Differences in brain potentials to open and closed class words: class and frequency effects.

Closed class (determiners, pronouns, conjunctions, prepositions etc. ) and open class (nouns, verbs, adjectives, adverbs) words have different linguistic functions and have been proposed to be processed by different neural systems. Here, event-related potentials (ERPs) were recorded in young German-speaking subjects while they read closed class and open class words flashed upon a video-screen. In the first experiment closed class words were sorted into four different frequency categories and open class words into three categories. The words were presented in a list with the subjects' task to detect occasional non-words. A centroparietal negativity (N400) with a peak latency of about 400 ms varied in amplitude as a function of frequency in both classes. The N400 in closed class items, however, was considerably smaller than that in open class words of similar frequency. A left anterior negativity (N280/LPN) showed some degree of frequency-sensitivity regardless of word class. Only for the very high frequency closed class words a frontal negativity with an onset of about 400 ms was obtained (N400-700). This N400-700 effect was replicated in the second study, in which medium frequency closed and open class words and very high frequency closed class words were presented at the fifth position of simple German sentences. It is suggested that neither N400 nor the left anterior negativity (N280/LPN) distinguish qualitatively between the two word classes and thus claims about different brain systems involved in the processing of open and closed class words are not substantiated electrophysiologically. The N400-700 effect is possibly related to specific grammatical functions of some closed class items, such as determiners.

Event-related potentials of verbal encoding into episodic memory: dissociation between the effects of subsequent memory performance and distinctiveness.

Episodic memory encoding and distinctiveness detection were examined using event-related potentials (ERP) in a single-trial word list learning paradigm with free recall following distraction. To manipulate distinctiveness, encoding of high- and very low-frequency words was contrasted. Amplitudes of the N400 and late positive component (LPC) were larger for low- than for high-frequency words, and ERPs were more positive for subsequently recalled than not recalled words. This subsequent memory effect was dissociated from the correlates of distinctiveness by polarity for the N400 and by time course for the LPC and dissociable into two effects. The data suggest that the first subsequent memory effect, which occurred for both word categories, is more directly related to episodic memory formation, whereas the second effect, which occurred for high-frequency words only, is related to processes influencing episodic encoding success indirectly.

Successful verbal encoding into episodic memory engages the posterior hippocampus: a parametrically analyzed functional magnetic resonance imaging study.

The medial temporal lobe (MTL) is essential for episodic memory encoding, as evidenced by memory deficits in patients with MTL damage. However, previous functional neuroimaging studies have either failed to show MTL activation during encoding or they did not differentiate between two MTL related processes: novelty assessment and episodic memory encoding. Furthermore, there is evidence that the MTL can be subdivided into subcomponents serving different memory processes, but the extent of this functional subdivision remains unknown. The aim of the present functional magnetic resonance imaging (fMRI) study was to investigate the role of the MTL in episodic encoding and to determine whether this function might be restricted to anatomical subdivisions of the MTL. Thirteen healthy volunteers performed a word list learning paradigm with free recall after distraction. Functional images acquired during encoding were analyzed separately for each participant by a voxel-wise correlation (Kendall's tau) between the time series of the T2*-signal intensity and the number of subsequently recalled words encoded during each particular scan. Of the 13 participants, 11 showed voxel clusters with statistically significant, positive correlations in the posterior part of the hippocampus. Across participants, an ANOVA on the number of voxels with significant, positive correlations within individually defined volumes of interest confirmed a statistically significant difference in activation for anterior versus posterior regions of the hippocampus. However, no differences between left and right hippocampal activation were revealed. Thus, these findings demonstrate that successful encoding into episodic memory engages neural circuits in the posterior part of the hippocampus.

How the brain processes complex words: an event-related potential study of German verb inflections.

Event-related brain potentials (ERPs) were recorded as German-speaking subjects read verbs in correct and incorrect participle forms. The critical words were presented in three different versions to three different groups of subjects, as part of a simple sentence, in a word list, and embedded in a story; for each version separate ERPs were recorded. Three types of verbs were investigated, regulars, irregulars and nonce verbs. We compared correct regular and irregular participles with incorrect ones; the latter had -(e)n on verbs that actually take -t participles (* getanz-en), or -(e)t on verbs that require -(e)n (* gelad-et). For the nonce verbs, we compared participles with the unexpected -(e)n ending with the expected -t participle forms. The ERP responses were very consistent across the three versions of the experiment: (i) incorrect irregular participles (* gelad-et) elicited a left frontotemporal negativity; (ii) incorrect regulars (* getanz-en) produced no differences to the correct ones; (iii) nonce verbs were associated with an N400 component but did not show a difference between expected and unexpected endings. We will interpret these findings with respect to psycholinguistic models of morphological processing and argue that the brain processes regularly inflected words differently from irregularly inflected ones, the latter by accessing full-form entries stored in memory and the former by a computational process that decomposes complex words into stems and affixes.

ERPs to encoding and recognition in two different inter-item association tasks.

EVENT-RELATED potentials were obtained during study and recognition of word pairs in an incidental learning paradigm. Word pairs were studied either by performing a semantic judgment separately for each word (non-associative encoding) or by creating a semantic association between the two words (associative encoding). Only word pairs encoded associatively elicited a reliable dm-effect with a right frontal maximum. Recognition of previously studied word pairs revealed two topographically and temporally distinct old/new effects: an earlier parietal effect which was only reliable for associatively encoded items and a right frontal effect which was of equal magnitude for word pairs from both encoding tasks. The findings suggest that ERP effects of distinct memory processes are differentially influenced by the encoding instructions.

Brain potentials indicate differences between regular and irregular German plurals.

Event-related brain potentials were recorded as 18 German-speaking subjects read sentences that contained as critical words German nouns in correct and incorrect plural forms. Two types of plurals were investigated: regular -s plurals (e.g. Karussell-s 'roundabouts') and irregular -(e)n plurals (Muskel-n "muscles'). We compared correct regular and irregular plurals with incorrect ones; the latter had -(e)n on nouns that actually take -s plurals (*Karussell-en), or -s on nouns that require -(e)n (*Muskel-s). ERPs showed different responses to regular and irregular plurals: incorrect irregular plurals (*Muskel-s) elicited a ramp-shaped left frontotemporal negativity, whereas incorrect regulars (*Karussell-en) produced a central phasic negativity with a maximum at 380 ms. This dissociation supports the view that regularly inflected words are processed differently from irregularly inflected ones.

Mental representations of morphologically complex words: an event-related potential study with adult humans.

Event related brain potentials (ERPs) were recorded from 13 subjects while performing a lexical decision task. Regular and irregular German principles were presented twice (identical repetition) or were primed by their related infinitives (morphological repetition). Regular participles primed by their infinitives displayed ERP waveforms similar to those evoked by identical repetitions, namely a large positivity with an onset latency of about 200 ms. Irregular participles, by contrast, showed no reliable morphological repetition effects. The results are discussed in terms of the controversy between approaches proposing a single representational mechanism for regular and irregular inflection and the "dual mechanism' account that suggests an associative memory for the storage of irregulars and a rule-based symbolic processor for generating regular forms.