RESUMEN
The language profile of patients suffering from Alzheimer's disease (AD) is characterized not only by lexicosemantic impairments but also by phonological deficits, as shown by an increasing number of neuropsychological studies. This study explored the functional neural correlates underlying phonological and lexicosemantic processing in AD. Using H(215)O PET functional brain imaging, a group of mild to moderate AD patients and a group of age-matched controls were asked to repeat four types of verbal stimuli: words, wordlike nonwords (WL+), non-wordlike nonwords (WL-) and simple vowels. The comparison between the different conditions allowed us to determine brain activation preferentially associated with lexicosemantic or phonological levels of language representations. When repeating words, AD patients showed decreased activity in the left temporo-parietal and inferior frontal regions relative to controls, consistent with distorted lexicosemantic representations. Brain activity was abnormally increased in the right superior temporal area during word repetition, a region more commonly associated with perceptual-phonological processing. During repetition of WL+ and WL- nonwords, AD patients showed decreased activity in the middle part of the superior temporal gyrus, presumably associated with sublexical phonological information; at the same time, AD patients showed larger activation than controls in the inferior temporal gyrus, typically associated with lexicosemantic levels of representation. Overall, the results suggest that AD patients use altered pathways to process phonological and lexicosemantic information, possibly related to a progressive loss of specialization of phonological and lexicosemantic neural networks.
Asunto(s)
Enfermedad de Alzheimer/complicaciones , Enfermedad de Alzheimer/diagnóstico por imagen , Corteza Cerebral/diagnóstico por imagen , Corteza Cerebral/patología , Trastornos del Lenguaje/diagnóstico por imagen , Trastornos del Lenguaje/etiología , Anciano , Anciano de 80 o más Años , Enfermedad de Alzheimer/fisiopatología , Trastornos de la Articulación/diagnóstico por imagen , Trastornos de la Articulación/etiología , Trastornos de la Articulación/fisiopatología , Mapeo Encefálico , Corteza Cerebral/fisiopatología , Femenino , Lóbulo Frontal/diagnóstico por imagen , Lóbulo Frontal/patología , Lóbulo Frontal/fisiopatología , Humanos , Lenguaje , Trastornos del Lenguaje/fisiopatología , Pruebas del Lenguaje , Masculino , Persona de Mediana Edad , Red Nerviosa/diagnóstico por imagen , Red Nerviosa/patología , Red Nerviosa/fisiopatología , Fonética , Tomografía de Emisión de Positrones , Valor Predictivo de las Pruebas , Semántica , Habla/fisiología , Percepción del Habla/fisiología , Lóbulo Temporal/diagnóstico por imagen , Lóbulo Temporal/patología , Lóbulo Temporal/fisiopatologíaRESUMEN
In two H2(15)O PET scan experiments, we investigated the cerebral correlates of explicit and implicit knowledge in a serial reaction time (SRT) task. To do so, we used a novel application of the Process Dissociation Procedure, a behavioral paradigm that makes it possible to separately assess conscious and unconscious contributions to performance during a subsequent sequence generation task. To manipulate the extent to which the repeating sequential pattern was learned explicitly, we varied the pace of the choice reaction time task-a variable that is known to have differential effects on the extent to which sensitivity to sequence structure involves implicit or explicit knowledge. Results showed that activity in the striatum subtends the implicit component of performance during recollection of a learned sequence, whereas the anterior cingulate/mesial prefrontal cortex (ACC/MPFC) supports the explicit component. Most importantly, we found that the ACC/MPFC exerts control on the activity of the striatum during retrieval of the sequence after explicit learning, whereas the activity of these regions is uncoupled when learning had been essentially implicit. These data suggest that implicit learning processes can be successfully controlled by conscious knowledge when learning is essentially explicit. They also supply further evidence for a partial dissociation between the neural substrates supporting conscious and nonconscious components of performance during recollection of a learned sequence.
Asunto(s)
Mapeo Encefálico , Corteza Cerebral/fisiología , Estado de Conciencia/fisiología , Aprendizaje Seriado/fisiología , Inconsciente en Psicología , Adolescente , Adulto , Concienciación/fisiología , Corteza Cerebral/diagnóstico por imagen , Conducta de Elección/fisiología , Femenino , Humanos , Masculino , Recuerdo Mental/fisiología , Red Nerviosa/diagnóstico por imagen , Red Nerviosa/fisiología , Radioisótopos de Oxígeno , Tomografía de Emisión de Positrones , Tiempo de Reacción/fisiología , Valores de ReferenciaRESUMEN
This study used positron emission tomography (PET) to identify the brain substrate of self-referential reflective activity and to investigate its relationship with brain areas that are active during the resting state. Thirteen healthy volunteers performed reflective tasks pertaining to three different matters (the self, another person, and social issues) while they were scanned. Rest scans were also acquired, in which subjects were asked to simply relax and not think in a systematic way. The mental activity experienced during each scan was assessed with rating scales. The results showed that, although self-referential thoughts were most frequent during the self-referential task, some self-referential reflective activity also occurred during rest. Compared to rest, performing the reflective tasks was associated with increased blood flow in the dorsomedial prefrontal cortex, the left anterior middle temporal gyrus, the temporal pole bilaterally, and the right cerebellum; there was a decrease of blood flow in right prefrontal regions and in medial and right lateral parietal regions. In addition, the ventromedial prefrontal cortex (VMPFC) (1) was more active during the self-referential reflective task than during the other two reflective tasks, (2) showed common activation during rest and the self-referential task, and (3) showed a correlation between cerebral metabolism and the amount of self-referential processing. It is suggested that the VMPFC is crucial for representing knowledge pertaining to the self and that this is an important function of the resting state.
Asunto(s)
Ego , Tomografía de Emisión de Positrones , Descanso/fisiología , Pensamiento/fisiología , Adulto , Mapeo Encefálico , Femenino , Humanos , MasculinoRESUMEN
In everyday social interactions, hearing our own first name captures our attention and gives rise to a sense of self-awareness, since it is one of the most socially self related stimulus. In the present study, we combined ERPs and PET scan methods to explore the cerebral mechanisms underlying the detection of our own name. While categorical analyses of PET data failed to reveal significant results, we found that the amplitude of the P3 component, elicited when hearing one's own name, correlates with regional cerebral blood changes in right superior temporal sulcus, precuneus and medial prefrontal cortex. Additionally, the latter was more correlated to the P3 obtained for the subject's name compared to that obtained for other first names. These results suggest that the medial prefrontal cortex plays the most prominent role in self-processing.
Asunto(s)
Encéfalo/diagnóstico por imagen , Encéfalo/fisiología , Potenciales Relacionados con Evento P300/fisiología , Percepción del Habla/fisiología , Adulto , Mapeo Encefálico , Discriminación en Psicología/fisiología , Electroencefalografía , Femenino , Humanos , Relaciones Interpersonales , Imagen por Resonancia Magnética , Masculino , Lóbulo Parietal/diagnóstico por imagen , Lóbulo Parietal/fisiología , Tomografía de Emisión de PositronesRESUMEN
In rats, the firing sequences observed in hippocampal ensembles during spatial learning are replayed during subsequent sleep, suggesting a role for posttraining sleep periods in the offline processing of spatial memories. Here, using regional cerebral blood flow measurements, we show that, in humans, hippocampal areas that are activated during route learning in a virtual town are likewise activated during subsequent slow wave sleep. Most importantly, we found that the amount of hippocampal activity expressed during slow wave sleep positively correlates with the improvement of performance in route retrieval on the next day. These findings suggest that learning-dependent modulation in hippocampal activity during human sleep reflects the offline processing of recent episodic and spatial memory traces, which eventually leads to the plastic changes underlying the subsequent improvement in performance.
Asunto(s)
Hipocampo/fisiología , Memoria/fisiología , Sueño/fisiología , Conducta Espacial/fisiología , Adulto , Mapeo Encefálico , Electroencefalografía/métodos , Electromiografía/métodos , Electrooculografía/métodos , Humanos , Masculino , Polisomnografía , Tomografía de Emisión de Positrones/métodos , Procesamiento de Señales Asistido por Computador , Vigilia/fisiologíaRESUMEN
The brain processes light information to visually represent the environment but also to detect changes in ambient light level. The latter information induces non-image-forming responses and exerts powerful effects on physiology such as synchronization of the circadian clock and suppression of melatonin. In rodents, irradiance information is transduced from a discrete subset of photosensitive retinal ganglion cells via the retinohypothalamic tract to various hypothalamic and brainstem regulatory structures including the hypothalamic suprachiasmatic nuclei, the master circadian pacemaker. In humans, light also acutely modulates alertness, but the cerebral correlates of this effect are unknown. We assessed regional cerebral blood flow in 13 subjects attending to auditory and visual stimuli in near darkness following light exposures (>8000 lux) of different durations (0.5, 17, 16.5, and 0 min) during the biological night. The bright broadband polychromatic light suppressed melatonin and enhanced alertness. Functional imaging revealed that a large-scale occipito-parietal attention network, including the right intraparietal sulcus, was more active in proportion to the duration of light exposures preceding the scans. Activity in the hypothalamus decreased in proportion to previous illumination. These findings have important implications for understanding the effects of light on human behavior.
Asunto(s)
Atención/fisiología , Encéfalo/irrigación sanguínea , Luz , Melatonina/sangre , Estimulación Acústica , Adulto , Análisis de Varianza , Atención/efectos de la radiación , Encéfalo/metabolismo , Humanos , Imagen por Resonancia Magnética , Estimulación Luminosa , Tomografía de Emisión de Positrones , Flujo Sanguíneo Regional/fisiología , Flujo Sanguíneo Regional/efectos de la radiaciónRESUMEN
The present study aimed to ascertain the neuroanatomical basis of an influential neuropsychological model for upper limb apraxia [Rothi LJ, et al. The Neuropsychology of Action. 1997. Hove, UK: Psychology Press]. Regional cerebral blood flow was measured in healthy volunteers using H2 15O PET during performance of four tasks commonly used for testing upper limb apraxia, i.e., pantomime of familiar gestures on verbal command, imitation of familiar gestures, imitation of novel gestures, and an action-semantic task that consisted in matching objects for functional use. We also re-analysed data from a previous PET study in which we investigated the neural basis of the visual analysis of gestures. First, we found that two sets of discrete brain areas are predominantly engaged in the imitation of familiar and novel gestures, respectively. Segregated brain activation for novel gesture imitation concur with neuropsychological reports to support the hypothesis that knowledge about the organization of the human body mediates the transition from visual perception to motor execution when imitating novel gestures [Goldenberg Neuropsychologia 1995;33:63-72]. Second, conjunction analyses revealed distinctive neural bases for most of the gesture-specific cognitive processes proposed in this cognitive model of upper limb apraxia. However, a functional analysis of brain imaging data suggested that one single memory store may be used for "to-be-perceived" and "to-be-produced" gestural representations, departing from Rothi et al.'s proposal. Based on the above considerations, we suggest and discuss a revised model for upper limb apraxia that might best account for both brain imaging findings and neuropsychological dissociations reported in the apraxia literature.
Asunto(s)
Apraxias/fisiopatología , Mapeo Encefálico , Cognición/fisiología , Gestos , Modelos Neurológicos , Adulto , Apraxias/diagnóstico por imagen , Brazo/fisiología , Encéfalo/anatomía & histología , Encéfalo/diagnóstico por imagen , Encéfalo/fisiología , Circulación Cerebrovascular/fisiología , Femenino , Humanos , Masculino , Pruebas Neuropsicológicas , Desempeño Psicomotor/fisiología , Tomografía Computarizada de EmisiónRESUMEN
BACKGROUND: The minimally conscious state (MCS) is a recently defined clinical condition; it differs from the persistent vegetative state (PVS) by the presence of inconsistent, but clearly discernible, behavioral evidence of consciousness. OBJECTIVE: To study auditory processing among patients who are in an MCS, patients who are in a PVS, and healthy control subjects. METHODS: By means of (15)O-radiolabeled water-positron emission tomography, we measured changes in regional cerebral blood flow induced by auditory click stimuli in 5 patients in an MCS, 15 patients in a PVS, and 18 healthy controls. RESULTS: In both patients in an MCS and the healthy controls, auditory stimulation activated bilateral superior temporal gyri (Brodmann areas 41, 42, and 22). In patients in a PVS, the activation was restricted to Brodmann areas 41 and 42 bilaterally. We also showed that, compared with patients in a PVS, patients in an MCS demonstrated a stronger functional connectivity between the secondary auditory cortex and temporal and prefrontal association cortices. CONCLUSIONS: Although assumptions about the level of consciousness in severely brain injured patients are difficult to make, our findings suggest that the cerebral activity observed in patients in an MCS is more likely to lead to higher-order integrative processes, thought to be necessary for the gain of conscious auditory perception.
Asunto(s)
Percepción Auditiva/fisiología , Lesiones Encefálicas/psicología , Estado de Conciencia/fisiología , Estado Vegetativo Persistente/psicología , Estimulación Acústica , Adulto , Anciano , Circulación Cerebrovascular/fisiología , Electroencefalografía , Femenino , Humanos , Masculino , Persona de Mediana Edad , Radioisótopos de Oxígeno , Estado Vegetativo Persistente/fisiopatología , Psicofisiología , Tomografía Computarizada de EmisiónRESUMEN
We have previously shown that several brain areas are activated both during sequence learning at wake and during subsequent rapid-eye-movements (REM) sleep (Nat. Neurosci. 3 (2000) 831-836), suggesting that REM sleep participates in the reprocessing of recent memory traces in humans. However, the nature of the reprocessed information remains open. Here, we show that regional cerebral reactivation during posttraining REM sleep is not merely related to the acquisition of basic visuomotor skills during prior practice of the serial reaction time task, but rather to the implicit acquisition of the probabilistic rules that defined stimulus sequences. Moreover, functional connections between the reactivated cuneus and the striatum--the latter being critical for implicit sequence learning--are reinforced during REM sleep after practice on a probabilistic rather than on a random sequence of stimuli. Our results therefore support the hypothesis that REM sleep is deeply involved in the reprocessing and optimization of the high-order information contained in the material to be learned. In addition, we show that the level of acquisition of probabilistic rules attained prior to sleep is correlated to the increase in regional cerebral blood flow during subsequent REM sleep. This suggests that posttraining cerebral reactivation is modulated by the strength of the memory traces developed during the learning episode. Our data provide the first experimental evidence for a link between behavioral performance and cerebral reactivation during REM sleep.
Asunto(s)
Encéfalo/fisiología , Aprendizaje/fisiología , Sueño REM/fisiología , Adulto , Encéfalo/diagnóstico por imagen , Humanos , Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Masculino , Desempeño Psicomotor/fisiología , Tiempo de Reacción/fisiología , Aprendizaje Seriado/fisiología , Técnicas Estereotáxicas , Tomografía Computarizada de EmisiónRESUMEN
The neural mechanisms underlying the antinociceptive effects of hypnosis are not well understood. Using positron emission tomography (PET), we recently showed that the activity in the anterior cingulate cortex (midcingulate area 24a') covaries with the hypnosis-induced reduction of affective and sensory responses to noxious thermal stimulation [Faymonville et al., Anesthesiology 92 (2000) 1257-1267]. In the present study, we assessed changes in cerebral functional connectivity related to the hypnotic state, compared to simple distraction and the resting state. Nineteen highly hypnotizable right-handed volunteers were studied using H2(15)O-PET. The experimental conditions were hot noxious or warm non-noxious stimulation of the right hand during resting state, mental imagery and hypnotic state. Using a psychophysiological interaction analysis, we identified brain areas that would respond to noxious stimulations under the modulatory action of the midcingulate cortex in, and only in, the hypnotic state. Hypnosis, compared to the resting state, reduced pain perception by 50%. Pain perception during rest and mental imagery was not significantly different. Analysis of PET data showed that the hypnotic state, compared to normal alertness (i.e., rest and mental imagery), significantly enhanced the functional modulation between midcingulate cortex and a large neural network encompassing bilateral insula, pregenual anterior cingulate cortex, pre-supplementary motor area, right prefrontal cortex and striatum, thalamus and brainstem. These findings point to a critical role for the midcingulate cortex in the modulation of a large cortical and subcortical network underlying its influence on sensory, affective, cognitive and behavioral aspects of nociception, in the specific context of hypnosis.
Asunto(s)
Corteza Cerebral/fisiología , Hipnosis/métodos , Imaginación/fisiología , Dimensión del Dolor/métodos , Descanso/fisiología , Adulto , Análisis de Varianza , Femenino , Humanos , Masculino , Tomografía Computarizada de Emisión/métodosRESUMEN
Landau-Kleffner syndrome (LKS) is a rare acquired aphasia occurring in otherwise healthy children, together with spike-wave discharges predominating over superior temporal regions and activated by sleep. Although the outcome of language abilities is variable, a residual impairment in verbal short-term memory (STM) is frequent. This STM deficit might be related to the persistent dysfunction of those temporal lobe regions where epileptic discharges were observed during the active phase of the disorder. We tested this hypothesis by measuring brain activation during immediate serial recall of lists of 4 words, compared to single word repetition, using H(2) (15)O positron emission tomography (PET), in 3 LKS patients after recovery and in 14 healthy controls. The patients (TG, JPH, and DC) had shown abnormally increased or decreased glucose metabolism in left or right superior temporal gyrus (STG) at different stages during the active phase of their disease. At the time of this study, the patients were 6-10 years from the active phase of LKS. Results showed that Patients JPH and DC had impaired performance in the STM condition, whereas TG showed near normal performance. PET data showed that JPH and DC activated significantly less than controls left and right posterior STG. TG, having near normal STM performance, showed increased activity in the posterior part of the right STG. These data suggest that impaired verbal STM at late outcome of LKS might indeed be related to a persistent decrease of activity in those posterior superior temporal gyri that were involved in the epileptic focus during the active phase.
Asunto(s)
Trastornos de la Articulación/etiología , Mapeo Encefálico , Síndrome de Landau-Kleffner/fisiopatología , Memoria a Corto Plazo/fisiología , Lóbulo Temporal/metabolismo , Estimulación Acústica , Adulto , Trastornos de la Articulación/diagnóstico por imagen , Femenino , Glucosa/metabolismo , Humanos , Síndrome de Landau-Kleffner/complicaciones , Síndrome de Landau-Kleffner/diagnóstico por imagen , Masculino , Recuerdo Mental/fisiología , Lóbulo Temporal/diagnóstico por imagen , Lóbulo Temporal/fisiopatología , Tomografía Computarizada de EmisiónRESUMEN
Using positron emission tomography (PET) and regional cerebral blood flow (rCBF) measurements, we investigated the cerebral correlates of consciousness in a sequence learning task through a novel application of the Process Dissociation Procedure, a behavioral paradigm that makes it possible to separately assess conscious and unconscious contributions to performance. Results show that the metabolic response in the anterior cingulate/mesial prefrontal cortex (ACC/MPFC) is exclusively and specifically correlated with the explicit component of performance during recollection of a learned sequence. This suggests a significant role for the ACC/MPFC in the explicit processing of sequential material.
Asunto(s)
Corteza Cerebral/fisiología , Aprendizaje/fisiología , Adolescente , Adulto , Corteza Cerebral/diagnóstico por imagen , Circulación Cerebrovascular/fisiología , Giro del Cíngulo/diagnóstico por imagen , Giro del Cíngulo/fisiología , Humanos , Masculino , Memoria/fisiología , Corteza Prefrontal/diagnóstico por imagen , Corteza Prefrontal/fisiología , Tiempo de Reacción/fisiología , Sinapsis/fisiología , Tomografía Computarizada de EmisiónRESUMEN
Positron emission tomography (PET) techniques represent a useful tool to better understand the residual brain function in vegetative state patients. It has been shown that overall cerebral metabolic rates for glucose are massively reduced in this condition. However, the recovery of consciousness from vegetative state is not always associated with substantial changes in global metabolism. This finding led us to hypothesize that some vegetative patients are unconscious not just because of a global loss of neuronal function, but rather due to an altered activity in some critical brain regions and to the abolished functional connections between them. We used voxel-based Statistical Parametric Mapping (SPM) approaches to characterize the functional neuroanatomy of the vegetative state. The most dysfunctional brain regions were bilateral frontal and parieto-temporal associative cortices. Despite the metabolic impairment, external stimulation still induced a significant neuronal activation (i.e., change in blood flow) in vegetative patients as shown by both auditory click stimuli and noxious somatosensory stimuli. However, this activation was limited to primary cortices and dissociated from higher-order associative cortices, thought to be necessary for conscious perception. Finally, we demonstrated that vegetative patients have impaired functional connections between distant cortical areas and between the thalami and the cortex and, more importantly, that recovery of consciousness is paralleled by a restoration of this cortico-thalamo-cortical interaction.