Interference with episodic memory retrieval following transcranial stimulation of the inferior but not the superior parietal lobule.

Although posterior parietal cortex (PPC) has been traditionally associated with spatial attention and sensorimotor functions, recent neuroimaging evidence has suggested the involvement of regions of left PCC (LPPC) in memory retrieval. Yet, the role of the parietal lobe in memory-related functions is still controversial. Here we investigated the causal involvement of different LPPC regions in episodic memory retrieval using repetitive transcranial magnetic stimulation (rTMS) during a task that provided both objective and subjective measures of item recognition and source memory. Stimulation sites were identified on the basis of a recent fMRI study showing the involvement of regions of the default mode network (DMN), such as the angular gyrus (AG) in the inferior parietal lobule (IPL), during search for relevant information in episodic memory, and regions of the dorsal attention network (DAN), such as the superior parietal lobule (SPL), during perceptual search. We predicted a selective disruption of memory performance following rTMS stimulation of the left AG relative to a sham condition or stimulation of the left SPL. We found a modest but significant decrease of sensitivity for item recognition when AG was directly compared to SPL, but not to sham stimulation. A stronger effect was however observed for the criterion of source memory judgments when comparing AG with both SPL and sham stimulation, suggesting that the rTMS over AG affects subjective aspects of source monitoring associated with the weighing of relevant retrieved information for source attribution.

Studying social cognition using near-infrared spectroscopy: the case of social Simon effect.

In order to understand the so-called "social brain," we need to monitor social interactions in face-to-face paradigms. Near-infrared spectroscopy (NIRS) is a promising technique to achieve this goal. We investigate the neuronal underpinnings of sharing a task in a proper social context. We record cortical activity by means of NIRS, while participants perform a joint Simon task. Different from other hemodynamic techniques, NIRS allows us to have both participants sit comfortably close to each other in a realistic and ecological environment. We found higher activation in the sensorimotor cortex while processing compatible trials as compared to incompatible ones referring to one's own action alternative. Strikingly, when the participant was not responding because it was the turn of the other member of the pair, the inferior parietal was activated. This study provides twofold findings: first, they suggest that the joint Simon effect relies more on shared attentional mechanisms than a proper mapping of the other's motor response. Second, they highlight the invaluable contribution NIRS can afford to social neuroscience in order to preserve ecological and naturalistic settings.

Negative BOLD effect on somato-motor inhibitory processing: an fMRI study.

Inhibiting inappropriate behavior and thoughts in the current context is an essential ability for humans, but the neural mechanisms for response inhibitory processing are a matter of continuous debate. The aim of this event-related functional magnetic resonance imaging (fMRI) study was to evaluate the negative blood oxygen level dependent (BOLD) effect on inhibitory processing during go/no-go paradigms. Fifteen subjects performed two different types of somatosensory go/no-go paradigm: (1) button press and (2) count. Go and no-go stimuli were presented with an even probability. We observed a common negative activation during Movement No-go and Count No-go trials in the right SFG, corresponding to BA 8. These findings suggest that the right SFG region was responsible for the negative BOLD effect on inhibitory processing, which was independent of the required response mode. We hypothesized several possible explanations for the deactivation of the SFG during no-go trials.

Somato-motor inhibitory processing in humans: an event-related functional MRI study.

Inhibiting inappropriate behavior and thoughts is an essential ability for humans, but the regions responsible for inhibitory processing are a matter of continuous debate. This is the first study of somatosensory go/nogo tasks using event-related functional magnetic resonance imaging (fMRI). Fifteen subjects preformed two different types of go/nogo task, i.e. (1) Movement and (2) Count, to compare with previous studies using visual go/nogo tasks, and confirm whether the inhibitory processing is dependent on sensory modalities. Go and nogo stimuli were presented with an even probability. Our data indicated that the response inhibition network involved the dorsolateral (DLPFC) and ventrolateral (VLPFC) prefrontal cortices, pre-supplementary motor area (pre-SMA), anterior cingulate cortex (ACC), inferior parietal lobule (IPL), insula, and temporoparietal junction (TPJ), which were consistent with previous results obtained using visual go/nogo tasks. These activities existed in both Movement and Count Nogo trials. Therefore, our results suggest that the network for inhibitory processing is not dependent on sensory modalities but reflects common neural activities. In addition, there were differences of activation intensity between Movement and Count Nogo trials in the prefrontal cortex, temporal lobe, and ACC. Thus, inhibitory processing would involve two neural networks, common and uncommon regions, depending on the required response mode.

Sequential activation of human oculomotor centers during planning of visually-guided eye movements: a combined fMRI-MEG study.

We used magneto-encephalography (MEG) to measure visually evoked activity in healthy volunteers performing saccadic eye movements to visual targets. The neuromagnetic activity was analyzed from regions of cortical activation identified in separate functional magnetic resonance imaging (fMRI) studies. The latency of visual responses significantly increased from the Middle Temporal region (MT+) to the Intraparietal Sulcus (IPS) to the Frontal Eye Field (FEF), and their amplitude was greater in the hemisphere contralateral to the visual target. Trial-to-trial variability of oculomotor reaction times correlated with visual response latency across cortical areas. These results support a feedforward recruitment of oculomotor cortical centers by visual information, and a model in which behavioral variability depends on variability at different neural stages of processing.

Pre- and poststimulus alpha rhythms are related to conscious visual perception: a high-resolution EEG study.

Conscious and unconscious visuospatial processes have been related to parietooccipital cortical activation as revealed by late visual-evoked potentials. Here the working hypothesis was that a specific pattern of pre- and poststimulus theta (about 4-6 Hz) and alpha (about 6-12 Hz) rhythms is differently represented during conscious compared with unconscious visuospatial processes. Electroencephalographic (EEG) data (128 channels) were recorded in normal adults during a visuospatial task. A cue stimulus appeared at the right or left (equal probability) monitor side for a "threshold time" inducing about 50% of correct recognitions. It was followed (2 s) by visual go stimuli at spatially congruent or incongruent position with reference to the cue location. Left (right) mouse button was clicked if the go stimulus appeared at the left (right) monitor side. Then, subjects said "seen" if they had detected the cue stimulus or "not seen" if missed (self-report). Sources of theta and alpha rhythms during seen and not seen EEG epochs were estimated by low-resolution electromagnetic brain topography software. Results showed that the prestimulus "low-band" (about 6-10 Hz) alpha rhythms in frontal, parietal, and occipital areas were stronger in power in the seen than in the not seen trials. After the visual stimulation, the power of the "high-band" (about 10-12 Hz) alpha rhythms in parietal and occipital areas decreased more in the seen than in the not seen trials. The present results suggest that visuospatial consciousness covary--presumably with a facilitatory effect--with the power of both pre- and poststimulus alpha rhythms.

Sources of cortical rhythms in adults during physiological aging: a multicentric EEG study.

This electroencephalographic (EEG) study tested whether cortical EEG rhythms (especially delta and alpha) show a progressive increasing or decreasing trend across physiological aging. To this aim, we analyzed the type of correlation (linear and nonlinear) between cortical EEG rhythms and age. Resting eyes-closed EEG data were recorded in 108 young (Nyoung; age range: 18-50 years, mean age 27.3+/-7.3 SD) and 107 elderly (Nold; age range: 51-85 years, mean age 67.3+/-9.2 SD) subjects. The EEG rhythms of interest were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). EEG cortical sources were estimated by low-resolution brain electromagnetic tomography (LORETA). Statistical results showed that delta sources in the occipital area had significantly less magnitude in Nold compared to Nyoung subjects. Similarly, alpha 1 and alpha 2 sources in the parietal, occipital, temporal, and limbic areas had significantly less magnitude in Nold compared to Nyoung subjects. These nine EEG sources were given as input for evaluating the type (linear, exponential, logarithmic, and power) of correlation with age. When subjects were considered as a single group there was a significant linear correlation of age with the magnitude of delta sources in the occipital area and of alpha 1 sources in occipital and limbic areas. The same was true for alpha 2 sources in the parietal, occipital, temporal, and limbic areas. In general, the EEG sources showing significant linear correlation with age also supported a nonlinear correlation with age. These results suggest that the occipital delta and posterior cortical alpha rhythms decrease in magnitude during physiological aging with both linear and nonlinear trends. In conclusion, this new methodological approach holds promise for the prediction of dementia in mild cognitive impairment by regional source rather than surface EEG data and by both linear and nonlinear predictors.

Donepezil effects on sources of cortical rhythms in mild Alzheimer's disease: Responders vs. Non-Responders.

Acetylcholinesterase inhibitors (AChEI) such as donepezil act in mild Alzheimer's disease (AD) by increasing cholinergic tone. Differences in the clinical response in patients who do or do not benefit from therapy may be due to different functional features of the central neural systems. We tested this hypothesis using cortical electroencephalographic (EEG) rhythmicity. Resting eyes-closed EEG data were recorded in 58 mild AD patients (Mini Mental State Examination [MMSE] range 17-24) before and approximately 1 year after standard donepezil treatment. Based on changes of MMSE scores between baseline and follow-up, 28 patients were classified as "Responders" (MMSEvar >or=0) and 30 patients as "Non-Responders" (MMSEvar <0). EEG rhythms of interest were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). Cortical EEG sources were studied with low-resolution brain electromagnetic tomography (LORETA). Before treatment, posterior sources of delta, alpha 1 and alpha 2 frequencies were greater in amplitude in Non-Responders. After treatment, a lesser magnitude reduction of occipital and temporal alpha 1 sources characterized Responders. These results suggest that Responders and Non-Responders had different EEG cortical rhythms. Donepezil could act by reactivating existing yet functionally silent cortical synapses in Responders, restoring temporal and occipital alpha rhythms.