On the modularity of recognition memory for object form and spatial location: a topographic ERP analysis.

Event-related potentials from 61 scalp sites were used to examine the brain processes subserving recognition memory for object forms and spatial locations. Subjects memorized line drawings of highly familiar objects and their spatial locations within a two-dimensional matrix. Prior to the test phases a cue indicated whether object-based or spatially-based recognition judgements were required. Recognition judgements were faster and more accurate for spatially-based than for object-based judgements. A variety of topographical differences in the ERP waveforms as a function of recognition task emerged: First, when the cue indicated that object-based judgements were required, negative slow wave activity extending for several hundred ms with a maximum at frontal recording sites was obtained. Conversely when spatially-based judgements were required, slow wave activity developed over parieto-occipital areas. Second, early portions of the old/new effects evoked by the test items (i.e. 300-600 ms after stimulus onset) showed a similar anterior-posterior dissociation as a function of recognition task. Third, for object-based, but not for spatially-based, judgements, late old/new effects (i.e. 700-1600 ms) were found with a clear maximum at right frontal recordings. The results are consistent with the view that functionally and anatomically different brain systems are involved in recognition memory for object form and spatial location. They further suggest that the retrieval of object forms involves conceptual semantic integration processes.

Double dissociation of processing temporal and spatial information in working memory.

Based on the converging evidence supporting the view of domain specific object and spatial working memory processes, the question was addressed whether the property of domain specificity holds equally for temporal information. Using a selective interference paradigm the objective was to test a dissociation of the processing of temporal duration and spatial location information in working memory of intact human subjects. Subjects performed a temporal and a spatial memory task in which they were required to indicate whether the study and the test stimuli were the same or different in duration (temporal memory) or in location (spatial memory) as primary tasks. Both primary tasks were combined with three types of interference tasks, a spatial classification memory task, a temporal classification memory task and a non-interference baseline task--to be performed in-between the presentation of study and test stimuli. Memory for temporal duration was shown to be impaired by the temporal classification task but not by the spatial classification task; memory for spatial position showed the opposite pattern of impairment. These data thus provide evidence for the view that temporal and spatial working memory contents are subject to selective interference, reflecting a functional dissociation in the processing of temporal duration and spatial location information. The results are interpreted as evidence for the domain specificity in the processing of temporal information in working memory.

Event-related potentials reveal topographical and temporal distinct neuronal activation patterns for spatial and object working memory.

This study examined whether working memory processes for object and spatial information are associated with different patterns of ERP activity. Subjects performed two versions of a delayed match-to-sample task in which either object forms or two-dimensional spatial configurations (S1) had to be encoded and retained in working memory for 6800 ms for comparison with a subsequent stimulus (S2). Event-related potentials (ERPs) were recorded from 29 electrode sites in the S1-S2 interval. Negative slow wave activity in the ERP varied with both, the type of memory task and the amount of materials held in working memory. When spatial information was maintained in working memory negative slow wave activity rapidly rose at recording sites overlying posterior parietal and occipital cortical areas. At these recording sites, slow wave increased in amplitude with increasing spatial memory load. For object information, load-sensitive negative slow wave activity was obtained approximately 2000 ms later than in the spatial task and it was focused to mid-frontal recording sites. Moreover, in the object memory task more pronounced negative slow wave activity was found at right inferior temporal recording sites indicating a larger involvement of the right temporal lobe in the processing of object as compared to spatial information. The results provide evidence for the notion that encoding and retention processes for object information and for spatial information can be functionally dissociated and involve differential patterns of neuronal activation rapidly shifting in time.

Slow cortical potentials during retention of object, spatial, and verbal information.

We used event related potentials (ERPs) to examine both the specificity and the timing of slow cortical scalp potentials (SPs) elicited by the retention of object, spatial, and verbal information in working memory (WM). Participants performed a modified delayed matching task in which a task cue presented in the middle of the delay interval indicated what type of information had to be retained for a subsequent comparison with the test stimulus. The first experiment used nameable objects and spatial locations as stimuli. The retrieval mode (visual vs. verbal) was manipulated by presenting either figural information or printed words as test stimuli. Transient ensembles of frontal and parieto-occipital slow waves with different scalp topographies for object and spatial information were evoked as a function of task cues. When words rather than objects were used as test stimuli highly similar, though more pronounced, fronto-parietal slow wave patterns were obtained. The second experiment using unfamiliar objects and non-nameable spatial locations indicated that neither the left frontal negative SP nor the posterior SPs are exclusively related to verbal working memory operations. The results indicate that a parietal negative SP reflects processes of spatial selective attention whereas a parieto-occipital positive SP indexes the retention of visual object information. Left frontal negative SPs are generated by a compound of higher order frontal control processes and vary as a function of information type.

Repetition related ERP effects in a visual object target detection task.

ERP responses to initial and repeated presentations of possible and impossible objects were recorded from 61 recording sites in a simple target detection task. In Experiment 1, the non-target objects were line drawings of possible and impossible 3-D geometric figures and the targets were line drawings of familiar everyday objects or combinations of parts of everyday objects. In Experiment 2, the non-target objects were everyday objects and the targets were possible and impossible 3-D geometric figures. In both experiments, at frontal sites, the repeated possible and impossible non-target items elicited less negative ERP waveforms relative to first presentations between 250 and 350-400 ms. At parieto-occipital sites, in both experiments, the repeated possible and impossible non-target items elicited less positive ERP waveforms than did first presentations beginning at about 300 ms. The briefly reduced frontal negativity to repeated items is consistent with familiarity arising from a facilitation of access to conceptual, semantic and visuo-spatial representations during object categorization. The polarity of the parieto-occipital effect was the reverse of what is usually found in stimulus repetition tasks, although it is consistent with earlier work using similar visual stimuli. It is interpreted as reflecting the availability of a newly formed representation (i.e., token) of the object just experienced.

Event related brain potentials and illusory memories: the effects of differential encoding.

This study investigates event related potentials (ERP) elicited by true and false recognition using words from different semantic categories. In Experiment 1, ERPs for true and false recognition were more positive than for correctly rejected NEW words starting around 300 ms after test word presentation (old/new ERP effects). ERP waveforms for true and false recognition revealed equal early (300-500 ms) fronto-medial old/new ERP effects, reflecting similar familiarity processes, but smaller parietal old/new ERP effects (500-700 ms) for false relative to true recognition, suggesting less active recollection. Interestingly, a subsequent performance based group comparison showed equivalent old/new ERP effects for true and false recognition for participants with high rates of false recognition. In contrast, false recognition failed to elicit an old/new ERP effect in a group with low false recognition rates. To examine whether this between group difference was driven by the differential use of information that studied words and semantically related non studied test words (LURE) have in common (conceptual similarity), we manipulated encoding strategy in Experiment 2. When encoding focused on conceptual similarity, comparable ERP-effects for true and false recognition were obtained, suggesting that both forms of recognition were equally based on familiarity and recollection processes. Conversely, when encoding was focused on item specific features, differences in brain activity for true and false recognition were obtained. The ERP data indicate that, in addition to the false recognition rate, strategic processes during encoding, such as processing conceptual features, are an important factor in determining electrophysiological differences between true and false recognition.

Event-related brain potentials dissociate visual working memory processes under categorial and identical comparison conditions.

Event-related potentials (ERPs) have been successfully employed to examine the functional and neuronal characteristics of working memory processes. In the present study, we examined the ERP waveforms in a delayed matching task to examine the cognitive processes underlying category and identity comparison and the effects of stimulus complexity. Subjects had to decide whether two visual stimuli are (a) physically identical (identical comparison condition, IC) or (b) identical, irrespective of their orientation (categorial comparison condition, CC). The stimuli were structured five-point patterns, which varied in complexity. For the ERPs elicited during the 1500 ms retention interval, the following pattern of results was obtained: Stimuli in the CC-condition elicited larger P300 components than in the IC-condition. In the IC-condition, the P300 was followed by a broadly distributed negative slow wave. Moreover, complex patterns elicited a posteriorily distributed negativity at 350 ms (N350), whereas the less complex patterns gave rise to a fronto-centrally distributed slow wave that started around 500 ms. These results suggest that S1 was more elaborately processed in the CC-condition, while the more complex figures were associated with an early classification process during the retention interval.

Dissociable brain mechanisms for inhibitory control: effects of interference content and working memory capacity.

In this study, event-related fMRI was used to examine whether the resolution of interference arising from two different information contents activates the same or different neuronal circuitries. In addition, we examined the extent to which these inhibitory control mechanisms are modulated by individual differences in working memory capacity. Two groups of participants with high and low working memory capacity [high span (HS) and low span (LS) participants, respectively] performed two versions of an item recognition task with familiar letters and abstract objects as stimulus materials. Interference costs were examined by means of the recent negative probe technique with otherwise identical testing conditions across both tasks. While the behavioral interference costs were of similar magnitude in both tasks, the underlying brain activation pattern differed between tasks: The object task interference-effects (higher activation in interference trials than in control trials) were restricted to the anterior intraparietal sulcus (IPS). Interference effects for familiar letters were obtained in the anterior IPS, the left postero-ventral and the right dorsolateral prefrontal cortex (PFC) as well as the precuneus. As the letters were more discernible than the objects, the results suggest that the critical feature for PFC and precuneus involvement in interference resolution is the saliency of stimulus-response mappings. The interference effects in the letter task were modulated by working memory capacity: LS participants showed enhanced activation for interference trials only, whereas for HS participants, who showed better performance and also lower interference costs in the letter task, the above-mentioned neuronal circuitry was activated for interference and control trials, thereby attenuating the interference effects. The latter results support the view that HS individuals allocate more attentional resources for the maintenance of task goals in the face of interfering information from preceding trials with familiar stimulus materials.

Syntactic parsing preferences and their on-line revisions: a spatio-temporal analysis of event-related brain potentials.

The present study investigates the processes involved in the recovery from temporarily ambiguous garden-path sentences. Event-related brain potentials (ERP) were recorded while subjects read German subject-object ambiguous relative and complement clauses. As both clause types are initially analyzed as subject-first structures, object-first structures require a revision which is more difficult for complement than for relative clauses. The hypothesis is tested that the revision process consists of two sub-processes, namely diagnosis and actual reanalysis. Applying a spatio-temporal principal component analysis to the ERP data, distinct positive sub-components presumably reflecting different sub-processes could be identified in the time range of the P300 and P600. It will be argued that the P600 is not a monolithic component, and that different sub-processes may be involved at varying time points depending on the type of garden-path sentence.

The P300 to novel and target events: a spatiotemporal dipole model analysis.

It has been reported that frontal lobe structures are critical for the generation of P300 activity in the human event-related potential (ERP) evoked by novel events. In this study spatial-temporal dipole model analyses were employed to obtain further evidence on the temporal dynamics and the approximate location of neuronal activity involved in P300 generation for target and novel events. A triple dipole configuration was found to describe the scalp recorded P300 activity for novel and target events with less than 2.5% residual variance. For the novel, but not the target dipole configuration a frontally oriented dipole was obtained whose temporal activity function suggests that the brain areas involved in the processing of novel events are engaged about 70 ms earlier than those relevant for target processing.

Attentive and pre-attentive periodicity analysis in auditory memory: an event-related brain potential study.

In an event-related potential (ERP) study we presented human subjects with streams of repetitive white noise (semi-periodic noise) under attend and ignore conditions to investigate whether the perception of the periodicity with short cycle-lengths is due to lower level, pre-attentive sensory memory processing or higher level, attentive working memory processing. The ERPs of both conditions reveal N1-like deflections that are time locked on the semi-periodic noise suggesting that the processing of the periodicity is due to a pre-attentive rather than an attentive process. The topography of the deflections suggests that its generators are located in the supratemporal plane. Additionally, the ERPs elicited by infrequent disruptions in the periodicity show differences between the conditions suggesting that the detection of disruptions in periodicity is facilitated by attention.

Mental comparison: P300 component of the ERP reflects the symbolic distance effect.

The symbolic distance effect was studied in a numerical comparison task by means of behavioural measures and event-related potentials (ERP). Two numbers in the range from 11 to 20 were presented simultaneously and the subjects had to indicate if the right side number was smaller or larger than the left side number. Reaction time, error rate and ERPs were selectively averaged according to the numerical differences between the two numbers. The symbolic distance effect was confirmed: Reaction times and error rates decreased significantly with increasing difference between the two numbers. A significant distance effect was also found for both P300 amplitude and latency. The larger the numerical difference the larger the P300 amplitude and the P300 latency tended to be shorter. This finding suggests that the numerical difference is effective already during stimulus encoding.

Gamma responses and ERPs in a visual classification task.

OBJECTIVE: We examined event-related potentials (ERPs) and gamma range EEG activity in a visual classification task to assess which variables affect these responses. METHODS: Ten subjects silently counted the occurrence of rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). By applying a time-frequency analysis to the data and selectively calculating topographical maps of certain frequencies. RESULTS: We were able to find 3 different types of gamma responses to Kanizsa figures: an early phase-locked gamma response at 40 Hz in the N100 time range, late phase-locked gamma activity (200-300 ms) at 40 Hz and a continuous phase-locked gamma response at 80 Hz due to the monitor refresh frequency. The two 40 Hz responses were significantly higher for Kanizsa figures than for non-Kanizsa figures and within the Kanizsa figures were higher for the target figure than for the non-target. CONCLUSION: The phase-locking of these two responses, previously found also as non-phase-locked activity, could be synchronized due to the monitor flicker frequency. Also, our findings suggest that the gamma responses are not solely associated with the binding of stimulus features, but reflect some processes related to target processing.

Transient global ischemia specifically modulates visual P300 scalp distribution.

OBJECTIVE: Latency, amplitude, and scalp topography of the visual P300 component was examined in patients who had suffered from transient global ischemia (TGI) due to cardiac arrest and in age matched clinical and healthy controls in order to investigate the diagnostic value of this component. METHOD: Event-related potentials (ERPs) were recorded from 19 scalp electrodes in a visual oddball paradigm. RESULTS: Mean latency of the P300 component was prolonged in both patient groups. Changes in scalp distribution of the P300, however, appear to be specific to anoxic-ischemic encephalopathy. In particular, a selective reduction of the P300 amplitudes at posterior recording sites was observed in TGI patients. Moreover, examination of the auditory P300 in TGI patients revealed that this selective change seems to be restricted to the visual modality. CONCLUSION: The results are discussed with respect to selective vulnerability of brain tissue to hypoxic-ischemic injury. After TGI a modality-specific subset of P300 generators, probably located in the transitional parieto-occipital and extrastriate occipital cortex, appears to be affected. It is also noted, that the visual P300 component could serve as an additional marker of TGI especially in patients who do not show neuropathological changes in structural brain images.

Semantic aspects of novelty detection in humans.

The ability to detect unusual and novel events is an important prerequisite for storage of information in memory. Non-tonal novel sounds that deviate from an ongoing auditory environment elicit a positive event-related potential (ERP) component, the so-called novel P3. Though there is converging evidence on the neuronal network engaged in novelty detection, little attention has been paid to the properties of novel sounds, such as their typicality or relationship to mental concepts. Here we report the ERPs evoked by two types of generically novel stimuli, namely identifiable (meaningful) and non-identifiable (non-meaningful) novel sounds. The ERP analysis revealed a novel P3 for both types of sounds. However, when subjects actively attended to the stimuli only identifiable novel sounds evoked a right-lateralized negativity (N4) that peaked shortly after the novel P3. We conclude that novelty detection not only includes the registration of deviancy but also fast access and identification of related semantic concepts.

Changes in spontaneous EEG activity indicate a special kind of information processing in concept learning.

Some findings in the literature suggest that a high amount of spectral power in spontaneous EEG alpha 1 band (7.5-10.0 Hz) may be associated with tasks demanding extensive monitoring of stimulus information and combining features for a match with internal concepts ("exploration"). EEG recordings were obtained from 9 subjects involved in a concept-learning task. They had to match visually presented objects with a concept built up by hypothesis and to respond by pressing a "yes" or "no" key. Epochs of EEG data were analyzed with epoch center-times at 500 and 250 ms before response execution. In the case of "yes" responses, when subjects could match their hypothesis positively with combined features of the presented object increased alpha 1 spectral power was found. Also alpha 1 power was larger immediately after disconfirming feedback than after confirming feedback. Additionally, alpha 2 spectral power (10.5-12.5 Hz) was found to be larger 250 ms after confirming feedback than after disconfirming feedback. It is argued that alpha 1 power seems to reflect those mental processes which are involving in combining features and matching them to a concept in mind. The increase in alpha 2 power after confirming feedback is interpreted in terms of general processing demands imposed during task performance.

Working memory constraints on syntactic ambiguity resolution as revealed by electrical brain responses.

Parsing strategies in temporarily ambiguous sentences were investigated in readers with different sentence memory capacities using event-related brain potentials (ERPs). Readers with a high memory span as well as readers with a low memory span were required to read subject and object relative sentences which were either ambiguous until the last word (late disambiguation) or were disambiguated by case marking either the clause initial pronoun (immediate disambiguation) or the noun phrase following it (early disambiguation). ERPs registered during sentence reading elicited the following effects: In the late disambiguation condition, high span readers, but not low span readers, displayed a more positive going wave at the disambiguating number marked auxiliary for the object relative sentences than for the subject relative sentences. This positivity is taken to reflect processes of revision that become necessary at the disambiguating element if the initial structure considered is a subject relative clause. When case marking was available in the clause initial at the relative pronoun, both high and low span readers showed a positivity at the disambiguating element for the object relative sentences, suggesting the immediate use of case marking information for revision. When case marking was available in the noun phrase following an ambiguous pronoun both groups showed no clear effect of revision at the disambiguating element, but only at the sentence final number marked auxiliary. This non-immediate use of the case marking information seems to be due to an inherent ambiguity in the German case marking system which interacts with the disambiguating element's position in the sentence. The combined data indicate that morphological information can be used immediately by high and low span readers to resolve syntactic ambiguity during sentence processing whenever the information given is clearly unambiguous. In addition they suggest that possible processing differences in ambiguity resolution between high and low span readers may only appear when the ambiguous regions are long.

Temporal structure of syntactic parsing: early and late event-related brain potential effects.

Event-related brain potentials (ERPs) were recorded from participants listening to or reading sentences that were correct, contained a violation of the required syntactic category, or contained a syntactic-category ambiguity. When sentences were presented auditorily (Experiment 1), there was an early left anterior negativity for syntactic-category violations, but not for syntactic-category ambiguities. Both anomaly types elicited a late centroparietally distributed positivity. When sentences were presented visually word by word (Experiment 2), again an early left anterior negativity was found only for syntactic-category violations, and both types of anomalies elicited a late positivity. The combined data are taken to be consistent with a 2-stage model of parsing, including a 1st stage, during which an initial phrase structure is built and a 2nd stage, during which thematic role assignment and, if necessary, reanalysis takes place. Disruptions to the 1st stage of syntactic parsing appear to be correlated with an early left anterior negativity, whereas disruptions to the 2nd stage might be correlated with a late posterior distributed positivity.

Magnetoencephalographic responses to illusory figures: early evoked gamma is affected by processing of stimulus features.

We examined evoked and induced responses in event-related fields and gamma activity in the magnetoencephalogram (MEG) during a visual classification task. The objective was to investigate the effects of target classification and the different levels of discrimination between certain stimulus features. We performed two experiments, which differed only in the subjects' task while the stimuli were identical. In Experiment 1, subjects responded by a button-press to rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). This task requires the processing of both stimulus features (colinearity and number of inducer disks). In Experiment 2, the four stimuli of Experiment 1 were used as standards and the occurrence of an additional stimulus without any feature overlap with the Kanizsa stimuli (a rare and highly salient red fixation cross) had to be detected. Discrimination of colinearity and number of inducer disks was not necessarily required for task performance. We applied a wavelet-based time-frequency analysis to the data and calculated topographical maps of the 40 Hz activity. The early evoked gamma activity (100-200 ms) in Experiment 1 was higher for targets as compared to standards. In Experiment 2, no significant differences were found in the gamma responses to the Kanizsa figures and non-Kanizsa figures. This pattern of results suggests that early evoked gamma activity in response to visual stimuli is affected by the targetness of a stimulus and the need to discriminate between the features of a stimulus.

Mental comparison of visually presented two-digit numbers: a P300 study.

Comparison processes were investigated in a multiple-stimulus paradigm with a pseudo-random sequence of visually presented numbers 11 to 20. The subjects' task was to compare each current number with the preceding one and to indicate whether it was larger or smaller. ERPs were selectively averaged for the 10 number stimuli, for three functional conditions according to the information the numbers provide about the ensuing response and for the differences between consecutive numbers. P300 amplitude averaged for each number stimulus showed a U-shaped trend with largest amplitudes for the numbers 11 and 20. It was found that P300 amplitudes change with the amount of information the current number is delivering for the ensuing response. This information delivery is related to the processing in the subsequent trial, as revealed by a negative correlation between P300 in the current trial and RT in the ensuing trial. Reaction times decreased significantly with increasing difference between the current number and the preceding one. This symbolic distance effect was not found for P300 parameters. The dissociation between RT and P300 data provides evidence for the assumption that under the present experimental conditions informational transaction leading to the distance effect occur after the elicitation of P300.