Claudia Poch

Hippocampal Theta-Phase Modulation of Replay Correlates with Configural-Relational Short-Term Memory Performance

There is now growing evidence that the hippocampus generates theta rhythms that can phase bias fast neural oscillations in the neocortex, allowing coordination of widespread fast oscillatory populations outside limbic areas. A recent magnetoencephalographic study showed that maintenance of configural-relational scene information in a delayed match-to-sample (DMS) task was associated with replay of that information during the delay period. The periodicity of the replay was coordinated by the phase of the ongoing theta rhythm, and the degree of theta coordination during the delay period was positively correlated with DMS performance. Here, we reanalyzed these data to investigate which brain regions were involved in generating the theta oscillations that coordinated the periodic replay of configural–relational information. We used a beamformer algorithm to produce estimates of regional theta rhythms and constructed volumetric images of the phase-locking between the local theta cycle and the instances of replay (in the 13–80 Hz band). We found that individual differences in DMS performance for configural-relational associations were related to the degree of phase coupling of instances of cortical reactivations to theta oscillations generated in the right posterior hippocampus and the right inferior frontal gyrus. This demonstrates that the timing of memory reactivations in humans is biased toward hippocampal theta phase.

Network reconfiguration and working memory impairment in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (mTLE) is the most prevalent form of focal epilepsy, and hippocampal sclerosis (HS) is considered the most frequent associated pathological finding. Recent connectivity studies have shown that abnormalities, either structural or functional, are not confined to the affected hippocampus, but can be found in other connected structures within the same hemisphere, or even in the contralesional hemisphere. Despite the role of hippocampus in memory functions, most of these studies have explored network properties at resting state, and in some cases compared connectivity values with neuropsychological memory scores. Here, we measured magnetoencephalographic responses during verbal working memory (WM) encoding in left mTLE patients and controls, and compared their effective connectivity within a frontotemporal network using dynamic causal modelling. Bayesian model comparison indicated that the best model included bilateral, forward and backward connections, linking inferior temporal cortex (ITC), inferior frontal cortex (IFC), and the medial temporal lobe (MTL). Test for differences in effective connectivity revealed that patients exhibited decreased ipsilesional MTL-ITC backward connectivity, and increased bidirectional IFC-MTL connectivity in the contralesional hemisphere. Critically, a negative correlation was observed between these changes in patients, with decreases in ipsilesional coupling among temporal sources associated with increases contralesional frontotemporal interactions. Furthermore, contralesional frontotemporal interactions were inversely related to task performance and level of education. The results demonstrate that unilateral sclerosis induced local and remote changes in the dynamic organization of a distributed network supporting verbal WM. Crucially, pre-(peri) morbid factors (educational level) were reflected in both cognitive performance and (putative) compensatory changes in physiological coupling.

Anterobasal Temporal Lobe Lesions Alter Recurrent Functional Connectivity within the Ventral Pathway during Naming

An increasing amount of evidence supports a crucial role for the anterior temporal lobe (ATL) in semantic processing. Critically, a selective disruption of the functional connectivity between left and right ATLs in patients with chronic aphasic stroke has been illustrated. The aim of the current study was to evaluate the consequences that lesions on the ATL have on the neurocognitive network supporting semantic cognition. Unlike previous work, in this magnetoencephalography study we selected a group of patients with small lesions centered on the left anteroventral temporal lobe before surgery. We then used an effective connectivity method (i.e., dynamic causal modeling) to investigate the consequences that these lesions have on the functional interactions within the network. This approach allowed us to evaluate the directionality of the causal interactions among brain regions and their associated connectivity strengths. Behaviorally, we found that semantic processing was altered when patients were compared with a strictly matched group of controls. Dynamic causal modeling for event related responses revealed that picture naming was associated with a bilateral frontotemporal network, encompassing feedforward and feedback connections. Comparison of specific network parameters between groups revealed that patients displayed selective network adjustments. Specifically, backward connectivity from anterior to posterior temporal lobe was decreased in the ipsilesional hemisphere, whereas it was enhanced in the contralesional hemisphere. These results reinforce the relevance of ATL in semantic memory, as well as its amodal organization, and highlight the role of feedback connections in enabling the integration of the semantic information.

Explicit processing of verbal and spatial features during letter-location binding modulates oscillatory activity of a fronto-parietal network

The present study investigated the binding of verbal and spatial features in immediate memory. In a recent study, we demonstrated incidental and asymmetrical letter-location binding effects when participants attended to letter features (but not when they attended to location features) that were associated with greater oscillatory activity over prefrontal and posterior regions during the retention period. We were interested to investigate whether the patterns of brain activity associated with the incidental binding of letters and locations observed when only the verbal feature is attended differ from those reflecting the binding resulting from the controlled/explicit processing of both verbal and spatial features. To achieve this, neural activity was recorded using magnetoencephalography (MEG) while participants performed two working memory tasks. Both tasks were identical in terms of their perceptual characteristics and only differed with respect to the task instructions. One of the tasks required participants to process both letters and locations. In the other, participants were instructed to memorize only the letters, regardless of their location. Time-frequency representation of MEG data based on the wavelet transform of the signals was calculated on a single trial basis during the maintenance period of both tasks. Critically, despite equivalent behavioural binding effects in both tasks, single and dual feature encoding relied on different neuroanatomical and neural oscillatory correlates. We propose that enhanced activation of an anterior-posterior dorsal network observed in the task requiring the processing of both features reflects the necessity for allocating greater resources to intentionally process verbal and spatial features in this task.

Aphasic seizures in patients with temporopolar and anterior temporobasal lesions: A video-EEG study

Studies of patients with temporal lobe epilepsy provide few descriptions of seizures that arise in the temporopolar and the anterior temporobasal brain region. Based on connectivity, it might be assumed that the semiology of these seizures is similar to that of medial temporal lobe epilepsy. However, accumulating evidence suggests that the anterior temporobasal cortex may play an important role in the language system, which could account for particular features of seizures arising here. We studied the electroclinical features of seizures in patients with circumscribed temporopolar and temporobasal lesions in order to identify specific features that might differentiate them from seizures that originate in other temporal areas. Among 172 patients with temporal lobe seizures registered in our epilepsy unit in the last 15 years, 15 (8.7%) patients had seizures caused by temporopolar or anterior temporobasal lesions (11 left-sided lesions). The main finding in our study is that patients with left-sided lesions had aphasia during their seizures as the most prominent feature. In addition, while all patients showed normal to high intellectual functioning in standard neuropsychological testing, semantic impairment was found in a subset of 9 patients with left-sided lesions. This case series demonstrates that aphasic seizures without impairment of consciousness can result from small, circumscribed left anterior temporobasal and temporopolar lesions. Thus, the presence of speech manifestation during seizures should prompt detailed assessment of the structural integrity of the basal surface of the temporal lobe in addition to the evaluation of primary language areas.

Visual object naming in patients with small lesions centered at the left temporopolar region

Naming is considered a left hemisphere function that operates according to a posterior–anterior specificity gradient, with more fine-grained information processed in most anterior regions of the temporal lobe (ATL), including the temporal pole (TP). Word finding difficulties are typically assessed using visual confrontation naming tasks, and have been associated with selective damage to ATL resulting from different aetiologies. Nonetheless, the role of the ATL and, more specifically, of the TP in the naming network is not completely established. Most of the accumulated evidence is based on studies on patients with extensive lesions, often bilateral. Furthermore, there is a considerable variability in the anatomical definition of ATL. To better understand the specific involvement of the left TP in visual object naming, we assessed a group of patients with an epileptogenic lesion centered at the TP, and compared their performance with that of a strictly matched control group. We also administered a battery of verbal and non-verbal semantic tasks that was used as a semantic memory baseline. Patients showed an impaired naming ability, manifesting in a certain degree of anomia and semantically related naming errors, which was influenced by concept familiarity. This pattern took place in a context of mild semantic dysfunction that was evident in different types and modalities of semantic tasks. Therefore, current findings demonstrate that a restricted lesion to the left TP can cause a significant deficit in object naming. Of importance, the observed semantic impairment was far from the devastating degradation observed in semantic dementia and other bilateral conditions.

Small temporal pole encephalocele: A hidden cause of “normal” MRI temporal lobe epilepsy

Small temporal pole encephalocele (STPE) can be the pathologic substrate of epilepsy in a subgroup of patients with noninformative magnetic resonance imaging (MRI). Herein, we analyzed the clinical, neurophysiologic, and radiologic features of the epilepsy found in 22 patients with STPE, and the frequency of STPE in patients with refractory focal epilepsy (RFE).

Time-Varying Effective Connectivity during Visual Object Naming as a Function of Semantic Demands

Accumulating evidence suggests that visual object understanding involves a rapid feedforward sweep, after which subsequent recurrent interactions are necessary. The extent to which recurrence plays a critical role in object processing remains to be determined. Recent studies have demonstrated that recurrent processing is modulated by increasing semantic demands. Differentially from previous studies, we used dynamic causal modeling to model neural activity recorded with magnetoencephalography while 14 healthy humans named two sets of visual objects that differed in the degree of semantic accessing demands, operationalized in terms of the values of basic psycholinguistic variables associated with the presented objects (age of acquisition, frequency, and familiarity). This approach allowed us to estimate the directionality of the causal interactions among brain regions and their associated connectivity strengths. Furthermore, to understand the dynamic nature of connectivity (i.e., the chronnectome; Calhoun et al., 2014) we explored the time-dependent changes of effective connectivity during a period (200–400 ms) where adding semantic-feature information improves modeling and classifying visual objects, at 50 ms increments. First, we observed a graded involvement of backward connections, that became active beyond 200 ms. Second, we found that semantic demands caused a suppressive effect in the backward connection from inferior frontal cortex (IFC) to occipitotemporal cortex over time. These results complement those from previous studies underscoring the role of IFC as a common source of top-down modulation, which drives recurrent interactions with more posterior regions during visual object recognition. Crucially, our study revealed the inhibitory modulation of this interaction in situations that place greater demands on the conceptual system.

Working memory retrieval differences between medial temporal lobe epilepsy patients and controls: A three memory layer approach

Multi-store models of working memory (WM) have given way to more dynamic approaches that conceive WM as an activated subset of long-term memory (LTM). The resulting framework considers that memory representations are governed by a hierarchy of accessibility. The activated part of LTM holds representations in a heightened state of activation, some of which can reach a state of immediate accessibility according to task demands. Recent neuroimaging studies have studied the neural basis of retrieval information with different states of accessibility. It was found that the medial temporal lobe (MTL) was involved in retrieving information within immediate access store and outside this privileged zone. In the current study we further explored the contribution of MTL to WM retrieval by analyzing the consequences of MTL damage to this process considering the state of accessibility of memory representations. The performance of a group of epilepsy patients with left hippocampal sclerosis in a 12-item recognition task was compared with that of a healthy control group. We adopted an embedded model of WM that distinguishes three components: the activated LTM, the region of direct access, and a single-item focus of attention. Groups did not differ when retrieving information from single-item focus, but patients were less accurate retrieving information outside focal attention, either items from LTM or items expected to be in the WM range. Analyses focused on items held in the direct access buffer showed that consequences of MTL damage were modulated by the level of accessibility of memory representations, producing a reduced capacity.

Selection within working memory based on a color retro-cue modulates alpha oscillations

Abstract Working Memory (WM) maintains flexible representations. Retrospective cueing studies indicate that selective attention can be directed to memory representations in WM improving performance. While most of the work has explored the neural substrates of orienting attention based on a spatial retro‐cue, behavioral studies show that a feature other than location can also improve WM performance. In the present work we explored the oscillatory underpinnings of orienting attention to a relevant representation held in WM guided by a feature value. We recorded EEG data in a group of 36 healthy human subjects (20 females) performing a WM task in which they had to memorize the orientation of four rectangles of different colors. After a maintenance period, a cue was presented indicating the color of the relevant item. We showed that directing attention to a memory item based on its color resulted in a modulation of posterior alpha activity, which appears as more desynchronization in the contralateral than in the ipsilateral hemisphere. Alpha lateralization is considered a neurophysiological marker of external and internal spatial attention. We propose that current findings support the idea that selection of a memory item based on a non‐location feature could be accomplished by a spatial attentional mechanism. Moreover, using a centrally presented color retro‐cue allowed us to surpass the confounds inherent to the use of spatial retro‐cues, supporting that the observed lateralized alpha results from an endogenous attentional mechanism. HighlightsWe recorded EEG during a retro‐cue change detection task.The relevant memory representation was cued based on a non‐spatial feature.Posterior alpha oscillations were spatially modulated after the non‐spatial retro‐cue.