Lionel Naccache

Bio: Lionel Naccache is an academic researcher from University of Paris. The author has contributed to research in topics: Consciousness & Minimally conscious state. The author has an hindex of 49, co-authored 175 publications receiving 15747 citations. Previous affiliations of Lionel Naccache include French Institute of Health and Medical Research & Pierre-and-Marie-Curie University.

The visual word form area: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients.

Abstract: A standard model of word reading postulates that visual information is initially processed by occipitotemporal areas contralateral to the stimulated hemifield, from whence it is subsequently transferred to the visual word form (VWF) system, a left inferior temporal region specifically devoted to the processing of letter strings. For stimuli displayed in the left visual field, this transfer proceeds from the right to the left hemisphere through the posterior portion of the corpus callosum. In order to characterize the spatial and temporal organization of these processes, reading tasks with split-field presentation were performed by five control subjects and by two patients suffering from left hemialexia following posterior callosal lesions. The subjects' responses were studied using behavioural measures and functional brain imaging techniques, providing both high spatial resolution (functional MRI, fMRI) and high temporal resolution (high-density event-related potentials, ERPs). Early visual processing was revealed as activations contralateral to stimulation, located by fMRI in the inferior occipitotemporal region and presumably coincident with area V4. A negative wave occurring 150-160 ms post-stimulus, also strictly contralateral to stimulation, was recorded over posterior electrodes. In contrast with these hemifield-dependent effects, the VWF system was revealed as a strictly left-hemispheric activation which, in control subjects, was identical for stimuli presented in the left or in the right hemifield and was located in the middle portion of the left fusiform gyrus. The electrical signature of the VWF system consisted of a unilateral sharp negativity, recorded 180-200 ms post-stimulus over left inferior temporal electrodes. In callosal patients, due to the inability of visual information to pass across the posterior part of the corpus callosum, the VWF system was activated only by stimuli presented in the right visual field. Similarly, a significant influence of the word/non-word status on ERPs recorded over the left hemisphere was discernible for either hemifield in controls, while it affected only right-hemifield stimuli in callosal patients. These findings provide direct support for the main components of the classical model of reading and help specify their timing and cerebral substrates.

Cerebral mechanisms of word masking and unconscious repetition priming.

Abstract: We used functional magnetic resonance imaging (fMRI) and event-related potentials (ERPs) to visualize the cerebral processing of unseen masked words. Within the areas associated with conscious reading, masked words activated left extrastriate, fusiform and precentral areas. Furthermore, masked words reduced the amount of activation evoked by a subsequent conscious presentation of the same word. In the left fusiform gyrus, this repetition suppression phenomenon was independent of whether the prime and target shared the same case, indicating that case-independent information about letter strings was extracted unconsciously. In comparison to an unmasked situation, however, the activation evoked by masked words was drastically reduced and was undetectable in prefrontal and parietal areas, correlating with participants' inability to report the masked words.

Imaging unconscious semantic priming

Abstract: Visual words that are masked and presented so briefly that they cannot be seen may nevertheless facilitate the subsequent processing of related words, a phenomenon called masked priming It has been debated whether masked primes can activate cognitive processes without gaining access to consciousness Here we use a combination of behavioural and brain-imaging techniques to estimate the depth of processing of masked numerical primes Our results indicate that masked stimuli have a measurable influence on electrical and haemodynamic measures of brain activity When subjects engage in an overt semantic comparison task with a clearly visible target numeral, measures of covert motor activity indicate that they also unconsciously apply the task instructions to an unseen masked numeral A stream of perceptual, semantic and motor processes can therefore occur without awareness

Working memory: looking back and looking forward

Abstract: The concept of working memory proposes that a dedicated system maintains and stores information in the short term, and that this system underlies human thought processes. Current views of working memory involve a central executive and two storage systems: the phonological loop and the visuospatial sketchpad. Although this basic model was first proposed 30 years ago, it has continued to develop and to stimulate research and debate. The model and the most recent results are reviewed in this article.

Imaging Cognition II: An Empirical Review of 275 PET and fMRI Studies

Abstract: Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have been extensively used to explore the functional neuroanatomy of cognitive functions. Here we review 275 PET and fMRI studies of attention (sustained, selective, Stroop, orientation, divided), perception (object, face, space/motion, smell), imagery (object, space/ motion), language (written/spoken word recognition, spoken/ no spoken response), working memory (verbal/numeric, object, spatial, problem solving), semantic memory retrieval (categorization, generation), episodic memory encoding (verbal, object, spatial), episodic memory retrieval (verbal, nonverbal, success, effort, mode, context), priming (perceptual, conceptual), and procedural memory (conditioning, motor, and nonmotor skill learning). To identify consistent activation patterns associated with these cognitive operations, data from 412 contrasts were summarized at the level of cortical Brodmann's areas, insula, thalamus, medial-temporal lobe (including hippocampus), basal ganglia, and cerebellum. For perception and imagery, activation patterns included primary and secondary regions in the dorsal and ventral pathways. For attention and working memory, activations were usually found in prefrontal and parietal regions. For language and semantic memory retrieval, typical regions included left prefrontal and temporal regions. For episodic memory encoding, consistently activated regions included left prefrontal and medial-temporal regions. For episodic memory retrieval, activation patterns included prefrontal, medial-temporal, and posterior midline regions. For priming, deactivations in prefrontal (conceptual) or extrastriate (perceptual) regions were consistently seen. For procedural memory, activations were found in motor as well as in non-motor brain areas. Analysis of regional activations across cognitive domains suggested that several brain regions, including the cerebellum, are engaged by a variety of cognitive challenges. These observations are discussed in relation to functional specialization as well as functional integration.

Bayesian T Tests for Accepting and Rejecting the Null Hypothesis

Abstract: Progress in science often comes from discovering invariances in relationships among variables; these invariances often correspond to null hypotheses. As is commonly known, it is not possible to state evidence for the null hypothesis in conventional significance testing. Here we highlight a Bayes factor alternative to the conventional t test that will allow researchers to express preference for either the null hypothesis or the alternative. The Bayes factor has a natural and straightforward interpretation, is based on reasonable assumptions, and has better properties than other methods of inference that have been advocated in the psychological literature. To facilitate use of the Bayes factor, we provide an easy-to-use, Web-based program that performs the necessary calculations.

The economy of brain network organization

Abstract: On the basis of data from brain network science, Bullmore and Sporns propose that brain organization is shaped by an economical trade-off between minimizing wiring cost and maximizing the efficiency of information transfer between neuronal populations and discuss this idea in the context of psychiatric and neurological disorders. The brain is expensive, incurring high material and metabolic costs for its size — relative to the size of the body — and many aspects of brain network organization can be mostly explained by a parsimonious drive to minimize these costs. However, brain networks or connectomes also have high topological efficiency, robustness, modularity and a 'rich club' of connector hubs. Many of these and other advantageous topological properties will probably entail a wiring-cost premium. We propose that brain organization is shaped by an economic trade-off between minimizing costs and allowing the emergence of adaptively valuable topological patterns of anatomical or functional connectivity between multiple neuronal populations. This process of negotiating, and re-negotiating, trade-offs between wiring cost and topological value continues over long (decades) and short (millisecond) timescales as brain networks evolve, grow and adapt to changing cognitive demands. An economical analysis of neuropsychiatric disorders highlights the vulnerability of the more costly elements of brain networks to pathological attack or abnormal development.