A real-time marker of object-based attention in the human brain. A possible component of a "gate-keeping mechanism" performing late attentional selection in the Ventro-Lateral Prefrontal Cortex.

The decision to process an incoming stimulus attentively - and to trigger a follow-up cascade of high-level processes - is strategic for the human brain as it becomes transiently unavailable to subsequent stimulus processing. In this study, we set to identify brain networks that carry out such evaluations. We therefore assessed the time-course of neural responses with intracerebral EEG in human patients during an attentional reading task, contrasting to-be-attended vs. to-be-ignored items. We measured High-Frequency Activity [50-150 â€‹Hz] as a proxy of population-level spiking activity and we identified a crucial component of a Gate-Keeping Mechanism bilateral in the mid-Ventro-Lateral Prefrontal Cortex (VLPFC), at the interplay of the Ventral and Dorsal Attention Networks, that selectively reacts before domain specialized cortical regions that engage in full stimulus analysis according to task demands.

Dynamics of parietofrontal networks underlying visuospatial short-term memory encoding.

Brain imaging studies in TEP, functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) have shown that visuospatial short-term memory tasks depend on dorsal parietofrontal networks. Knowing the spatiotemporal dynamics of this network would provide further understanding of the neural bases of the encoding process. We combined magnetoencephalography (MEG) with EEG and fMRI techniques to study this network in a task, in which participants had to judge the symmetry in position of two dots, presented either simultaneously ("immediate comparison") or successively ("memorization" of a first dot and "delayed comparison", after 3 s, with a second dot). With EEG, larger amplitude was observed in the parietocentral P3b component (350-500 ms) in the immediate and "delayed comparisons" than in "memorization" condition, where topography at this time was more anterior and right lateralized. MEG provided a more accurate localization and temporal variations of sources, revealing a strong M4 component at 450 ms in the "memorization" condition, with two sources localized in parietal and right premotor regions. These localizations are consistent with both fMRI foci and EEG cortical current source densities (CSD), but only MEG revealed the strong increase in premotor region at 450 ms related to "memorization". These combined results suggest that EEG P3B and MEG M4 components reflect two different dynamics in parietofrontal networks: the parietocentral P3b indexes a decision mechanism during the immediate and "delayed comparisons", whereas the MEG M4 component, with a larger right premotor source, reflects the encoding process in visuospatial short-term memory.

Is amygdalohippocampectomy really selective in medial temporal lobe epilepsy? A study using positron emission tomography with (18)fluorodeoxyglucose.

PURPOSE: Selective amygdalohippocampectomy (SAH) is a surgical technique effective for the treatment of medial temporal lobe epilepsy, which selectively removes the epileptogenic hippocampus and amygdala but spares the temporal neocortex. However, the benefit of SAH in terms of functional outcome is debated. In this study, we aimed to assess the metabolic consequences of SAH. METHODS: Volumetric magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (PET) studies were performed in nine patients with medial temporal lobe epilepsy associated with hippocampal sclerosis before and after SAH. Regions of interest were delineated on MRIs and then replaced on PET images using an automatic 3D image registration. We calculated absolute metabolic rates of glucose and normalized metabolic values in each region of interest. RESULTS: The comparison between preoperative and postoperative metabolic values showed a statistically significant worsening of the hypometabolism on the ipsilateral temporal pole on the superior and the hippocampal levels (p < 0.05 and 0.0045, respectively). A postoperative increase of the metabolic activity also was noted in the contralateral anterior hippocampus (p < 0.05) and the orbitofrontal cortex bilaterally (p < 0.002 and 0.001, respectively) CONCLUSIONS: SAH functional benefit is controversial. SAH worsened significantly the hypometabolism of a temporal structure that was not surgically removed (i.e., the temporal pole), and it improved postoperatively the metabolic activity in the contralateral hippocampus and the orbitofrontal cortex. Whether this postoperative improvement is linked to the selectivity of the surgical procedure must be further clarified.