Biomedical subjects

J Streb

Publications and source records attributed to J Streb.

3 recordsLinked to original sources

PubMed2000

Individual cortical current density reconstructions of the semantic N400 effect: using a generalized minimum norm model with different constraints (L1 and L2 norm).

Event-related brain potentials were recorded to study whether verbs and nouns activate topographically distinct cortical generators. Fifteen subjects performed a primed lexical decision task with verb/verb and noun/noun pairs. The relatedness between prime and target items was varied in three steps (unrelated, moderately, and strongly related) and the EEG was recorded from 124 scalp electrodes. The topography of cortical sources of the N400 effect was evaluated by standardized differences scores and by cortical current source estimates which were constrained by the individual MRI-determined cortex anatomy. A behavioral priming effect and a substantial N400 effect was found for both word categories. However, the topography of the grand average N400 effect of verbs and nouns did not differ, neither for raw nor for standardized amplitudes. Cortical current source estimates of the N400 effect revealed a very broad and scattered distribution of active locations with pronounced interindividual differences. Cortical current source estimates obtained with the L1-norm and L2-norm model, respectively, differed in the distribution of sources over the cortex but converged on the same "hot spots." The data give no indication that the N400 effect is generated by word category-specific networks which have a different topography. The marked individual differences are discussed with respect to the involved processes and the current source estimation procedures.

Adult↗

PubMed1999

Event-related responses to pronoun and proper name anaphors in parallel and nonparallel discourse structures.

The present study investigated event-related potential (ERP) effects of pronoun and proper name anaphors in both parallel and nonparallel discourse structures. Thirty-seven students processed 400 semantically different text passages. Each trial consisted of two sentences and a comprehension question. The first sentence introduced a protagonist who was referred to by an anaphoric word in the second sentence. The anaphoric word was either a pronoun or a repetition of the proper name of the protagonist and had either the same or a different syntactic role as its antecedent (subject or object). The sentences were presented word by word as rapid serial visual display. Event-related potentials were recorded from 61 scalp electrodes. In agreement with the parallel function strategy, nonparallel discourse structures required longer decision times and exhibited higher error rates than parallel structures. The ERPs revealed two effects: First, pronoun anaphors evoked a more pronounced negativity than proper name anaphors between 270 and 420 ms latency over the frontal cortex electrodes. Another relative negativity occurred between 510 and 600 ms over the parietal cortex electrodes. Second, anaphors in nonparallel positions were accompanied by a more pronounced negativity over the parietal cortex. These data support the idea that an anaphor in nonparallel position triggers extra processing steps, probably search processes in working memory which integrate currently encountered information with previously activated representations.

Adult↗

PubMed1997

[Memory traces in EEG].

The paper gives a brief overview of five experimental approaches in which memory processes were studied by means of event-related brain potentials (ERPs). Some of the results were already published in English (Study 1), while others are new and will be reported in greater length as full paper elsewhere (Studies 2, 3, 4, and 5). Study 1 revealed that retrieval of information from episodic long-term memory is accompanied by a systematic slow negative potential. The topography of this slow wave depends on the quality of the reactivated information (spatial vs. verbal), and its amplitude reflects the difficulty of the retrieval process. In experiment 2 ERPs were recorded while subjects acquired either explicit or implicit knowledge about a sequential stimulus-response pattern. The data suggest that explicit learners who posses verbalizable knowledge about sequential dependencies have formed both perceptual and motor representations, while implicit learners have formed motor representations only. In study 3 fact retrieval in mental arithmetic was activated by a verification task. Incongruent solutions evoked an arithmetic N400-effect whose amplitude varied with the associative distance between an expected and an actually perceived solution to a multiplication problem. In study 4 ERPs were recorded during mental rotation tasks. A set of experiments revealed that mental rotation is always accompanied by a systematic negative variation over the parietal cortex. The amplitude of this "rotation specific negativity" increases with an increasing angular disparity between a perceived sign and its normal upright template. It was shown that this negativity is functionally distinct from a P300-complex which is often superimposed on it within the same latency window. Finally, study 5 examined ERPs in a sentence reading task in which grammatically legal but infrequent sentence constructions had to be processed. A left-anterior negativity was observed whenever an explicit case marker (the definite article in German) signalled a nominal phrase at a noncanonical position. The LAN phenomenon appears to be a manifestation of a syntax processor which performes a first-pass formal analysis of a sentence and which possibly allocates working memory resources whenever a word cannot be assigned immediately to an expected propositional role.

Brain↗