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Biomedical subjects

N J Shah

Publications and source records attributed to N J Shah.

At least 19 recordsLinked to original sources
PubMed2004

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping.

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.

Algorithms↗
PubMed2004

A functional magnetic resonance imaging study of local/global processing with stimulus presentation in the peripheral visual hemifields.

When stimuli are presented in the left or right visual fields, hemispheric specialization for global and local processing in occipital areas is attenuated. Using functional magnetic resonance imaging, we investigated how this attenuation is compensated for when information must cross the corpus callosum to reach the areas specialized for global and local processing. We presented hierarchically nested letters (e.g. a large E made of smaller E's) to the right or the left visual hemifield while subjects fixated centrally. In half the trials, subjects indicated whether the global aspect and in the other half whether the local aspect of the stimulus matched a pre-specified target letter. Visual hemifield presentations showed the expected contralateral activations of occipital cortex. The main effects of locally or globally directed attention did not show any differential occipital activations, but the right anterior cingulate cortex was activated differentially during local processing. Region-of-interest-based analyses showed increased neural activity in left posterior occipital cortex during local processing when stimuli were presented in the left hemifield. During global processing with stimulus presentation to the right hemifield, the right posterior occipital cortex was activated. Activation of right anterior cingulate cortex during local processing is likely to reflect the suppression of global processing precedence in order to select correctly the local stimulus level. The activations in left (local) and right (global) occipital areas are likely to reflect the top-down augmentation of stimulus information that has been degraded by callosal crossing in order to access the hemisphere specialized for local or global processing.

Adult↗
PubMed2004

Left and right superior parietal lobule in tactile object discrimination.

Tactile object discrimination is one of the major manual skills of humans. While the exploring finger movements are not perceived explicitly, attention to the movement-evoked kinaesthetic information gates the tactile perception of object form. Using event-related functional magnetic resonance imaging in seven healthy subjects we found one area in the right superior parietal cortex, which was specifically activated by kinaesthetic attention during tactile object discrimination. Another area with similar location in the left hemisphere was related to the maintenance of tactile information for subsequent object discrimination. We conclude that kinaesthetic information is processed in the anterior portion of the superior parietal cortex (aSPL) with a right hemispheric predominance for discrimination and a left hemispheric predominance for information maintenance.

Adult↗
PubMed2003

Error reduction and parameter optimization of the TAPIR method for fast T1 mapping.

A methodology is presented for the reduction of both systematic and random errors in T(1) determination using TAPIR, a Look-Locker-based fast T(1) mapping technique. The relations between various sequence parameters were carefully investigated in order to develop recipes for choosing optimal sequence parameters. Theoretical predictions for the optimal flip angle were verified experimentally. Inversion pulse imperfections were identified as the main source of systematic errors in T(1) determination with TAPIR. An effective remedy is demonstrated which includes extension of the measurement protocol to include a special sequence for mapping the inversion efficiency itself.

Algorithms↗
PubMed2002

Topographic segregation and convergence of verbal, object, shape and spatial working memory in humans.

This functional magnetic resonance imaging study investigates commonalties and differences in working memory (WM) processes employing different types of stimuli. We specifically sought to characterize topographic convergence and segregation with respect to prefrontal cortex involvement using verbal, spatial, real object and shape memory items in a two-back WM task. Both the dorsolateral and ventrolateral prefrontal cortices are conjointly activated across all stimulus types. No stimulus-specific differences in the activation patterns of the prefrontal cortex could be demonstrated giving support to the view of an amodal prefrontal involvement during WM processes. However, extra-frontal regions specialized on feature processing and involved in the preprocessing of the stimuli were selectively activated by these different subtypes of WM. These selectively activated regions are assigned to parts of the ventral and dorsal stream.

Adult↗
PubMed2002

Does dichotic listening probe temporal lobe functions?

OBJECTIVE: To explore cortical hemodynamic responses using fMRI in the context of three dichotic listening tasks. BACKGROUND: Dichotic listening is a widely used behavioral technique indicating brain laterality during which subjects are presented with two different auditory signals at the same time, one arriving at each ear. fMRI offers the potential to explore the hemodynamic response during dichotic listening and to relate the behavioral indices with these cortical measures. METHOD: fMRI was performed for 10 right-handed normal subjects listening to consonant-vowel syllable pairs with the task of detecting a "target" syllable by pressing a button. The target stimulus appeared equally often in the left and right ear. The subjects were instructed to either concentrate on the stimuli presented in both ears (DIV) or only in the left ear (FL) or right ear (FR). In addition, a control condition was used during which the syllables were presented binaurally. Hemodynamic responses were measured by applying whole-head echo planar imaging techniques and statistically analyzed by using statistical parametric mapping (SPM99) software. RESULTS: During dichotic listening, there were generally extended activations in frontotemporal networks. For the DIV condition, the authors found strong bilateral activations in the inferior frontal gyrus, Broca's area, the left middle frontal gyrus, and in the left superior temporal gyrus. During the FL condition, there was an additional cluster in the right inferior frontal gyrus. For the FR condition, there were stronger activations in Broca's area and the left superior temporal gyrus. CONCLUSIONS: These findings were taken as evidence that dichotic listening is more demanding, requiring more processing capacity distributed in frontotemporal networks. The behavioral measures of dichotic listening were not simply a function of temporal lobe activation. Rather, the cortical activations support the notion that different processing strategies controlled by different neural structures are applied during dichotic listening.

Adult↗
PubMed2002

Neural activity in human primary motor cortex areas 4a and 4p is modulated differentially by attention to action.

The mechanisms underlying attention to action are poorly understood. Although distracted by something else, we often maintain the accuracy of a movement, which suggests that differential neural mechanisms for the control of attended and nonattended action exist. Using functional magnetic resonance imaging (fMRI) in normal volunteers and probabilistic cytoarchitectonic maps, we observed that neural activity in subarea 4p (posterior) within the primary motor cortex was modulated by attention to action, while neural activity in subarea 4a (anterior) was not. The data provide the direct evidence for differential neural mechanisms during attended and unattended action in human primary motor cortex.

Adult↗
PubMed2001

The transfer of a timing pattern to the untrained human hand investigated with functional magnetic resonance imaging.

The study investigates cortical hemodynamic responses during continuation tapping using auditory pacing stimuli in five healthy right-handed subjects using functional magnetic resonance imaging. The tasks required the use of either the same finger for synchronization of the tapping movement and for continuation, or to use the contralateral finger for continuation. Results show, that using the contralateral finger increases regional cerebral blood flow in motor areas such as the anterior cerebellar hemispheres and vermis, in the cingulate motor area, but also in the posterior cingulum, when compared to using the same finger. The complementary comparison shows increased regional blood flow in the left hippocampus. The results suggest that in addition to pure executive functions, higher cognitive functions localized in these areas are involved in the transfer of interval timing.

Acoustic Stimulation↗
PubMed2001

The motion aftereffect: more than area V5/MT? Evidence from 15O-butanol PET studies.

The motion aftereffect is a perceptual phenomenon which has been extensively investigated both psychologically and physiologically. Neuroimaging techniques have recently demonstrated that area V5/MT is activated during the perception of this illusion. The aim of this study was to test the hypothesis if a more broadly distributed network of brain regions subserves the motion aftereffect. To identify the neuronal structures involved in the perception of the motion aftereffect, regional cerebral blood flow (rCBF) measurements with positron emission tomography were performed in six normal volunteers. Data were analysed using SPM96. The motion-sensitive visual areas including area V5/MT were activated in both hemispheres. Additionally, the lateral parietal cortex bilaterally, the right dorsolateral prefrontal cortex, the anterior cingulate cortex and the left cerebellum showed significant increases in rCBF values during the experience of the waterfall illusion. In a further reference condition with identical attentional demand but no perception of a motion aftereffect elevated rCBF were found in these regions as well. In conclusion, our findings support the notion that the perceptual illusion of motion arises exclusively in the motion-sensitive visual area V5/MT. In addition, a more widespread network of brain regions including the prefrontal and parietal cortex is activated during the waterfall illusion which represents a non-motion aftereffect-specific subset of brain areas but is involved in more basic attentional processing and cognition.

Adult↗
PubMed2001

Shared k-space echo planar imaging with keyhole.

Time-dependent phenomena are of great interest, and researchers have sought to shed light on these processes with MRI, particularly in vivo. In this work, a new hybrid technique based on EPI and using the concept of keyhole imaging is presented. By sharing peripheral k-space data between images and acquiring the keyhole more frequently, it is shown that the spatial resolution of the reconstructed images can be maintained. The method affords a higher temporal resolution and is more robust against susceptibility and chemical-shift artifacts than single-shot EPI. The method, termed shared k-space echo planar imaging with keyhole (shared EPIK), has been implemented on a standard clinical scanner. Technical details, simulation results, phantom images, in vivo images, and fMRI results are presented. These results indicate that the new method is robust and may be used for dynamic MRI applications. Magn Reson Med 45:109-117, 2001.

Artifacts↗
PubMed2001

Fast T(1) mapping with volume coverage.

Four different sequences which enable high-resolution, multislice T(1) relaxation-time mapping are presented. All these sequences are based on the Look-Locker method with differences arising from the use of either a saturation-recovery or inversion-recovery module prior to data acquisition with a full k-space or banded k-space acquisition scheme. The methods were implemented on a standard clinical scanner and the accuracy of the T(1) results was evaluated against spectroscopic measurements. The accuracy of the T(1) maps validated by phantom imaging measurements is around 1% for species which relax with T(1) times that mimic gray/white matter (T(1) < or = 1000 ms). Additionally, the inherent multislice, multipoint capability of the methods is demonstrated. Finally, in vivo results of the human brain obtained using the faster method are presented. The fastest data acquisition was achieved with a saturation-recovery, banded k-space method where k-space was divided into three segments; an overall acquisition time of around 5 min (for species with T(1) < or = 1 sec) was achieved for a T(1) map which can, in principle, provide whole-brain coverage with a matrix size of 256 x 256 at multiple time-points. Magn Reson Med 46:131-140, 2001.

Brain↗
PubMed2001

Focused and nonfocused attention in verbal and emotional dichotic listening: an FMRI study.

Functional magnetic resonance imaging (fMRI) was used to identify cortical regions which are involved in two dichotic listening tasks. During one task the subjects were required to allocate attention to both ears and to detect a specific target word (phonetic task), while during a second task the subjects were required to detect a specific emotional tone (emotional task). During three attentional conditions of each task, the subjects were required to focus attention to the right (FR) or left ear (FL), while during a third condition subjects were required to allocate attention to both ears simultaneously. In 11 right-handed male subjects, these dichotic listening tasks evoked strong activations in a temporofrontal network involving auditory cortices located in the temporal lobe and prefrontal brain regions. Hemodynamic responses were measured in the following regions of interest: Heschl's gyrus (HG), the planum polare (PP), the planum temporale (PT), the anterior superior temporal sulcus (aSTS), the posterior superior temporal sulcus (pSTS), and the inferior frontal gyrus region (IFG) of both hemispheres. The following findings were obtained: (1) the degree of activation in HG and PP depends on the direction of attention. In particular it was found that selectively attending to right-ear input led to increased activity specifically in the left HG and PP and attention to left ear input increased right-sided activity in these structures; (2) hemodynamic responses in the PT, aSTS, pSTS, and IFG were not modulated by the different focused-attention conditions; (3) hemodynamic responses in HG and PP in the nonforced conditions were the sum activation of the forced conditions; (4) there was no general difference between the phonetic and emotion tasks in terms of hemodynamic responses; (5) hemodynamic responses in the PT and pSTS were strongly left-lateralized, reflecting the specialization of these brain regions for language processing. These findings are discussed in the context of current theories of hemispheric specialization.

Adult↗
PubMed2001

Deriving numerosity and shape from identical visual displays.

We presented identical displays of three to five dots in a functional magnetic resonance imaging (fMRI) experiment with normal volunteers. Two distinct directed attention tasks were performed on these displays: In one condition, subjects assessed the numerosity of the display; in the other condition, they assessed the shape of the display. Decisions based on numerosity activated differentially striate and extrastriate visual processing areas as well as left inferior frontal cortex. Decisions based on shape derived from arrangement activated differentially temporoparietal cortex bilaterally, medial posterior cingulate cortex, and left dorsolateral prefrontal cortex. These divergent neural activations in response to identical stimuli suggest that attentional mechanisms are deployed in very different ways in rapid enumeration of visual objects and in linking spatially discrete elements to one form.

Adult↗
PubMed2001

Mind reading: neural mechanisms of theory of mind and self-perspective.

Human self-consciousness as the metarepresentation of ones own mental states and the so-called theory of mind (TOM) capacity, which requires the ability to model the mental states of others, are closely related higher cognitive functions. We address here the issue of whether taking the self-perspective (SELF) or modeling the mind of someone else (TOM) employ the same or differential neural mechanisms. A TOM paradigm was used and extended to include stimulus material that involved TOM and SELF capacities in a two-way factorial design. A behavioral study in 42 healthy volunteers showed that TOM and SELF induced differential states of mind: subjects assigned correctly first or third person pronouns when providing responses to the stimuli. Following the behavioral study, we used functional magnetic resonance imaging (fMRI) in eight healthy, right-handed males to study the common and differential neural mechanisms underlying TOM and SELF. The main factor TOM led to increased neural activity in the anterior cingulate cortex and left temporopolar cortex. The main factor SELF led to increased neural activity in the right temporoparietal junction and in the anterior cingulate cortex. A significant interaction of both factors TOM and SELF was observed in the right prefrontal cortex. These divergent neural activations in response to TOM and SELF suggest that these important differential mental capacities of human self-consciousness are implemented at least in part in distinct brain regions. Press

Adult↗
PubMed2001

A new method for fast multislice T(1) mapping.

A sequence for T(1) relaxation-time mapping which enables high-resolution, multislice imaging in short acquisition times is presented. The sequence is based on the Look-Locker method and employs a magnetization-preparation module prior to data acquisition with a banded k-space data collection scheme. The method was implemented on a standard clinical scanner and the accuracy of the T(1) results was evaluated against spectroscopic measurements. The accuracy of the T(1) maps validated by phantom imaging measurements is <3% for slow-relaxing compartments (T(1) congruent with 2000 ms) and is around 1% for faster-relaxing species (T(1) < or = 1200 ms). Additionally, the inherent multislice, multipoint capability of the method is demonstrated. Multislice, multipoint in vivo results of the human brain obtained using this method are presented. An acquisition time of approximately 8 min was achieved for a T(1) map, which, in principle, can provide whole-brain coverage with 25 slices, a matrix size of 256 x 256, and 12 time points. The speed of the sequence is derived through optimized interleaving of slices and time points, together with the acquisition of multiple echoes, which are used to fill a 3-segment k-space.

Brain↗
PubMed2001

Polymodal motion processing in posterior parietal and premotor cortex: a human fMRI study strongly implies equivalencies between humans and monkeys.

In monkeys, posterior parietal and premotor cortex play an important integrative role in polymodal motion processing. In contrast, our understanding of the convergence of senses in humans is only at its beginning. To test for equivalencies between macaque and human polymodal motion processing, we used functional MRI in normals while presenting moving visual, tactile, or auditory stimuli. Increased neural activity evoked by all three stimulus modalities was found in the depth of the intraparietal sulcus (IPS), ventral premotor, and lateral inferior postcentral cortex. The observed activations strongly suggest that polymodal motion processing in humans and monkeys is supported by equivalent areas. The activations in the depth of IPS imply that this area constitutes the human equivalent of macaque area VIP.

Acoustic Stimulation↗
PubMed2001

The neural correlates of person familiarity. A functional magnetic resonance imaging study with clinical implications.

Neural activity was measured in 10 healthy volunteers by functional MRI while they viewed familiar and unfamiliar faces and listened to familiar and unfamiliar voices. The familiar faces and voices were those of people personally known to the subjects; they were not people who are more widely famous in the media. Changes in neural activity associated with stimulus modality irrespective of familiarity were observed in modules previously demonstrated to be activated by faces (fusiform gyrus bilaterally) and voices (superior temporal gyrus bilaterally). Irrespective of stimulus modality, familiarity of faces and voices (relative to unfamiliar faces and voices) was associated with increased neural activity in the posterior cingulate cortex, including the retrosplenial cortex. Our results suggest that recognizing a person involves information flow from modality-specific modules in the temporal cortex to the retrosplenial cortex. The latter area has recently been implicated in episodic memory and emotional salience, and now seems to be a key area involved in assessing the familiarity of a person. We propose that disturbances in the information flow described may underlie neurological and psychiatric disorders of the recognition of familiar faces, voices and persons (prosopagnosia, phonagnosia and Capgras delusion, respectively).

Acoustic Stimulation↗