Biomedical subjects

A C Nobre

Publications and source records attributed to A C Nobre.

9 recordsLinked to original sources

PubMed1995

Functional magnetic resonance imaging of sensory and motor cortex: comparison with electrophysiological localization.

Functional magnetic resonance (MR) imaging was performed using a 1.5-tesla MR system to localize sensorimotor cortex. Six neurologically normal subjects were studied by means of axial gradient-echo images with a motor task and one or more sensory tasks: 1) electrical stimulation of the median nerve; 2) continuous brushing over the thenar region; and 3) pulsed flow of compressed air over the palm and digits. An increased MR signal was observed in or near the central sulcus, consistent with the location of primary sensory and motor cortex. Four patients were studied using echo planar imaging sequences and motor and sensory tasks. Three patients had focal refractory seizures secondary to a lesion impinging on sensorimotor cortex. Activation seen on functional MR imaging was coextensive with the location of the sensorimotor area determined by evoked potentials and electrical stimulation. Functional MR imaging provides a useful noninvasive method of localization and functional assessment of sensorimotor cortex.

Adult↗

PubMed1995

Language-related field potentials in the anterior-medial temporal lobe: I. Intracranial distribution and neural generators.

Field potentials were recorded from intracranial electrodes in humans to study language-related processing. Subjects viewed sentences in which each word was presented successively in the center of a video monitor. Half of the sentences ended normally, while the other half ended with a semantically anomalous word. The anomalous sentence-ending words elicited a large negative field potential with a peak latency near 400 msec, which was focally distributed bilaterally in the anterior medial temporal lobe (AMTL), anterior to the hippocampus and near the amygdala. Subdural electrodes positioned near the collateral sulcus just inferior and lateral to the amygdala recorded a positive field potential at the same latency. This spatial distribution of voltage suggested that this language-sensitive field potential was generated in the neocortex near the collateral sulcus and anterior fusiform gyrus. Additional task-related field potentials were recorded in the hippocampus. The AMTL field potential at 400 msec shares characteristics with the N400 potential recorded from scalp electrodes that has been associated with semantic processing.

Action Potentials↗

PubMed1995

Language-related field potentials in the anterior-medial temporal lobe: II. Effects of word type and semantic priming.

Field potentials were recorded from intracranial electrodes in humans to study the role of the anterior medial temporal lobe (AMTL) in language-related processing. Subjects viewed lists of words in which orthography and word type varied, or in which words were primed by semantic associates. Large negative field potentials were elicited within the AMTL by isolated words. The amplitude and intracranial distribution of these AMTL field potentials were consistent with those in our previous study in which anomalous sentence-ending words were used as stimuli (McCarthy et al., in press). The neocortex, in the region of the collateral sulcus and anterior fusiform gyrus, was identified as the likely neural generator of this field potential. The AMTL field potential was diminished by semantic priming, and was larger for words with semantic content than for words serving grammatical function. Orthographically illegal nonwords did not elicit this field potential. The N400 scalp event-related potential (ERP) has been shown to respond in the same manner to these task manipulations (Nobre and McCarthy, 1994), and, thus, the AMTL field potential was proposed to contribute to the generation of N400. The possible roles in language processing reflected by the AMTL field potential were considered.

Action Potentials↗

PubMed1994

Word recognition in the human inferior temporal lobe.

Studies of primates and of patients with brain lesions have shown that the visual system represents the external world in regions and pathways specialized to compute visual features and attributes. For example, object recognition is performed by a ventral pathway located in the inferior portion of the temporal lobe. We studied visual processing of words and word-like stimuli (letter-strings) by recording field potentials directly from the human inferior temporal lobe. Our results showed that two discrete portions of the fusiform gyrus responded preferentially to letter-strings. A region of the posterior fusiform gyrus responded equally to words and non-words, and was unaffected by the semantic context in which words were presented. In contrast, a region of the anterior fusiform gyrus was sensitive to these stimulus dimensions. These regions were distinct from areas that responded to other types of complex visual stimuli, including faces and coloured patterns, and thus form a functionally specialized stream within the ventral visual pathway.

Adult↗

PubMed1994

Functional magnetic resonance imaging of human prefrontal cortex activation during a spatial working memory task.

High-speed magnetic resonance (MR) imaging was used to detect activation in the human prefrontal cortex induced by a spatial working memory task modeled on those used to elucidate neuronal circuits in nonhuman primates. Subjects were required to judge whether the location occupied by the current stimulus had been occupied previously over a sequence of 14 or 15 stimuli presented in various locations. Control tasks were similar in all essential respects, except that the subject's task was to detect when one of the stimuli presented was colored red (color detection) or when a dot briefly appeared within the stimulus (dot detection). In all tasks, two to three target events occurred randomly. The MR signal increased in an area of the middle frontal gyrus corresponding to Brodmann's area 46 in all eight subjects performing the spatial working memory task. Right hemisphere activation was greater and more consistent than left. The MR signal change occurred within 6-9 sec of task onset and declined within a similar period after task completion. An increase in MR signal was also noted in the control tasks, but the magnitude of change was less than that recorded in the working memory task. These differences were replicated when testing was repeated in five of the original subjects. The localization of spatial working memory function in humans to a circumscribed area of the middle frontal gyrus supports the compartmentalization of working memory functions in the human prefrontal cortex and the localization of spatial memory processes to comparable areas in humans and nonhuman primates.

Color Perception↗

PubMed1994

Qualitative mapping of cerebral blood flow and functional localization with echo-planar MR imaging and signal targeting with alternating radio frequency.

PURPOSE: To create qualitative maps of cerebral blood flow (CBF) with the EPISTAR (echo-planar imaging and signal targeting with alternating radio frequency) technique. MATERIALS AND METHODS: The EPISTAR technique was performed in a pig model of hypercapnia and then tested in 26 volunteers by using various paradigms for cortical activation. Echo-planar images were acquired with and without use of a radio-frequency inversion pulse applied to inflowing arterial spins. A qualitative map of CBF was then created by subtracting the image obtained without the radio-frequency pulse from that obtained with the radio-frequency pulse. RESULTS: Progressively more distal portions of the tagged vessels were seen as the inflow time was lengthened until cortical enhancement was seen for inflow times of approximately 1 second or longer. Signal intensity increases from rest to sensorimotor activation ranged from 13% to 193%. CBF changes in the motor strip, primary visual cortex, and the motor area for eye movements were well localized to the cortical gray matter ribbon. CONCLUSION: The EPISTAR technique is a rapid, noninvasive means for creating qualitative maps of CBF.

Animals↗

PubMed1994

Face recognition in human extrastriate cortex.

1. Twenty-four patients with electrodes chronically implanted on the surface of extrastriate visual cortex viewed faces, equiluminant scrambled faces, cars, scrambled cars, and butterflies. 2. A surface-negative potential, N200, was evoked by faces but not by the other categories of stimuli. N200 was recorded only from small regions of the left and right fusiform and inferior temporal gyri. Electrical stimulation of the same region frequently produced a temporary inability to name familiar faces. 3. The results suggest that discrete regions of inferior extrastriate visual cortex, varying in location between individuals, are specialized for the recognition of faces. These "face modules" appear to be intercalated among other functionally specific small regions.

Adolescent↗

PubMedDate not supplied

Modulation of semantic processing by spatial selective attention.

The effects of spatial selective attention upon ERPs associated with the processing of word stimuli were investigated. While subjects maintained central eye fixation, ERPs were recorded to words presented to the left and right visual fields. In each of 6 runs, subjects focussed attention to alternate fields to perform a category-detection task. Pairs of semantically related and repeated words were embedded in the word lists presented to the attended and unattended visual fields. Consistent with prior studies, the P1-N1 visual ERP was larger when elicited by words in attended spatial locations. A large negative slow wave identified as N400 was elicited by attended, but not unattended, words. For attended words, N400 was smaller for semantically primed or repeated words. We concluded that spatial selective attention can modulate the degree to which words are processed, and that the cognitive processes associated with N400 are not automatic.

Adolescent↗

PubMedDate not supplied

Electrophysiological studies of color processing in human visual cortex.

Electrophysiological recordings from human visual cortex were carried out with electrodes chronically implanted in 13 patients for localization of an epileptogenic focus. Visual evoked potentials (VEPs) elicited by red or blue checkerboard stimuli were recorded using an adaptation stimulus-test stimulus design in which color was the most salient feature. A "significant color effect," defined as a statistically significant effect of the adaptation stimulus on test stimulus VEPs evoked by the same or a different color, was determined for various cortical regions: medial lingual gyrus, 20%; lateral lingual gyrus, 38%; posterior fusiform gyrus, 50%; anterior fusiform gyrus, 0%; inferior temporal gyrus, 5%; occipital pole, 30%; lateral surface of non-visual cortex, 6%; inferior parietal and temporal cortex, 5%. The time course of the significant color effects suggests that wave length-selective neuronal activity occurs initially at the first stage of cortical processing in the medial lingual gyrus, followed by progressively later activation of the lateral lingual gyrus, the posterior fusiform gyrus, and the inferior temporal gyrus. In two patients, stimulation of the lateral lingual and fusiform gyri elicited color sensations in the contralateral half-field, whereas stimulation of the medial lingual and cuneate gyri evoked retinotopically appropriate quadrantic "shimmering" devoid of color. These results suggest that a region of inferior occipital cortex, primarily the posterior portion of the fusiform gyrus, is involved in color perception and may be homologous with area V4 in monkeys. There is also a region of dorsolateral surface cortex which exhibits a fairly high percentage of significant color effects and when stimulated may evoke sensations of color. This region may be the same as the dorsolateral region thought to be involved in selective attention to color.

Adult↗