A.A. Wijers

Inhibition, response mode, and stimulus probability: a comparative event-related potential study

OBJECTIVES In the present study, effects of response mode (finger movement or counting) and stimulus probability on inhibitory processes were studied. METHODS Electroencephalographic activity was registered in a visual go/nogo paradigm. Subjects either responded manually to go stimuli or counted silently the occurrence of each go stimulus in different conditions. In both response mode conditions, response probability was varied. RESULTS For finger movement and counting, similar N2 and P3 go/nogo effects were found. The amplitude of the nogo N2 and nogo P3 varied as a negative function of nogo stimulus probability. The go P3 varied as a negative function of go stimulus probability. In the manual condition, however, the descending flank of the go N2 at anterior electrode sites was more negative in the 0.50go and 0.75go probability trials than in the 0.25go probability trials. CONCLUSIONS The results of the present study confirm the hypothesis that differences between go and nogo event-related potentials are not dependent on overt movement-related potentials. It could be speculated that the probability effect on the N2 amplitude in go trials in the manual condition has to be explained in terms of a modulation of the strength of motoric preparation processes varying as a positive function of the probability of the go stimulus.

Localizing components of a complex task: sentence processing and working memory

THREE areas of the left hemisphere play different roles in sentence comprehension. An area of posterior middle and superior temporal gyrus shows activation correlated with the structural complexity of a sentence, suggesting that this area supports processing of sentence structure. The lateral anterior temporal gyrus is more activated bilaterally by all sentence conditions than by word lists; thus the function of the area probably does not directly support processing of structure but rather processing of words specific to a sentence context. Left inferior frontal cortex also shows activation related to sentence complexity but is also more activated in word list processing than in simple sentences; this region may thus support a form of verbal working memory which maintains sentence structural information as well as lexical items.

Response priming in a go/nogo task: do we have to explain the go/nogo N2 effect in terms of response activation instead of inhibition?

OBJECTIVES In the present study, we examined the effects of response priming on the event-related potentials (ERPs) evoked by target stimuli in a go/nogo task. METHODS In each trial, subjects were presented a cue and a target stimulus. The cue informed subjects about the following target in that trial, and therefore, also about the kind of response (right-hand response, left-hand response, no overt response) potentially to be given in that trial. RESULTS The traditional N2 and P3 go/nogo effects were replicated: the ERPs to nogo targets were negative compared to the ERPs evoked by go targets in the N2 latency range at frontal electrode sites, and the nogo P3s were more anteriorly distributed than the go P3s. Comparing the ERPs evoked by nogo targets, we found the P3, but not the N2, to be modulated by response priming. CONCLUSIONS These results seem to indicate that the P3, but not the N2, is associated with response inhibition, or with an evaluation/decision process with regard to the expected and/or given response. It could be speculated that the traditional go/nogo N2 effect has to be explained in terms of response activation instead of response inhibition.

An event-related brain potential correlate of visual short-term memory

Event-related potentials (ERPs) were recorded as 12 subjects performed a delayed matching to sample task. We presented two bilateral abstract shapes and cued spatially which had to be memorized for a subsequent matching task: left, right or both. During memorization a posterior slow negative ERP wave developed over the hemisphere contralateral to the memorized shape. This effect was similar in high and low memory load trials while the memory figures were visible (for 1000 ms). As the figures disappeared (for 1500 ms), the effect persisted only in the low memory load conditions. We suggest that the contralateral negativity reflects a visual short-term memory process and that capacity limitation in the high memory load condition causes this process to collapse.

Memory-search and visual spatial attention - an event-related brain potential analysis

Event-related brain potentials (ERPs) were recorded from subjects as they attended to one diagonal of a visual display. The task was to respond only to memory set items (targets) at the attended diagonal and to ignore stimuli at the other diagonal. The probability that the display contained either an attended or an unattended target was 0.30. The spatial separation between attended and unattended stimuli was 1.6°. The ERP elicited by stimuli at the unattended diagonal contained a sequence of phasic components. The early N170 and P250 components were elicited by the onset of the display and the later components N480 and P550 by the offset of the display. The presence of masks delayed N170 and P250. The ERPs elicited by attended non-targets, in addition, contained an increased N350 (Cz, Fz) and P410 (P3a, Pz, Cz). The ERPs elicited by attended and unattended non-targets started to differ after 200 msec. This finding suggested that selection is relatively late if selection must be based on a conjunction of features (location and orientation) and if the spatial separation between attended and unattended stimuli is small. Memory-set size affected the ERPs after 250 msec. The ERPs elicited by attended stimuli contained a broadly distributed (Fz, Cz, Pz) negative endogeneous component. The amplitude of this component was related to memory-set size. Finally, the ERPs elicited by attended targets contained a large P3b (Pz, Oz) with a peak latency around 600 msec. The ERP results suggested the existence of three processing stages: (1) orienting to the attended stimuli; (2) controlled search, and (3) target decision.

Cortisol-Induced Increases of Plasma Oxytocin Levels Predict Decreased Immediate Free Recall of Unpleasant Words

Cortisol and oxytocin have been shown to interact in both the regulation of stress responses and in memory function. In the present study we administered cortisol to 35 healthy female subjects in a within-subject double-blind placebo-controlled design, while measuring oxytocin levels, adrenocorticotropic hormone (ACTH) levels, and free recall of pleasant and of unpleasant words. We found that cortisol administration suppressed ACTH levels and (1) induced a decrease in oxytocin associated with ACTH suppression and (2) an increase in oxytocin that was independent from ACTH suppression. This cortisol-induced increase in plasma oxytocin was associated with a selective decrease in immediate free recall of unpleasant words from primacy positions. The present results add to evidence that cortisol-induced increases in oxytocin could mediate some of the effects of stress and cortisol on memory, and possibly play a role in the regulation of the hypothalamo-pituitary–adrenal stress response. This mechanism could significantly impact affective and social behaviors, in particular during times of stress.

Neural Differentiation of Lexico-Syntactic Categories or Semantic Features? Event-Related Potential Evidence for Both

Event-related potentials (ERPs) were used to investigate whether processing differences between nouns and verbs can be accounted for by the differential salience of visual-perceptual and motor attributes in their semantic specifications. Three subclasses of nouns and verbs were selected, which differed in their semantic attribute composition (abstract, high visual, high visual and motor). Single visual word presentation with a recognition memory task was used. While multiple robust and parallel ERP effects were observed for both grammatical class and attribute type, there were no interactions between these. This pattern of effects provides support for lexicalsemantic knowledge being organized in a manner that takes account both of category-based (grammatical class) and attribute-based distinctions.

Color selection and location selection in ERPs : differences, similarities and 'neural specificity'

It was hypothesized that color selection consists of two stages. The first stage represents a feature specific selection in neural populations specialized in processing color. The second stage constitutes feature non-specific selections, related to executive attentional processes and/or motor processes. This hypothesis was tested by investigating the effects of selectively attending to a specific color, location, or conjunction of location and color on the ERPs elicited by briefly flashed gratings. The gratings differed on three dimensions: color (red or blue), location in the visual field (4.4 degrees to the left or right of fixation) and form (target or non-target). Subjects had to respond to the presentation of target gratings in the attended category. Color selection was reflected in an enhanced parietal positivity in the 150-190 ms interval. Source analyses suggested that this color selection positivity might be generated in the basal occipital cortex, possibly human V4, an area of the brain specialized in color processing. The effect was separated from the P1 spatial attention effect both in topography and sources. Color selection was also reflected in a contralateral occipitotemporal negativity, which resembled the N1 spatial attention effect both in timing and topography. And finally, color selection was reflected in an N2b component. This N2b was similar in timing, topography and sources to the N2bs elicited by location selection and conjunction selection. We suggested that the N2b reflects feature non-specific selection processes, elicited by a range of attended stimuli, and possibly reflects activity in the anterior cingulate cortex. The NP80 was unaffected by attention to color and/or location and localized in striate cortex.

Sentence comprehension and word repetition: A positron emission tomography investigation

Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words (in a different context, when subjects did not attempt to learn the initial lists) led to significant patterns of both increased blood flow (left putamen and right caudate) and decreased blood flow (left posterior temporal lobe). Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

VISUAL-SEARCH AND SPATIAL ATTENTION - ERPS IN FOCUSED AND DIVIDED ATTENTION CONDITIONS

ERPs and performance were measured in divided and focussed attention visual search tasks. In focussed attention tasks, to-be-attended and to-be-ignored letters were presented simultaneously. We varied display load, mapping conditions and display size. RT, P3b-latency and negativity in the ERP associated with controlled search all increased with display load. Each of these measures showed selectivity of controlled search, in that they decreased with focussing of attention. An occipital N230, on the other hand, was not sensitive to focussing of attention, but was primarily affected by display load. ERPs to both attended and unattended targets in focussed attention conditions showed and N2 compared to nontargets, suggesting that both automatic and controlled letter classifications are possible. These effects were not affected by display size. Consistent mapping resulted in shorter RT and P3b-latency in divided attention conditions, compared to varied mapping conditions, but had no effect in focussed attention conditions.