Intracortical Directed Connectivity for Information Retention in Visual-Spatial Working Memory.

A group of 27 healthy young adults solved a task involving the working memory (WM) activation, consisting of the comparison between the spatial orientations of two sequentially presented square-wave luminance gratings. We investigated the effective (directed) connectivity patterns between the frontal and postcentral cortical regions related to the visual system. The connectivity was assessed using vector autoregression modeling of EEG. It was shown that the strength of the top-down right-hemispheric connectivity patterns directed from the frontal cortex to the visual areas in θ frequency was significantly lower at the stage of stimulus retention in the WM than at the stage of stimulus anticipation. On the contrary, in the α band the descending influences were slightly more intense. The results of the study showed the frequency-dependent dynamics of the descending influences of the frontal cortex on visual areas and confirm that the frontal cortex plays the role of a controlling and modulating center in the brain system underlying WM.

Alpha-band functional connectivity during cued versus implicit modality-specific anticipatory attention: EEG-source coherence analysis.

The anticipation of future events based on a background experience is one of the main components of any goal-directed behavior. Anticipatory attention can be either voluntary (explicit) or involuntary (implicit). We presumed that these two types of anticipatory attention differed in terms of cortical functional organization. We examined this assumption with an experimental model consisting of three experimental sessions (cued attention, implicit learning, and baseline) that were equal in terms of stimuli, motor responses, and cognitive task. Participants were asked to discriminate the temporal order of stimuli within a pair presented in either the visual or auditory sensory modality. Prestimulus functional connectivity was assessed via alpha-band coherence computed in the source space for preselected regions of interests. Functional links between the cortices of the frontoparietal control system increased during the cued attention condition and did not increase during the implicit anticipation condition. The buildup of implicit anticipation was accompanied by the strengthening of functional links between the intraparietal, ventral premotor, and presupplementary motor areas. It was discovered that both cued and implicit types of anticipation were underlain by functional modality-specific cortical links.

[Delayed Copying of Unfamiliar Contour Shapes: Does Reaction Time Decrease with Growing Delay Reflect a Change in Internal Representation Of Fothcoming Movement?].

Group of 24 adults has performed the delayed reproduction (copying) of unfamiliar contour shapes (trajectory templates). Templates were shown for 250 ms and the participants were asked to reproduce them upon detecting acoustical go signal (short click). Go signal was delayed relatively to the end of a visual template exposure by T = 0, 500, 1000, 2000 or 4000 ms. Block design of the experiment was used when delay T was held constant within a block consisted of 32 trials. We analyzed reaction time (RT), mean movement time (MT) along a single segment of trajectory, and the mean dwell time (DT) in the vertices of the template. It is shown that RT does not depend monotonically on the delay T showing a decrease at T ≤ 1000 ms and increase at T ≤ 2000 ms. The RT of T curve is well described by a simple additive model that includes a linearly growing and an exponentially decaying terms. The linear growth reflects the foreperiod effect, i.e., decreasing capacity to predict an exact moment of go signal with the growing delay T [Niemi, Naatanen, 1981; Meulenbroek, Van Galen, 1988]. We suggested that exponentially decaying term may be caused by the internal representation of a trajectory undergoing some transformation during retention in the working memory.

[Cortical functional connectivity during retention of affective pictures in working memory: EEG-source theta coherence analysis].

A pattern of cortical functional connectivity in the source space was studied in a group of right-handed adult participants (N = 44:17 women, 27 men, aged M = 29.61 ± 6.45 years) who retained in their working memory (WM) traces of realistic pictures of positive, neutral, and negative emotional valence while in their working memory (WM) while performing same different task in which participants had to compare an etalon picture against a target picture that followed after a specified delay. A coherence (COH) between pairs of cortical sources chosen in advance according to fMRI data was estimated in the theta frequency range for the period of time preceding the etalon stimulus, distinct sets of functional links are found. The links of the first type that presumably reflect the involvement of sustained attention were between the dorsal anterior cingulate cortex, the prefrontal areas, and temporal areas of the right hemispheres. When compared to the rest period, links of this type showed strengthening not only during the retention period but also during the period preceding the etalon picture. The links of the second type presumably reflecting a progressive neocortex-to-hippocampus functional integration with increasing memory load and strengthened exclusively during retention period. Those links were between parietal, temporal and prefrontal cortices in the lateral surface of both hemispheres with the additional inclusion of the posterior cingulate cortex and the medial parietal cortex in the left hemisphere. An impact of emotional valence onto the strength and topography of the functional links of the second type was found. In the left hemisphere, an increase in the strength of cortical interaction was more pronounced for pictures of positive valence than for pictures of either neutral or negative valences. When compared to the pictures of neutral valence, the retention of pictorial information of both positive and negative valence showed some extraneous integration of the cortical areas for the theta rhythm. This finding might be related to the additional load exerted by emotionally colored pictures onto the mechanisms of short-time retention of visual information.

[Event-Related Potentials Accompanying Processing Referentially Ambiguous Pronouns in Russian].

The study is based on the analysis of event-related potentials (ERPs) accompanying processing referentially ambiguous pronouns under condition when disambiguation is necessary for effective task performance. Par- ticipants were asked to match the pronoun in the second sentence with its antecedent (the noun phrase it is related to) in the first sentence in two conditions: experimental (two possible antecedents) and control (only one possible antecedent). Processing referentially ambiguous pronouns as compared to the control condition elicited an Nref effect - a diffuse ERP deflection 300-400 ms poststimulus that was earlier observed in Dutch and Chinese. Moreover, in contrast to previous results, no P600 effect - late positivity associated with ac- ceptability judgment under condition of referential ambiguity - was found. Our data in comparison with re- sults of previous studies indicate that strategies in processing referentially ambiguous pronouns (acceptability judgment) are determined not only by experimental task but also by absence/presence of anomalous trials in the experimental materials.

[Delayed Copying of Unfamiliar Contour Shapes: Analysis of Potentials Related to Stimuli].

The high-density EEG was recorded and ERPs related to showing unfamiliar contour shapes and delivering imperative signal (a short sound) were estimated in the task requiring delayed motor reproduction of these shapes. A total of 22 right-handed adults participated in the experiment. They performed 5-blocks of trials corresponding-to 5 different delays (T= 0; 500, 1000, 2000 4000 ms) between the imperative signal relative to the end of the contour trajectory presentation. An ERP analysis showed that, unlike ERPs related to the contour shape presentation, those related to the imperative signal delivery do change with growing delay T. A subsequent analysis of cortical sources of the ERPs related to the imperative stimulus showed corresponding pronounced grows of reactivity of orbito-frontal cortex of the right hemisphere and a symmetrical bilateral grows of reactivity of dorsal parts of the sensorimotor cortex. The reported findings are discussed in the framework of the proposal according to which the internal representation of a trajectory undergoes a transi- tion from a sensory-specific format towards more abstract neither sensory- nor motor-specific format.

[A delayed motor production of open chains of linear strokes presented visually in static and dynamic modes: a comparison between 9 to 11 years old children and adults].

The production of drawing movements was studied in 29 right-handed children of 9-to-11 years old. The movements were the sequences of horizontal and vertical linear stokes conjoined at right angle (open polygonal chains) referred to throughout the paper as trajectories. The length of a trajectory varied from 4 to 6. The trajectories were presented visually to a subject in static (linedrawing) and dynamic (moving cursor that leaves no trace) modes. The subjects were asked to draw (copy) a trajectory in response to delayed go-signal (short click) as fast as possible without lifting the pen. The production latency time, the average movement duration along a trajectory segment, and overall number of errors committed by a subject during trajectory production were analyzed. A comparison of children's data with similar data in adults (16 subjects) shows the following. First, a substantial reduction in error rate is observed in the age range between 9 and 11 years old for both static and dynamic modes of trajectory presentation, with children of 11 still committing more error than adults. Second, the averaged movement duration shortens with age while the latency time tends to increase. Third, unlike the adults, the children of 9-11 do not show any difference in latency time between static and dynamic modes of visual presentation of trajectories. The difference in trajectory production between adult and children is attributed to the predominant involvement of on-line programming in children and pre-programming in adults.

[Cortical Functional Connectivity during Cued vs. Implicit Modality-Specific Anticipatory Attention: EEG-Source Alpha Coherence Analysis].

The brain functional organization was studied in a group of healthy right-handed adults (N= 16, mean age = 23 ± 5.7) during preparation for visual and auditory sensory tasks in two conditions: (1) participants waited for appearance of either a visual or an auditory stimulus after being cued about its sensory modality (the cued anticipatory attention) or (2) they developed implicit anticipation of stimulus in the course of repetitive exposure to the same sequence of visual and auditory stimuli pairs. In both cases, participants were asked to discriminate the temporal order of stimuli presentation within a pair of either visual or auditory modality. The functional connectivity was assessed via alpha coherence computed in the source space for preselected regions of interests. For both visual and auditory modalities, increase of strength of functional links among cortical areas involved in the fronto-parietal attention system is found during the cued attention when compared to nonspecific sustained attention. An increase is also observed in the connection strengths between sensory-specific and associative (parietal and prefrontal) areas. In visual modality, the buildup of implicit anticipation was accompanied by the strengthening of functional links between the ventral premotor cortex and caudal (parietal and occipital) areas of the right hemisphere. In the case of auditory task, the increase of connection strengths within fronto-temporal cortical areas was observed. These areas included the rostral supplementary motor areas, ventral premotor cortices and primary auditory cortices.

Tracking neurons recorded from tetrodes across time.

Tetrodes allow isolation of multiple neurons at a single recording site by clustering spikes. Due to electrode drift and perhaps due to time-varying neuronal properties, positions and shapes of clusters change in time. As data is typically collected in sequential files, to track neurons across files one has to decide which clusters from different files belong to the same neuron. We report on a semi-automated neuron tracking procedure that uses computed similarities between the mean spike waveforms of the clusters. The clusters with the most similar waveforms are assigned to the same neuron, provided their similarity exceeds a threshold. To set this threshold, we calculate two distributions: of within-file similarities, and of best matches in the across adjacent file similarities. The threshold is set to the value that optimally separates the two distributions. We compare different measures of similarity (metrics) by their ability to separate these distributions. We find that these metrics do not differ drastically in their performance, but that taking into account the cross-channel noise correlation significantly improves performance of all metrics. We also demonstrate the method on an independent dataset and show that neurons, as assigned by the procedure, have consistent physiological properties across files.

Probing feature selectivity of neurons in primary visual cortex with natural stimuli.

One way to characterize neural feature selectivity is to model the response probability as a nonlinear function of the output of a set of linear filters applied to incoming signals. Traditionally these linear filters are measured by probing neurons with correlated Gaussian noise ensembles and calculating correlation functions between incoming signals and neural responses. It is also important to derive these filters in response to natural stimuli, which have been shown to have strongly non-Gaussian spatiotemporal correlations. An information-theoretic method has been proposed recently for reconstructing neural filters using natural stimuli in which one looks for filters whose convolution with the stimulus ensemble accounts for the maximal possible part of the overall information carried the sequence of neural responses. Here we give a first-time demonstration of this method on real neural data, and compare responses of neurons in cat primary visual cortex driven with natural stimuli, noise ensembles, and moving gratings. We show that the information-theoretic method achieves the same quality of filter reconstruction for natural stimuli as that of well-established white-noise methods. Major parameters of neural filters derived from noise ensembles and natural stimuli, as well as from moving gratings are consistent with one another. We find that application of the reverse correlation method to natural stimuli ensembles leads to significant distortions in filters for a majority of studied cells with non-zero reverse-correlation filter.

Interhemispheric interaction in children of 7-8: analysis of EEG coherence and finger tapping parameters.

To investigate interhemispheric interaction (IHI), the EEGs of 42 children between the ages of 7-8 years were recorded while they were performing, as fast as possible, both unimanual and bimanual rhythmic tapping tasks. As an EEG index of IHI we used coherence values (Coh) in the alpha band in the pairs of leads F3/F4, C3/C4, P3/P4, O1/O2, and T3/T4 during resting and tapping. The difference between left hand (L-L) and right hand (R-R) intertap intervals in comparable conditions was though to reflect IHI. Interhemispheric alpha Coh increase was only evident in bimanual rhythmic tasks, and was confined to the C3/C4 and P3/P4 regions. Coh in these brain areas correlated negatively with the time difference between the L-L and R-R intertap intervals. The results show that interhemispheric alpha Coh. can be regarded as an index of IHI during bimanual tapping, and support the clinical hypothesis that bimanual versus unimanual tapping might reflect interhemispheric or callosal function.