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Biomedical subjects

Lotfi Merabet

Publications and source records attributed to Lotfi Merabet.

6 recordsLinked to original sources
PubMed2006

An atypical presentation of visual hallucinatory experiences following prolonged blindness.

We report a patient with long-standing blindness experiencing both simple and complex visual hallucinations secondary to a cortical arteriovenous malformation (AVM). The hallucinations were located in the right visual field corresponding to the contra-lateral site of cortical damage. This case contributes to our understanding of neurophysiological mechanisms underlying visual hallucinations and ongoing research investigating the phenomenology of hallucinations with respect to the cause and localization of neural damage.

Aged↗
PubMed2004

Feeling by sight or seeing by touch?

We have addressed the role of occipital and somatosensory cortex in a tactile discrimination task. Sight-ed and congenitally blind subjects rated the roughness and distance spacing for a series of raised dot patterns. When judging roughness, intermediate dot spacings were perceived as being the most rough, while distance judgments generated a linear relation. Low-frequency rTMS applied to somatosensory cortex disrupted roughness without affecting distance judgments, while rTMS to occipital cortex disrupted distance but not roughness judgments. We also tested an early blind patient with bilateral occipital cortex damage. Her performance on the roughness determination task was normal; however, she was greatly impaired with distance judgments. The findings suggest a double-dissociation effect in which roughness and distance are primarily processed in somatosensory and occipital cortex, respectively. The differential effect of rTMS on task performance and corroborative clinical evidence suggest that occipital cortex is engaged in tactile tasks requiring fine spatial discrimination.

Adult↗
PubMed2004

Unconscious modulation of motor cortex excitability revealed with transcranial magnetic stimulation.

The neuronal effects of sensory events that do not enter conscious awareness have been reported in numerous pathological conditions and in normal subjects. In the present study, unconscious modulation of corticospinal excitability was probed in healthy volunteers with transcranial magnetic stimulation (TMS). TMS-induced motor evoked potentials (MEPs) were collected from the first dorsal interosseus muscle while subjects performed a masked semantic priming task that has been shown to elicit covert motor cortex activations. Our data show that the amplitude of the MEPs is modulated by an unseen prime, in line with temporal patterns revealed with event related potentials. These data confirm previous reports showing specific motor neural responses associated with an unseen visual stimulus and establish TMS as a valuable tool in the study of the neural correlates of consciousness.

Adult↗
PubMed2004

Modulation of right motor cortex excitability without awareness following presentation of masked self-images.

The neural substrates of self-awareness have been studied with a variety of neurophysiological and behavioral tools. In the present study, unconscious modulation of corticospinal excitability following presentation of self-images was probed with transcranial magnetic stimulation (TMS). TMS-induced motor evoked potentials (MEP) were collected from the contralateral first dorsal interosseus (FDI) muscle while subjects viewed masked pictures of their own face. MEP amplitudes were compared to those obtained when pictures of strangers were masked. Masked self-images induced a relative increase in corticospinal excitability when TMS was applied to the right primary motor cortex. These results demonstrate the utility of TMS to probe unconscious processing and support the notion of hemispheric asymmetry in the processing of self-images.

Adult↗
PubMedDate not supplied

Behavioral and neuroplastic changes in the blind: evidence for functionally relevant cross-modal interactions.

The study of blind individuals provides insight into the brain re-organization and behavioral compensations that occur following sensory deprivation. While behavioral studies have yielded conflicting results in terms of performance levels within the remaining senses, deafferentation of visual cortical areas through peripheral blindness results in clear neuroplastic changes. Most striking is the activation of occipital cortex in response to auditory and tactile stimulation. Indeed, parts of the "unimodal" visual cortex are recruited by other sensory modalities to process sensory information in a functionally relevant manner. In addition, a larger area of the sensorimotor cortex is devoted to the representation of the reading finger in blind Braille readers. The "visual" function of the deafferented occipital cortex is also altered, where transcranial magnetic stimulation-induced phosphenes can be elicited in only 20% of blind subjects. The neural mechanisms underlying these changes remain elusive but recent data showing rapid cross-modal plasticity in blindfolded, sighted subjects argue against the establishment of new connections to explain cross-modal interactions in the blind. Rather, latent pathways that participate in multisensory percepts in sighted subjects might be unmasked and may be potentiated in the event of complete loss of visual input. These issues have important implications for the development of visual prosthesis aimed at restoring some degree of vision in the blind.

Adaptation, Psychological↗