Large-vessel BOLD contamination is a serious impediment to localization of neural activity in high-resolution fMRI studies. A new method is presented which estimates and removes the fraction of BOLD signal that arises from oriented vessels, such as cerebral and pial veins in a voxel, by measuring their influence on the phase angle of the complex valued fMRI time series. A maximum likelihood estimator based on a linear least-squares fit of the BOLD signal phase to the BOLD signal magnitude in a voxel is shown to efficiently suppress the BOLD effect from these larger veins, whose activation is not well colocalized with the neural response. In high-resolution in vivo fMRI data at 4 T, it is estimated that the method is sensitive to the phase changes in the cerebral, larger intracortical, and pial veins. The technique requires no special pulse sequence modifications or acquisition strategies, and is computationally fast and intrinsically robust.

Brain↗

PubMed2002

In vivo brain (31)P-MRS: measuring the phospholipid resonances at 4 Tesla from small voxels.

An optimized phosphorous ((31)P) three-dimensional chemical-shift imaging (3D-CSI) protocol was developed at 4 T to study the phospholipid metabolism from discrete regions in the human brain without the need for (1)H-decoupling or nuclear Overhauser enhancement (NOE). In this study, a spherically bound, weighted average, random point omission 3D-CSI technique was developed and tested, based on methods proposed in the literature. The technique yields a significant (p < 0.001, two-tailed, 5% confidence level) increase in signal-to-noise (SNR) efficiency over conventional 3D-CSI (phantom 32%), without an increase in voxel bleedthrough. An automated time-domain fitting procedure utilizing prior spectral knowledge quantified the individual brain phospholipid metabolites from 15 cm(3) effective (8.0 cm(3) nominal) volumes from the left/right-parieto-occipital cortex and left/right thalamus in 10 normal volunteers. Individual constituents from the phosphomonoester (PME) region; phosphoethanolamine (PEth), phosphocholine (PCh) and the phosphodiester (PDE) region; glycerophosphoethanolamine (GPEth), glycerophosphocholine (GPCh) and membrane phospholipids (MP) were separately quantified to assess the precision of our method at 4 T against previous (1)H-decoupled (31)P-MRS brain studies at lower fields and much larger voxels. Derived concentrations (mM/l tissue) for PEth, PCh, GPEth, GPCh and MP in the left-parieto-occipital cortex were 0.81 +/- 0.21, 0.46 +/- 0.14, 0.74 +/- 0.30, 1.15 +/- 0.43 and 1.54 +/- 0.95 mM, respectively, and 0.94 +/- 0.16, 0.46 +/- 0.17, 0.83 +/- 0.22, 1.14 +/- 0.40 and 1.26 +/- 0.78 mM for the right parieto-occipital cortex. Derived concentrations (mM/l tissue) for PEth, PCh, GPEth, GPCh and MP in the left-thalamus were 0.69 +/- 0.18, 0.42 +/- 0.16, 0.63 +/- 0.20, 1.05 +/- 0.42 and 0.93 +/- 0.56 mM, respectively, and 0.68 +/- 0.24, 0.34 +/- 0.18, 0.60 +/- 0.23, 1.09 +/- 0.36 and 0.74 +/- 0.48 mM for the right-thalamus. This is the first study to our knowledge that has been able to quantify each of these individual phospholipid metabolites from such small voxels in the brain within a clinically reasonable scan time and without (1)H-decoupling or NOE.

Adult↗

PubMed2002

Noise reduction in BOLD-based fMRI using component analysis.

Principle Component Analysis (PCA) and Independent Component Analysis (ICA) were used to decompose the fMRI time series signal and separate the BOLD signal change from the structured and random noise. Rather than using component analysis to identify spatial patterns of activation and noise, the approach we took was to identify PCA or ICA components contributing primarily to the noise. These noise components were identified using an unsupervised algorithm that examines the Fourier decomposition of each component time series. Noise components were then removed before subsequent reconstruction of the time series data. The BOLD contrast sensitivity (CS(BOLD)), defined as the ability to detect a BOLD signal change in the presence of physiological and scanner noise, was then calculated for all voxels. There was an increase in CS(BOLD) values of activated voxels after noise reduction as a result of decreased image-to-image variability in the time series of each voxel. A comparison of PCA and ICA revealed significant differences in their treatment of both structured and random noise. ICA proved better for isolation and removal of structured noise, while PCA was superior for isolation and removal of random noise. This provides a framework for using and evaluating component analysis techniques for noise reduction in fMRI.

Adult↗

PubMed2002

Haptic study of three-dimensional objects activates extrastriate visual areas.

In humans and many other primates, the visual system plays the major role in object recognition. But objects can also be recognized through haptic exploration, which uses our sense of touch. Nonetheless, it has been argued that the haptic system makes use of 'visual' processing to construct a representation of the object. To investigate possible interactions between the visual and haptic systems, we used functional magnetic resonance imaging to measure the effects of cross-modal haptic-to-visual priming on brain activation. Subjects studied three-dimensional novel clay objects either visually or haptically before entering the scanner. During scanning, subjects viewed visually primed, haptically primed, and non-primed objects. They also haptically explored non-primed objects. Visual and haptic exploration of non-primed objects produced significant activation in several brain regions, and produced overlapping activation in the middle occipital area (MO). Viewing visually and haptically primed objects produced more activation than viewing non-primed objects in both area MO and the lateral occipital area (LO). In summary, haptic exploration of novel three-dimensional objects produced activation, not only in somatosensory cortex, but also in areas of the occipital cortex associated with visual processing. Furthermore, previous haptic experience with these objects enhanced activation in visual areas when these same objects were subsequently viewed. Taken together, these results suggest that the object-representation systems of the ventral visual pathway are exploited for haptic object perception.

Adult↗

PubMed2002

Human fMRI evidence for the neural correlates of preparatory set.

We used functional magnetic resonance imaging (fMRI) to study readiness and intention signals in frontal and parietal areas that have been implicated in planning saccadic eye movements-the frontal eye fields (FEF) and intraparietal sulcus (IPS). To track fMRI signal changes correlated with readiness to act, we used an event-related design with variable gap periods between disappearance of the fixation point and appearance of the target. To track changes associated with intention, subjects were instructed before the gap period to make either a pro-saccade (look at target) or an anti-saccade (look away from target). FEF activation increased during the gap period and was higher for anti- than for pro-saccade trials. No signal increases were observed during the gap period in the IPS. Our findings suggest that within the frontoparietal networks that control saccade generation, the human FEF, but not the IPS, is critically involved in preparatory set, coding both the readiness and intention to perform a particular movement.

Brain Mapping↗

PubMed2002

High resolution fMRI of ocular dominance columns within the visual cortex of human amblyopes.

Non-human primate models suggest that amblyopia has a neural basis in the form of a massive reduction in binocular neurons, and in some cases, a shift in ocular dominance of neural activity toward the unaffected eye. To date, the resolution of neuroimaging has been insufficient to investigate the neural basis of ocular dominance in human amblyopia. We used high spatial resolution (0.5 x 0.5 x 3 mm) functional magnetic resonance imaging (fMRI) to obtain maps of ocular dominance within the visual cortex of adult human amblyopes. fMRI maps of ocular dominance were similar in appearance to maps reported in the literature. For each of six adults with early-onset amblyopia, the number of map pixels corresponding to the unaffected eye was greater than the number corresponding to the amblyopic eye. This shift in ocular dominance was not seen for the two adults with later-onset amblyopia, suggesting that a shift in ocular dominance of neural activity occurs only if amblyopia onset is within the critical period of brain development. Our findings demonstrate how fMRI can non-invasively investigate the neural substrates underlying human amblyopia at the cortical column level.

Adult↗

PubMed2002

Glutamate and glutamine measured with 4.0 T proton MRS in never-treated patients with schizophrenia and healthy volunteers.

OBJECTIVE: This in vivo (1)H magnetic resonance spectroscopy study examined levels of glutamate, glutamine, and N-acetylaspartate in patients experiencing their first episode of schizophrenia. METHOD: Localized in vivo (1)H spectra were acquired at 4.0 T from the left anterior cingulate and thalamus of 21 never-treated patients with schizophrenia and 21 comparable healthy volunteers. RESULTS: The level of glutamine was significantly higher in the left anterior cingulate cortex and thalamus of the patients with schizophrenia than in the healthy subjects. No differences were found between groups in the levels of other metabolites in the anterior cingulate or thalamus. CONCLUSIONS: Higher than normal glutamine levels in the left anterior cingulate and thalamus provide in vivo evidence of greater than normal glutamatergic activity proposed by glutamatergic models of schizophrenia. In contrast to other studies in chronically ill patients, no differences were seen in the levels of N-acetylaspartate in either location, suggesting that the findings in patients with chronic schizophrenia may be related to the effect of medication or the progression of the illness.

Adult↗

PubMed2002

Region-specific changes in phospholipid metabolism in chronic, medicated schizophrenia: (31)P-MRS study at 4.0 Tesla.

BACKGROUND: Membrane phospholipid abnormalities in people with schizophrenia, measured with (31)P magnetic resonance spectroscopy ((31)P-MRS), have been previously reported in brain regions involved in this disorder. AIMS: In this 4.0 Tesla (31)P-MRS study of people with schizophrenia, membrane phospholipid metabolism was examined in brain regions previously inaccessible due to their small volumes. METHOD: Three-dimensional chemical-shift imaging (3D-CSI) examined 15 cc volumes in 12 brain regions in 11 people with chronic schizophrenia and 11 healthy control volunteers. RESULTS: Glycerophosphoethanolamine was decreased in the anterior cingulate, right prefrontal cortex and left thalamus, but increased in the left hippocampus and cerebellum in those with schizophrenia. Phosphoethanolamine and glycerophosphocholine were decreased in the right prefrontal region and phosphocholine was decreased in the anterior cingulate. No significant difference in membrane phospholipid levels existed between groups in the parieto-occipital and posterior cingulate regions. CONCLUSIONS: Altered membrane phospholipid metabolism was demonstrated in all regions implicated in schizophrenia.

Adult↗

PubMedDate not supplied

Behavioral and neuroimaging evidence for a contribution of color and texture information to scene classification in a patient with visual form agnosia.

A common notion is that object perception is a necessary precursor to scene perception. Behavioral evidence suggests, however, that scene perception can operate independently of object perception. Further, neuroimaging has revealed a specialized human cortical area for viewing scenes that is anatomically distinct from areas activated by viewing objects. Here we show that an individual with visual form agnosia, D.F., who has a profound deficit in object recognition but spared color and visual texture perception, could still classify scenes and that she was fastest when the scenes were presented in the appropriate color. When scenes were presented as black-and-white images, she made a large number of errors in classification. Functional magnetic resonance imaging revealed selective activation in the parahippocampal place area (PPA) when D.F. viewed scenes. Unlike control observers, D.F. demonstrated higher activation in the PPA for scenes presented in the appropriate color than for black-and-white versions. The results demonstrate that an individual with profound form vision deficits can still use visual texture and color to classify scenes-and that this intact ability is reflected in differential activation of the PPA with colored versions of scenes.

Adult↗