Recent research has shown that brain potentials time-locked to fixations in natural reading can be similar to brain potentials recorded during rapid serial visual presentation (RSVP). We attempted two replications of Hagoort, Hald, Bastiaansen, and Petersson [Hagoort, P., Hald, L., Bastiaansen, M., & Petersson, K. M. Integration of word meaning and world knowledge in language comprehension. Science, 304, 438–441, 2004] to determine whether this correspondence also holds for oscillatory brain responses. Hagoort et al. reported an N400 effect and synchronization in the theta and gamma range following world knowledge violations. Our first experiment ( n = 32) used RSVP and replicated both the N400 effect in the ERPs and the power increase in the theta range in the time–frequency domain. In the second experiment ( n = 49), participants read the same materials freely while their eye movements and their EEG were monitored. First fixation durations, gaze durations, and regression rates were increased, and the ERP showed an N400 effect. An analysis of time–frequency representations showed synchronization in the delta range (1–3 Hz) and desynchronization in the upper alpha range (11–13 Hz) but no theta or gamma effects. The results suggest that oscillatory EEG changes elicited by world knowledge violations are different in natural reading and RSVP. This may reflect differences in how representations are constructed and retrieved from memory in the two presentation modes.

A central question concerning word recognition is whether linguistic categories are processed in continuous or categorical ways, in particular, whether regular and irregular inflection is stored and processed by the same or by distinct systems. Here, we contribute to this issue by contrasting regular (regular stem, regular suffix) with semi-irregular (regular stem, irregular suffix) and irregular (irregular stem, irregular suffix) participle formation in a visual priming experiment on German verb inflection. We measured ERPs and RTs and manipulated the inflectional and meaning relatedness between primes and targets. Inflected verb targets (e.g., leite , “head”) were preceded either by themselves, by their participle ( geleitet , “headed”), by a semantically related verb in the same inflection as the target ( führe , “guide”) or in the participle form ( geführt , “guided”), or by an unrelated verb in the same inflection ( nenne , “name”). Results showed that behavioral and ERP priming effects were gradually affected by verb regularity. Regular participles produced a widely distributed frontal and parietal effect, irregular participles produced a small left parietal effect, and semi-irregular participles yielded an effect in-between these two in terms of amplitude and topography. The behavioral and ERP effects further showed that the priming because of participles differs from that because of semantic associates for all verb types. These findings argue for a single processing system that generates participle priming effects for regular, semi-irregular, and irregular verb inflection. Together, the findings provide evidence that the linguistic categories of verb inflection are processed continuously. We present a single-system model that can adequately account for such graded effects.

Remembering is more than an activation of a memory trace. As retrieval cues are often not uniquely related to one specific memory, cognitive control should come into play to guide selective memory retrieval by focusing on relevant while ignoring irrelevant information. Here, we investigated, by means of EEG and fMRI, how the memory system deals with retrieval interference arising when retrieval cues are associated with two material types (faces and spatial positions), but only one is task-relevant. The topography of slow EEG potentials and the fMRI BOLD signal in posterior storage areas indicated that in such situations not only the relevant but also the irrelevant material becomes activated. This results in retrieval interference that triggers control processes mediated by the medial and lateral PFC, which are presumably involved in biasing target representations by boosting the task-relevant material. Moreover, memory-based conflict was found to be dissociable from response conflict that arises when the relevant and irrelevant materials imply different responses. The two types of conflict show different activations in the medial frontal cortex, supporting the claim of domain-specific prefrontal control systems.

Many of our daily decisions are memory based, that is, the attribute information about the decision alternatives has to be recalled. Behavioral studies suggest that for such decisions we often use simple strategies (heuristics) that rely on controlled and limited information search. It is assumed that these heuristics simplify decision-making by activating long-term memory representations of only those attributes that are necessary for the decision. However, from behavioral studies alone, it is unclear whether using heuristics is indeed associated with limited memory search. The present study tested this assumption by monitoring the activation of specific long-term-memory representations with fMRI while participants made memory-based decisions using the “take-the-best” heuristic. For different decision trials, different numbers and types of information had to be retrieved and processed. The attributes consisted of visual information known to be represented in different parts of the posterior cortex. We found that the amount of information required for a decision was mirrored by a parametric activation of the dorsolateral PFC. Such a parametric pattern was also observed in all posterior areas, suggesting that activation was not limited to those attributes required for a decision. However, the posterior increases were systematically modulated by the relative importance of the information for making a decision. These findings suggest that memory-based decision-making is mediated by the dorsolateral PFC, which selectively controls posterior storage areas. In addition, the systematic modulations of the posterior activations indicate a selective boosting of activation of decision-relevant attributes.

The present study used functional magnetic resonance imaging to delineate cortical networks that are activated when objects or spatial locations encoded either visually (visual encoding group, n = 10) or haptically (haptic encoding group, n = 10) had to be retrieved from long-term memory. Participants learned associations between auditorily presented words and either meaningless objects or locations in a 3-D space. During the retrieval phase one day later, participants had to decide whether two auditorily presented words shared an association with a common object or location. Thus, perceptual stimulation during retrieval was always equivalent, whereas either visually or haptically encoded object or location associations had to be reactivated. Moreover, the number of associations fanning out from each word varied systematically, enabling a parametric increase of the number of reactivated representations. Recall of visual objects predominantly activated the left superior frontal gyrus and the intraparietal cortex, whereas visually learned locations activated the superior parietal cortex of both hemispheres. Retrieval of haptically encoded material activated the left medial frontal gyrus and the intraparietal cortex in the object condition, and the bilateral superior parietal cortex in the location condition. A direct test for modality-specific effects showed that visually encoded material activated more vision-related areas (BA 18/19) and haptically encoded material more motor and somatosensory-related areas. A conjunction analysis identified supramodal and material-unspecific activations within the medial and superior frontal gyrus and the superior parietal lobe including the intraparietal sulcus. These activation patterns strongly support the idea that code-specific representations are consolidated and reactivated within anatomically distributed cell assemblies that comprise sensory and motor processing systems.

We report a series of event-related potential experiments designed to dissociate the functionally distinct processes involved in the comprehension of highly restricted lexical-semantic relations (antonyms). We sought to differentiate between influences of semantic relatedness (which are independent of the experimental setting) and processes related to predictability (which differ as a function of the experimental environment). To this end, we conducted three ERP studies contrasting the processing of antonym relations ( black-white ) with that of related ( black-yellow ) and unrelated ( black-nice ) word pairs. Whereas the lexical-semantic manipulation was kept constant across experiments, the experimental environment and the task demands varied: Experiment 1 presented the word pairs in a sentence context of the form The opposite of X is Y and used a sensicality judgment. Experiment 2 used a word pair presentation mode and a lexical decision task. Experiment 3 also examined word pairs, but with an antonymy judgment task. All three experiments revealed a graded N400 response (unrelated > related > antonyms), thus supporting the assumption that semantic associations are processed automatically. In addition, the experiments revealed that, in highly constrained task environments, the N400 gradation occurs simultaneously with a P300 effect for the antonym condition, thus leading to the superficial impression of an extremely “reduced” N400 for antonym pairs. Comparisons across experiments and participant groups revealed that the P300 effect is not only a function of stimulus constraints (i.e., sentence context) and experimental task, but that it is also crucially influenced by individual processing strategies used to achieve successful task performance.

The present study investigated the neurophysiological processes underlying associative long-term memory retrieval of objects and spatial positions by means of a modified fan paradigm with cued recall and two neuroimaging methods (electroencephalogram [EEG] and functional magnetic resonance imaging). In an acquisition phase, either one stimulus or two stimuli became associated with a noun. During retrieval, probe stimuli comprising noun pairs were presented, and participants had to recall the respective associations and decided whether the nouns are linked to each other via a commonly associated stimulus. With this design, the quality and quantity of recalled associations was systematically varied, whereas the triggering stimuli and response requirements were held constant in all experimental conditions. Recall time proved to be directly related to the number of associations fanning out from a retrieval cue. Correspondingly, the hemodynamic response (blood oxygen level-dependent [BOLD] signal) and the amplitude of slow negative EEG potentials increased monotonically with the number of associations in both left anterior and bilateral posterior cortical areas. These effects were consistently observed with content-specific topographies for the two distinct materials. Furthermore, the multimethod approach revealed a close temporal link between response times and event-related slow potential changes on the one side and a close topographical and amplitude correspondence between slow potentials and BOLD signal changes on the other. The integrated results suggest that the neuronal dynamics of associative memory retrieval are equivalent for different types of associations, but that the structural basis is clearly content-specific.

Slow, DC-like event-related brain potentials (ERPs) were recorded from the scalp of 30 healthy young adults to test the hypothesis that distinct cortical areas are activated when different types of information are retrieved from long-term memory. Three groups of 10 subjects each were first trained with associations between either pictures and spatial positions (spatial condition), pictures and color patches (color condition), or nouns and nouns (verbal condition). All three experimental conditions were completely analogous with respect to the established associative structure, the learning procedure, the performance criterion, and the retrieval test that followed 1 day after learning. Slow event-related brain potentials being recorded during retrieval had a significantly distinct topography. The maximum of a DC-like negative wave was found in the verbal condition over the left frontal, in the spatial condition over the parietal, and in the color condition over the right occipital to temporal cortex. These results are consistent with the idea that memory representations are either “down-loaded” into or directly reactivated within those cortical processing modules in which the same material was handled during perception. Response times, on the other hand, revealed no difference between the three retrieval conditions. In each case RT increased monotonically, if more items had to be scanned. Thus, while the ERPs suggest the involvement of different cortical processors during memory search the response times suggest that a sequentially operating scanning mechanism applies to all of them.

The objective of the present study was to delineate brain-electrical correlates of semantic and syntactic integration processes during language comprehension. Twenty-eight subjects were engaged in a lexical decision task. The target item (a legal word or a pseudo word) was always preceded by a prime consisting of a sentence fragment that provided a particular context. With respect to the prime a word target could be either a correct completion, a violation of a semantic selection restriction, or a violation of a syntactic subcategorization rule. An N400-like wave was elicited by both types of deviations. Syntactic anomalies evoked a negative shift predominantly over the anterior scalp with a preponderance over the left hemisphere, while semantic anomalies were accompanied by a much more widespread negativity with the maximum over posterior temporal areas. The amplitude of the semantic vie lation effect was found to be related to the strength of the priming constraints. The topographic difference is consistent with the idea that syntactic and semantic aspects of comprehension are handled by different neural subsystems.