Elvira Khachatryan

Representation of steady-state visual evoked potentials elicited by luminance flicker in human occipital cortex: An electrocorticography study

&NA; Despite the widespread use of steady‐state visual evoked potentials (SSVEPs) elicited by luminance flicker in clinical and research settings, their spatial and temporal representation in the occipital cortex largely remain elusive. We performed intracranial‐EEG recordings in response to targets flickering at frequencies from 11 to 15 Hz using a subdural electrode grid covering the entire right occipital cortex of a human subject, and we were able to consistently locate the gazed stimulus frequency at the posterior side of the primary visual cortex (V1). Peripheral flickering, undetectable in scalp‐EEG, elicited activations in the interhemispheric fissure at locations consistent with retinotopic maps. Both foveal and peripheral activations spatially coincided with activations in the high gamma band. We detected localized alpha synchronization at the lateral edge of V2 during stimulation and transient post‐stimulation theta band activations at the posterior part of the occipital cortex. Scalp‐EEG exhibited only a minor occipital post‐stimulation theta activation, but a strong transient frontal activation. HighlightsThe spatiotemporal representation of SSVEP in the occipital cortex is largely unclear.The fundamental frequency is represented at the posterior part of the primary visual cortex.The spatial representation of the second harmonic varies with the stimulation frequency.Simultaneous foveal and peripheral flickering stimuli are processed independently.SSVEP stimulation elicits localized alpha band activations at the lateral edge of V2.

Sentence Context Prevails Over Word Association in Aphasia Patients with Spared Comprehension: Evidence from N400 Event-Related Potential

Behavioral and event-related potential (ERP) studies on aphasia patients showed that lexical information is not lost but rather its integration into the working context is hampered. Studies have been conducted on the processing of sentence-level information (meaningful versus meaningless) and of word-level information (related versus unrelated) in aphasia patients, but we are not aware of any study that assesses the relationship between the two. In healthy subjects the processing of a single word in a sentence context has been studied using the N400 ERP. It was shown that, even when there is only a weak expectation of a final word in a sentence, this expectation will dominate word relatedness. In order to study the effect of semantic relatedness between words in sentence processing in aphasia patients, we conducted a crossed-design ERP study, crossing the factors of word relatedness and sentence congruity. We tested aphasia patients with mild to minimum comprehension deficit and healthy young and older (age-matched with our patients) controls on a semantic anomaly judgment task when simultaneously recording EEG. Our results show that our aphasia patient’s N400 amplitudes in response to the sentences of our crossed-design study were similar to those of our age-matched healthy subjects. However, we detected an increase in the N400 ERP latency in those patients, indicating a delay in the integration of the new word into the working context. Additionally, we observed a positive correlation between comprehension level of those patients and N400 effect in response to meaningful sentences without word relatedness contrasted to meaningless sentences without word relatedness.

Language processing in bilingual aphasia: a new insight into the problem

There is increasing evidence that a bilingual person should not be considered as two monolinguals in a single body, a view that has gradually been adopted in the diagnosis and treatment of bilingual aphasia. However, its investigation is complicated due to the large variety in possible language combinations, pre- and postmorbid language proficiencies, and age of second language acquisition. Furthermore, the tests and tasks used to assess linguistic capabilities differ in almost every study, hindering a direct comparison of their outcomes. Behavioral, electrophysiological, and neuroimaging data from healthy population show that the processing of second language domains (semantics, syntax, morphology) depends on factors such as age and method of acquisition, proficiency level and environment in which the second language was acquired. A number of single and multiple case reports that rely on behavioral testing of bilingual aphasics replicate these results. Additionally, they show that the patients performance depends on the size and location of the lesion, as well as language typology and morphological characteristics. Furthermore, the impairment and recovery patterns and recovery generalization from treated to untreated language depend on the lexical and orthographic distances between the two languages. For healthy bilinguals, language processing is usually studied in comparison to monolinguals. We advocate that a good starting point for identifying patterns specific for bilingual aphasia is to compare patient studies of bilinguals and monolinguals.

ERP Response Unveils Effect of Second Language Manipulation on First Language Processing

Lexical access in bilinguals has been considered either selective or non-selective and evidence exists in favor of both hypotheses. We conducted a linguistic experiment to assess whether a bilingual’s language mode influences the processing of first language information. We recorded event related potentials during a semantic priming paradigm with a covert manipulation of the second language (L2) using two types of stimulus presentations (short and long). We observed a significant facilitation of word pairs related in L2 in the short version reflected by a decrease in N400 amplitude in response to target words related to the English meaning of an inter-lingual homograph (homograph-unrelated group). This was absent in the long version, as the N400 amplitude for this group was similar to the one for the control-unrelated group. We also interviewed the participants whether they were aware of the importance of L2 in the experiment. We conclude that subjects participating in the long and short versions were in different language modes: closer to monolingual mode for the long and closer to bilingual mode for the short version; and that awareness about covert manipulation of L2 can influence the language mode, which in its turn influences the processing of the first language.

Amplitude of N400 component unaffected by lexical priming for moderately constraining sentences

The N400 is an event-related potential (ERP) that reflects the processing of semantics in the brain. When reading sentences, the N400 amplitude is modulated by both the cloze probability of the sentence and the association strength between individual words. When contradicted in strongly constraining sentences, that is, the beginning of the sentence builds a strong expectation of the final word; the cloze probability overrules the effect of association strength. We evidence that this is also the case for non-constraining sentences, such as the ones with low to moderate cloze probabilities. Our results give the evidences that if the sentence generates even weak to moderate expectations about the final word, word association plays almost no role in the processing of this word.

Cortical distribution of N400 potential in response to semantic priming with visual non-linguistic stimuli

We conducted a study on visual semantic priming using related and unrelated image pairs while simultaneously recording electroencephalography (EEG) from 27 scalp electrodes and electrocorticography (ECoG) from a mixture of deep brain and subdural grid/strip electrodes in the left and right hippocampus, the right temporo-basal and temporo-lateral cortices, and the left temporal cortex. The EEG data showed a clear centro-parietal, bi-hemispheric N400 effect in response to unrelated image-pairs compared to related ones. Although with ECoG the N400 effect was more widely spread across both hemispheres, compared to linguistic stimuli, it was relatively localized within each ECoG grid as it was present only in some electrodes and, in some cases, even had its polarity reversed. We advocate this could be due to some grids gauging dipoles at different positions when covering sulci and gyri.

Language processing in bilingual aphasia: A new insight into the problem

There is increasing evidence that a bilingual person should not be considered as two monolinguals in a single body, a view that has gradually been adopted in the diagnosis and treatment of bilingual aphasia. However, its investigation is complicated due to the large variety in possible language combinations, pre- and postmorbid language proficiencies, and age of second language acquisition. Furthermore, the tests and tasks used to assess linguistic capabilities differ in almost every study, hindering a direct comparison of their outcomes. Behavioral, electrophysiological, and neuroimaging data from healthy population show that the processing of second language domains (semantics, syntax, morphology) depends on factors such as age and method of acquisition, proficiency level and environment in which the second language was acquired. A number of single and multiple case reports that rely on behavioral testing of bilingual aphasics replicate these results. Additionally, they show that the patients performance depends on the size and location of the lesion, as well as language typology and morphological characteristics. Furthermore, the impairment and recovery patterns and recovery generalization from treated to untreated language depend on the lexical and orthographic distances between the two languages. For healthy bilinguals, language processing is usually studied in comparison to monolinguals. We advocate that a good starting point for identifying patterns specific for bilingual aphasia is to compare patient studies of bilinguals and monolinguals.

Effect of word association on linguistic event-related potentials in moderately to mildly constraining sentences

The processing of word associations in sentence context depends on several factors. EEG studies have shown that when the expectation of the upcoming word is high (high semantic constraint), the within-sentence word association plays a negligible role, whereas in the opposite case, when there is no expectation (as in pseudo-sentences), the role of word association becomes more pronounced. However, what happens when the expectations are not high (mild to moderate semantic constraint) is not yet clear. By adopting a cross-factorial design, crossing sentence congruity with within-sentence word association, our EEG recordings show that association comes into play during semantic processing of the word only when the sentence is meaningless. We also performed an exploratory source localization analysis of our EEG recordings to chart the brain regions putatively implicated in processing the said factors and showed its complementarity to EEG temporal analysis. This study furthers our knowledge on sentence processing and the brain networks involved in it.

A new insight into sentence comprehension: The impact of word associations in sentence processing as shown by invasive EEG recording

ABSTRACT The effect of word association on sentence processing is still a matter of debate. Some studies observe no effect while others found a dependency on sentence congruity or an independent effect. In an attempt to separate the effects of sentence congruity and word association in the spatio‐temporal domain, we jointly recorded scalp‐ and invasive‐EEG (iEEG). The latter provides highly localized spatial (unlike scalp‐EEG) and high temporal (unlike fMRI) resolutions. We recorded scalp‐ and iEEG in three patients with refractory epilepsy. The stimuli consisted of 280 sentences with crossed factors of sentence congruity and within sentence word‐association. We mapped semantic retrieval processes involved in sentence comprehension onto the left temporal cortex and both hippocampi, and showed for the first time that certain localized regions participate in the processing of word‐association in sentence context. Furthermore, simultaneous recording of scalp‐ and iEEG gave us a direct overview of signal change due to its propagation across the head tissues. HIGHLIGHTSThe brain mapping of sentence comprehension is still in progress.Invasive EEG provides the combination of high spatial and temporal resolutions.We mapped semantic processing of sentences on the left temporal cortex.Certain brain areas process word‐associations independent of sentence meaning.Scalp and invasive EEGs combined shows propagation of EEG signal through tissues.

Decoding Steady-State Visual Evoked Potentials From Electrocorticography

We report on a unique electrocorticography (ECoG) experiment in which Steady-State Visual Evoked Potentials (SSVEPs) to frequency- and phase-tagged stimuli were recorded from a large subdural grid covering the entire right occipital cortex of a human subject. The paradigm is popular in EEG-based Brain Computer Interfacing where selectable targets are encoded by different frequency- and/or phase-tagged stimuli. We compare the performance of two state-of-the-art SSVEP decoders on both ECoG- and scalp-recorded EEG signals, and show that ECoG-based decoding is more accurate for very short stimulation lengths (i.e., less than 1 s). Furthermore, whereas the accuracy of scalp-EEG decoding benefits from a multi-electrode approach, to address interfering EEG responses and noise, ECoG decoding enjoys only a marginal improvement as even a single electrode, placed over the posterior part of the primary visual cortex, seems to suffice. This study shows, for the first time, that EEG-based SSVEP decoders can in principle be applied to ECoG, and can be expected to yield faster decoding speeds using less electrodes.