Zsófia Anna Gaál

Spectral and complexity features of the EEG changed by visual input in a case of subcortical stroke compared to healthy controls

OBJECTIVE To compare spectral and complexity characteristics of the EEG in a unique case of subcortical infarct to those seen in healthy controls. METHODS Absolute and relative frequency spectra, theta/beta ratio, the brain symmetry index (BSI), Omega-complexity and synchronization likelihood were calculated of the EEG recorded in eyes closed and eyes open conditions. RESULTS Increased absolute delta, theta, and Omega-complexity in these frequency bands, higher theta/beta ratios, and decreased relative beta activity were found in the side of the infarct. The BSI localized the excess of slow, and decrease of fast frequency activity to the area of ischemia. Following eyes opening the increase of fast and decrease of slow frequencies, the increase of Omega-complexity in the alpha and beta bands, and the decrease of synchronization likelihood for the fast frequency bands were reduced in the side of the infarct. CONCLUSIONS The subcortical infarct caused ipsilaterally increased slow, and decreased fast frequency activity accompanied by decreased synchronization of slow, increased synchronization of fast frequencies. Reduced reactivity in the ischemic side was particularly apparent for complexity measures. SIGNIFICANCE Complexity indices of the EEG are sensitive complementary measures of electrophysiological changes caused by local lesions such as subcortical stroke.

Age-dependent changes of auditory evoked potentials—Effect of task difficulty

The goal of this study was to evaluate the patterns of age-dependent changes of P3 components of auditory event-related potentials exploring the effects of task difficulty. The participants (age span: 19-68 years, n=55, divided into five age groups) took part in an easy and in a difficult two-tone oddball frequency discrimination task with speed or accuracy instructions, and in a novelty oddball task. The latency of the P3 components increased with aging. While in the easy task a linear P3b latency increase could be seen, in the difficult tasks (difficult frequency discrimination or distracting novel stimuli) an accelerated latency increase was observed for the P3b and P3a. In the two-tone oddball paradigm age had no effect on P3b amplitude, but in the novelty oddball task the amplitude of P3 potentials decreased with age. These results indicate that distracting stimuli increase task demands, and in difficult tasks decay can be observed more easily due to the accumulation of various processing mechanisms characterizing aging.

EEG synchronization characteristics of functional connectivity and complex network properties of memory maintenance in the delta and theta frequency bands

Task-dependent changes of nonlinear-linear synchronization features and graph theoretical properties of the delta and theta frequencies were analyzed in the present EEG study that were related to episodic memory maintenance processes. Synchronization was found to increase with respect to both the delta and theta bands within the frontal and parietal areas and also between these regions. Results of graph theoretical analysis indicated a task-related shift towards small-world network topology in the theta band.

EEG network connectivity changes in mild cognitive impairment — Preliminary results

Resting state EEGs were compared between patients with amnestic subtype of mild cognitive impairment (aMCI) and matched elderly controls at two times over a one year period. The study aimed at investigating the role of functional connectivity between and within different brain regions in relation to the progression of cognitive deficit in MCI. The EEG was recorded in two sessions during eyes closed and eyes open resting conditions. Functional brain connectivity was investigated based on the measurement of phase synchronization in different frequency bands. Delta and theta synchronization characteristics indicated decreased level of local and large-scale connectivity in the patients within the frontal, between the frontal and temporal, and frontal and parietal brain areas which was more pronounced 1year later. As a consequence of opening the eyes connectivity in the alpha1 band within the parietal lobe decreased compared to the eyes closed condition but only in the control group. The lack of alpha1 band reactivity following eye opening could reliably differentiate patients from controls. Our preliminary results support the notion that the functional disconnection between distant brain areas is a characteristic feature of MCI, and may prove to be predictive in terms of the progression of this condition.

Spectral characteristics and linear-nonlinear synchronization changes of different EEG frequency bands during the CNV.

During the CNV recorded in a simple auditory working memory task, task-specific decrease of the relative delta band and a transient increase of the absolute theta band were seen, accompanied by an increase of the absolute alpha1 and alpha2 bands in the posterior region. The decreased delta power probably corresponds to increased task-evoked arousal, whereas the transient theta power increase corresponds to working memory demand and possibly to the orienting response. The increased alpha1 and alpha2 power may be a manifestation of a top-down mechanism revealing control over the execution of a response. The area-specific, task-related, and frequency-dependent changes of EEG complexity measures indicate frontally increasing complexity during the early part of the CNV in the beta frequency bands, which underscores the importance of this region in the mechanisms of anticipatory behavior.

Task-switching training and transfer: age-related effects on late ERP components

We used task-switching (TS) paradigms to study how cognitive training can compensate age-related cognitive decline. Thirty-nine young (age span: 18–25 years) and 40 older (age span: 60–75 years) women were assigned to training and control groups. The training group received 8 one-hour long cognitive training sessions in which the difficulty level of TS was individually adjusted. The other half of the sample did not receive any intervention. The reference task was an informatively cued TS paradigm with nogo stimuli. Performance was measured on reference, near-transfer, and far-transfer tasks by behavioral indicators and event-related potentials (ERPs) before training, 1 month after pretraining, and in case of older adults, 1 year later. The results showed that young adults had better pretraining performance. The reference task was too difficult for older adults to form appropriate representations as indicated by the behavioral data and the lack of P3b components. But after training older adults reached the level of performance of young participants, and accordingly, P3b emerged after both the cue and the target. Training gain was observed also in near-transfer tasks, and partly in far-transfer tasks; working memory and executive functions did not improve, but we found improvement in alerting and orienting networks, and in the execution of variants of TS paradigms. Behavioral and ERP changes remained preserved even after 1 year. These findings suggest that with an appropriate training procedure older adults can reach the level of performance seen in young adults and these changes persist for a long period. The training also affects the unpracticed tasks, but the transfer depends on the extent of task similarities.

Visual mismatch negativity to vanishing parts of objects in younger and older adults

We investigated visual mismatch negativity (vMMN) to vanishing parts of continuously present objects by comparing the event-related potentials (ERPs) to infrequently (deviant) and frequently (standard) disappearing parts of the objects. This paradigm both excludes low-level stimulus-specific adaptation differences between the responses to deviants and standards, and increases the ecological validity of the stimuli. In comparison to frequently disappearing parts of the stimulus objects, infrequently vanishing parts elicited posterior negative event-related brain activity (vMMN). However, no vMMN emerged to the reappearance of the same parts of the objects. We compared the ERPs of an older and a younger sample of participants. In the 120–180 ms time period vMMN was similar in the two age groups, but in the 180–220 ms time period vMMN emerged only in the younger participants. We consider this difference as an index of more elaborate automatic processing of infrequent stimulus changes in younger adults.

Task-optimal auditory attention set restored as fast in older as in younger adults after distraction

The present study investigated how fast younger and older adults recovered from a distracted attentional state induced by rare, unpredictable sound events. The attentional state was characterized by the auditory N1 event-related potential (ERP), which is enhanced for sound events in the focus of attention. Younger (19-26 years) and older (62-74 years) adults listened to continuous tones containing rare pitch changes (glides) and short gaps. Glides and gaps could be separated in 150ms, 250ms, 650ms or longer and the task was gap detection while ignoring glides. With longer glide-gap separations similar N1 enhancements were observable in both groups suggesting that the duration of the distracted sensory state was not affected by aging. Older adults responded, however, slower at short glide-gap separations which indicated that distraction at subsequent levels of processing may have nonetheless more impact in older than in younger adults.

Age-related processing strategies and go–nogo effects in task-switching: An ERP study

We studied cognitive and age-related changes in three task-switching (TS) paradigms: (1) informatively cued TS with go stimuli, (2) informatively cued TS with go and nogo stimuli, (3) non-informatively cued TS with go and nogo stimuli. This design allowed a direct comparison, how informative and non-informative cues influenced preparatory processes, and how nogo stimuli changed the context of the paradigm and cognitive processing in different aging groups. Beside the behavioral measures [reaction time (RT), error rate], event-related potentials (ERPs) were registered to the cue and target stimuli in young (N = 39, mean age = 21.6 ± 1.6 years) and older (N = 40, mean age = 65.7 ± 3.2 years) adults. The results provide evidence for declining performance in the older group: they had slower RT, less hits, more erroneous responses, higher mixing costs and decreased amplitude of ERP components than the participants of the younger group. In the task without the nogo stimuli young adults kept the previous task-set active that could be seen in shorter RT and larger amplitude of cue-locked late positivity (P3b) in task repeat (TR) trials compared to task switch trials. If both go and nogo stimuli were presented, similar RTs and P3b amplitudes appeared in the TR and TS trials. In the complex task situations older adults did not evolve an appropriate task representation and task preparation, as indicated by the lack of cue-locked P3b, CNV, and target-locked P3b. We conclude that young participants developed explicit representation of task structures, but the presence of nogo stimuli had marked effects on such representation. On the other hand, older people used only implicit control strategy to solve the task, hence the basic difference between the age groups was their strategy of task execution.

Automatic Change Detection in Older and Younger Women: A Visual Mismatch Negativity Study

Background: In comparison to controlled (attentional) processing, relatively little is known about the age-related changes of the earlier (preattentive) processes. An event-related potential (ERP) index of preattentive (automatic) visual processing, the visual mismatch negativity (vMMN) is a good candidate for analyzing age-related differences in the automatic processing of visual events. Objective: So far results concerning age-related changes in vMMN have been equivocal. Our aim was to develop a method resulting in a reliable vMMN in a paradigm short enough to use in the applied field. Methods: We investigated an older (mean age: 66.4 years, n = 15) and a younger (mean age: 22.4 years, n = 15) group of healthy women. ERPs were obtained for checkerboard onset patterns in a passive oddball condition (during which participants performed a tracking task). One of the checkerboards was frequent (standard; p = 0.8), and the other was rare (deviant; p = 0.2). Results: vMMN emerged over posterior locations in the latency range of 100–300 ms in both age groups. The amplitude of the earlier part of the vMMN was similar in the older and the younger participants, but latency was longer in the older group. The later part of the vMMN was slightly diminished in the elderly. Conclusion: Automatic detection of violated sequential regularities, reflected by the vMMN, emerged in the two age groups (earlier vMMN). However, detection of stimulus change, a preattentive visual process delayed in the elderly, and identification of the specific change was compromised in the older participants.