RESUMEN
The well-known arcuate fasciculus that connects the posterior superior temporal region with the language production region in the ventrolateral frontal cortex constitutes the classic peri-Sylvian dorsal stream of language. A second temporofrontal white matter tract connects ventrally the anterior to intermediate lateral temporal cortex with frontal areas via the extreme capsule. This temporofrontal extreme capsule fasciculus (TFexcF) constitutes the ventral stream of language processing. The precise origin, course, and termination of this pathway has been examined in invasive tract tracing studies in macaque monkeys, but there have been no standard protocols for its reconstruction in the human brain using diffusion imaging tractography. Here we provide a protocol for the dissection of the TFexcF in vivo in the human brain using diffusion magnetic resonance imaging (MRI) tractography which provides a solid basis for exploring its functional role. A key finding of the current dissection protocol is the demonstration that the TFexcF is left hemisphere lateralized. Furthermore, using the present dissection protocol, we demonstrate that the TFexcF is related to lexical retrieval scores measured with the category fluency test, in contrast to the classical arcuate fasciculus (the dorsal language pathway) that was also dissected and was related to sentence repetition.
Asunto(s)
Imagen de Difusión por Resonancia Magnética , Lóbulo Frontal , Humanos , Vías Nerviosas/diagnóstico por imagen , Lóbulo Frontal/diagnóstico por imagen , Imagen de Difusión Tensora , Lóbulo Temporal/diagnóstico por imagenRESUMEN
The relationship between the local morphological features that define the entorhinal and parahippocampal cortex in the medial temporal region of the human brain and activation as measured during a navigation task with functional magnetic resonance imaging was examined individually in healthy participants. Two functional activation clusters were identified one within the caudal end of the collateral sulcus proper and the other in the parahippocampal extension of the collateral sulcus, clearly establishing the activation in the posterior parahippocampal cortex. A third activation cluster was identified where the anterior segment of the collateral sulcus proper gives way to the posterior segment, demonstrating also activation within the middle parahippocampal cortex. No activation was observed in the entorhinal cortex that lies medial to the rhinal sulcus or in the anterior part of the parahippocampal cortex along the anterior branch of the collateral sulcus proper. The activations could also be clearly differentiated from the cortex of the fusiform and lingual gyri that lie laterally and posteriorly. These findings demonstrated specific activation in the middle and posterior part of the parahippocampal cortex when information necessary for navigation was retrieved from a previously established cognitive map and demonstrate that the sulci that comprise the collateral sulcal complex represent important landmarks that can provide an accurate localization of activation foci along the parahippocampal cortex and allow identification of subdivisions involved in the processing of spatial information.
Asunto(s)
Giro Parahipocampal/anatomía & histología , Giro Parahipocampal/fisiología , Navegación Espacial/fisiología , Adulto , Mapeo Encefálico , Femenino , Humanos , Imagen por Resonancia Magnética , Masculino , Adulto JovenRESUMEN
Conditional associative sensori-motor learning (i.e. the acquisition of specific arbitrary sensori-motor mappings) involves several processes that depend upon the integrity of the fronto-striatal system. The specific role of the different components of the fronto-striatal system in this type of learning is still unclear and was examined in the present functional Magnetic Resonance Imaging (fMRI) study in humans. The subjects had to learn by trial and error arbitrary associations between visual stimuli and motor responses in an experimental paradigm designed to dissociate between the neuronal substrates specifically involved in the selection of the appropriate response and in the analysis of the feedback obtained during the learning and post-learning periods. First, the results demonstrate that the dorsal premotor (PMd) cortex is the critical structure for the acquisition and execution of arbitrary mappings of visual stimuli to motor responses. Second, they reveal an important shift in activation from the cognitive fronto-striatal network (involving the caudate nucleus, the dorsolateral prefrontal cortex, and the PMd) to the motor fronto-striatal network (involving the putamen and the PMd) as we move from initial learning of sensori-motor relations to the post-learning selection of the responses. Finally, they show that feedback processing, but not response selection, increased activity in the anterior cingulate and orbitofrontal cortical regions, demonstrating the selective involvement of these limbic frontal regions in the processing of the consequences of a given action. Altogether our data suggest that, in conditional visuo-motor learning, the associations are critically regulated by the dorsal premotor cortex and the striatum, with additional brain areas contributing to specific aspects of the learning and performance of such associations.
Asunto(s)
Encéfalo/fisiología , Retroalimentación Sensorial/fisiología , Aprendizaje/fisiología , Imagen por Resonancia Magnética , Adulto , Femenino , Humanos , Masculino , Actividad Motora , Estimulación LuminosaRESUMEN
The study of architectonic differentiation in the cortex is advanced by the articulation of objective definitions based on clear features of the cortical architecture. We adopted areal density profiles as a means of sampling the cortex. On the profiles, we isolated and quantified the density of individual cortical layers. These features may serve as criteria in objective definitions in a way that builds on qualitative observations found in the classical literature. A preprocessing procedure was introduced to overcome artefacts in the density profiles caused by the partial overlap of neighboring neuronal layers and cortical folding. We applied this method to the medial half of the orbital frontal cortex in specimens drawn from 10 human postmortem brain hemispheres. The measurements successfully confirmed the existence of several qualitatively observed areas (architectonic areas 14c, 14r, 11m, 11 and 13). The selection of specific sampling parameters was justified on the basis of simultaneous measurements of the cortical morphology which demonstrate its influence on the appearance of the cortical layers. We also examined the robustness of the measuring procedure by analyzing the outcome of varying systematically the sampling parameters. We describe here a novel method of sampling the cortex for architectonic analysis and demonstrate its application on histological sections obtained from the medial half of the human orbitofrontal cortex.
Asunto(s)
Lóbulo Frontal/anatomía & histología , Interpretación de Imagen Asistida por Computador/métodos , Análisis de Varianza , Recuento de Células , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Fotomicrografía , Células PiramidalesRESUMEN
Developments in functional neuroimaging in normal human subjects, such as functional magnetic resonance imaging (fMRI), have permitted the mapping of several visual areas of the human brain and have already provided provisional identification of some of the visual areas that were first described in nonhuman primates. However, the lack of a detailed description of the sulcal patterns of the human occipital lobe makes it difficult to establish clear relationships between sulcal landmarks and identified visual areas with functional neuroimaging. In the present study we used magnetic resonance images to investigate the morphological variation of the human occipital sulci in both the left and right hemispheres of 40 normal adult human brains. We identified 11 occipital sulci, the parieto-occipital fissure and the temporo-occipital incisure, and their corresponding gray matter voxels were marked in the magnetic resonance volumes which had been transformed into the Montreal Neurological Institute standard proportional stereotaxic space. Probability maps were then constructed for each occipital sulcus. These probability maps provide a quantitative measure of the variability of the occipital sulci in standard stereotaxic space and are a useful tool to identify the location of voxels of other magnetic resonance imaging images transformed in the same stereotaxic space.
Asunto(s)
Mapeo Encefálico/métodos , Lóbulo Occipital/anatomía & histología , Lóbulo Occipital/fisiología , Técnicas Estereotáxicas , Adulto , Femenino , Lateralidad Funcional/fisiología , Humanos , Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Masculino , Modelos Anatómicos , Modelos Estadísticos , Caracteres Sexuales , Corteza Visual/anatomía & histología , Corteza Visual/fisiologíaRESUMEN
Do lesions of the fornix or the hippocampus impair the performance of spatial conditional associative learning tasks, and to what extent does damage to these brain structures result in comparable deficits in this type of spatial behavior? The available evidence is not clear. In the present study, rats with lesions of the fornix, hippocampus, and normal control animals were trained on two spatial-visual conditional learning tasks in which they had to form arbitrary associations between visual stimuli and the context in which these stimuli were embedded. In one condition, rats were required to choose stimulus X in place A and stimulus Y in place B, and there was no overlap in the contents of the two scenes. In the other condition, the animal approached the same scene from two different directions and had to select stimulus X when the scene was viewed from perspective A and to select stimulus Y when the scene was viewed from perspective B. Rats with fornix transection were able to learn both conditional tasks at a rate comparable to that of normal control animals, but rats with hippocampal damage were severely impaired under both conditions. The findings extend the range of tasks known to be sensitive to damage of the hippocampus. In addition, the results argue that the fornix is not necessary for the acquisition of certain spatial conditional learning tasks and that this brain structure cannot be used as an indicator of hippocampal dysfunction under all learning situations.
Asunto(s)
Aprendizaje por Asociación/fisiología , Fórnix/fisiología , Hipocampo/fisiología , Conducta Espacial/fisiología , Análisis de Varianza , Animales , Conducta Animal , Conducta Exploratoria/fisiología , Fórnix/lesiones , Hipocampo/lesiones , Masculino , Estimulación Luminosa/métodos , RatasRESUMEN
The anterior thalamic region is intimately linked anatomically and functionally with the hippocampus, which is critical for various forms of spatial learning. Rats with lesions to the anterior thalamic nuclei and a control group were trained on a visual-spatial conditional associative learning task in which they had to learn to go to one of two locations depending on the particular visual cue presented on each trial; the rats approached the cues from different directions. The animals were subsequently tested on a spatial working memory task, the eight-arm radial maze. Performance on both these tasks had previously been shown to be impaired by hippocampal lesions. Rats with anterior thalamic damage were able to acquire the conditional associative task at a rate comparable to that of the control animals, but were impaired on the radial maze task. The finding of a dissociation between the effects of lesions of the anterior thalamic nuclei on two different classes of behavior known to be associated with hippocampal function suggest that while different neural stations within the extended hippocampal circuit may all play a role in spatial learning, the role of each of these regions in such learning may be more selective than previously considered.
Asunto(s)
Núcleos Talámicos Anteriores/fisiología , Conducta Animal/fisiología , Condicionamiento Operante/fisiología , Animales , Memoria a Corto Plazo/fisiología , Ratas , Conducta Espacial/fisiología , Percepción Visual/fisiologíaRESUMEN
The efferent association fibers from the caudal part of the prefrontal cortex to posterior cortical areas course via several pathways: the three components of the superior longitudinal fasciculus (SLF I, SLF II, and SLF III), the arcuate fasciculus (AF), the fronto-occipital fasciculus (FOF), the cingulate fasciculus (CING F), and the extreme capsule (Extm C). Fibers from area 8Av course via FOF and SLF II, merging in the white matter of the inferior parietal lobule (IPL) and terminating in the caudal intraparietal sulcus (IPS). A group of these fibers turns ventrally to terminate in the caudal superior temporal sulcus (STS). Fibers from the rostral part of area 8Ad course via FOF and SLF II to the IPS and IPL and via the AF to the caudal superior temporal gyrus and STS. Some fibers from the rostral part of area 8Ad are conveyed to the medial parieto-occipital region via FOF, to the STS via Extm C, and to the caudal cingulate gyrus via CING F. Fibers from area 8B travel via SLF I to the supplementary motor area and area 31 in the caudal dorsal cingulate region and via the CING F to cingulate areas 24 and 23 and the cingulate motor areas. Fibers from area 9/46d course via SLF I to the superior parietal lobule and medial parieto-occipital region, via SLF II to the IPL. Fibers from area 9/46v travel via SLF III to the rostral IPL and the frontoparietal opercular region and via the CING F to the cingulate gyrus.
Asunto(s)
Vías Eferentes/anatomía & histología , Macaca/anatomía & histología , Corteza Prefrontal/anatomía & histología , Aminoácidos/química , Aminoácidos/metabolismo , Animales , Vías Eferentes/metabolismo , Isótopos/química , Isótopos/metabolismo , Macaca/metabolismo , Imagen por Resonancia Magnética , Corteza Prefrontal/metabolismoRESUMEN
The effects of cylindrical orifice length and diameter on the flow rate of three commonly used pharmaceutical direct compression diluents (lactose, dibasic calcium phosphate dihydrate and pregelatinised starch) were investigated, besides the powder particle characteristics (particle size, aspect ratio, roundness and convexity) and the packing properties (true, bulk and tapped density). Flow rate was determined for three different sieve fractions through a series of miniature tableting dies of different orifice diameter (0.4, 0.3 and 0.2 cm) and thickness (1.5, 1.0 and 0.5 cm). It was found that flow rate decreased with the increase of the orifice length for the small diameter (0.2 cm) but for the large diameter (0.4 cm) was increased with the orifice length (die thickness). Flow rate changes with the orifice length are attributed to the flow regime (transitional arch formation) and possible alterations in the position of the free flowing zone caused by pressure gradients arising from the flow of self-entrained air, both above the entrance in the die orifice and across it. Modelling by the conventional Jones-Pilpel non-linear equation and by two machine learning algorithms (lazy learning, LL, and feed-forward back-propagation, FBP) was applied and predictive performance of the fitted models was compared. It was found that both FBP and LL algorithms have significantly higher predictive performance than the Jones-Pilpel non-linear equation, because they account both dimensions of the cylindrical die opening (diameter and length). The automatic relevance determination for FBP revealed that orifice length is the third most influential variable after the orifice diameter and particle size, followed by the bulk density, the difference between bulk and tapped densities and the particle convexity.
Asunto(s)
Excipientes Farmacéuticos/química , Comprimidos/química , Algoritmos , Fosfatos de Calcio/química , Lactosa/química , Microfluídica , Redes Neurales de la Computación , Tamaño de la Partícula , Polvos , Almidón/química , Tecnología Farmacéutica , Factores de TiempoRESUMEN
We used positron emission tomography (PET) to investigate brain regions associated with odor imagery. Changes in regional cerebral blood flow (CBF) during odor imagery were compared with changes during nonspecific expectation of olfactory stimuli and with those during odor perception. Sixty-seven healthy volunteers were screened for their odor imagery (with a paradigm developed in a previous study), and 12 of them, assessed to be "good odor imagers," participated in the neuroimaging part of the study. Imagination of odors was associated with increased activation in several olfactory regions in the brain: the left primary olfactory cortical (POC) region including piriform cortex, the left secondary olfactory cortex or posterior orbitofrontal cortex (OFC), and the rostral insula bilaterally. Furthermore, blood flow in two regions within the right orbitofrontal cortex correlated significantly with the behavioral measure of odor imagery during scanning. Overall, the findings indicated that neural networks engaged during odor perception and imagery overlap partially.
Asunto(s)
Encéfalo/diagnóstico por imagen , Imaginación/fisiología , Odorantes , Adolescente , Adulto , Conducta/fisiología , Circulación Cerebrovascular/fisiología , Femenino , Humanos , Interpretación de Imagen Asistida por Computador , Masculino , Vías Olfatorias/irrigación sanguínea , Vías Olfatorias/diagnóstico por imagen , Tomografía de Emisión de Positrones , Análisis de Regresión , Mecánica Respiratoria , Estimulación QuímicaRESUMEN
Rats with lesions of the hippocampus, the mammillary region, the anterior thalamic nuclei, and normal control animals were trained on a conditional associative learning task in which they had to learn to make one of two motor responses (i.e., turn left or right), depending on which one of two visual cues was presented. Damage to the hippocampus severely impaired performance of this task. By contrast, rats with lesions of the mammillary region or the anterior thalamic nuclei were able to acquire the task at a rate comparable to that of the normal animals. These findings demonstrate that hippocampal lesions impair the ability to form arbitrary associations between visual cues and kinesthetic responses (body turns) and, furthermore, suggest that the hippocampus does not rely on input from its major subcortical targets for learning such visual-kinesthetic associations.
Asunto(s)
Núcleos Talámicos Anteriores/fisiología , Aprendizaje por Asociación/fisiología , Condicionamiento Psicológico/fisiología , Hipocampo/fisiología , Tubérculos Mamilares/fisiología , Vías Nerviosas/fisiología , Animales , Señales (Psicología) , Desnervación , Cinestesia/fisiología , Discapacidades para el Aprendizaje/patología , Discapacidades para el Aprendizaje/fisiopatología , Aprendizaje por Laberinto/fisiología , Estimulación Luminosa , Desempeño Psicomotor/fisiología , RatasRESUMEN
OBJECTIVE: The gross morphology and morphometry of the hippocampus, fornix, and corpus callosum in patients with severe non-missile traumatic brain injury (nmTBI) without obvious neuroradiological lesions was examined and the volumes of these structures were correlated with performance on memory tests. In addition, the predictability of the length of coma from the selected anatomical volumes was examined. METHOD: High spatial resolution T1 weighted MRI scans of the brain (1 mm3) and neuropsychological evaluations with standardised tests were performed at least 3 months after trauma in 19 patients. RESULTS: In comparison with control subjects matched in terms of gender and age, volume reduction in the hippocampus, fornix, and corpus callosum of the nmTBI patients was quantitatively significant. The length of coma correlated with the volume reduction in the corpus callosum. Immediate free recall of word lists correlated with the volume of the fornix and the corpus callosum. Delayed recall of word lists and immediate recall of the Rey figure both correlated with the volume of the fornix. Delayed recall of the Rey figure correlated with the volume of the fornix and the right hippocampus. CONCLUSION: These findings demonstrate that in severe nmTBI without obvious neuroradiological lesions there is a clear hippocampal, fornix, and callosal volume reduction. The length of coma predicts the callosal volume reduction, which could be considered a marker of the severity of axonal loss. A few memory test scores correlated with the volumes of the selected anatomical structures. This relationship with memory performance may reflect the diffuse nature of the damage, leading to the disruption of neural circuits at multiple levels and the progressive neural degeneration occurring in TBI.
Asunto(s)
Lesiones Encefálicas/complicaciones , Lesiones Encefálicas/patología , Coma/fisiopatología , Cuerpo Calloso/patología , Fórnix/patología , Hipocampo/patología , Trastornos de la Memoria/etiología , Adulto , Anciano , Estudios de Casos y Controles , Femenino , Humanos , Imagen por Resonancia Magnética , Masculino , Persona de Mediana Edad , Pruebas Neuropsicológicas , Índice de Severidad de la Enfermedad , Factores de TiempoRESUMEN
Our aim was to quantify with functional magnetic resonance imaging (fMRI) changes in brain activity in concussed athletes and compare the results with those of normal control subjects. Regional brain activations associated with a working memory task were obtained from a group of concussed athletes (15 symptomatic, 1 asymptomatic) and eight matched control subjects, using blood oxygen level dependent (BOLD) fMRI. The average percent signal change from baseline to working memory condition in each region of interest was computed. Symptomatic concussed athletes demonstrated task-related activations in some but not all the regions of interest, even when they performed as well as the control subjects. Furthermore, several concussed athletes had additional increases in activity outside the regions of interest, not seen in the control group. Quantitative analysis of BOLD signals within regions of interest revealed that, in general, concussed athletes had different BOLD responses compared to the control subjects. The task-related activation pattern of the one symptom-free athlete was comparable to that of the control group. We also repeated the study in one athlete whose symptoms had resolved. On the first study, when he was still symptomatic, less task-related activations were observed. On follow-up, once his symptoms had disappeared, the task-related activations became comparable to those of the control group. These results demonstrate the potential of fMRI, in conjunction with the working memory task, to identify an underlying pathology in symptomatic concussed individuals with normal structural imaging results.
Asunto(s)
Traumatismos en Atletas/patología , Traumatismos en Atletas/fisiopatología , Conmoción Encefálica/patología , Conmoción Encefálica/fisiopatología , Adulto , Estudios de Seguimiento , Lateralidad Funcional/fisiología , Humanos , Imagen por Resonancia Magnética , Masculino , Memoria a Corto Plazo/fisiología , Oxígeno/sangreRESUMEN
We examined odor imagery by looking for its effects on detection of weak odors. Seventy-two healthy subjects performed a forced-choice odor detection task in one of three conditions: after being told to imagine an odor (odor imagery), after being told to imagine an object (visual imagery), or without having received imagery instructions (no-imagery control). For the two imagery conditions, the presented and imagined stimuli were either the same (matched) or different (mismatched). There was a significant difference between detection in the matched and mismatched conditions for odor imagery, but not for visual imagery. We conclude that our paradigm does measure odor imagery and that the effect of imagery on detection is both content- and modality-specific. Further, the difference between conditions was due to lower detection with mismatched odor imagery than without imagery, indicating that interference underlies the effect.
Asunto(s)
Discriminación en Psicología , Imaginación , Odorantes , Olfato/fisiología , Percepción Visual , Adolescente , Adulto , Femenino , Humanos , MasculinoRESUMEN
Williams syndrome (WS) is characterised by a defined genetic aetiology and a specific cognitive profile. It provides an opportunity to examine associations between neuroanatomy, behaviour, and genetics. High-resolution T1-weighted MRI of the brain of 12 patients with WS and 12 normal control subjects were used to estimate the shape and volume of the corpus callosum (CC), as well as the voxel intensity values as a measure its water content. The CC of patients with WS was more convex than that of normal control subjects and overall smaller in volume, particularly in the splenium and in the caudal part of the callosal body. In addition, there were higher ratio values of voxel intensity (i.e. less water content) in the mid-section of the body and the caudal part of the body of the CC. These combined features indicate an aberrant development of the CC in patients with WS and document some of the anatomical abnormalities that may underlie some of the cognitive impairments observed in subjects with WS.
Asunto(s)
Agenesia del Cuerpo Calloso , Trastornos del Conocimiento/patología , Cuerpo Calloso/patología , Malformaciones del Sistema Nervioso/patología , Síndrome de Williams/patología , Adolescente , Adulto , Antropometría , Corteza Cerebral/anomalías , Corteza Cerebral/patología , Corteza Cerebral/fisiopatología , Niño , Trastornos del Conocimiento/genética , Trastornos del Conocimiento/fisiopatología , Cuerpo Calloso/fisiopatología , Femenino , Lateralidad Funcional/fisiología , Humanos , Imagen por Resonancia Magnética , Masculino , Malformaciones del Sistema Nervioso/genética , Malformaciones del Sistema Nervioso/fisiopatología , Percepción Espacial/fisiología , Síndrome de Williams/genética , Síndrome de Williams/fisiopatologíaRESUMEN
Rats with lesions of the fornix, the hippocampus, or normal control animals were trained on a visual-spatial conditional associative learning task in which they had to learn to go to a particular location based on the presence of a specific visual cue; the rats approached the cues from different directions. Animals with damage of the fornix were able to learn the task at a rate comparable to that of the control animals. The performance of the hippocampal rats was significantly impaired as compared with the control group. Both the fornix and the hippocampal animals were significantly impaired on a spatial working memory task, the eight-arm radial maze. These findings suggest that, under certain conditions, a functional dissociation exists between the effects of damage to the fornix or the hippocampus and that the fornix may be only selectively involved in spatial learning and memory.
Asunto(s)
Aprendizaje por Asociación/fisiología , Encefalopatías/psicología , Condicionamiento Psicológico/fisiología , Fórnix , Hipocampo , Animales , Conducta Animal/fisiología , Encefalopatías/inducido químicamente , Señales (Psicología) , Agonistas de Aminoácidos Excitadores , Ácido Iboténico , Masculino , Aprendizaje por Laberinto/fisiología , Memoria/fisiología , Ratas , Percepción Espacial/fisiología , Percepción Visual/fisiologíaRESUMEN
A comparison of the cytoarchitecture of the human and the macaque monkey ventrolateral prefrontal cortex demonstrated a region in the monkey that exhibits the architectonic characteristic of area 45 in the human brain. This region occupies the dorsal part of the ventrolateral prefrontal convexity just below area 9/46v. Rostroventral to area 45 in the human brain lies a large cortical region labelled as area 47 by Brodmann. The ventrolateral component of this region extending as far as the lateral orbital sulcus has architectonic characteristics similar to those of the ventrolateral prefrontal region labelled by Walker as area 12 in the macaque monkey. We designated this region in both the human and the monkey ventrolateral prefrontal cortex as area 47/12. Thus, area 47/12 designates the specific part of the zone previously labelled as area 47 in the human brain that has the same overall architectonic pattern as that of Walker's area 12 in the macaque monkey brain. The cortical connections of these two areas were examined in the monkey by injecting fluorescent retrograde tracers. Although both area 45 and area 47/12 as defined here had complex multimodal input, they could be differentiated in terms of some of their inputs. Retrograde tracers restricted to area 47/12 resulted in heavy labelling of neurons in the rostral inferotemporal visual association cortex and in temporal limbic areas (i.e. perirhinal and parahippocampal cortex). In contrast, injections of tracers into dorsally adjacent area 45 demonstrated strong labelling in the superior temporal gyrus (i.e. the auditory association cortex) and the multimodal cortex in the upper bank of the superior temporal sulcus.
Asunto(s)
Macaca mulatta/anatomía & histología , Vías Nerviosas/fisiología , Neuronas/citología , Neuronas/fisiología , Corteza Prefrontal/citología , Corteza Prefrontal/fisiología , Anciano , Animales , Mapeo Encefálico , Colorantes , Colorantes Fluorescentes , Humanos , Macaca mulatta/fisiología , MasculinoRESUMEN
It has been argued that saccade generation is supported by two systems, a'where' system that decides the direction and extent of an impending saccade, and a 'when' system that is involved in the timing of the release of fixation. We evaluated the contributions of these systems to saccade latencies, and used functional MRI to identify the neural substrates of these systems. We found that advance knowledge of the direction and the timing of an impending target movement had both overlapping and discrete effects on saccade latencies and on neural activation. Knowledge of either factor decreased regular saccade latencies. However, knowledge of target direction increased the number of predictive and express saccades while knowledge of target timing did not. The brain activation data showed that advance knowledge of the direction or the timing of the target movement activated primarily overlapping structures. The precentral gyrus, in the region of the frontal eye fields, was more active in conditions in which some aspect of the target movement was predictable than in saccade control and fixation conditions. In the basal ganglia, activation discriminated between advance knowledge of target timing and target direction. The lenticular nuclei were more active when only target timing was known in advance, while the caudate was more active when only target direction was known in advance. These data suggest that the neural structures supporting the 'where' and 'when' systems are highly overlapping, although there is some dissociation sub-cortically. Knowledge of target timing and target direction converge in precentral gyrus, a region where there is strong evidence of context-dependent modulation of neural activity.
Asunto(s)
Ganglios Basales/fisiología , Movimientos Sacádicos/fisiología , Percepción Visual/fisiología , Adulto , Parpadeo , Femenino , Movimientos de la Cabeza , Humanos , Imagen por Resonancia Magnética , Masculino , Pruebas Neuropsicológicas , Factores de TiempoRESUMEN
Modulation of cortico-cortical connectivity in specific neural circuits might underlie some of the behavioural effects observed following repetitive transcranial magnetic stimulation (rTMS) of the human frontal cortex. This possibility was tested by applying rTMS to the left mid-dorsolateral frontal cortex (MDL-FC) and subsequently measuring functional connectivity of this region with positron emission tomography (PET) and TMS. The results showed a strong rTMS-related modulation of brain activity in the fronto-cingulate circuit. These results were confirmed in a parallel experiment in the rat using electrical stimulation and field-potential recordings. Future studies are needed to provide a direct link between the rTMS-induced modulation of cortical connectivity and its effects on specific behaviours.
Asunto(s)
Mapeo Encefálico , Circulación Cerebrovascular/fisiología , Giro del Cíngulo/fisiología , Vías Nerviosas/fisiología , Corteza Prefrontal/fisiología , Estimulación Magnética Transcraneal , Adolescente , Adulto , Animales , Estimulación Eléctrica , Emociones/fisiología , Femenino , Lateralidad Funcional/fisiología , Giro del Cíngulo/anatomía & histología , Giro del Cíngulo/diagnóstico por imagen , Humanos , Masculino , Vías Nerviosas/anatomía & histología , Vías Nerviosas/diagnóstico por imagen , Corteza Prefrontal/anatomía & histología , Corteza Prefrontal/diagnóstico por imagen , Ratas , Ratas Sprague-Dawley , Tomografía Computarizada de EmisiónRESUMEN
The present study examined the role of the prefrontal cortex in retrieval processing using functional magnetic resonance imaging in human subjects. Ten healthy subjects were scanned while they performed a task that required retrieval of specific aspects of visual information. In order to examine brain activity specifically associated with retrieval, we designed a task that had retrieval and control conditions that were perfectly matched in terms of depth of encoding, decision making and postretrieval monitoring and differed only in terms of whether retrieval was required. In the retrieval condition, based on an instructional cue, the subjects had to retrieve either the particular stimulus that was previously presented or its location. In the control condition, the cue did not instruct retrieval but shared with the instructional cues the function of alerting the subjects of the impending test phase. The comparison of activity between the retrieval and control conditions demonstrated a significant and selective increase in activity related to retrieval processes within the ventrolateral prefrontal cortical region, more specifically within area 47/12. These activity increases were bilateral but stronger in the right hemisphere. The present study by strictly controlling the level of encoding, postretrieval monitoring, and decision making has demonstrated a specific increase in the ventrolateral prefrontal region that could be clearly related to active retrieval processing, i.e. the active selection of particular stored visual representations.