No effect of target probability on P3b amplitudes.

Event probability has been traditionally regarded as the major determinant of P3b amplitudes, with amplitudes increasing when stimuli are less likely. Here we show in a simple variant of the continuous performance task that this "oddball effect" does not universally apply. Stimuli were a continuous series of (A or B) -> (X or Y) pairs, with the letter X requiring a key-press response and occurring in 80% of trials after A and in 20% after B (vice versa the Y). P3b amplitudes were equally large with probable and improbable occurrence of X. This was in contrast to visual Mismatch Negativity which was consistently larger with less probable stimuli, and also in contrast to no-go P3 amplitudes, which were larger with improbable than probable Y. The only effect on P3b amplitude was due to stimulus onset asynchronies (SOA): P3b was larger with SOAs of 2000 ms compared to 1500 ms. This result dovetails with previous evidence in the oddball task that the main determinant of the oddball effect is not event probability but rather time interval between stimuli. The absence of probability effects on P3b was in sharp contrast to the presence of these effects on no-go P3. Implications are discussed for theories about the psychological meaning of the P3b component.

Effects of relevance and response frequency on P3b amplitudes: Review of findings and comparison of hypotheses about the process reflected by P3b.

Diverse psychological correlates have been ascribed to "P300," the conspicuous P3b component of event-related potentials (ERPs) recorded in many laboratory tasks. Traditionally, hypotheses on P3b have conceived of this component being independent from implementing the response to the present stimulus. This has changed in the recent decade when P3b has been related to aspects of the decision process. The present review first focusses on effects of the classic variables stimulus frequency and relevance on P3b amplitude. It turns out that already these effects are related to response requirements because effects of stimulus frequency actually are effects of frequency of response-defined stimulus categories and effects of relevance may be defined as effects of graduating the response requirements. Then, constructs and hypotheses on psychological functions reflected by P3b are evaluated for their abilities in explaining those effects. The tested constructs are information, relevance, and capacity, and the hypotheses are priming, cognitive processing, memory storage, context updating, closure, response facilitation, decision, stimulus-response (S-R) link reactivation, and conscious representations. S-R link reactivation hypothesis performed best, closely followed by memory storage and closure hypotheses. To make further progress, more studies should conduct tests between competing hypotheses.

Get Set or Get Distracted? Disentangling Content-Priming and Attention-Catching Effects of Background Lure Stimuli on Identifying Targets in Two Simultaneously Presented Series.

In order to study the changing relevance of stimulus features in time and space, we used a task with rapid serial presentation of two stimulus streams where two targets ("T1" and "T2") had to be distinguished from background stimuli and where the difficult T2 distinction was impeded by background stimuli presented before T1 that resemble T2 ("lures"). Such lures might actually have dual characteristics: Their capturing attention might interfere with target identification, whereas their similarity to T2 might result in positive priming. To test this idea here, T2 was a blue digit among black letters, and lures resembled T2 either by alphanumeric category (black digits) or by salience (blue letters). Same-category lures were expected to prime T2 identification whereas salient lures would impede T2 identification. Results confirmed these predictions, yet the precise pattern of results did not fit our conceptual framework. To account for this pattern, we speculate that lures serve to confuse participants about the order of events, and the major factor distinguishing color lures and digit lures is their confusability with T2. Mechanisms of effects were additionally explored by measuring event-related EEG potentials. Consistent with the assumption that they attract more attention, color lures evoked larger N2pc than digit lures and affected the ensuing T1-evoked N2pc. T2-evoked N2pc was indistinguishably reduced by all kinds of preceding lures, though. Lure-evoked mesio-frontal negativity increased from first to third lures both with digit and color lures and, thereby, might have reflected expectancy for T1.

Left-Hemisphere Delay of EEG Potentials Evoked by Standard Letter Stimuli During Rapid Serial Visual Presentation: Indicating Right-Hemisphere Advantage or Left-Hemisphere Load?

During rapid serial visual presentation (RSVP), two streams of letters simultaneously presented in the left and right visual fields (LVF and RVF) evoke visual potentials (VEPs) of EEG a few milliseconds earlier at the right (RH) than the left hemisphere (LH). This small LH VEP lag might be attributed to a RH advantage in initial processing of rapidly changing stimuli or to larger load of the LH by its specialized processing of letters from both visual fields simultaneously. In the present study, the two-stream condition was compared in two experiments to conditions with smaller instantaneous verbal load, namely with stimuli presented either solely or slightly earlier in the LVF or RVF. The RH advantage hypothesis predicts a LH VEP lag very similar to the standard two-stream condition when comparing between LH and RH VEPs contralateral to the single or earlier stream. The LH load hypothesis predicts shorter VEP latencies at the LH in the one-stream and earlier-stream than in the two-stream condition, resulting in an absent LH lag in those conditions. Results tended to be more in line with these latter predictions suggesting that in RSVP the LH might be more involved in partial processing of letters in search for target features. However, since the RH advantage hypothesis could not be reliably rejected these results might indicate a complex interplay between both hemispheres. This interplay would exploit the abilities of either hemisphere during the demanding processing of rapidly presented letters, both the LH advantage in letter processing and the RH advantage in visual perception at initial stages.

The oddball effect on P3 disappears when feature relevance or feature-response mappings are unknown.

The P3b component of human event-related EEG potentials is larger with rare than frequent task-relevant stimuli. In a previous study, this oddball effect was much reduced when stimulus-response (S-R) mappings were still undefined at stimulus presentation (being later provided by response prompts). This reduction may reflect P3b's dependence on transmitted information which might be any relevant information (informational value hypothesis) or, more specifically, information about how to respond (S-R link hypothesis). To distinguish between these two hypotheses and clarify their differences from classical stimulus evaluation hypothesis, we added a second dimension by presenting colored letters, with both colors and letters varying between a rare and a frequent alternative. Response prompts, presented half a second later, were, in different blocks, constant or variable across trials with respect to S-R mapping and with respect to the relevant dimension (color or letter). With partial information, when only one of these two factors is known at stimulus presentation (by being constant across trials), the hypotheses differ in their predictions. The oddball effect will be abolished according to S-R link hypothesis because knowledge of both factors is needed to determine the response, but will only be reduced according to informational value hypothesis and be fully maintained according to stimulus evaluation hypothesis. In fact, oddball effects only occurred with knowledge of both factors, i.e., if both the relevant dimension and its mapping to responses were constant across trials. These results confirm the preeminent role of knowledge about responses for eliciting P3.

Dynamic coupling between slow waves and sleep spindles during slow wave sleep in humans is modulated by functional pre-sleep activation.

Co-existent sleep spindles and slow waves have been viewed as a mechanism for offline information processing. Here we explored if the temporal synchronization between slow waves and spindle activity during slow wave sleep (SWS) in humans was modulated by preceding functional activations during pre-sleep learning. We activated differentially the left and right hemisphere before sleep by using a lateralized variant of serial response time task (SRTT) and verified these inter-hemispheric differences by analysing alpha and beta electroencephalographic (EEG) activities during learning. The stability and timing of coupling between positive and negative phases of slow waves and sleep spindle activity during SWS were quantified. Spindle activity was temporally synchronized with both positive (up-state) and negative (down-state) slow half waves. Synchronization of only the fast spindle activity was laterally asymmetric after learning, corresponding to hemisphere-specific activations before sleep. However, the down state was associated with decoupling, whereas the up-state was associated with increased coupling of fast spindle activity over the pre-activated hemisphere. These observations provide original evidence that (1) the temporal grouping of fast spindles by slow waves is a dynamic property of human SWS modulated by functional pre-sleep activation patterns, and (2) fast spindles synchronized by slow waves are functionally distinct.

On Why Targets Evoke P3 Components in Prediction Tasks: Drawing an Analogy between Prediction and Matching Tasks.

P3 is the most conspicuous component in recordings of stimulus-evoked EEG potentials from the human scalp, occurring whenever some task has to be performed with the stimuli. The process underlying P3 has been assumed to be the updating of expectancies. More recently, P3 has been related to decision processing and to activation of established stimulus-response associations (S/R-link hypothesis). However, so far this latter approach has not provided a conception about how to explain the occurrence of P3 with predicted stimuli, although P3 was originally discovered in a prediction task. The present article proposes such a conception. We assume that the internal responses right or wrong both become associatively linked to each predicted target and that one of these two response alternatives gets activated as a function of match or mismatch of the target to the preceding prediction. This seems similar to comparison tasks where responses depend on the matching of the target stimulus with a preceding first stimulus (S1). Based on this idea, this study compared the effects of frequencies of first events (predictions or S1) on target-evoked P3s in prediction and comparison tasks. Indeed, frequencies not only of targets but also of first events had similar effects across tasks on target-evoked P3s. These results support the notion that P3 evoked by predicted stimuli reflects activation of appropriate internal "match" or "mismatch" responses, which is compatible with S/R-link hypothesis.

Sleep Spindles in the Right Hemisphere Support Awareness of Regularities and Reflect Pre-Sleep Activations.

Study Objectives: The present study explored the sleep mechanisms which may support awareness of hidden regularities. Methods: Before sleep, 53 participants learned implicitly a lateralized variant of the serial response-time task in order to localize sensorimotor encoding either in the left or right hemisphere and induce implicit regularity representations. Electroencephalographic (EEG) activity was recorded at multiple electrodes during both task performance and sleep, searching for lateralized traces of the preceding activity during learning. Sleep EEG analysis focused on region-specific slow (9-12 Hz) and fast (13-16 Hz) sleep spindles during nonrapid eye movement sleep. Results: Fast spindle activity at those motor regions that were activated during learning increased with the amount of postsleep awareness. Independently of side of learning, spindle activity at right frontal and fronto-central regions was involved: there, fast spindles increased with the transformation of sequence knowledge from implicit before sleep to explicit after sleep, and slow spindles correlated with individual abilities of gaining awareness. These local modulations of sleep spindles corresponded to regions with greater presleep activation in participants with postsleep explicit knowledge. Conclusions: Sleep spindle mechanisms are related to explicit awareness (1) by tracing the activation of motor cortical and right-hemisphere regions which had stronger involvement already during learning and (2) by recruitment of individually consolidated processing modules in the right hemisphere. The integration of different sleep spindle mechanisms with functional states during wake collectively supports the gain of awareness of previously experienced regularities, with a special role for the right hemisphere.

Lateralization of spatial rather than temporal attention underlies the left hemifield advantage in rapid serial visual presentation.

In bilateral rapid serial visual presentation (RSVP), the second of two targets, T1 and T2, is better identified in the left visual field (LVF) than in the right visual field (RVF). This LVF advantage may reflect hemispheric asymmetry in temporal attention or/and in spatial orienting of attention. Participants performed two tasks: the "standard" bilateral RSVP task (Exp.1) and its unilateral variant (Exp.1 & 2). In the bilateral task, spatial location was uncertain, thus target identification involved stimulus-driven spatial orienting. In the unilateral task, the targets were presented block-wise in the LVF or RVF only, such that no spatial orienting was needed for target identification. Temporal attention was manipulated in both tasks by varying the T1-T2 lag. The results showed that the LVF advantage disappeared when involvement of stimulus-driven spatial orienting was eliminated, whereas the manipulation of temporal attention had no effect on the asymmetry. In conclusion, the results do not support the hypothesis of hemispheric asymmetry in temporal attention, and provide further evidence that the LVF advantage reflects right hemisphere predominance in stimulus-driven orienting of spatial attention. These conclusions fit evidence that temporal attention is implemented by bilateral parietal areas and spatial attention by the right-lateralized ventral frontoparietal network.

How handedness influences perceptual and attentional processes during rapid serial visual presentation.

Stimulus-driven orienting of visual attention is lateralized to the right hemisphere (RH). This lateralization has been studied in the dual-stream rapid serial visual presentation task (dual RSVP). In this task a second target (T2), hard to discern by being embedded in one of two lateral streams of rapidly changing distractors, is better identified on the left than on the right. This phenomenon is called the left visual field advantage (LVFA). Furthermore, in terms of event related potentials (ERPs), the N2pc component and visual evoked potentials (VEPs) are evoked earlier at the RH than at the left hemisphere (LH). All previous dual RSVP experiments were performed on right-handers. In the present study it was investigated how the LVFA and its neural correlates are modulated by handedness. To that end, the size of the LVFA and ERPs (VEPs and N2pc) were compared between right- and left-handers. VEPs were evoked earlier at the RH than the LH in right-handers but not in left-handers. Besides this effect, handedness modulated neither the size of the LVFA nor T2-evoked N2pc. Thus, the LVFA seems to be independent of handedness. Rather than for lateralization of attention, handedness might be relevant for lateralization of early visual perception processes.

Effects on P3 of spreading targets and response prompts apart.

When key-press responses to targets have to be withheld until the presentation of response prompts, target-evoked P3 amplitudes are reduced and so is the P3 difference between rare and frequent targets (the oddball effect on P3). Recently we showed that this even applied when go-signals followed targets by 100ms. Here we aimed at replicating this result with more fine-grained temporal resolution in 100ms steps from 0ms to 500ms, and dissecting the P3 complex to stimulus- and response-related portions by applying residue iteration decomposition (RIDE). Frequent and rare target stimuli (in random series) were followed by go signals (and occasional no-go signals), with block-wise fixed stimulus-onset asynchronies (SOAs) from 0ms to 500ms. Target-evoked P3 amplitudes decreased monotonically across SOAs. Part of this decrease might have been due to an overlapping Contingent Negative Variation (CNV) prior to go signals, increasing across SOAs. When CNV was subtracted out by forming rare-frequent difference waveforms, oddball-P3 was largest at SOA 0, smallest at SOA 500, and equally large at SOAs 100-400. According to RIDE, it was P3's response-related part that was increased at SOA0. These results may be interpreted in terms of the stimulus-response-link reactivation hypothesis of P3.

A right hemisphere advantage at early cortical stages of processing alphanumeric stimuli. Evidence from electrophysiology.

This study investigates hemispheric asymmetry evoked by non-target alphanumeric stimuli in a bilateral rapid serial visual presentation (RSVP) task. Our indicators of asymmetry are shorter latencies and larger amplitudes of the right hemisphere (RH) P1 and N1 components of visual evoked potentials (VEPs). This VEP asymmetry might reflect either a RH advantage, possibly in early perceptual processing, or for familiar stimuli, or for directing attention, or might be a paradoxical reflection of left hemisphere specialization in letter processing. Experiment 1 showed that the VEP asymmetry decreased, though remained present, with unfamiliar stimuli (Tibetan letters), as compared to familiar stimuli (Latin letters and Arabic digits). Experiment 2 showed that while leftward and rightward attentional biases affected the relation between hemispheres contra- and ipsilateral to attended visual fields, the VEP asymmetry remained independent of attention. As the most parsimonious explanation, the primary cause of the VEP asymmetry seems to be a general predominance of the RH in early perceptual processing.

Visual and non-visual motion information processing during pursuit eye tracking in schizophrenia and bipolar disorder.

Despite many reports on visual processing deficits in psychotic disorders, studies are needed on the integration of visual and non-visual components of eye movement control to improve the understanding of sensorimotor information processing in these disorders. Non-visual inputs to eye movement control include prediction of future target velocity from extrapolation of past visual target movement and anticipation of future target movements. It is unclear whether non-visual input is impaired in patients with schizophrenia. We recorded smooth pursuit eye movements in 21 patients with schizophrenia spectrum disorder, 22 patients with bipolar disorder, and 24 controls. In a foveo-fugal ramp task, the target was either continuously visible or was blanked during movement. We determined peak gain (measuring overall performance), initial eye acceleration (measuring visually driven pursuit), deceleration after target extinction (measuring prediction), eye velocity drifts before onset of target visibility (measuring anticipation), and residual gain during blanking intervals (measuring anticipation and prediction). In both patient groups, initial eye acceleration was decreased and the ability to adjust eye acceleration to increasing target acceleration was impaired. In contrast, neither deceleration nor eye drift velocity was reduced in patients, implying unimpaired non-visual contributions to pursuit drive. Disturbances of eye movement control in psychotic disorders appear to be a consequence of deficits in sensorimotor transformation rather than a pure failure in adding cognitive contributions to pursuit drive in higher-order cortical circuits. More generally, this deficit might reflect a fundamental imbalance between processing external input and acting according to internal preferences.

Leftward bias in orienting to and disengaging attention from salient task-irrelevant events in rapid serial visual presentation.

When embedded in the left or right stream of rapidly changing distractors, the second target (T2) is systematically better identified on the left than on the right. This left visual field advantage (LVFA) was recently attributed to better abilities of the right hemisphere in stimulus-driven orienting of attention: it was almost absent when salient uninformative cues were valid (presented in the T2 stream), and increased with invalid (different-stream) cues. However, cue-evoked negativity of event related potentials being earlier at the right than at the left hemisphere suggested that cues also are unequally processed, thereby possibly contributing to increased LVFA after invalid cues. This might occur through easier directing of attention toward left than right cues and/or through harder disengaging of attention from left than right cues. Alternatively, the increase in the LVFA could be caused by larger spatial distance between cues and T2 with invalid cues than with neutral cues presented at fixation. In order to test these hypotheses, an additional stream of stimuli at the vertical midline was used to separate the processing of lateral cues from the processing of lateral T2. If left cues increase the LVFA then this bias should be larger after invalid lateral than invalid midline cues and also midline T2 should be more impaired after left than right cues. These expectations were confirmed. Furthermore, increased negative amplitudes evoked by right cues suggest that orienting was more difficult toward right than left cues, and increased amplitudes of a following positivity suggest that disengaging attention was more difficult from left than right cues. Overall, these results suggest asymmetric abilities of the hemispheres in attentional processing of both task-relevant and salient task-irrelevant events.

Rebalancing Spatial Attention: Endogenous Orienting May Partially Overcome the Left Visual Field Bias in Rapid Serial Visual Presentation.

In dynamically changing environments, spatial attention is not equally distributed across the visual field. For instance, when two streams of stimuli are presented left and right, the second target (T2) is better identified in the left visual field (LVF) than in the right visual field (RVF). Recently, it has been shown that this bias is related to weaker stimulus-driven orienting of attention toward the RVF: The RVF disadvantage was reduced with salient task-irrelevant valid cues and increased with invalid cues. Here we studied if also endogenous orienting of attention may compensate for this unequal distribution of stimulus-driven attention. Explicit information was provided about the location of T1 and T2. Effectiveness of the cue manipulation was confirmed by EEG measures: decreasing alpha power before stream onset with informative cues, earlier latencies of potentials evoked by T1-preceding distractors at the right than at the left hemisphere when T1 was cued left, and decreasing T1- and T2-evoked N2pc amplitudes with informative cues. Importantly, informative cues reduced (though did not completely abolish) the LVF advantage, indicated by improved identification of right T2, and reflected by earlier N2pc latency evoked by right T2 and larger decrease in alpha power after cues indicating right T2. Overall, these results suggest that endogenously driven attention facilitates stimulus-driven orienting of attention toward the RVF, thereby partially overcoming the basic LVF bias in spatial attention.

Synchronization of fronto-parietal beta and theta networks as a signature of visual awareness in neglect.

In the neglect syndrome, the perceptual deficit for contra-lesional hemi-space is increasingly viewed as a dysfunction of fronto-parietal cortical networks, the disruption of which has been described in neuroanatomical and hemodynamic studies. Here we exploit the superior temporal resolution of electroencephalography (EEG) to study dynamic transient connectivity of fronto-parietal circuits at early stages of visual perception in neglect. As reflected by inter-regional phase synchronization in a full-field attention task, two functionally distinct fronto-parietal networks, in beta (15-25Hz) and theta (4-8Hz) frequency bands, were related to stimulus discrimination within the first 200 ms of visual processing. Neglect pathology was specifically associated with significant suppressions of both beta and theta networks engaging right parietal regions. These connectivity abnormalities occurred in a pattern that was distinctly different from what was observed in right-hemisphere lesion patients without neglect. Also, both beta and theta abnormalities contributed additively to visual awareness decrease, quantified in the Behavioural Inattention Test. These results provide evidence for the impairment of fast dynamic fronto-parietal interactions during early stages of visual processing in neglect pathology. Also, they reveal that different modes of fronto-parietal dysfunction contribute independently to deficits in visual awareness at the behavioural level.

Go and no-go P3 with rare and frequent stimuli in oddball tasks: A study comparing key-pressing with counting.

The P3 component of event-related potentials (ERPs) is large at posterior scalp sites with rare go stimuli (go-P3) and at anterior sites with rare no-go stimuli (no-go P3). Most hypotheses on P3, including our S-R link reactivation notion, imply that these characteristics are independent of specific response modes. This assumption was here investigated by comparing ERPs between key-pressing and covert counting responses in oddball tasks that required responses to either frequent or rare stimuli. Replicating previous results, topographic differences between parietal rare go-P3 and fronto-central rare no-go P3 were much reduced with counting compared to key-press responses. Besides, while go-P3 was generally more positive than no-go P3 in the key-press tasks (except for fronto-central no-go P3 with rare stimuli) the reverse was true in the counting tasks. Thus, the characteristic posterior and fronto-central go and no-go topographies appear to be specific to hand movements. Moreover, P3s evoked in counting tasks might not simply be P3 proper, uncontaminated by movement-related potentials, but rather the go/no-go logic might differ between counting and key-pressing, with the oddball effects being affected by specific processes implemented for these particular response modes.

Effects of response delays and of unknown stimulus-response mappings on the oddball effect on P3.

P3b is a prominent component of human event-related EEG potentials. P3b has been related to consciousness, encoding into memory, and updating of strategic schemata, among others, yet evidence has also been provided for its close relationship with deciding how to respond to the presented stimuli. P3b is large with rarely occurring stimuli and small with frequent ones. Here, we investigate the extent to which this oddball effect depends on selecting and executing responses. Participants pressed one of two keys in response to one of two letters, one of which was presented rarely and one frequently. Information about letter-key mapping was provided by a second stimulus. In different blocks, this mapping stimulus was either constant across trials or varied randomly, and either preceded or followed the letter. The oddball effect was reduced when responses were delayed (by waiting for the constant mapping stimulus following the letter) and was further reduced when responses could not be assigned to the letters (because letters were followed by varying mapping stimuli). This evidence suggests that P3b is closely related to decision processes, possibly reflecting reactivation of stimulus-response links.

Do Rare Stimuli Evoke Large P3s by Being Unexpected? A Comparison of Oddball Effects Between Standard-Oddball and Prediction-Oddball Tasks.

The P3 component of event-related potentials increases when stimuli are rarely presented. It has been assumed that this oddball effect (rare-frequent difference) reflects the unexpectedness of rare stimuli. The assumption of unexpectedness and its link to P3 amplitude were tested here. A standard- oddball task requiring alternative key-press responses to frequent and rare stimuli was compared with an oddball-prediction task where stimuli had to be first predicted and then confirmed by key-pressing. Oddball effects in the prediction task depended on whether the frequent or the rare stimulus had been predicted. Oddball effects on P3 amplitudes and error rates in the standard oddball task closely resembled effects after frequent predictions. This corroborates the notion that these effects occur because frequent stimuli are expected and rare stimuli are unexpected. However, a closer look at the prediction task put this notion into doubt because the modifications of oddball effects on P3 by expectancies were entirely due to effects on frequent stimuli, whereas the large P3 amplitudes evoked by rare stimuli were insensitive to predictions (unlike response times and error rates). Therefore, rare stimuli cannot be said to evoke large P3 amplitudes because they are unexpected. We discuss these diverging effects of frequency and expectancy, as well as general differences between tasks, with respect to concepts and hypotheses about P3b's function and conclude that each discussed concept or hypothesis encounters some problems, with a conception in terms of subjective relevance assigned to stimuli offering the most consistent account of these basic effects.

Is P3 a strategic or a tactical component? Relationships of P3 sub-components to response times in oddball tasks with go, no-go and choice responses.

P3 (viz. P300) is a most prominent component of event-related EEG potentials recorded during task performance. There has been long-standing debate about whether the process reflected by P3 is tactical or strategic, i.e., required for making the present response or constituting some overarching process. Here, we used residue iteration decomposition (RIDE) to delineate P3 subcomponents time-locked to responses and tested for the temporal relations between P3 components and response times (RTs). Data were obtained in oddball tasks (i.e., tasks presenting two stimuli, one rarely and one frequently) with rare and frequent go, no-go, or choice responses (CRs). As usual, rare-go P3s were large at Pz and rare no-go P3s at FCz. Notably, P3s evoked with rare CRs were large at either site, probably comprising both go and no-go P3. Throughout, with frequent and rare responses, P3 latencies coincided with RTs. RIDE decomposed P3 complexes into a large CPz-focused C-P3 and an earlier Pz-focused response-locked R-P3. R-P3 had an additional large fronto-central focus with rare CRs, modeling the no-go-P3 part, suggesting that the process reflected by no-go P3 is tightly time-locked to making the alternative response. R-P3 coincided with the fast RTs to frequent stimuli and C-P3 coincided with the slower RTs to rare stimuli. Thus, the process reflected by C-P3 might be required for responding to rare events, but not to frequent ones. We argue that these results are nevertheless compatible with a tactical role of P3 because responses may not be contingent on stimulus analysis with frequent stimuli.