BACKGROUND
we are only beginning to understand the neural mechanisms of decision-making although decision-making plays a pivotal role in translating perception into action  <cit> .
adaptive performance requires decision processes capable of identifying perceptual situations demanding specific responses.
performance emerges from interaction between an organism's goals  and stimuli that impact on that organism .
decision processes therefore must be able to resolve possible conflicts between stimulus-driven  and goal-contingent  control over performance.
this study aims at describing the temporal dynamics of neural processes related to component processes of perceptual decision-making  in humans.
the issue of stimulus-driven versus goal-contingent control has traditionally been important for theoretical formulations of selective attention  <cit> .
reflexive and voluntary factors interact to allow for selectivity of visual attention  <cit> .
several authors argued that visual selective attention is predominantly stimulus-driven  <cit> .
this view proposes that salient visual events attract attention irrespective of the goals of the observer.
however, observers often adopt a deliberate voluntary attentional set for target-compatible features when they are searching for particular visual target stimuli  <cit> .
purely goal-contingent capture of visual attention has been demonstrated when to-be-ignored non-target stimuli  possess target-defining features  <cit> .
theories of attention have generally been divided between early-selection theories, which propose that attention influences perceptual processes, and late-selection theories, which propose that attention operates only after perception is complete  <cit> .
this study was specifically designed to investigate the dynamic coordination of neural processes related to perceptual decision-making.
visual feature analysis, selective attention and conflict-related processing were analyzed by event-related brain potentials .
erps provide online measures of cortical processing of events with excellent temporal resolution, even when the events do not require behavioral responses  <cit> .
the best-described erp index of attention is the p3b component which is a parietal positivity, peaking at approximately  <dig> ms after stimulus onset.
it is larger to infrequent stimuli, particularly when these stimuli are targets.
the p3b can be elicited by stimuli that differ in their probability of occurrence, but also in the amount of goal-relevant information presented by the stimulus .
there are two additional families of earlier erp components which also show goal-contingent enhancement to target stimuli.
firstly, feature-based attention as assessed by erps affects processing in visual cortical areas  <cit> .
this modulation, termed selection negativity , was found for a variety of visual features, including color, shape, and spatial frequency.
the sn is observed over the parieto-occipital scalp, beginning at around  <dig> ms and lasting  <dig> ms or more  <cit> .
the cortical sources of the sn are probably located in the corresponding feature-selective areas of the extrastriate cortex  <cit> .
secondly, prefrontal positivities at about the same latency were described, variously termed p2a  <cit> , p3f  <cit> , or frontal selection positivity   <cit> , which is enhanced to visual target stimuli  <cit> .
there is little evidence regarding which cognitive process the p2a reflects .
potts  <cit>  proposed that the p2a reflects stimulus evaluation in the service of the task demands, i.e., an early, perhaps preliminary, target identification mechanism.
distinctive distractor-contingent components have also been described in various studies.
firstly, infrequent stimuli that are irrelevant to the task , but that are more salient than the targets, evoke the 'novelty p3' or p3a  <cit> , which peaks earlier than the p3b and has a more frontocentral scalp distribution than the p3b.
the p3a was conceived as a correlate of attentional orienting to salient distractors  <cit> .
this idea finally led to the attention switching model of the p3a which proposes that this erp component reflects involuntary switching of attention to deviant events  <cit> .
the attention switching model specifically holds that the task irrelevance of the deviant stimulus is an antecedent condition for eliciting the p3a.
debener et al.
<cit>  showed that the task irrelevance of the deviant stimulus is not an antecedent condition for eliciting the p3a.
we have shown that the p3a is a neural correlate of shifts between attentional sets in multidimensional visual selection tasks  <cit> .
secondly, stimulus displays containing response-incompatible distractors, as in flanker, stroop or certain no-nogo tasks, elicit the n2c which has a midline frontal scalp distribution, and peaks around 250– <dig> ms post-stimulus in tasks that utilize simple stimuli  <cit> .
it may be generated in the medial prefrontal cortex .
the n2c amplitude is related to the degree of response conflict in correct-response trials, even when no overt signs of response conflict are present.
we and others have suggested that the n2c may reflect the detection and suppression of to-be-inhibited responses  <cit> .
we examined cortical activity  during go-nogo choices in humans.
prefrontal p2a, parietal p3b, posterior sn, prefrontal n2c and frontocentral p3a were analyzed in the context of a newly developed cross-dimensional perceptual decision-making task.
perceptual decisions that are based on the conjunction of cross-dimensional stimulus features, such as color and form, have been examined in several studies before .
figure  <dig> illustrates our cross-dimensional decision-making task.
the task made use of a set of four stimuli.
this set was produced by factorially combining two features of two dimensions .
each participant was instructed that one stimulus was the target .
it represented the object that demanded a behavioral response.
there were three different types of non-target stimuli, a color-overlap distractor , a form-overlap distractor , and a standard non-target stimulus .
in the example of figure  <dig>  the red ellipse served as the target stimulus, the red rectangle was the color-overlap distractor, the blue ellipse was the form-overlap distractor, and the blue rectangle equaled the standard non-target stimulus.
the ability to change over time is the very essence of attention  <cit> .
we manipulated the discriminability between stimulus colors at three levels in order to challenge the adaptive dynamics of selective attention.
perceptual decisions should be primarily based on object color if color is a preeminent feature of the to-be-identified targets.
the form feature should gain importance when the discriminability between target and distractor colors decreases.
we recorded erps to examine whether or not color-based and form-based effects, in particular on sn, n2c and p3a, are in accordance with the predicted selection priorities.
RESULTS
response speed and accuracy
accuracy and response speed are documented in table  <dig>  participants performed the required classification at a near-perfect level, as revealed by the hit rates  as well as by the correct rejection rates .
a one-way distinctiveness anova was performed on the hit rates  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p =  <dig> , ε =  <dig> ) and on the average correct rejection rates  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p =  <dig> , ε =  <dig> ).
accuracy was generally near-perfect, without any modulation by the perceptual distinctiveness of the color dimension.
response times were prolonged in the perceptual hard condition compared to the other perceptual distinctiveness conditions.
the readily identifiable slowdown was confirmed by a one-way distinctiveness anova on the response times  =  <dig> , mse =  <dig>  η <dig> =  <dig> , p <  <dig> , ε =  <dig> ).
simple contrasts revealed that response speed was slower in the perceptual hard condition than in each of the remaining perceptual distinctiveness conditions  =  <dig> , p <  <dig> ; perceptual hard vs. intermediate condition: f  =  <dig> , p <  <dig> ).
rt, response time ; hits, ; cr, correct rejections ; c+f+, target stimulus; c+f-, color-overlap distractor; c-f+, form-overlap distractor; c-f-, standard non-target stimulus; avg, average across c+ f-, c-f+, and c-f-, i.e., across all non-target stimuli.
p2a and p3b
the target stimuli , but not the non-target stimuli , evoked a readily identifiable p2a  with prefrontal maximum.
the presence of a target-p2a at fz was confirmed by a two-way stimulus category  × distinctiveness anova  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ), with no effect of perceptual distinctiveness or of the interaction between stimulus category and distinctiveness.
sn
difference sn waves  were computed by subtracting the standard non-target sn from the color-overlap distractor sn to further parse the color-related attentional amplification at occipitoparietal electrodes .
inspection of figure  <dig> reveals that perceptual distinctiveness apparently affected the peak latency of the color-related dsn, but that this experimental manipulation may not have affected the peak amplitude of the color-related dsn.
the observation about the peak latencies was confirmed by a three-way distinctiveness × hemisphere  × position  anova that was performed on the dsn peak latencies.
perceptual distinctiveness  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ), hemisphere  =  <dig> , mse =  <dig>  η <dig> =  <dig> , p <  <dig> ), electrode position  =  <dig> , mse =  <dig>  η <dig> =  <dig> , p <  <dig> ) proved significant .
simple contrasts revealed that the color-related dsn peaks in the perceptual easy and intermediate conditions did not differ significantly, f  =  <dig> , mse =  <dig> , η <dig> <  <dig> , p =  <dig> , whereas the color-related dsn peak was delayed in the perceptual hard condition  in comparison to the perceptual easy condition , f  =  <dig> , mse =  <dig>  η <dig> =  <dig> , p <  <dig> , and to the intermediate  condition, f  =  <dig> , mse =  <dig>  η <dig> =  <dig> , p <  <dig> .
the observation about the peak amplitudes was confirmed by a three-way distinctiveness × hemisphere  × position  anova that was performed on the color-related dsn peak amplitudes.
solely the effect of electrode position  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ) proved significant.
all other main or interaction effects failed to reach significance.
form-related difference sn waves  are shown in figure  <dig>  latencies and amplitudes of the form-related dsns could not be reliably determined due to the relatively weak signal.
color-related and form-related dsn mean amplitudes  were compared in a four-way stimulus category  × distinctiveness × hemisphere  × position  anova.
stimulus category, f  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig>  , and electrode position, f  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , but no other main or interaction effect, proved significant.
n2c and p3a
a two-way stimulus category × distinctiveness anova was performed on the p3a amplitude at cz .
stimulus category  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ), perceptual distinctiveness  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ) as well as the interaction category × distinctiveness  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ) exerted significant effects on the p3a amplitude.
separate distinctiveness anovas on the p3a amplitude at cz were performed in each perceptual distinctiveness condition to further parse the two-way interaction.
p3a amplitudes differed between the various non-target stimuli in the easy  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ) and intermediate  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> , ε =  <dig> ) distinctiveness conditions, whereas no amplitude difference was discernible in the hard distinctiveness condition  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p =  <dig> , ε =  <dig> ).
simple contrasts further revealed that the color-overlap distractor evoked a more prominent p3a compared to the standard non-target stimulus , f  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ,  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ) in the easy distinctiveness condition .
simple contrasts revealed that the color-overlap distractor evoked a more prominent p3a compared to the standard non-target stimulus , f  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ,  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ) in the intermediate distinctiveness condition .
finally, simple contrasts revealed that the standard non-target stimulus  evoked a similar p3a compared to the color-overlap distractor, f  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p =  <dig> ,  =  <dig> , mse =  <dig> , η <dig> =  <dig> , p <  <dig> ) in the hard distinctiveness condition .
discussion
erps revealed neural processes related to visual feature analysis, selective attention and conflict-related processing during cross-dimensional go-nogo choices.
firstly, target identification was accompanied by the emergence of both, p2a and p3b.
the relative feature salience was reflected in p2a amplitude that was recorded at frontolateral electrodes in response to feature-overlap distractors.
secondly, target-compatible features captured attention more than target-incompatible features as revealed by dsns.
we conclude that intra-dimensional attentional capture was goal-contingent.
color-related dsns were more prominent than form-related dsns, suggesting that the color property was possibly more preeminent than the form property.
there were no measurable changes of cross-dimensional selection priorities when color discriminability was altered.
thirdly, peak latencies of color-related dsns provide a chronometric measure of the duration of attention-related neural processing.
fourthly, erps recorded over the frontocentral scalp  showed that color-overlap distractors required more late selection than form-overlap distractors.
finally, the need for additional late selection of color-overlap distractors was severely reduced when color discriminability decreased.
the theoretical relevance of these results is discussed below.
it is well-known that the p3b emerges in response to target stimuli in oddball tasks .
the p3b and the prefrontal p2a showed similar sensitivity to target stimuli .
it has been proposed that the p2a is related to the perceptual analysis of stimuli, possibly reflecting a cross-dimensional feature detection process  <cit> .
alternatively, recent papers promoted the role of top-down processing in visual object recognition .
target sensitivity of the p2a may contribute to top-down facilitation of object recognition.
a partially analyzed version of the visual input is projected from visual areas to the prefrontal cortex  <cit> .
this process, in turn, facilitates object recognition through intracortical feedback by limiting the number of objects that need to be considered .
feature salience was reflected in the amplitude of the p2a, evoked at frontolateral electrodes by color-overlap and form-overlap distractors.
in particular, when colors were easy to distinguish , color-related p2a  was enlarged as compared to form-related p2a .
however, when color discriminability decreased , color-related p2a was attenuated relative to form-related p2a .
these results are compatible with the idea that p2a reflects neural processing related to an early, preliminary target identification process  <cit> .
the crossover interaction between the color-related and form-related p2a amplitudes and color discriminability conditions suggests that adaptive changes of selection priorities – based on the relative salience of feature dimensions – are part of this processing.
as outlined in the introduction, the issue of reflexive versus voluntary control has traditionally been important for theories of selective attention  <cit> .
a behavioral study by folk and colleagues  <cit>  is an important precursor of our study because distractor properties and the properties used to find the target were analyzed in both studies.
when subjects in folk et al.
's study looked for abrupt onset targets, abrupt-onset distractors, but not color distractors, captured attention.
when subjects were looking for color targets, color distractors, but not abrupt-onset distractors, captured attention.
folk et al.
<cit>  concluded that not the stimulus properties of the distractors per se, but the relationship of distractor properties and target-defining properties, determines attentional capture.
folk et al.
<cit>  proposed that cognitive goals determine attentional control settings in advance .
the appearance of stimuli matching that setting capture attention  without further involvement of other cognitive processes  <cit> .
this idea is the theoretical background of analyzing sns and dsns in our study.
the attentional control setting necessarily specified two cross-dimensional target properties, namely its color and form.
the various types of distractors  differed only with regard to the relationship of distractor properties and target-defining properties.
for example, color-related dsn reflects goal-contingent attentional capture induced by the presence of a target-compatible color-feature in the c+f--distractor in comparison to the presence of a target-incompatible color-feature in the c-f--distractor.
comparisons were made using identical physical stimuli under different conditions to isolate purely attentional effects .
the feature-related dsns thus reflect purely goal-contingent, rather than reflexive, attentional capture by target-compatible distractor properties.
our dsn results are in general agreement with those of many earlier erp studies  <cit>  and functional magnetic resonance imaging  studies  <cit>  examining top-down attentional modulation.
the analysis of feature-related dsns suggests that color was probably the prior-ranking property of the attentional control setting.
no evidence for dynamic adaptation of selection priorities in accordance with the relative salience of the feature dimensions could be discerned from the dsns.
this result contrasts with frontolateral p2a findings, suggesting that occipitoparietal dsn and prefrontal p2a are dissociable components of the erp.
the latency of the peak of the color-related dsn provides a chronometric measure of the duration of attention-related neural processing.
on average, the color-related dsn peaks occurred  <dig> ms ,  <dig> ms  and  <dig> ms  after stimulus onset.
thus, the color-related dsn peak latency effect yields a full explanation of the effect of the manipulation of color distinctiveness on average response times which were  <dig> ms ,  <dig>  and  <dig> ms .
we propose that the peak of the color-related dsn provides a suitable approach towards chronometrically localizing  <cit>  the neural effects of manipulations of color discriminability.
the chronometric effect  lies in-between early sensory encoding  and components of the erp that are related to late selection .
its cognitive correlate is most likely the goal-contingent attentional modulation of late perceptual analysis, in this case color analysis.
a number of alternative chronometric measures for late perceptual analysis based on erps has recently been described  <cit> .
both feature-overlap distractors, but not the standard distractor, possessed the potential to provoke response conflicts, due to the coexistence of "go" and "nogo" signals .
as outlined in the introduction, the neural mechanisms of conflict-related processing can be analyzed by comparing n2c and p3a in response to color-overlap and form-overlap distractors, respectively, with n2c and p3a in response to the standard distractor.
hansen and hillyard  <cit>  distinguished three functional modes of multidimensional selection.
firstly, they proposed a hierarchically dependent mode, in which the selection in one dimension  depends on whether another dimension  has a target-compatible feature.
in this mode, erps to target-compatible and target-incompatible features of one dimension would show no difference if the feature of the other dimension is target-incompatible.
with respect to n2c and p3a, these findings were obtained in easy and intermediate color distinctiveness conditions: the amplitudes of both erp components in response to the form-overlap distractor  and the standard distractor  were indistinguishable.
this result suggests that stimuli have received no goal-contingent processing if the color feature was target-incompatible.
in contrast, if the color feature was target-compatible, stimuli have received further goal-contingent processing, as revealed by enhanced n2c and p3a amplitudes in response to color-overlap distractors  as compared to the corresponding amplitudes in response to the remaining distractor types.
further, hansen and hillyard  <cit>  proposed an independent selection mode, in which the selection of one feature does not depend on another feature being target-compatible.
the independent selection mode was not supported by our data.
finally, a holistic mode was proposed, in which selection is based on the conjunction of relevant features.
in this mode, erps in response to all stimuli are identical except for those possessing the relevant conjunction of features .
our n2c and p3a results suggest that this selection mode occurred in the hard color distinctiveness condition: the amplitudes of both erp components were indistinguishable for the form-overlap distractor , for the color-overlap distractor , and for the standard distractor .
the bottom panel of figure  <dig> illustrates perceptual decision-making when colors were hard to distinguish.
color-related and form-related perceptual analyses proceed at comparable rates.
there is no temporal gap between color-related and form-related decisions.
as a consequence, early selection of all distractors is sufficient because no stimulus has been processed to response preparation.
we assume synchronous color-related and form-related decisions in the condition in which colors were hard to distinguish.
hansen and hillyard's  <cit>  model and our decision timing model are not mutually exclusive.
the decision timing model adopts the assumptions underlying the hierarchical dependent mode, which may be considered as a description of asynchronously occurring decisions, whereas the holistic mode may be considered as a description of synchronous decisions.
however, the decision timing model provides a particularly economical explanation of our n2c and p3a results.
qualitatively different modes of selection are not assumed.
the decision timing model postulates response decisions within parallel perceptual channels, and it explains our n2c and p3a results.
the decision timing model assumes that visual objects are first segregated perceptually into dimensional modules , processed in parallel.
it also assumes that these modules are endowed with separable response-decision processes .
according to the model, each dimensional module registers the features from that dimension, and makes response decisions based on these features.
conflicts between the decisions  are associated with the appearance of the n2c and the p3a, depending upon the temporal asynchrony between the decisions .
however, we do not claim that the n2c and the p3a are similarly related to conflict-related processing.
the available p3a studies  <cit>  clearly point into the direction that the p3a is functionally related to attention switching.
given this, it is possible that the p3a reflects a dimensional attention switch that may be required when the dimensional decisions are in conflict with each other.
if so, it is important that conflict-related dimensional attention switches were purely goal-contingent , thereby challenging the attentional switching model  <cit>  of the p3a which posits that the p3a reflects involuntary attention switches to deviant events.
finally, the available evidence about the functional significance of the n2c clearly suggests that this erp component is related to the resolution of the response conflict itself  <cit> .
CONCLUSIONS
a comprehensive explanation of the temporal dynamics of neural processing related to perceptual decisions should be based on a decomposition of behavioral response times into its constituent parts  <cit> .
we stripped cross-dimensional perceptual decision-making down to its bare essentials.
we showed that traditional erp measures provide valuable tools to empirically constrain the neural chronometry of perceptual decision-making.
we capitalized mainly on the excellent temporal resolution of erps as well as on the fact that erps are evoked by events that do not require behavioral responses  <cit> .
the erp findings were integrated into a simple decision timing model of cross-dimensional perceptual decision-making which offers plenty opportunities to guide future studies.
recent studies in monkeys – based on single and multiple unit recordings  <cit>  – and in humans – based on fmri  <cit>  – have revealed possible neural networks for perceptual-decision making.
dorsolateral prefrontal and posterior parietal contributions are critically involved in selecting actions based on instantaneous stimulus information  <cit> .
the left dorsolateral prefrontal cortex is particularly important for perceptual decision-making  <cit> .
it becomes increasingly apparent that a frontoparietal network  <cit>  is crucial for perceptual decision-making.
we have identified erp measures of neural processing supporting component processes of perceptual decision-making.
distinct classes of erp components were related to target identification , attentional selection  and conflict-related processing .
it seems feasible to disentangle the functional connectivity within the frontoparietal decision-making network by combining the chronometric erp measures with fmri-based spatial activation data  <cit> .

 we are only beginning to understand the neural mechanisms of decision-making although decision-making plays a pivotal role in translating perception into action. adaptive performance requires decision processes capable of identifying perceptual situations demanding specific responses. performance emerges from interaction between an organism's goals and stimuli that impact on that organism. decision processes therefore must be able to resolve possible conflicts between stimulus-driven and goal-contingent control over performance. this study aims at describing the temporal dynamics of neural processes related to component processes of perceptual decision-making in humans. the issue of stimulus-driven versus goal-contingent control has traditionally been important for theoretical formulations of selective attention. reflexive and voluntary factors interact to allow for selectivity of visual attention. several authors argued that visual selective attention is predominantly stimulus-driven. this view proposes that salient visual events attract attention irrespective of the goals of the observer. however, observers often adopt a deliberate voluntary attentional set for target-compatible features when they are searching for particular visual target stimuli. purely goal-contingent capture of visual attention has been demonstrated when to-be-ignored non-target stimuli possess target-defining features. theories of attention have generally been divided between early-selection theories, which propose that attention influences perceptual processes, and late-selection theories, which propose that attention operates only after perception is complete. this study was specifically designed to investigate the dynamic coordination of neural processes related to perceptual decision-making. visual feature analysis, selective attention and conflict-related processing were analyzed by event-related brain potentials. erps provide online measures of cortical processing of events with excellent temporal resolution, even when the events do not require behavioral responses. the best-described erp index of attention is the p3b component which is a parietal positivity, peaking at approximately ms after stimulus onset. it is larger to infrequent stimuli, particularly when these stimuli are targets. the p3b can be elicited by stimuli that differ in their probability of occurrence, but also in the amount of goal-relevant information presented by the stimulus. there are two additional families of earlier erp components which also show goal-contingent enhancement to target stimuli. firstly, feature-based attention as assessed by erps affects processing in visual cortical areas. this modulation, termed selection negativity, was found for a variety of visual features, including color, shape, and spatial frequency. the sn is observed over the parieto-occipital scalp, beginning at around ms and lasting ms or more. the cortical sources of the sn are probably located in the corresponding feature-selective areas of the extrastriate cortex. secondly, prefrontal positivities at about the same latency were described, variously termed p2a, p3f, or frontal selection positivity, which is enhanced to visual target stimuli. there is little evidence regarding which cognitive process the p2a reflects. potts proposed that the p2a reflects stimulus evaluation in the service of the task demands, i.e., an early, perhaps preliminary, target identification mechanism. distinctive distractor-contingent components have also been described in various studies. firstly, infrequent stimuli that are irrelevant to the task, but that are more salient than the targets, evoke the'novelty p3' or p3a, which peaks earlier than the p3b and has a more frontocentral scalp distribution than the p3b. the p3a was conceived as a correlate of attentional orienting to salient distractors. this idea finally led to the attention switching model of the p3a which proposes that this erp component reflects involuntary switching of attention to deviant events. the attention switching model specifically holds that the task irrelevance of the deviant stimulus is an antecedent condition for eliciting the p3a. debener et al. showed that the task irrelevance of the deviant stimulus is not an antecedent condition for eliciting the p3a. we have shown that the p3a is a neural correlate of shifts between attentional sets in multidimensional visual selection tasks. secondly, stimulus displays containing response-incompatible distractors, as in flanker, stroop or certain no-nogo tasks, elicit the n2c which has a midline frontal scalp distribution, and peaks around 250– ms post-stimulus in tasks that utilize simple stimuli. it may be generated in the medial prefrontal cortex. the n2c amplitude is related to the degree of response conflict in correct-response trials, even when no overt signs of response conflict are present. we and others have suggested that the n2c may reflect the detection and suppression of to-be-inhibited responses. we examined cortical activity during go-nogo choices in humans. prefrontal p2a, parietal p3b, posterior sn, prefrontal n2c and frontocentral p3a were analyzed in the context of a newly developed cross-dimensional perceptual decision-making task. perceptual decisions that are based on the conjunction of cross-dimensional stimulus features, such as color and form, have been examined in several studies before. figure illustrates our cross-dimensional decision-making task. the task made use of a set of four stimuli. this set was produced by factorially combining two features of two dimensions. each participant was instructed that one stimulus was the target. it represented the object that demanded a behavioral response. there were three different types of non-target stimuli, a color-overlap distractor, a form-overlap distractor, and a standard non-target stimulus. in the example of figure the red ellipse served as the target stimulus, the red rectangle was the color-overlap distractor, the blue ellipse was the form-overlap distractor, and the blue rectangle equaled the standard non-target stimulus. the ability to change over time is the very essence of attention. we manipulated the discriminability between stimulus colors at three levels in order to challenge the adaptive dynamics of selective attention. perceptual decisions should be primarily based on object color if color is a preeminent feature of the to-be-identified targets. the form feature should gain importance when the discriminability between target and distractor colors decreases. we recorded erps to examine whether or not color-based and form-based effects, in particular on sn, n2c and p3a, are in accordance with the predicted selection priorities.  response speed and accuracy accuracy and response speed are documented in table participants performed the required classification at a near-perfect level, as revealed by the hit rates as well as by the correct rejection rates. a one-way distinctiveness anova was performed on the hit rates =, mse =, η =, p =, ε = ) and on the average correct rejection rates =, mse =, η =, p =, ε = ). accuracy was generally near-perfect, without any modulation by the perceptual distinctiveness of the color dimension. response times were prolonged in the perceptual hard condition compared to the other perceptual distinctiveness conditions. the readily identifiable slowdown was confirmed by a one-way distinctiveness anova on the response times =, mse = η =, p <, ε = ). simple contrasts revealed that response speed was slower in the perceptual hard condition than in each of the remaining perceptual distinctiveness conditions =, p <; perceptual hard vs. intermediate condition: f =, p < ). rt, response time; hits,; cr, correct rejections; c+f+, target stimulus; c+f-, color-overlap distractor; c-f+, form-overlap distractor; c-f-, standard non-target stimulus; avg, average across c+ f-, c-f+, and c-f-, i.e., across all non-target stimuli. p2a and p3b the target stimuli, but not the non-target stimuli, evoked a readily identifiable p2a with prefrontal maximum. the presence of a target-p2a at fz was confirmed by a two-way stimulus category × distinctiveness anova =, mse =, η =, p < ), with no effect of perceptual distinctiveness or of the interaction between stimulus category and distinctiveness. sn difference sn waves were computed by subtracting the standard non-target sn from the color-overlap distractor sn to further parse the color-related attentional amplification at occipitoparietal electrodes. inspection of figure reveals that perceptual distinctiveness apparently affected the peak latency of the color-related dsn, but that this experimental manipulation may not have affected the peak amplitude of the color-related dsn. the observation about the peak latencies was confirmed by a three-way distinctiveness × hemisphere × position anova that was performed on the dsn peak latencies. perceptual distinctiveness =, mse =, η =, p <, ε = ), hemisphere =, mse = η =, p < ), electrode position =, mse = η =, p < ) proved significant. simple contrasts revealed that the color-related dsn peaks in the perceptual easy and intermediate conditions did not differ significantly, f =, mse =, η <, p =, whereas the color-related dsn peak was delayed in the perceptual hard condition in comparison to the perceptual easy condition, f =, mse = η =, p <, and to the intermediate condition, f =, mse = η =, p <. the observation about the peak amplitudes was confirmed by a three-way distinctiveness × hemisphere × position anova that was performed on the color-related dsn peak amplitudes. solely the effect of electrode position =, mse =, η =, p < ) proved significant. all other main or interaction effects failed to reach significance. form-related difference sn waves are shown in figure latencies and amplitudes of the form-related dsns could not be reliably determined due to the relatively weak signal. color-related and form-related dsn mean amplitudes were compared in a four-way stimulus category × distinctiveness × hemisphere × position anova. stimulus category, f =, mse =, η =, p <, and electrode position, f =, mse =, η =, p <, but no other main or interaction effect, proved significant. n2c and p3a a two-way stimulus category × distinctiveness anova was performed on the p3a amplitude at cz. stimulus category =, mse =, η =, p <, ε = ), perceptual distinctiveness =, mse =, η =, p <, ε = ) as well as the interaction category × distinctiveness =, mse =, η =, p <, ε = ) exerted significant effects on the p3a amplitude. separate distinctiveness anovas on the p3a amplitude at cz were performed in each perceptual distinctiveness condition to further parse the two-way interaction. p3a amplitudes differed between the various non-target stimuli in the easy =, mse =, η =, p <, ε = ) and intermediate =, mse =, η =, p <, ε = ) distinctiveness conditions, whereas no amplitude difference was discernible in the hard distinctiveness condition =, mse =, η =, p =, ε = ). simple contrasts further revealed that the color-overlap distractor evoked a more prominent p3a compared to the standard non-target stimulus, f =, mse =, η =, p <, =, mse =, η =, p < ) in the easy distinctiveness condition. simple contrasts revealed that the color-overlap distractor evoked a more prominent p3a compared to the standard non-target stimulus, f =, mse =, η =, p <, =, mse =, η =, p < ) in the intermediate distinctiveness condition. finally, simple contrasts revealed that the standard non-target stimulus evoked a similar p3a compared to the color-overlap distractor, f =, mse =, η =, p =, =, mse =, η =, p < ) in the hard distinctiveness condition. discussion erps revealed neural processes related to visual feature analysis, selective attention and conflict-related processing during cross-dimensional go-nogo choices. firstly, target identification was accompanied by the emergence of both, p2a and p3b. the relative feature salience was reflected in p2a amplitude that was recorded at frontolateral electrodes in response to feature-overlap distractors. secondly, target-compatible features captured attention more than target-incompatible features as revealed by dsns. we conclude that intra-dimensional attentional capture was goal-contingent. color-related dsns were more prominent than form-related dsns, suggesting that the color property was possibly more preeminent than the form property. there were no measurable changes of cross-dimensional selection priorities when color discriminability was altered. thirdly, peak latencies of color-related dsns provide a chronometric measure of the duration of attention-related neural processing. fourthly, erps recorded over the frontocentral scalp showed that color-overlap distractors required more late selection than form-overlap distractors. finally, the need for additional late selection of color-overlap distractors was severely reduced when color discriminability decreased. the theoretical relevance of these results is discussed below. it is well-known that the p3b emerges in response to target stimuli in oddball tasks. the p3b and the prefrontal p2a showed similar sensitivity to target stimuli. it has been proposed that the p2a is related to the perceptual analysis of stimuli, possibly reflecting a cross-dimensional feature detection process. alternatively, recent papers promoted the role of top-down processing in visual object recognition. target sensitivity of the p2a may contribute to top-down facilitation of object recognition. a partially analyzed version of the visual input is projected from visual areas to the prefrontal cortex. this process, in turn, facilitates object recognition through intracortical feedback by limiting the number of objects that need to be considered. feature salience was reflected in the amplitude of the p2a, evoked at frontolateral electrodes by color-overlap and form-overlap distractors. in particular, when colors were easy to distinguish, color-related p2a was enlarged as compared to form-related p2a. however, when color discriminability decreased, color-related p2a was attenuated relative to form-related p2a. these results are compatible with the idea that p2a reflects neural processing related to an early, preliminary target identification process. the crossover interaction between the color-related and form-related p2a amplitudes and color discriminability conditions suggests that adaptive changes of selection priorities – based on the relative salience of feature dimensions – are part of this processing. as outlined in the introduction, the issue of reflexive versus voluntary control has traditionally been important for theories of selective attention. a behavioral study by folk and colleagues is an important precursor of our study because distractor properties and the properties used to find the target were analyzed in both studies. when subjects in folk et al.'s study looked for abrupt onset targets, abrupt-onset distractors, but not color distractors, captured attention. when subjects were looking for color targets, color distractors, but not abrupt-onset distractors, captured attention. folk et al. concluded that not the stimulus properties of the distractors per se, but the relationship of distractor properties and target-defining properties, determines attentional capture. folk et al. proposed that cognitive goals determine attentional control settings in advance. the appearance of stimuli matching that setting capture attention without further involvement of other cognitive processes. this idea is the theoretical background of analyzing sns and dsns in our study. the attentional control setting necessarily specified two cross-dimensional target properties, namely its color and form. the various types of distractors differed only with regard to the relationship of distractor properties and target-defining properties. for example, color-related dsn reflects goal-contingent attentional capture induced by the presence of a target-compatible color-feature in the c+f--distractor in comparison to the presence of a target-incompatible color-feature in the c-f--distractor. comparisons were made using identical physical stimuli under different conditions to isolate purely attentional effects. the feature-related dsns thus reflect purely goal-contingent, rather than reflexive, attentional capture by target-compatible distractor properties. our dsn results are in general agreement with those of many earlier erp studies and functional magnetic resonance imaging studies examining top-down attentional modulation. the analysis of feature-related dsns suggests that color was probably the prior-ranking property of the attentional control setting. no evidence for dynamic adaptation of selection priorities in accordance with the relative salience of the feature dimensions could be discerned from the dsns. this result contrasts with frontolateral p2a findings, suggesting that occipitoparietal dsn and prefrontal p2a are dissociable components of the erp. the latency of the peak of the color-related dsn provides a chronometric measure of the duration of attention-related neural processing. on average, the color-related dsn peaks occurred ms, ms and ms after stimulus onset. thus, the color-related dsn peak latency effect yields a full explanation of the effect of the manipulation of color distinctiveness on average response times which were ms, and ms. we propose that the peak of the color-related dsn provides a suitable approach towards chronometrically localizing the neural effects of manipulations of color discriminability. the chronometric effect lies in-between early sensory encoding and components of the erp that are related to late selection. its cognitive correlate is most likely the goal-contingent attentional modulation of late perceptual analysis, in this case color analysis. a number of alternative chronometric measures for late perceptual analysis based on erps has recently been described. both feature-overlap distractors, but not the standard distractor, possessed the potential to provoke response conflicts, due to the coexistence of "go" and "nogo" signals. as outlined in the introduction, the neural mechanisms of conflict-related processing can be analyzed by comparing n2c and p3a in response to color-overlap and form-overlap distractors, respectively, with n2c and p3a in response to the standard distractor. hansen and hillyard distinguished three functional modes of multidimensional selection. firstly, they proposed a hierarchically dependent mode, in which the selection in one dimension depends on whether another dimension has a target-compatible feature. in this mode, erps to target-compatible and target-incompatible features of one dimension would show no difference if the feature of the other dimension is target-incompatible. with respect to n2c and p3a, these findings were obtained in easy and intermediate color distinctiveness conditions: the amplitudes of both erp components in response to the form-overlap distractor and the standard distractor were indistinguishable. this result suggests that stimuli have received no goal-contingent processing if the color feature was target-incompatible. in contrast, if the color feature was target-compatible, stimuli have received further goal-contingent processing, as revealed by enhanced n2c and p3a amplitudes in response to color-overlap distractors as compared to the corresponding amplitudes in response to the remaining distractor types. further, hansen and hillyard proposed an independent selection mode, in which the selection of one feature does not depend on another feature being target-compatible. the independent selection mode was not supported by our data. finally, a holistic mode was proposed, in which selection is based on the conjunction of relevant features. in this mode, erps in response to all stimuli are identical except for those possessing the relevant conjunction of features. our n2c and p3a results suggest that this selection mode occurred in the hard color distinctiveness condition: the amplitudes of both erp components were indistinguishable for the form-overlap distractor, for the color-overlap distractor, and for the standard distractor. the bottom panel of figure illustrates perceptual decision-making when colors were hard to distinguish. color-related and form-related perceptual analyses proceed at comparable rates. there is no temporal gap between color-related and form-related decisions. as a consequence, early selection of all distractors is sufficient because no stimulus has been processed to response preparation. we assume synchronous color-related and form-related decisions in the condition in which colors were hard to distinguish. hansen and hillyard's model and our decision timing model are not mutually exclusive. the decision timing model adopts the assumptions underlying the hierarchical dependent mode, which may be considered as a description of asynchronously occurring decisions, whereas the holistic mode may be considered as a description of synchronous decisions. however, the decision timing model provides a particularly economical explanation of our n2c and p3a results. qualitatively different modes of selection are not assumed. the decision timing model postulates response decisions within parallel perceptual channels, and it explains our n2c and p3a results. the decision timing model assumes that visual objects are first segregated perceptually into dimensional modules, processed in parallel. it also assumes that these modules are endowed with separable response-decision processes. according to the model, each dimensional module registers the features from that dimension, and makes response decisions based on these features. conflicts between the decisions are associated with the appearance of the n2c and the p3a, depending upon the temporal asynchrony between the decisions. however, we do not claim that the n2c and the p3a are similarly related to conflict-related processing. the available p3a studies clearly point into the direction that the p3a is functionally related to attention switching. given this, it is possible that the p3a reflects a dimensional attention switch that may be required when the dimensional decisions are in conflict with each other. if so, it is important that conflict-related dimensional attention switches were purely goal-contingent, thereby challenging the attentional switching model of the p3a which posits that the p3a reflects involuntary attention switches to deviant events. finally, the available evidence about the functional significance of the n2c clearly suggests that this erp component is related to the resolution of the response conflict itself.  a comprehensive explanation of the temporal dynamics of neural processing related to perceptual decisions should be based on a decomposition of behavioral response times into its constituent parts. we stripped cross-dimensional perceptual decision-making down to its bare essentials. we showed that traditional erp measures provide valuable tools to empirically constrain the neural chronometry of perceptual decision-making. we capitalized mainly on the excellent temporal resolution of erps as well as on the fact that erps are evoked by events that do not require behavioral responses. the erp findings were integrated into a simple decision timing model of cross-dimensional perceptual decision-making which offers plenty opportunities to guide future studies. recent studies in monkeys – based on single and multiple unit recordings – and in humans – based on fmri – have revealed possible neural networks for perceptual-decision making. dorsolateral prefrontal and posterior parietal contributions are critically involved in selecting actions based on instantaneous stimulus information. the left dorsolateral prefrontal cortex is particularly important for perceptual decision-making. it becomes increasingly apparent that a frontoparietal network is crucial for perceptual decision-making. we have identified erp measures of neural processing supporting component processes of perceptual decision-making. distinct classes of erp components were related to target identification, attentional selection and conflict-related processing. it seems feasible to disentangle the functional connectivity within the frontoparietal decision-making network by combining the chronometric erp measures with fmri-based spatial activation data. 
we recorded event-related potentials during a visual target-identification task that required go-nogo choices. erp measures contribute to a comprehensive explanation of the temporal dynamics of component processes of perceptual decision-making. peak latencies of the color-related sn provided a chronometric measure of the duration of attention-related neural processing. color discriminability was manipulated in three conditions to determine the effects of color distinctiveness on component processes of decision-making. target identification was accompanied by the emergence of prefrontal p2a and p3b. we propose a simple model of cross-dimensional perceptual decision-making. erps recorded over the frontocentral scalp revealed that color-overlap distractors, more than form-overlap distractors, required additional late selection. the need for additional response selection induced by color-overlap distractors was severely reduced when color discriminability decreased. no changes of cross-dimensional selection priorities were measurable when color discriminability was altered. selection negativity revealed that target-compatible features captured attention more than target-incompatible features, suggesting that intra-dimensional attentional capture was goal-contingent.
 decision-making is a fundamental capacity which is crucial to many higher-order psychological functions. we recorded event-related potentials during a visual target-identification task that required go-nogo choices. targets were identified on the basis of cross-dimensional conjunctions of particular colors and forms. color discriminability was manipulated in three conditions to determine the effects of color distinctiveness on component processes of decision-making.  target identification was accompanied by the emergence of prefrontal p2a and p3b. selection negativity revealed that target-compatible features captured attention more than target-incompatible features, suggesting that intra-dimensional attentional capture was goal-contingent. no changes of cross-dimensional selection priorities were measurable when color discriminability was altered. peak latencies of the color-related sn provided a chronometric measure of the duration of attention-related neural processing. erps recorded over the frontocentral scalp revealed that color-overlap distractors, more than form-overlap distractors, required additional late selection. the need for additional response selection induced by color-overlap distractors was severely reduced when color discriminability decreased.  we propose a simple model of cross-dimensional perceptual decision-making. the temporal synchrony of separate color-related and form-related choices determines whether or not distractor processing includes post-perceptual stages. erp measures contribute to a comprehensive explanation of the temporal dynamics of component processes of perceptual decision-making. 