Theta Phase Coupling Research Articles

Functional connectivity and gray matter deficits within the auditory attention circuit in first-episode psychosis.

Selective attention deficits in first episode of psychosis (FEP) can be indexed by impaired attentional modulation of auditory M100. It is unknown if the pathophysiology underlying this deficit is restricted to auditory cortex or involves a distributed attention network. We examined the auditory attention network in FEP. MEG was recorded from 27 FEP and 31 matched healthy controls (HC) while alternately ignoring or attending tones. A whole-brain analysis of MEG source activity during auditory M100 identified non-auditory areas with increased activity. Time-frequency activity and phase-amplitude coupling were examined in auditory cortex to identify the attentional executive carrier frequency. Attention networks were defined by phase-locking at the carrier frequency. Spectral and gray matter deficits in the identified circuits were examined in FEP. Attention-related activity was identified in prefrontal and parietal regions, markedly in precuneus. Theta power and phase coupling to gamma amplitude increased with attention in left primary auditory cortex. Two unilateral attention networks were identified with precuneus seeds in HC. Network synchrony was impaired in FEP. Gray matter thickness was reduced within the left hemisphere network in FEP but did not correlate with synchrony. Several extra-auditory attention areas with attention-related activity were identified. Theta was the carrier frequency for attentional modulation in auditory cortex. Left and right hemisphere attention networks were identified, with bilateral functional deficits and left hemisphere structural deficits, though FEP showed intact auditory cortex theta phase-gamma amplitude coupling. These novel findings indicate attention-related circuitopathy early in psychosis potentially amenable to future non-invasive interventions.

Sustained attention is enhanced by disrupted inter-regional frontal theta phase coupling

Sustained attention is enhanced by disrupted inter-regional frontal theta phase coupling

Theta low-gamma phase amplitude coupling in the human orbitofrontal cortex increases during a conflict-processing task

Objective. The human orbitofrontal cortex (OFC) is involved in automatic response inhibition and conflict processing, but the mechanism of frequency-specific power changes that control these functions is unknown. Theta and gamma activity have been independently observed in the OFC during conflict processing, while theta–gamma interactions in other brain areas have been noted primarily in studies of memory. Within the OFC, it is possible that theta–gamma phase amplitude coupling (PAC) drives conflict processing. This study aims to characterize the coupled relationship between theta and gamma frequency bands in the OFC during conflict processing using a modified Stroop task. Approach. Eight epilepsy patients implanted with OFC stereotactic electroencephalography electrodes participated in a color-word modified Stroop task. PAC between theta phase and gamma amplitude was assessed to determine the timing and magnitude of neural oscillatory changes. Group analysis was conducted using a non-parametric cluster-permutation t-test on coherence values. Main results. Theta-low gamma (LG) PAC significantly increased in five out of eight patients during successful trials of the incongruent condition compared with the congruent condition. Significant increases in theta-LG PAC were most prominent during cue processing 200–800 ms after cue presentation. On group analysis, trial-averaged mean theta-LG PAC was statistically significantly greater in the incongruent condition compared to the congruent condition (p < 0.001, Cohen’s d = 0.51). Significance. For the first time, we report that OFC theta phase and LG amplitude coupling increases during conflict resolution. Given the delayed onset after cue presentation, OFC theta-LG PAC may contribute to conflict processing after conflict detection and before motor response. This explanation follows the hypothesis that global theta waves modulate local gamma signals. Understanding this relationship within the OFC will help further elucidate the neural mechanisms of human conflict resolution.

Different theta frameworks coexist in the rat hippocampus and are coordinated during memory-guided and novelty tasks.

Hippocampal firing is organized in theta sequences controlled by internal memory processes and by external sensory cues, but how these computations are coordinated is not fully understood. Although theta activity is commonly studied as a unique coherent oscillation, it is the result of complex interactions between different rhythm generators. Here, by separating hippocampal theta activity in three different current generators, we found epochs with variable theta frequency and phase coupling, suggesting flexible interactions between theta generators. We found that epochs of highly synchronized theta rhythmicity preferentially occurred during behavioral tasks requiring coordination between internal memory representations and incoming sensory information. In addition, we found that gamma oscillations were associated with specific theta generators and the strength of theta-gamma coupling predicted the synchronization between theta generators. We propose a mechanism for segregating or integrating hippocampal computations based on the flexible coordination of different theta frameworks to accommodate the cognitive needs.

S153. IMPAIRED THETA PHASE-COUPLING BETWEEN HIPPOCAMPUS AND MEDIAL PREFRONTAL CORTEX IN SCHIZOPHRENIA

BackgroundA genetic schizophrenia (Scz) mouse model has impaired hippocampal-prefrontal synchrony during spatial working memory (Sigurdsson et al., 2000, Nature). These Df(16)A(+/-) mice (models of the human 22q11.2 microdeletion) have impaired mPFC phase coupling to hippocampal theta (1–8 Hz) oscillations, which predicts performance. This deficit has never been demonstrated in human Scz subjects, however, and its mechanistic basis is unclear.Methods18 Scz (8 unmedicated) and 26 age, gender and IQ-matched controls performed a spatial memory task whilst undergoing magnetoencephalography (MEG), and a ‘memory integration’ task dependent on hippocampal function (without MEG). A partly overlapping group of 33 Scz and 29 controls underwent positron emission tomography (PET) to measure the availability of GABAARs expressing the α5 subunit (concentrated on hippocampal somatostatin interneurons).ResultsWe demonstrate – in the spatial memory task, during memory recall – that theta power increases in left medial temporal lobe (mTL) are impaired in Scz, as is theta phase coupling between mPFC and mTL. Importantly, the latter cannot be explained by theta power changes, head movement, antipsychotics, cannabis use, or IQ, and is not found in other frequency bands. Moreover, mPFC-mTL theta coupling correlated strongly with performance in controls, but not in Scz, who were mildly impaired at the spatial memory task and no better than chance on the memory integration task. Use of antipsychotic medication may ameliorate this mPFC-HC theta coupling deficit. Finally, mTL regions showing reduced theta phase coupling in Scz MEG participants overlapped substantially with areas of diminished α5-GABAAR availability in the wider Scz PET sample.DiscussionThese results indicate that impaired theta phase coupling between hippocampus and mPFC could underlie hippocampal-prefrontal dysconnectivity in schizophrenia, and impairments in the cognitive domains that depend on communication between these areas. They also imply α5-GABAARs (and the cells that express them) have a role in the phase coupling process.

Theta oscillations support the interface between language and memory

Recent evidence shows that hippocampal theta oscillations, usually linked to memory and navigation, are also observed during online language processing, suggesting a shared neurophysiological mechanism between language and memory. However, it remains to be established what specific roles hippocampal theta oscillations may play in language, and whether and how theta mediates the communication between the hippocampus and the perisylvian cortical areas, generally thought to support language processing. With whole-head magnetoencephalographic (MEG) recordings, the present study investigated these questions with two experiments. Using a violation paradigm, extensively used for studying neural underpinnings of different aspects of linguistic processing, we found increased theta power (4–8 ​Hz) in the hippocampal formation, when participants read a semantically incorrect vs. correct sentence ending. Such a pattern of results was replicated using different sentence stimuli in another cohort of participants. Importantly, no significant hippocampal theta power increase was found when participants read a semantically correct but syntactically incorrect sentence ending vs. a correct sentence ending. These findings may suggest that hippocampal theta oscillations are specifically linked to lexical-semantic related processing, and not general information processing in sentence reading. Furthermore, we found significantly transient theta phase coupling between the hippocampus and the left superior temporal gyrus, a hub area of the cortical network for language comprehension. This transient theta phase coupling may provide an important channel that links the memory and language systems for the generation of sentence meaning. Overall, these findings help specify the role of hippocampal theta in language, and provide a novel neurophysiological mechanism at the network level that may support the interface between memory and language.

Impaired theta phase coupling underlies frontotemporal dysconnectivity in schizophrenia.

Frontotemporal dysconnectivity is a key pathology in schizophrenia. The specific nature of this dysconnectivity is unknown, but animal models imply dysfunctional theta phase coupling between hippocampus and medial prefrontal cortex (mPFC). We tested this hypothesis by examining neural dynamics in 18 participants with a schizophrenia diagnosis, both medicated and unmedicated; and 26 age, sex and IQ matched control subjects. All participants completed two tasks known to elicit hippocampal-prefrontal theta coupling: a spatial memory task (during magnetoencephalography) and a memory integration task. In addition, an overlapping group of 33 schizophrenia and 29 control subjects underwent PET to measure the availability of GABAARs expressing the α5 subunit (concentrated on hippocampal somatostatin interneurons). We demonstrate-in the spatial memory task, during memory recall-that theta power increases in left medial temporal lobe (mTL) are impaired in schizophrenia, as is theta phase coupling between mPFC and mTL. Importantly, the latter cannot be explained by theta power changes, head movement, antipsychotics, cannabis use, or IQ, and is not found in other frequency bands. Moreover, mPFC-mTL theta coupling correlated strongly with performance in controls, but not in subjects with schizophrenia, who were mildly impaired at the spatial memory task and no better than chance on the memory integration task. Finally, mTL regions showing reduced phase coupling in schizophrenia magnetoencephalography participants overlapped substantially with areas of diminished α5-GABAAR availability in the wider schizophrenia PET sample. These results indicate that mPFC-mTL dysconnectivity in schizophrenia is due to a loss of theta phase coupling, and imply α5-GABAARs (and the cells that express them) have a role in this process.

A causal role for the precuneus in network-wide theta and gamma oscillatory activity during complex memory retrieval.

Complex memory of personal events is thought to depend on coordinated reinstatement of cortical representations by the medial temporal lobes (MTL). MTL-cortical theta and gamma coupling is believed to mediate such coordination, but which cortical structures are critical for retrieval and how they influence oscillatory coupling is unclear. We used magnetoencephalography (MEG) combined with continuous theta burst stimulation (cTBS) to (i) clarify the roles of theta and gamma oscillations in network-wide communication during naturalistic memory retrieval, and (ii) understand the causal relationship between cortical network nodes and oscillatory communication. Retrieval was associated with MTL-posterior neocortical theta phase coupling and theta-gamma phase-amplitude coupling relative to a rest period. Precuneus cTBS altered MTL-neocortical communication by modulating theta and gamma oscillatory coupling. These findings provide a mechanistic account for MTL-cortical communication and demonstrate that the precuneus is a critical cortical node of oscillatory activity, coordinating cross-regional interactions that drive remembering.

S126. A study on the electrophysiological mechanism of human hippocampus in propofol-induced amnesia: An ECoG study

Introduction General anesthesia is known to be a drug-induces, reversible condition that includes specific behavioral and physiological traits, such as unconsciousness, amnesia, analgesia, and akinesia (Brown et al., 2010). Among those characteristic features, the underlying mechanisms of propofol-induces amnesia still remains unknown. Propofol is a commonly used anesthetic drug and is known to induce anesthetic state through enhancing inhibition at GABAA receptors. The hippocampal formation occupies a central position for generation of memory (Perouansky and Pearce, 2011). From an electrophysiological perspective, hippocampal formation is a central theta rhythm generator controlled by GABAergic hippocampal interneurons (Buzsaki, 2006). Taken together, we hypothesized that enhancement of inhibition at GABAA receptors induced by propofol influences on theta oscillations of hippocampal formation which leads to propofol-induced amnesia. To address this hypothesis, we investigated an electrocorticography (ECoG) signals obtained from mesial temporal lobe epilepsy patients (MTLE) with hippocampal sclerosis (HS). Methods We recruited five MTLE with HS. Because we are especially interested in the hippocampal theta oscillations, we only included the patients who had depth-electrodes over hippocampal formation. Two patients had the right HS and 3 patients had the left HS. ECoG signals were recorded during 3 different awaked states such as, resting-state, sleep state and word memory task. In addition, ECoG were recorded under general anesthesia with 3 different propofol concentrations of 5 μ g/ml, 4 μ g/ml and 3 μ g/ml. ECoG signals were epoched for 2 min. Bad channel removal, drift removal, re-referencing (common average reference), and low-pass filter of 100 Hz were applied. We evaluated relative theta power, theta/beta ratio, and cross-frequency phase-amplitude coupling analysis between theta (4–7 Hz) and beta (13–30 Hz) frequency band. Modulation index representing theta phase and beta amplitude coupling was derived. Results Hippocampal formation consistently showed the reduced relative theta power during propofol-induced state compared with those in the resting-state, sleep and memory task conditions. The theta/beta ratios of hippocampal formation during propofol-induced state and during memory task showed similar levels between them, but, lower than those during resting-state and sleep conditions. It is notable that the modulation index of theta phase and beta amplitude at hippocampal formation was higher during propofol-induced state than other conditions. Conclusion Our results suggest that this specific theta-beta coupling occurred at hippocampal formation during propofol-induced state could be a possible electrophysiological mechanism for explaining propofol-induced amnesia. Acknowledgements This study was funded by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. 2015R1D1A1A02061486 ).

Disruption and rescue of interareal theta phase coupling and adaptive behavior

Rescuing executive functions in people with neurological and neuropsychiatric disorders has been a major goal of psychology and neuroscience for decades. Innovative computer-training regimes for executive functions have made tremendous inroads, yet the positive effects of training have not always translated into improved cognitive functioning and often take many days to emerge. In the present study, we asked whether it was possible to immediately change components of executive function by directly manipulating neural activity using a stimulation technology called high-definition transcranial alternating current stimulation (HD-tACS). Twenty minutes of inphase stimulation over medial frontal cortex (MFC) and right lateral prefrontal cortex (lPFC) synchronized theta (∼6 Hz) rhythms between these regions in a frequency and spatially specific manner and rapidly improved adaptive behavior with effects lasting longer than 40 min. In contrast, antiphase stimulation in the same individuals desynchronized MFC-lPFC theta phase coupling and impaired adaptive behavior. Surprisingly, the exogenously driven impairments in performance could be instantly rescued by reversing the phase angle of alternating current. The results suggest executive functions can be rapidly up- or down-regulated by modulating theta phase coupling of distant frontal cortical areas and can contribute to the development of tools for potentially normalizing executive dysfunction in patient populations.

Medial Prefrontal-Medial Temporal Theta Phase Coupling in Dynamic Spatial Imagery.

Hippocampal-medial prefrontal interactions are thought to play a crucial role in mental simulation. Notably, the frontal midline/medial pFC (mPFC) theta rhythm in humans has been linked to introspective thought and working memory. In parallel, theta rhythms have been proposed to coordinate processing in the medial temporal cortex, retrosplenial cortex (RSc), and parietal cortex during the movement of viewpoint in imagery, extending their association with physical movement in rodent models. Here, we used noninvasive whole-head MEG to investigate theta oscillatory power and phase-locking during the 18-sec postencoding delay period of a spatial working memory task, in which participants imagined previously learned object sequences either on a blank background (object maintenance), from a first-person viewpoint in a scene (static imagery), or moving along a path past the objects (dynamic imagery). We found increases in 4- to 7-Hz theta power in mPFC when comparing the delay period with a preencoding baseline. We then examined whether the mPFC theta rhythm was phase-coupled with ongoing theta oscillations elsewhere in the brain. The same mPFC region showed significantly higher theta phase coupling with the posterior medial temporal lobe/RSc for dynamic imagery versus either object maintenance or static imagery. mPFC theta phase coupling was not observed with any other brain region. These results implicate oscillatory coupling between mPFC and medial temporal lobe/RSc theta rhythms in the dynamic mental exploration of imagined scenes.

Effects of Dopamine and Serotonin Systems on Modulating Neural Oscillations in Hippocampus-Prefrontal Cortex Pathway in Rats.

Theta and gamma oscillations are believed to play an important role in cognition and memory, and their phase coupling facilitates the information transmission in hippocampal-cortex network. In a rat model of chronic stress, the phase coupling of both theta and gamma oscillations between ventral hippocampal CA1 (vCA1) and medial prefrontal cortex (mPFC) was found to be disrupted, which was associated with the impaired synaptic plasticity in the pathway. However, little was known about the mechanisms underlying the process. In order to address this issue, both dopamine and serotonin as monoaminergic neurotransmitters were involved in this study, since they were crucial factors in pathological basis of depressive disorder. Local field potentials (LFPs) were recorded simultaneously at both vCA1 and mPFC regions under anesthesia, before and after the injection of dopamine D1 receptor antagonist and 5-HT1A receptor agonist, respectively. The results showed that the blockage of D1 receptor could lead to depression-like decrement on theta phase coupling. In addition, the activation of 5-HT1A receptor enhanced vCA1-mPFC coupling on gamma oscillations, and attenuated CA1 theta-fast gamma cross frequency coupling. These data suggest that the theta phase coupling between vCA1 and mPFC may be modulated by dopamine system that is an underlying mechanism of the cognitive dysfunction in depression. Besides, the serotonergic system is probably involved in the regulation of gamma oscillations coupling in vCA1-mPFC network.

Synaptic plasticity-related neural oscillations on hippocampus–prefrontal cortex pathway in depression

It is believed that phase synchronization facilitates neural communication and neural plasticity throughout the hippocampal–cortical network, and further supports cognition and memory. The pathway from the ventral hippocampus to the medial prefrontal cortex (mPFC) is thought to play a significant role in emotional memory processing. Therefore, the information transmission on the pathway was hypothesized to be disrupted in the depressive state, which could be related to its impaired synaptic plasticity. In this study, local field potentials (LFPs) from both ventral CA1 (vCA1) and mPFC were recorded in both normal and chronic unpredictable stress (CUS) model rats under urethane anesthesia. LFPs of all rats were recorded before and after the long-term potentiation (LTP) induced on the vCA1–mPFC pathway in order to figure out the correlation of oscillatory synchronization of LFPs and synaptic plasticity. Our results showed the vCA1-to-mPFC unidirectional phase coupling of the theta rhythm, rather than the power of either region, was significantly enhanced by LTP induction, with less enhancement in the CUS model rats compared to that in the normal rats. In addition, theta phase coupling was positively correlated with synaptic plasticity on vCA1–mPFC pathway. Moreover, the theta-slow gamma phase–amplitude coupling in vCA1 was long-term enhanced after high frequency stimulation. These results suggest that the impaired synaptic plasticity in vCA1–mPFC pathway could be reflected by the attenuated theta phase coupling and theta–gamma cross frequency coupling of LFPs in the depression state.

EEG theta phase coupling during executive control of visual working memory investigated in individuals with schizophrenia and in healthy controls.

In healthy humans, it has been shown that executive functions are associated with increased frontal-midline EEG theta activity and theta phase coupling between frontal and posterior brain regions. In individuals with schizophrenia, central executive functions are supposed to be heavily impaired. Given that theta phase coupling is causally involved in central executive functions, one would expect that patients with an executive function deficit should display abnormal EEG theta synchronization. We therefore investigated executive functioning in 21 healthy controls and 21 individuals with schizophrenia while they performed a visuospatial delayed match to sample task. The task required either high executive demands (manipulation of content in working memory [WM]) or low executive demands (retention of WM content). In addition, WM load (one vs. three items) was varied. Results indicated higher frontal theta activity for manipulation processes than for retention processes in patients with schizophrenia, as compared with healthy controls, independently of WM load. Furthermore, individuals with schizophrenia revealed a reduction in theta phase coupling during early stages of the delay period for retention, as well as for manipulation processes at high-WM loads. Deviations in theta phase coupling in individuals with schizophrenia were mainly characterized by aberrant fronto-posterior connections, but also by attenuated posterior connections during manipulation of high-WM load. To conclude, fronto-parietal theta coupling seems to be substantially involved in executive control, whereas frontal theta activity seems to reflect general task demands, such as deployment of attentional resources during WM.

Corticostriatal coordination through coherent phase-amplitude coupling.

The corticostriatal axis is the main input stage of the basal ganglia and is crucial for their role in motor behavior. Synchronized oscillations might mediate interactions between cortex and striatum during behavior, yet direct evidence remains sparse. Here, we show that, during motor behavior, low- and high-frequency oscillations jointly couple cortex and striatum via cross-frequency interactions. We investigated neuronal oscillations along the corticostriatal axis in rats during rest and treadmill running. We found prominent theta and gamma oscillations in cortex and striatum, the peak frequencies of which scaled with motor demand. Theta and gamma oscillations were functionally coupled through phase-amplitude coupling. Furthermore, theta oscillations were phase coupled between structures. Together, local phase-amplitude coupling and corticostriatal theta phase coupling mediated the temporal correlation of gamma bursts between the cortex and striatum. The coordination of fast oscillations through coherent phase-amplitude coupling may be a general mechanism to regulate neuronal interactions along the corticostriatal axis and beyond.

Medial prefrontal theta phase coupling during spatial memory retrieval

Memory retrieval is believed to involve a disparate network of areas, including medial prefrontal and medial temporal cortices, but the mechanisms underlying their coordination remain elusive. One suggestion is that oscillatory coherence mediates inter-regional communication, implicating theta phase and theta-gamma phase-amplitude coupling in mnemonic function across species. To examine this hypothesis, we used non-invasive whole-head magnetoencephalography (MEG) as participants retrieved the location of objects encountered within a virtual environment. We demonstrate that, when participants are cued with the image of an object whose location they must subsequently navigate to, there is a significant increase in 4–8 Hz theta power in medial prefrontal cortex (mPFC), and the phase of this oscillation is coupled both with ongoing theta phase in the medial temporal lobe (MTL) and perceptually induced 65–85 Hz gamma amplitude in medial parietal cortex. These results suggest that theta phase coupling between mPFC and MTL and theta-gamma phase-amplitude coupling between mPFC and neocortical regions may play a role in human spatial memory retrieval. © 2014 The Authors. Hippocampus Published by Wiley Periodicals, Inc.

Human Retrosplenial Cortex Displays Transient Theta Phase Locking with Medial Temporal Cortex Prior to Activation during Autobiographical Memory Retrieval

The involvement of retrosplenial cortex (RSC) in human autobiographical memory retrieval has been confirmed by functional brain imaging studies, and is supported by anatomical evidence of strong connectivity between the RSC and memory structures within the medial temporal lobe (MTL). However, electrophysiological investigations of the RSC and its interaction with the MTL have mostly remained limited to the rodent brain. Recently, we reported a selective increase of high-frequency broadband (HFB; 70-180 Hz) power within the human RSC during autobiographical retrieval, and a predominance of 3-5 Hz theta band oscillations within the RSC during the resting state. In the current study, we aimed to explore the temporal dynamics of theta band interaction between human RSC and MTL during autobiographical retrieval. Toward this aim, we obtained simultaneous recordings from the RSC and MTL in human subjects undergoing invasive electrophysiological monitoring, and quantified the strength of RSC-MTL theta band phase locking. We observed significant phase locking in the 3-4 Hz theta range between the RSC and the MTL during autobiographical retrieval. This theta band phase coupling was transient and peaked at a consistent latency before the peak of RSC HFB power across subjects. Control analyses confirmed that theta phase coupling between the RSC and MTL was not seen for other conditions studied, other sites of recording, or other frequency ranges of interest (1-20 Hz). Our findings provide the first evidence of theta band interaction between the human RSC and MTL during conditions of autobiographical retrieval.

The Importance of Timing in Segregated Theta Phase-Coupling for Cognitive Performance

Functional cortical circuits for central executive functions have been shown to emerge by theta (~6 Hz) phase-coupling of distant cortical areas. It has been repeatedly shown that frontoparietal theta coupling at ~0° relative phase is associated with recognition, encoding, short-term retention, and planning; however, a causal link has not been demonstrated so far. Here we used transcranial alternating current stimulation simultaneously applied at 6 Hz over left prefrontal and parietal cortices with a relative 0° ("synchronized" condition) or 180° ("desynchronized" condition) phase difference or a placebo stimulation condition, whereas healthy subjects performed a delayed letter discrimination task. We show that exogenously induced frontoparietal theta synchronization significantly improves visual memory-matching reaction times as compared to placebo stimulation. In contrast, exogenously induced frontoparietal theta desynchronization deteriorates performance. The present findings provide for the first time evidence of causality of theta phase-coupling of distant cortical areas for cognitive performance in healthy humans. Moreover, the results demonstrate the suitability of transcranial alternating current stimulation to artificially induce coupling or decoupling of behaviorally relevant brain rhythms between segregated cortical regions.

The relationship between theta-gamma coupling and spatial memory ability in older adults

Although theta-gamma coupling is known to reflect memory-related processes, it is unclear whether it has a relationship with memory ability. Here, we investigated the relationship between theta phase and gamma power coupling (TGC) and scores obtained regarding memory tasks. Thirty-one older subjects performed a spatial delayed match-to-sample task (SDMST) during EEG recording. We also evaluated performance on the delayed figure recall (DFR) and delayed verbal recall (DVR) tasks regarding memory ability. Partial correlation analysis (controlled for age) demonstrated that increases in TGC in the parietal area were significantly correlated with increases in the DFR scores obtained. In addition, TGC was significantly correlated with the accuracy rate of the SDMST. Our results indicate that TGC could be a useful biophysiological marker of memory ability.

When attention meets working memory: Shifted visuo-spatial attention amplifies theta:gamma phase synchronisation in the posterior parietal cortex

Introduction: When a visual target has to be detected one should hold a template of it in working memory (WM) in order to perform a memory match when a stimulus occurs. Here the question is addressed whether a shift of spatial attention amplifies this memory match, and how this is manifested in the human brain. It was shown that visual attention is associated with oscillatory brain activity at gamma frequency (around 40 Hz) and that theta activity (around 5 Hz) reflects WM processes. There is recent evidence that gamma and theta oscillations might be phase coupled during memory processes in animals as well as in the human EEG. Here, we investigate whether a shift of spatial attention leads to increased phase synchronisation of theta and gamma activity. Methods: A Posner task was run in which a central cue, directing attention to one visual hemifield, was presented. The cue could be valid or invalid. Subsequently, one of two peripheral targets was presented in the left or right visual field. Subjects had to indicate which target was shown. EEG was recorded during the task. In response to a target, induced gamma amplitude was increased at posterior parietal cortex contralateral to the stimulated hemifield. Using a beamformer algorithm, bilateral neural sources of posterior gamma activity were localised for each subject individually (Fig. 1). Ongoing scalp EEG was transformed into source space. Gabor expansion was used to obtain frequency information of source activity, and 40:n Hz phase synchronisation was calculated on a single trial basis. Results: Target presentation elicited higher phase synchronisation between 40 Hz and oscillatory activity between 1 and 10 Hz (with its maximum at 5 Hz) at the contralateral posterior source compared to ipsilateral between 100 and 200 ms poststimulus. This phenomenon was significantly more pronounced when spatial attention was shifted to the hemifield where the target was presented, i.e. in the valid cue condition than in the invalid condition (Fig. 2). Conclusion: Here, we show that a match between a visual target and its memory template is reflected by increased gamma:theta phase coupling in the posterior parietal cortex. We could further show that attention does not only amplify early visual processing as it was often discussed, but it even enhances the matching of visual information with WM contents.