PubMed Health⌕ Search

Biomedical subjects

P T Fox

Publications and source records attributed to P T Fox.

At least 19 recordsLinked to original sources
PubMed1991

Localization of a human system for sustained attention by positron emission tomography.

Positron emission tomographic (PET) studies of human attention have begun to dissect isolable components of this complex higher brain function, including a midline attentional system in a region of the anterior cingulate cortex. The right hemisphere may play a special part in human attention; neglect, an important phenomenon associated with damage to attentional systems, is more severe, extensive and long-lasting after lesions to the right hemisphere. Here we use PET measurements of brain blood flow in healthy subjects to identify changes in regional brain activity during simple visual and somatosensory tasks of sustained attention or vigilance. We find localized increases in blood flow in the prefrontal and superior parietal cortex primarily in the right hemisphere, regardless of the modality or laterality of sensory input. The anterior cingulate was not activated during either task. These data localize the vigilance aspects of normal human attention to sensory stimuli, thereby clarifying the biology underlying asymmetries of attention to such stimuli that have been reported in clinical lesions.

Adult↗
PubMed1991

Does inter-subject variability in cortical functional organization increase with neural 'distance' from the periphery?

In mapping the functional anatomy of the human brain, anatomical variability is a recurring concern. The degree to which the functional organization of any one subject or group of subjects is more generally predictive is largely unknown. We have previously reported that the inter-subject variability of primary visual, somatosensory and motor cortices is small (4-8 mm). Many have suggested, however, that higher-order brain areas will be considerably more variable. For this reason we assessed the anatomical variability of several brain areas participating in language perception and production. In 10 anatomically normal subjects undergoing evaluation for partial complex epilepsy we applied a previously described battery of lexical tasks; intra-subject image averaging was used to minimize the effects of variations in response magnitude. We found inter-subject anatomical variability to be uniformly consistent, with no appreciable effect of distance from the neural periphery.

Adolescent↗
PubMed1991

Physiological ROI definition by image subtraction.

Quantitative analysis of functional images requires a strategy for reducing to tractable size the overwhelming amount of numeric data contained therein. Region-of-interest (ROI) selection is the most widely used means of image-data reduction, but it has many limitations. Spatial bias, introduced by selection of regions as being "of interest," is probably the greatest limitation of ROI analyses. Change-distribution analysis is a new data-analysis strategy that eliminates this a priori selection bias in a way that can increase the sensitivity, specificity, and localization precision. All image pixels are surveyed for changes from the control condition. Only areas of change are sampled and contribute to statistical analysis. Change-distribution analysis has been validated for within-subject pairs of images, but it is potentially applicable in a wide variety of imaging protocols.

Brain↗
PubMed1990

Activation of extrastriate and frontal cortical areas by visual words and word-like stimuli.

Visual presentation of words activates extrastriate regions of the occipital lobes of the brain. When analyzed by positron emission tomography (PET), certain areas in the left, medial extrastriate visual cortex were activated by visually presented pseudowords that obey English spelling rules, as well as by actual words. These areas were not activated by nonsense strings of letters or letter-like forms. Thus visual word form computations are based on learned distinctions between words and nonwords. In addition, during passive presentation of words, but not pseudowords, activation occurred in a left frontal area that is related to semantic processing. These findings support distinctions made in cognitive psychology and computational modeling between high-level visual and semantic computations on single words and describe the anatomy that may underlie these distinctions.

Adult↗
PubMed1989

Neuroanatomical correlates of anticipatory anxiety.

Positron emission tomographic measurements of regional blood flow, a marker of local neuronal activity, were used to investigate the neuroanatomical correlates of a normal emotion. Healthy volunteers were studied before, during, and after anticipation of a painful electric shock. During anticipatory anxiety, there were significant blood flow increases in bilateral temporal poles, the same regions recently implicated in a lactate-induced anxiety attack in patients with panic disorder. Thus, the temporal poles seem to be involved in normal and pathological forms of human anxiety.

Anxiety↗
PubMed1989

Neuroanatomical correlates of a lactate-induced anxiety attack.

Positron emission tomographic measurements of regional blood flow were used to assess local neuronal activity in patients with panic disorder and in normal control subjects before and during the infusion of sodium lactate. A new technique for the analysis of positron emission tomographic data was employed to identify significant changes in regional blood flow associated with lactate infusion in the panicking patients, nonpanicking patients, and controls. Lactate-induced panic was associated with significant blood flow increases bilaterally in the temporal poles; bilaterally in insular cortex, claustrum, or lateral putamen; bilaterally in or near the superior colliculus; and in or near the left anterior cerebellar vermis. Lactate infusion was not associated with significant changes in regional blood flow in the nonpanicking patients or control subjects. Thus, the identified regions seemed to be involved in an anxiety attack.

Adult↗
PubMed1989

A highly accurate method of localizing regions of neuronal activation in the human brain with positron emission tomography.

Functional mapping of the human brain with positron emission tomography (PET) can best be performed by obtaining multiple short measurements of cerebral blood flow in a single sitting. In this manner regional changes in blood flow accompanying the increased neuronal activity from a movement, sensation, or even cognition task, have been identified. However, localizing a functional region with PET has been severely limited by the poor resolving properties of PET devices. Using a new method of data analysis we recently reported the mapping of visual field stimuli on human visual cortex with surprisingly high reliability as measured by the low standard deviation in positions across different subjects (as low as 1 mm). In this work the analysis technique enabling such high-resolution functional brain mapping is fully described. Additionally, simulations are presented to illustrate its advantages and limitations.

Brain↗
PubMed1989

Noninvasive functional brain mapping by change-distribution analysis of averaged PET images of H215O tissue activity.

Change-distribution analysis and intersubject averaging of subtracted positron emission tomography (PET) images are new techniques for detecting, localizing, and quantifying state-dependent focal transients in neuronal activity. We previously described their application to cerebral blood flow images (intravenous bolus H215O, Kety autoradiographic model). We now describe their application to images of H215O regional tissue activity without conversion to units of blood flow. The sensitivity and specificity of response detection and the accuracy of response localization were virtually identical for the two types of images. Response magnitude expressed in percent change from rest was slightly, but consistently smaller in tissue-activity images. Response magnitude expressed in z-score was the same for the two-image types. Most research and clinical applications of functional brain mapping can employ images of H215O tissue activity (intravenous bolus, 40-sec nondynamic scan) without conversion to units of blood flow. This eliminates arterial blood sampling, thereby simplifying and minimizing the invasivity of the PET procedure.

Adult↗
PubMed1988

Nonoxidative glucose consumption during focal physiologic neural activity.

Brain glucose uptake, oxygen metabolism, and blood flow in humans were measured with positron emission tomography, and a resting-state molar ratio of oxygen to glucose consumption of 4.1:1 was obtained. Physiological neural activity, however, increased glucose uptake and blood flow much more (51 and 50 percent, respectively) than oxygen consumption (5 percent) and produced a molar ratio for the increases of 0.4:1. Transient increases in neural activity cause a tissue uptake of glucose in excess of that consumed by oxidative metabolism, acutely consume much less energy than previously believed, and regulate local blood flow for purposes other than oxidative metabolism.

Brain↗
PubMed1988

Localization of cognitive operations in the human brain.

The human brain localizes mental operations of the kind posited by cognitive theories. These local computations are integrated in the performance of cognitive tasks such as reading. To support this general hypothesis, new data from neural imaging studies of word reading are related to results of studies on normal subjects and patients with lesions. Further support comes from studies in mental imagery, timing, and memory.

Attention↗
PubMed1988

Enhanced detection of focal brain responses using intersubject averaging and change-distribution analysis of subtracted PET images.

Intersubject averaging and change-distribution analysis of subtracted positron emission tomographic (PET) images were developed and tested. The purpose of these techniques is to increase the sensitivity and objectivity of functional mapping of the human brain with PET. To permit image averaging, all primary tomographic images were converted to anatomically standardized three-dimensional images using stereotactic anatomical localization and interslice interpolation. Image noise, measured in control-minus-control subtractions, was strongly suppressed by averaging. Signal-to-noise ratio, measured in stimulus-minus-control subtractions (hand vibration minus eyes-closed rest), rose steadily with averaging, confirming the accuracy of our method of anatomical standardization. Distribution analysis of CBF change images (outlier detection by gamma-2 statistic) was assessed as an omnibus test for state-dependent changes in regional neuronal activity. Sensitivity in detecting the somatosensory response rose steadily with averaging, increasing from 50% in individual images to 100% when three or more images were averaged. Specificity was 100% at all averaging levels. Although described here as a technique for functional brain mapping with H2(15O) CBF images, image averaging, and change-distribution analysis are more generally applicable techniques, not limited to a single purpose or tracer.

Adolescent↗
PubMed1988

The effect of carotid artery disease on the cerebrovascular response to physiologic stimulation.

Eight of 16 patients with severe carotid artery disease, but no evidence of functional or structural brain damage, had abnormal regional cerebral blood flow (rCBF) responses to physiologic stimulation of sensorimotor cortex (unilateral reduction in six, abnormal diffuse increase in two). Thus, in patients with cerebrovascular disease the rCBF responses during physiologic tasks may not be reliable indicators of local neuronal activity.

Brain↗
PubMed1987

Regional asymmetries of cerebral blood flow, blood volume, and oxygen utilization and extraction in normal subjects.

Positron emission tomography (PET) and 15O-labeled radiotracers were used to measure regional CBF, cerebral blood volume (CBV), CMRO2, and oxygen extraction in 32 right-handed subjects at rest. Mean left hemispheric CBF (46.2 +/- 6.8 ml/100 g/min) and CMRO2 (2.60 +/- 0.59 ml/100 g/min) were significantly lower than right hemispheric values (47.4 +/- 7.2 and 2.66 +/- 0.61 ml/100 g/min, respectively; p less than 0.0001 for both), whereas left and right hemispheric CBV and oxygen extraction were not significantly different. We further investigated these asymmetries by comparing left- and right-sided values for specific cortical and subcortical regions. We found that left-sided CBF and CMRO2 were significantly lower than right-sided values for sensorimotor, occipital, and superior temporal regions, whereas only left-sided CBF values were lower for anterior cingulum. CBV was asymmetric for the anterior cingulate and mid-frontal regions, and oxygen extraction was asymmetric for the sensorimotor area. No asymmetries were observed in inferior parietal cortex, thalamus, putamen, or pallidum. Knowledge of these normal physiological asymmetries is essential for proper interpretation of PET studies of physiology and pathology. Furthermore, the ability to detect asymmetries with PET may lead to a better understanding of the lateralization of specific functions in the human brain.

Adult↗
PubMed1987

Mapping human somatosensory cortex with positron emission tomography.

Positron emission tomography measurements of regional cerebral blood flow were used to detect focal neuronal activation in the first somatosensory cortex (SI) of humans induced by cutaneous vibratory stimulation. Intravenously administered water labeled with oxygen-15 (H2(15)O) was used as a blood flow tracer to obtain five stimulated-state and two resting-state blood flow images in each of eight normal volunteers. Three cutaneous surfaces were tested: lips, fingers, and toes. Intense, highly focal SI responses were seen during all 39 stimulated-state trials. The SI responses from the three stimulation sites were anatomically distinct and formed a medial-to-lateral homonculus in every subject. Response magnitudes (increase in local blood flow) and response locales (expressed as proportionately measured bicommissural stereotaxic coordinates) were highly consistent among subjects and on repeated trials for each subject. These findings suggest that eliciting cerebral blood flow responses by cutaneous vibration provides a safe, rapid, and reproducible tool for locating and assessing the functional status of somatosensory cortex, and offers potential clinical and research utility. This study has established normative values for future applications of this experimental paradigm.

Brain Mapping↗
PubMed1987

Retinotopic organization of human visual cortex mapped with positron-emission tomography.

The retinotopic organization of primary visual cortex was mapped in normal human volunteers. Positron-emission tomographic measurements of regional cerebral blood flow were employed to detect focal functional brain activation. Oxygen-15-labeled water, delivered by intravenous bolus, was used as the blood flow tracer to allow multiple stimulated-state (n = 5) and control-state (n = 3) measurements to be acquired for each of 7 subjects. Responses were identified by applying a maximum-detection algorithm to subtraction-format images of the stimulus-induced change in cerebral blood flow. Response locales were described using a standardized system of stereotactic coordinates. Changes in stimulus location (macular, perimacular, peripheral, upper-field, lower-field) caused systematic, highly significant changes in response locale within visual cortex. Discrete extrastriate visual responses were also observed.

Adult↗
PubMed1986

Focal physiological uncoupling of cerebral blood flow and oxidative metabolism during somatosensory stimulation in human subjects.

Coupling between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) was studied using multiple sequential administrations of 15O-labeled radiotracers (half-life, 123 sec) and positron emission tomography. In the resting state an excellent correlation (mean r, 0.87) between CBF and CMRO2 was found when paired measurements of CBF and CMRO2 from multiple (30-48) brain regions were tested in each of 33 normal subjects. Regional uncoupling of CBF and CMRO2 was found, however, during neuronal activation induced by somatosensory stimulation. Stimulus-induced focal augmentation of cerebral blood flow (29% mean) far exceeded the concomitant local increase in tissue metabolic rate (mean, 5%), when resting-state and stimulated-state measurements were obtained in each of 9 subjects. Stimulus duration had no significant effect on response magnitude or on the degree of CBF-CMRO2 uncoupling observed. Dynamic, physiological regulation of CBF by a mechanism (neuronal or biochemical) dependent on neuronal firing per se, but independent of the cerebral metabolic rate of oxygen, is hypothesized.

Adult↗