Biomedical subjects

Lotfi B Merabet

Publications and source records attributed to Lotfi B Merabet.

8 recordsLinked to original sources

PubMed2006

Combined activation and deactivation of visual cortex during tactile sensory processing.

The involvement of occipital cortex in sensory processing is not restricted solely to the visual modality. Tactile processing has been shown to modulate higher-order visual and multisensory integration areas in sighted as well as visually deprived subjects; however, the extent of involvement of early visual cortical areas remains unclear. To investigate this issue, we employed functional magnetic resonance imaging in normally sighted, briefly blindfolded subjects with well-defined visuotopic borders as they tactually explored and rated raised-dot patterns. Tactile task performance resulted in significant activation in primary visual cortex (V1) and deactivation of extrastriate cortical regions V2, V3, V3A, and hV4 with greater deactivation in dorsal subregions and higher visual areas. These results suggest that tactile processing affects occipital cortex via two distinct pathways: a suppressive top-down pathway descending through the visual cortical hierarchy and an excitatory pathway arising from outside the visual cortical hierarchy that drives area V1 directly.

Adult↗

PubMed2006

Attentional modulation of emotional stimulus processing: an fMRI study using emotional expectancy.

We used emotional expectancy to study attentional modulation in the processing of emotional stimuli. During functional magnetic resonance imaging (fMRI), volunteers saw emotional and neutral expectancy cues signaling the subsequent presentation of corresponding emotional or neutral pictorial stimuli. As a control, emotional and neutral pictures were presented without preceding expectancy cue, resulting in a 2 x 2 factorial design with the factors "expectancy" and "emotion." Statistical analysis revealed a significant positive interaction effect between these factors in the medial prefrontal cortex (MPFC, Brodmann area [BA] 9/10), amygdala, and dorsal midbrain. In all these regions, expectancy augmented the neural response to emotional but not to neutral pictures. Time course analysis of raw data suggests that this augmented activation was not preceded by baseline increases in MPFC and amygdala during the period of emotional expectancy. In a post-scanning session, the paradigm was presented for a second time to allow emotional intensity rating. Again, a significant interaction between expectancy and emotion was observed, with intensity ratings specifically enhanced in emotional photographs preceded by expectancy. There was a positive correlation between intensity ratings and blood oxygenation level-dependent (BOLD) signals in the left amygdala. We conclude that specific components of the emotion network show enhanced activation in response to emotional stimuli when these are preceded by expectancy. This enhancement effect is not present in neutral pictures and might parallel accentuated subjective feeling states.

Adult↗

PubMed2005

Dissociable networks for the expectancy and perception of emotional stimuli in the human brain.

William James posited that comparable brain regions were implicated in the anticipation and perception of a stimulus; however, dissociable networks (at least in part) may also underlie these processes. Recent functional neuroimaging studies have addressed this issue by comparing brain systems associated with the expectancy and perception of visual, tactile, nociceptive, and reward stimuli. In the present fMRI study, we addressed this issue in the domain of pictorial emotional stimuli (IAPS). Our paradigm involved the experimental conditions emotional expectancy, neutral expectancy, emotional picture perception, and neutral picture perception. Specifically, the emotional expectancy cue was uncertain in that it did not provide additional information regarding the positive or negative valence of the subsequent picture. Neutral expectancy and neutral picture perception served as control conditions, allowing the identification of expectancy and perception effects specific for emotion processing. To avoid contamination of the perception conditions by the preceding expectancy periods, 50% of the pictorial stimuli were presented without preceding expectancy cues. We found that the emotional expectancy cue specifically produced activation in the supracallosal anterior cingulate, cingulate motor area, and parieto-occipital sulcus. These regions were not significantly activated by emotional picture perception which recruited a different neuronal network, including the amygdala, insula, medial and lateral prefrontal cortex, cerebellum, and occipitotemporal areas. This dissociation may reflect a distinction between anticipatory and perceptive components of emotional stimulus processing.

Adult↗

PubMed2005

What blindness can tell us about seeing again: merging neuroplasticity and neuroprostheses.

Significant progress has been made in the development of visual neuroprostheses to restore vision in blind individuals. Appropriate delivery of electrical stimulation to intact visual structures can evoke patterned sensations of light in those who have been blind for many years. However, success in developing functional visual prostheses requires an understanding of how to communicate effectively with the visually deprived brain in order to merge what is perceived visually with what is generated electrically.

Animals↗

PubMed2005

The plastic human brain cortex.

Plasticity is an intrinsic property of the human brain and represents evolution's invention to enable the nervous system to escape the restrictions of its own genome and thus adapt to environmental pressures, physiologic changes, and experiences. Dynamic shifts in the strength of preexisting connections across distributed neural networks, changes in task-related cortico-cortical and cortico-subcortical coherence and modifications of the mapping between behavior and neural activity take place in response to changes in afferent input or efferent demand. Such rapid, ongoing changes may be followed by the establishment of new connections through dendritic growth and arborization. However, they harbor the danger that the evolving pattern of neural activation may in itself lead to abnormal behavior. Plasticity is the mechanism for development and learning, as much as a cause of pathology. The challenge we face is to learn enough about the mechanisms of plasticity to modulate them to achieve the best behavioral outcome for a given subject.

Blindness↗

PubMed2004

Visual hallucinations during prolonged blindfolding in sighted subjects.

The authors report the occurrence of visual hallucinations of varying complexity in 13 normal subjects after sudden, complete, and prolonged visual deprivation. The subjects were all healthy individuals with no history of cognitive dysfunction, psychosis, or ocular pathology. They wore a specially designed blindfold for a period of five consecutive days (96 hours) and were asked to record their daily experiences using a hand-held microcassette recorder. Ten (77%) of the subjects reported visual hallucinations, which were both simple (bright spots of light) and complex (faces, landscapes, ornate objects). The onset of hallucinations was generally after the first day of blindfolding. Subjects were insightful as to their unreal nature. These results indicate that rapid and complete visual deprivation is sufficient to induce visual hallucinations in normal subjects.

Adult↗

PubMed2003

Suppression of complex visual hallucinatory experiences by occipital transcranial magnetic stimulation: a case report.

Abstract We report a patient with visual hallucinations and illusions along with an associated visual field defect after bilateral ischemic damage to his occipital visual cortex. These hallucinations were long-standing and of both simple and complex (well-formed) type. Application of low frequency (1 Hz) repetitive Transcranial Magnetic Stimulation (rTMS) to the occipital cortex led to a complete cessation of visual hallucinatory symptoms. The use of TMS to probe the neurophysiology, and possibly alleviate, visual hallucinatory experiences is discussed.

Brain Mapping↗

PubMed2003

Transcranial magnetic stimulation as an investigative tool in the study of visual function.

Transcranial magnetic stimulation (TMS) is a novel and powerful probe to study the relationship between human brain function and behavior. TMS is being widely used to investigate memory, language, attention, learning, and motor function and is even being utilized therapeutically in the treatment of depression. Some of the earliest applications of TMS have been directed toward the investigation of human visual perception. For example, a strong TMS pulse delivered to the occipital cortex in a sighted or even blind individual can evoke the sensation of perceiving light (visual phosphenes). TMS can also be used to suppress visual perception and investigate the timing of visual information processing. Furthermore, the functional connectivity between different brain areas can be mapped using TMS, thus establishing a causal link between visual cortical function and visual perception. The present article is meant as an overview of the technique of TMS and a review of the literature as it pertains to the study of visual function. The application of TMS in the diagnosis as well as possible therapeutic use in various visual disorders is also discussed.

Electromagnetic Fields↗