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H A Rowley

Publications and source records attributed to H A Rowley.

At least 37 records · Page 2Linked to original sources
PubMed1998

Somatosensory cortex: a comparison of the response to noxious thermal, mechanical, and electrical stimuli using functional magnetic resonance imaging.

In the present study, functional magnetic resonance imaging (fMRI) was used to examine pain perception in humans. Three types of noxious stimuli were presented: electric shock (20.8 mA, 2 Hz), heat (48 degrees C), and mechanical, as well as a control tactile stimulus. The significance of activation at the level of the voxel was determined using correlation analysis. Significant region of interest (ROI) activation was determined by comparing the percentage of active voxels in each ROI to activation in a control ROI in the visual cortex. In response to tactile and shock stimuli, consistent activation was seen in the postcentral gyrus, parietal operculum, and ipsilateral cerebellar cortex. No significant cortical activation was detected in response to noxious heat or mechanical stimulation when compared to nonpainful intensity levels. The data did not indicate adaptation, although further study in this area is necessary. Stationary noxious thermal and mechanical stimulation are "pure" noxious stimuli, while electrical stimulation influenced nociceptive and nonnociceptive receptors. Lack of detectable activation in response to pure noxious stimuli supports the idea that nociceptive and nonnociceptive fibers are interspersed in the somatosensory cortex. Conflicting results from recent functional imaging studies of pain perception regarding cortical activation indicate that it is essential to consider both the tactile and nociceptive components of the stimuli used, the spatial extent of stimulation, and the possibility of adaptation to the response. Furthermore, these results suggest that subtractive or correlative methods may not be sufficiently sensitive to image the activity of nociceptive cells, which are sparsely distributed throughout the somatosensory cortex.

Adolescent↗
PubMed1997

Learning transfer and neuronal plasticity in humans trained in tactile discrimination.

Adult humans were unilaterally trained in a tactile discrimination task of sequentially applied multi-finger stimuli. Magnetic source imaging (MSI) was performed before and after the training to evaluate use-dependent neuronal plasticity. All subjects showed fast improvements in performance and complete transfer of the learned task. MSI recordings revealed an unilateral decrease in current dipole strength in the somatosensory system contralateral to the trained hand. Attenuation of sensory evoked fields and a complete learning transfer indicate learning in associative and secondary cortices rather than perceptual plasticity operating on neuronal populations involved in early sensory processing. This findings are discussed with respect to an equivalent animal model and to learning specificity and generalization.

Adult↗
PubMed1997

Processing of vowels in supratemporal auditory cortex.

The auditory evoked neuromagnetic fields elicited by synthesized vowels of two different fundamental frequencies F0 were recorded in six subjects over the left and right temporal cortices using a 37-channel biomagnetometer. Single equivalent current dipole modeling of the fields elicited by all vowel types localized activity to a well-circumscribed area in supratemporal auditory cortex in both hemispheres. There were hemisphere asymmetries in the amplitude and latency of the M100 response. We also observed changes in M100 latency related to vowel type, but not to F0. There was no clear effect of vowel type or F0 on dipole localization for the M100, but a possible vowel type by latency interaction. These M100 data provide further evidence that vowels are processed independently of their pitch.

Adult↗
PubMed1997

Magnetoencephalography in partial epilepsy: clinical yield and localization accuracy.

The goals of this study were to determine (1) the yield of magnetoencephalography (MEG) according to epilepsy type, (2) if MEG spike sources colocalize with focal epileptogenic pathology, and (3) if MEG can identify the epileptogenic zone when scalp ictal electroencephalogram (EEG) or magnetic resonance imaging (MRI) fail to localize it. Twenty-two patients with mesial temporal (10 patients), neocortical temporal (3 patients), and extratemporal lobe epilepsy (9 patients) were studied. A 37-channel biomagnetometer was used for simultaneously recording MEG with EEG. During the typical 2-3-hour MEG recording session, interictal epileptiform activity was observed in 16 of 22 patients. MEG localization yield was greater in patients with neocortical epilepsy (92%) than in those with mesial temporal lobe epilepsy (50%). In 5 of 6 patients with focal epileptogenic pathology, MEG spike sources were colocalized with the lesions. In 11 of 12 patients with nonlocalizing (ambiguous abnormalities or normal) MRI, MEG spike sources were localized in the region of the epileptogenic zone as ultimately defined by all clinical and EEG information (including intracranial EEG). In conclusion, MEG can reliably localize sources of spike discharges in patients with temporal and extratemporal lobe epilepsy. MEG sometimes provides noninvasive localization data that are not otherwise available with MRI or conventional scalp ictal EEG.

Adolescent↗
PubMed1997

Presurgical multimodality neuroimaging in electroencephalographic lateralized temporal lobe epilepsy.

The purpose of this study was to compare 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET), hippocampal volumetry (HV), T2 relaxometry, and proton magnetic resonance spectroscopic imaging (1H-MRSI) in the presurgical neuroimaging lateralization of patients with nonlesional, electroencephalogram (EEG)-defined unilateral temporal lobe epilepsy (TLE). Twenty-five patients were prospectively studied, along with age-matched controls. T2 relaxometry examinations were performed in 13 patients. Comparison of FDG-PET, HV, and 1H-MRSI was possible in 23 patients. FDG-PET lateralized 87% of patients, HV 65%, N-acetyl aspartate (NAA)/(choline [Cho] + creatine [Cr]) 61%, and [NAA] 57%. Combined HV and NAA/(Cho + Cr) results lateralized 83% of the patients, a value similar to PET. Of 10 patients with normal magnetic resonance imaging (MRI) scans, 2 were lateralized with HV, 6 with FDG-PET, 4 with NAA/(Cho + Cr), and 3 with [NAA]. T2 relaxometry lateralized no patients without hippocampal atrophy. Bilateral abnormality was present in 29 to 33% of patients with 1H-MRSI measures and 17% with HV. Only hippocampal atrophy correlated with postoperative seizure-free outcome. FDG-PET remains the most sensitive imaging method to correlate with EEG-lateralized TLE. Both FDG-PET and 1H-MRSI can lateralize patients with normal MRI, but only the presence of relative unilateral hippocampal atrophy is predictive of seizure-free outcome. Bilaterally abnormal MRI and 1H-MRSI measures do not preclude good surgical outcome.

Adult↗
PubMed1997

Mapping of the sensorimotor cortex: functional MR and magnetic source imaging.

PURPOSE: To assess the reliability and comparability of functional MR imaging and magnetic source imaging for mapping the somatosensory cortex. METHODS: Parallel studies were performed in eight volunteer subjects in whom both hemispheres were measured with the use of painless tactile stimulation of the tip of each index finger. Magnetic source imaging was performed using a 37-channel biomagnetometer; evoked magnetic fields were analyzed using the single-equivalent dipole representation to ascertain the neuronal source. Functional MR imaging was performed on a 1.5-T MR unit. Blocks of images during periods of rest and activation were acquired using gradient-echo echo-planar imaging. Correlation analysis identified pixels in which signal intensity correlated with the stimulus function. A subsequent requirement for spatial connectivity of activation was imposed to reduce the random occurrence of pixels satisfying the correlation criteria. RESULTS: Using temporal and spatial statistical criteria for activation, we found that functional MR imaging showed activation in 1 of 16 hemispheres. In three cases, this was accompanied by activity either frontally or ipsilateral to the stimulus. Magnetic source imaging showed parietal contralateral location in all 16 cases. Where successful localization was achieved with both methods, the separation between sources appeared to be between 1 and 4 cm. Functional MR imaging localizations tended to lie more superficially than the magnetic source imaging localizations. Performance of a simple motor task, rather than use of somatosensory stimulation, resulted in a cortical signal change detectable with a similar functional MR imaging approach in all cases, suggesting the more robust nature of this stimulus. CONCLUSIONS: Functional mapping of the somatosensory cortex can be achieved with functional MR imaging or magnetic source imaging. Functional MR imaging yields more spurious locations and fails to show localization more often. If neuronal signal propagation pathways are of interest, the temporal resolution of functional MR imaging alone may be inadequate. A combination of magnetic source imaging and functional MR imaging may allow improved sensitivity, fewer false-positive results, and high spatial and temporal resolution.

Adult↗
PubMed1997

Magnetic source imaging as a tool for presurgical functional brain mapping.

This article describes magnetic source imaging, a newly-developing technology which can be used to noninvasively map eloquent cortex prior to surgical procedures. It is based on large-array detection of extracranial magnetic fields arising from neuronal currents evoked by peripheral stimulation. A range of evoked and spontaneous neuromagnetic activities are discussed along with clinical examples of the utility of the technique and comparison to other brain mapping techniques, such as positron emission tomography, functional magnetic resonance imaging, EEG, and ECoG.

Adolescent↗
PubMed1996

Comparison of late components in simultaneously recorded event-related electrical potentials and event-related magnetic fields.

We have attempted to define the late components of the event-related magnetic field (ERF) and to relate them to the late components of the event-related electrical potential (ERP). Simultaneous multichannel electroencephalogram (EEG) and magnetoencephalogram (MEG) were recorded in 13 subjects during an auditory oddball paradigm in two series of experiments. EEG responses to frequent tones consisted of the N1 and P2 components of the auditory vertex potential. Responses to rare tones consisted of N1, apparent P2, N2 and P3 components. All EEG components were best seen in the midline and were highly reproducible for all subjects. MEG responses to frequent tones consisted of N1m and P2m components that were highly reproducible only when recorded over the temporal region. By contrast, the ERF to rare tones was less well defined and only the N1m component could be identified satisfactorily. There was little consistent activity in the MEG at the time of occurrence of the N2 and P3 components of the ERP.

Acoustic Stimulation↗
PubMed1996

Noninvasive somatosensory monitoring of the injured inferior alveolar nerve using magnetic source imaging.

PURPOSE: This study evaluates magnetoencephalography (MEG) as an objective monitor for the evaluation of post-traumatic inferior alveolar nerve injuries. MATERIALS AND METHODS: Six patients with unilateral inferior alveolar nerve injuries were assessed using conventional sensory examination techniques. All damaged nerves, and their contralateral controls, were then reexamined using MEG technology. Regions of somatosensory-induced magnetic activity were superimposed on three-dimensional magnetic resonance imaging (MRI) scans to give cortical magnetic source images (MSI). All patients subsequently underwent surgical exploration of the damaged nerves. RESULTS: All six patients had no sensitivity to conventional testing on the damaged side. On evaluation with MEG, all six control nerves had an appropriate cortical signal in response to repetitive lip stimulation. Somatosensory stimulation of two of the six damaged nerves resulted in cortical magnetic field changes. Surgical exploration showed that the four nerves with negative MEG tests had discontinuity defects. In contrast, the two patients with positive MEG signals had intact nerves. CONCLUSION: This technology may differentiate between intact but damaged nerves and transected nerves.

Adult↗
PubMed1996

Task-induced asymmetry of the auditory evoked M100 neuromagnetic field elicited by speech sounds.

The auditory evoked neuromagnetic fields elicited by synthesized speech sounds (consonant-vowel syllables) were recorded in six subjects over the left and right temporal cortices using a 37-channel SQUID-based magnetometer. The latencies and amplitudes of the peaks of the M100 evoked responses were bilaterally symmetric for passively presented stimuli. In contrast, when subjects were asked to discriminate among the same syllabic stimuli, the amplitude of the M100 increased in the left and decreased in the right temporal cortices. Single equivalent current dipole modeling of the activity elicited by all stimulus-types localized to a well-circumscribed area in supratemporal auditory cortex. The results suggest that attentional modulation affects the two supratemporal cortices in a differential manner. Task-conditioned attention to speech sounds is reflected in lateralized supratemporal cortical responses possibly concordant with hemispheric language dominance.

Acoustic Stimulation↗
PubMed1995

Correlation of functional magnetic source imaging with intraoperative cortical stimulation in neurosurgical patients.

This study compared noninvasive preoperative functional imaging by using magnetoencephalography (MEG) with intraoperative direct cortical stimulation in ten patients undergoing neurosurgery. The goal was to assess the accuracy and reliability of MEG-based functional imaging in these patients as a possible replacement or adjunct for direct cortical stimulation with electrocorticography. Objective comparison of intraoperative mapping with preoperative MEG procedures was achieved by intraoperative recording of mapped cortical locations for motor responses using an interactive image-guided surgical device, the ISG viewing wand, with which mapping points could be marked on a previously acquired (MRI) set. In all ten patients, at least one stimulation site elicited a response during both MEG and intraoperative mapping. The central sulcus ipsilateral to the lesion was only directly visible on high-resolution MRIs in 3/10 cases and equivocally in 2/10. Coregistered with MRI to form magnetic source images (MSIs), MEG predictions of the postcentral gyrus were possible in all 10 cases. In all 10 cases, these were in agreement with intraoperative estimation of the precentral gyrus. Functional mapping of somatosensory cortex was achieved noninvasively in surgical patients by using MSI. The accuracy, compared with cortical stimulation, was always sufficient to define motor and somatosensory strips.

Adult↗
PubMed1995

MR in partial epilepsy: value of high-resolution volumetric techniques.

PURPOSE: To assess the utility of high-resolution volumetric MR examinations with thin partition size for patients with simple partial epilepsies that were well located with electrical and clinical criteria. METHODS: Fifteen patients with normal standard MR findings were studied with three-dimensional Fourier transform volumetric MR examinations using thin (1 to 1.5 mm) partition size. Imaging was done in the coronal plane, then reformatted manually on an independent console with each gyrus analyzed in the planes parallel and perpendicular to its axis. RESULTS: Cortical abnormalities were detected in 8 of the 15 patients in the study. Surgical resection of the affected cortex in 2 patients showed polymicrogyria in one and dysplastic cortical organization in the other. CONCLUSION: In this preliminary study, 3-D Fourier transform volumetric MR examinations with thin partition size appeared to be useful in identifying cortical dysplasias in patients with localized simple partial epilepsies.

Adolescent↗
PubMed1995

Functional localization by magnetoencephalography.

Magnetoencephalography combines the attributes of real-time millisecond temporal resolution with precise anatomic localization. In addition to characterizing spontaneous epileptiform discharges and slow wave activity, magnetoencephalography may be used to map sensorimotor, auditory, and visual functions. Magnetoencephalography functional data are registered to three-dimensional magnetic resonance imaging to provide a useful way to analyze the relationship of structure to function and to assist in surgical planning. Magnetoencephalography is noninvasive and can be done with rapid turn-around time, but it is expensive and is not optimal for all brain areas or functions. Future integration and cross-analysis of magnetoencephalography with other functional mapping modalities will likely prove to be complementary and of even greater use.

Auditory Cortex↗
PubMed1991

Cladosporium trichoides cerebral phaeohyphomycosis in a liver transplant recipient. Report of a case.

Cerebral phaeohyphomycosis (also referred to as cerebral chromomycosis), one of the diseases caused by the dematiaceous (black) fungi, is most commonly caused by Cladosporium trichoides (referred to by some as Xylohypha bantiana) and is a rare disease, with 31 culture-proven cases reported to date. Although most cases have occurred in immunocompetent hosts, recent experimental evidence suggests that host immunosuppression may predispose patients to the disease. The authors report a case of fatal cerebral phaeohyphomycosis in a liver transplant patient, the first to occur in a transplant patient of any type, to the best of the authors' knowledge. This case provides support for the hypothesis that immunosuppressed patients may be at increased risk for development of this disease.

Brain Diseases↗
PubMed1989

Thalamomesencephalic strokes after cocaine abuse.

Three young patients developed strokes of rostral midbrain and thalamus shortly following cocaine abuse. Two had infarctions and one had a hemorrhage, but none had clear risk factors other than cocaine for this relatively uncommon type of stroke. Toxicologic analysis confirmed isolated cocaine use in each patient. In the two cases of infarction studied angiographically, one had normal findings and the other had focal narrowing of the P1 segments of the posterior cerebral arteries bilaterally. Since the P1 segment has a uniquely sparse perivascular sympathetic supply, we suggest that direct adrenergic-mediated vasoconstriction is not critical to the production of cocaine-associated stroke.

Adult↗