Hippocampal gamma oscillations increase with memory load

Complex span tasks and hippocampal recruitment during working memory

The working memory (WM) system is vital to performing everyday functions that require attentive, non-automatic processing of information. However, its interaction with long term memory (LTM) is highly debated. Here, we used fMRI to examine whether a popular complex WM span task, thought to force the displacement of to-be-remembered items in the focus of attention to LTM, recruited medial temporal regions typically associated with LTM functioning to a greater extent and in a different manner than traditional neuroimaging WM tasks during WM encoding and maintenance. fMRI scans were acquired while participants performed the operation span (OSPAN) task and an arithmetic task. Results indicated that performance of both tasks resulted in significant activation in regions typically associated with WM function. More importantly, significant bilateral activation was observed in the hippocampus, suggesting it is recruited during WM encoding and maintenance. Right posterior hippocampus activation was greater during OSPAN than arithmetic. Persitimulus graphs indicate a possible specialization of function for bilateral posterior hippocampus and greater involvement of the left for WM performance. Recall time-course activity within this region hints at LTM involvement during complex span.

Category-Specific Visual Responses: An Intracranial Study Comparing Gamma, Beta, Alpha, and ERP Response Selectivity

The specificity of neural responses to visual objects is a major topic in visual neuroscience. In humans, functional magnetic resonance imaging (fMRI) studies have identified several regions of the occipital and temporal lobe that appear specific to faces, letter strings, scenes, or tools. Direct electrophysiological recordings in the visual cortical areas of epileptic patients have largely confirmed this modular organization, using either single-neuron peri-stimulus time-histogram or intracerebral event-related potentials (iERP). In parallel, a new research stream has emerged using high-frequency gamma-band activity (50–150 Hz) (GBR) and low-frequency alpha/beta activity (8–24 Hz) (ABR) to map functional networks in humans. An obvious question is now whether the functional organization of the visual cortex revealed by fMRI, ERP, GBR, and ABR coincide. We used direct intracerebral recordings in 18 epileptic patients to directly compare GBR, ABR, and ERP elicited by the presentation of seven major visual object categories (faces, scenes, houses, consonants, pseudowords, tools, and animals), in relation to previous fMRI studies. Remarkably both GBR and iERP showed strong category-specificity that was in many cases sufficient to infer stimulus object category from the neural response at single-trial level. However, we also found a strong discrepancy between the selectivity of GBR, ABR, and ERP with less than 10% of spatial overlap between sites eliciting the same category-specificity. Overall, we found that selective neural responses to visual objects were broadly distributed in the brain with a prominent spatial cluster located in the posterior temporal cortex. Moreover, the different neural markers (GBR, ABR, and iERP) that elicit selectivity toward specific visual object categories present little spatial overlap suggesting that the information content of each marker can uniquely characterize high-level visual information in the brain.

Striatal and extrastriatal dopamine D2/D3 receptors in schizophrenia evaluated with [18F]fallypride positron emission tomography

BACKGROUND

Alterations in dopamine D(2)/D(3) receptor binding have been reported in schizophrenia, and a meta-analysis of imaging studies has shown a modest elevation in striatum. Newer radioligands now allow the assessment of these receptors in extrastriatal regions. We used positron emission tomography with [(18)F]fallypride to evaluate D(2)/D(3) receptors in both striatal and extrastriatal regions in schizophrenia.

METHODS

Twenty-one patients with schizophrenia and 22 matched healthy control subjects were scanned with an ECAT EXACT HR+ camera. Two-tissue compartment modeling and the reference tissue method gave binding potentials relative to nondisplaceable uptake, total plasma concentration, and free plasma concentration. These were compared between groups in five striatal and eight extrastriatal regions. Several regional volumes were lower in the patient group, and positron emission tomography data were corrected for partial volume effects.

RESULTS

Binding potential values differed in three regions between groups. Values for binding potential relative to nondisplaceable uptake from two-tissue compartment modeling in patients and control subjects, respectively, were 28.7 ± 6.8 and 25.3 ± 4.3 in postcommissural caudate, 2.9 ± .7 and 2.6 ± .4 in thalamus, and 1.8 ± .5 and 2.1 ± .7 in uncus. Loss of D(2)/D(3) receptors with age was found in striatal and extrastriatal regions and was greater in neocortex.

CONCLUSIONS

Our study found selective alterations in D(2)/D(3) receptors in striatal and extrastriatal regions, consistent with some but not all previously published reports. As previously shown for the striatum, a more sensitive imaging approach for studying the role of dopamine in the pathophysiology of schizophrenia might be assessment of neurotransmitter levels rather than D(2)/D(3) receptor levels in extrastriatal regions.

Spatially distributed patterns of oscillatory coupling between high-frequency amplitudes and low-frequency phases in human iEEG

Spatially distributed coherent oscillations provide temporal windows of excitability that allow for interactions between distinct neuronal groups. It has been hypothesized that this mechanism for neuronal communication is realized by bursts of high-frequency oscillations that are phase-coupled to a low-frequency spatially distributed coupling oscillation. This mechanism requires multiple physiologically different interacting sources, one generating the low-frequency coupling oscillation and the others generating phase-coupled high-frequency oscillations. Using human intracranial EEG (iEEG) data, we provide evidence for multiple oscillatory patterns, as characterized on the basis of their spatial maps (topographies) and their frequency spectra. In fact, we show that the spatial maps for the coupling oscillations are much more widespread than the ones for the associated phase-coupled bursts. Second, in the majority of the patterns of phase-amplitude coupling (PAC), phase-coupled bursts of high-frequency activity are synchronized across brain areas. Third and last, working memory operations affect the PAC strength in a heterogeneous way: in some PAC patterns, working memory operations increase their strength, whereas in others they decrease it.

Direct brain recordings fuel advances in cognitive electrophysiology

Electrocorticographic brain recordings in patients with surgically implanted electrodes have recently emerged as a powerful tool for examining the neural basis of human cognition. These recordings measure the electrical activity of the brain directly, and thus provide data with higher temporal and spatial resolution than other human neuroimaging techniques. Here we review recent research in this area and in particular we explain how electrocorticographic recordings have provided insight into the neural basis of human working memory, episodic memory, language, and spatial cognition. In some cases this research has identified patterns of human brain activity that were unexpected on the basis of studies in animals.

Synchronous neural activity and memory formation

Accumulating evidence suggests that the synchronization of neuronal activity plays an important role in memory formation. In particular, several recent studies have demonstrated that enhanced synchronous activity within and among medial temporal lobe structures is correlated with increased memory performance in humans and animals. Modulations in rhythmic synchronization in the gamma-frequency (30-100 Hz) and theta-frequency (4-8 Hz) bands have been related to memory performance, and interesting relationships have been described between these oscillations that suggest a mechanism for inter-areal coupling. Neuronal synchronization has also been linked to spike timing-dependent plasticity, a cellular mechanism thought to underlie learning and memory. The available evidence suggests that neuronal synchronization modulates memory performance as well as potential cellular mechanisms of memory storage.