Trait anxiety and the neural efficiency of manipulation in working memory

The present study investigates the effects of trait anxiety on the neural efficiency of working memory component functions (manipulation vs. maintenance) in the absence of threat-related stimuli. For the manipulation of affectively neutral verbal information held in working memory, high- and low-anxious individuals (N = 46) did not differ in their behavioral performance, yet trait anxiety was positively related to the neural effort expended on task processing, as measured by BOLD signal changes in fMRI. Higher levels of anxiety were associated with stronger activation in two regions implicated in the goal-directed control of attention--that is, right dorsolateral prefrontal cortex (DLPFC) and left inferior frontal sulcus--and with stronger deactivation in a region assigned to the brain's default-mode network--that is, rostral-ventral anterior cingulate cortex. Furthermore, anxiety was associated with a stronger functional coupling of right DLPFC with ventrolateral prefrontal cortex. We interpret our findings as reflecting reduced processing efficiency in high-anxious individuals and point out the need to consider measures of functional integration in addition to measures of regional activation strength when investigating individual differences in neural efficiency. With respect to the functions of working memory, we conclude that anxiety specifically impairs the processing efficiency of (control-demanding) manipulation processes (as opposed to mere maintenance). Notably, this study contributes to an accumulating body of evidence showing that anxiety also affects cognitive processing in the absence of threat-related stimuli

The cognitive neuroscience of visual working memory

Visual working memory allows us to temporarily maintain and manipulate visual information in order to solve a task. The study of the brain mechanisms underlying this function began more than half a century ago, with Scoville and Milner’s (1957) seminal discoveries with amnesic patients. This timely collection of papers brings together diverse perspectives on the cognitive neuroscience of visual working memory from multiple fields that have traditionally been fairly disjointed: human neuroimaging, electrophysiological, behavioural and animal lesion studies, investigating both the developing and the adult brain

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

Space representation for eye movements is more contralateral in monkeys than in humans

Contralateral hemispheric representation of sensory inputs (the right visual hemifield in the left hemisphere and vice versa) is a fundamental feature of primate sensorimotor organization, in particular the visuomotor system. However, many higher-order cognitive functions in humans show an asymmetric hemispheric lateralization—e.g., right brain specialization for spatial processing—necessitating a convergence of information from both hemifields. Electrophysiological studies in monkeys and functional imaging in humans have investigated space and action representations at different stages of visuospatial processing, but the transition from contralateral to unified global spatial encoding and the relationship between these encoding schemes and functional lateralization are not fully understood. Moreover, the integration of data across monkeys and humans and elucidation of interspecies homologies is hindered, because divergent findings may reflect actual species differences or arise from discrepancies in techniques and measured signals (electrophysiology vs. imaging). Here, we directly compared spatial cue and memory representations for action planning in monkeys and humans using event-related functional MRI during a working-memory oculomotor task. In monkeys, cue and memory-delay period activity in the frontal, parietal, and temporal regions was strongly contralateral. In putative human functional homologs, the contralaterality was significantly weaker, and the asymmetry between the hemispheres was stronger. These results suggest an inverse relationship between contralaterality and lateralization and elucidate similarities and differences in human and macaque cortical circuits subserving spatial awareness and oculomotor goal-directed actions

The oscillatory mechanisms of working memory maintenance

Working memory (WM) is a cognitive process which allows for maintenance of information that is no longer perceived. Although theoretical models have recognized that working memory involves interactions across cell assemblies in multiple brain areas, the exact neural mechanisms which support this process remain unknown. In this thesis I investigate the neural dynamics in the human hippocampus, the ventral, dorsal and frontal cortex as well as the long-range network connectivity across these brain areas to understand how such a distributed network allows for maintenance of various information pieces in WM. The results described here support a model in which working memory relies on dynamic interactions across frequencies (the cross-frequency coupling, CFC) in a distributed network of cortical areas coordinated by the prefrontal cortex. In particular, maintenance of information during a delay period selectively involves the hippocampus, dorsal and ventral visual stream as well as the prefrontal cortex each of which represents different features. The hippocampus contributes to this large network specifically by representing multiple items in working memory. In two independent experiments I observed that the low-frequency activity (a marker of neural inhibition) was linearly reduced across memory loads. Importantly, the hippocampus showed very prominent low-frequency power during maintenance of a single item suggesting that during this condition the neural processing was strongly inhibited. In turn, the broadband gamma activity was linearly increasing as a function of memory load. This pattern of results may be interpreted as reflecting an increased involvement of the hippocampus in representing longer sequences. Importantly, the low-frequency decrease was not static but fluctuated periodically between two different modes. One of the modes was characterized by the load-dependent power decreases and reduced cross-frequency coupling (memory activation mode) whereas the other mode was reflected by the load-independent high levels of power and increased coupling strength (load-independent mode). Crucially, these modes were temporally organized by the phase of an endogenous delta rhythm forming a “hierarchy of oscillations”. This periodicity was essential for the successful performance. Finally, during the memory activation mode the WM capacity limit was inter-individually correlated with the peak frequency change as predicted by the multiplexing model of WM. All these effects were subsequently replicated in an independent dataset. These results suggest that the hippocampus is involved in WM maintenance showing periodic fluctuations between two different oscillatory modes. Parameters of the hippocampal iEEG signal correlate with individual WM capacity, specifically during the memory activation mode. The ventral and dorsal visual stream each contributes to the distributed WM network by representing configuration and spatial information, respectively. Specifically, the alpha power in the ventral visual stream was decreased during maintenance of face identities. In turn, the alpha power was desynchronized in the dorsal visual stream while participants were maintaining face orientations. This shows that the alpha power double dissociates between the feature specific networks in the ventral and dorsal visual stream. These effects are further interpreted as reflecting selective involvement of the dorsal and ventral visual pathway depending on the maintained features. Importantly, each of the visual streams was selectively synchronized with the prefrontal cortex depending on the memory condition and the alpha power. This corroborates a central prediction from the gating by inhibition model which assumes that the increased alpha power serves as the mechanism for gating of information by inhibiting task redundant pathways. Moreover, during maintenance of information the phase of alpha modulated the amplitude of high-frequency activity both in the dorsal and ventral visual stream. Additionally,the low-frequency phase in the prefrontal cortex modulated high-frequency activity both in the dorsal and ventral visual stream. These results suggest that both the dorsal and ventral visual streams are selectively involved during maintenance of distinct features (i.e. face orientation and identity, respectively). They also indicate that the prefrontal cortex selectively gets synchronized with the visual regions depending on the alpha power in that region and the maintained feature. Finally, the activity in the prefrontal cortex influences processing across long distance as evident from changes in the phase synchrony with the visual cortical areas and by modulating gamma power in the visual cortical regions. It is also noted that the ventrolateral prefrontal cortex (vlPFC) contains information regarding abstract rules (i.e. response mapping). In particular, using a multivariate decoding approach I found that the local field potentials recorded from the vlPFC dissociate between different types of responses. At the same time I observed no evidence for the load-dependent or stimulus-specific changes in that brain region. The null effect should be treated with caution. Nevertheless, the current results suggest that the vlPFC may contribute to working memory by processing of abstract rules such as a mapping between the stimulus and the response. Furthermore, I found that the alpha power dependent duty cycle in the vlPFC constrains the duration of the gamma burst which has been suggested as a mechanism for neural inhibition. This finding is important because such a property of the alpha activity has never been observed in a brain region other than the primary sensory cortex. Together, the results presented in this thesis support a model according to which the working memory is a complex and highly dynamic process engaging hierarchies of oscillations across multiple cortical regions. In particular, the hippocampus is important for the multi-item WM. The dorsal and ventral visual streams are relevant for distinct visual features. Finally, the prefrontal cortex represents abstract rules and influences processing in other cortical regions likely providing a top down control over these regions

Post-training load-related changes of auditory working memory: An EEG study

Working memory (WM) refers to the temporary retention and manipulation of information, and its capacity is highly susceptible to training. Yet, the neural mechanisms that allow for increased performance under demanding conditions are not fully understood. We expected that post-training efficiency in WM performance modulates neural processing during high load tasks. We tested this hypothesis, using electroencephalography (EEG) (N = 39), by comparing source space spectral power of healthy adults performing low and high load auditory WM tasks. Prior to the assessment, participants either underwent a modality-specific auditory WM training, or a modality-irrelevant tactile WM training, or were not trained (active control). After a modality-specific training participants showed higher behavioral performance, compared to the control. EEG data analysis revealed general effects of WM load, across all training groups, in the theta-, alpha-, and beta-frequency bands. With increased load theta-band power increased over frontal, and decreased over parietal areas. Centro-parietal alpha-band power and central beta-band power decreased with load. Interestingly, in the high load condition a tendency toward reduced beta-band power in the right medial temporal lobe was observed in the modality-specific WM training group compared to the modality-irrelevant and active control groups. Our finding that WM processing during the high load condition changed after modality-specific WM training, showing reduced beta-band activity in voice-selective regions, possibly indicates a more efficient maintenance of task-relevant stimuli. The general load effects suggest that WM performance at high load demands involves complementary mechanisms, combining a strengthening of task-relevant and a suppression of task-irrelevant processing

Non-Verbal Working Memory: A functional near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI) comparison

Ulike hjernenettverk virker å bli aktivert for verbal og ikke-verbal visuell og romlig informasjon i arbeidsminnet. Det finnes et bredt spenn av forskning på det visuell-romlige arbeidsminnet. Likevel, har en tilnærming som benytter seg av objekter med flere integrerte egenskaper og en klar spesifikasjon på den verbale dimensjonen blitt mindre anvendt. Det ikke-verbale arbeidsminnet for visuell-romlig informasjon har blitt mer oversett enn det verbale arbeidsminnet. Derfor søker denne studien å adressere de nevrale nettverkene som utnyttes for ikke-verbal arbeidsminneprestasjon. Hjerneaktivitet ble målt fra totalt 12 deltagere mens de utførte en nylig komponert ikke-verbal arbeidsminneoppgave. Både funksjonell nær-infrarød spektroskopi (fNIRS) og funksjonell magnetisk resonans avbildning (fMRI) ble benyttet for formålet. Resultatene indikerte at det ikke-verbale arbeidsminnet involverer høyre-lateraliserte hjerneaktiveringer, hvor frontoparietale nettverk og visuelle traséer ble benyttet for prestasjon. Disse funnene tilbyr viktig tilførsel til den nevrale nettverksmodellen av det ikke-verbale arbeidsminnet. Dermed virker oppgaven å teste konseptet effektivt. fNIRS og fMRI ble også brukt for å måle resting-state fra de samme deltagerne. Resultatene uttrykket forskjeller mellom de to øktene ved både fMRI og fNIRS. Endringene i konnektivitet kan reflektere effekter av oppgaven på resting-state.Masteroppgave i psykologiMAPSYK360INTL-SVINTL-MEDINTL-PSYKINTL-HFINTL-KMDINTL-JUSINTL-MNMAPS-PSY

Top-down modulation and memory deficits: Neural enhancement in the context of aging

Top-down modulation from a broader perspective suggests that some effortful control over posterior brain regions occurs. This study examined the extent to which age-related differences in top-down modulation could explain age-related memory decline. The theory of top-down modulation suggests that neural transmission during encoding requires the enhancement of relevant information and suppression of irrelevant information for efficacious neural function. Enhancement of attention to stimuli should be greater under higher task demands. In this study, we compared cortical modulation in a less effortful facial encoding task to cortical modulation in a more effortful facial encoding task. One-hundred-thirty older adults (mean age = 66.43 yrs) and 30 younger adults (mean age = 24.13 yrs) completed 2 tasks of facial encoding using functional magnetic resonance imaging (fMRI), a structural magnetic resonance imaging (MRI) scan, and a computer administered test of facial recognition outside the scanning environment. Activity in the fusiform face area was extracted when participants were told to view faces in the first encoding task and remember faces in the second encoding task. An enhancement index reflected the change in neural activity in the fusiform face area moving from the view faces task to the remember faces task. As predicted, levels of neural enhancement in the fusiform face area significantly predicted older and younger adult participants’ ability to correctly discriminate between faces they had and had not previously seen. Against predictions, the level of fusiform face area enhancement did not differ between younger and older adults. Thus, differences in enhancement levels are not driving age-related differences in facial recognition discrimination ability. In functional connectivity analysis, the idea that connections between the “top” and “bottom” components of the memory encoding network were examined. Consistent with predictions, the functional connection between right ventrolateral prefrontal cortex and the fusiform face area related with recognition. Taken together these data suggest that sensory enhancement is a critical component of efficacious memory encoding processes, but top-down enhancement of sensory activity does not adequately explain age-related decrement in memory performance

Shaping Functional Architecture by Oscillatory Alpha Activity: Gating by Inhibition

In order to understand the working brain as a network, it is essential to identify the mechanisms by which information is gated between regions. We here propose that information is gated by inhibiting task-irrelevant regions, thus routing information to task-relevant regions. The functional inhibition is reflected in oscillatory activity in the alpha band (8–13 Hz). From a physiological perspective the alpha activity provides pulsed inhibition reducing the processing capabilities of a given area. Active processing in the engaged areas is reflected by neuronal synchronization in the gamma band (30–100 Hz) accompanied by an alpha band decrease. According to this framework the brain could be studied as a network by investigating cross-frequency interactions between gamma and alpha activity. Specifically the framework predicts that optimal task performance will correlate with alpha activity in task-irrelevant areas. In this review we will discuss the empirical support for this framework. Given that alpha activity is by far the strongest signal recorded by EEG and MEG, we propose that a major part of the electrophysiological activity detected from the working brain reflects gating by inhibition