Models of forced-choice decision making rely heavily on the notion that the brain accumulates information for one stimulus versus others over some period of time, with the resulting choice being determined by the relative weight of this information at a decision stage (Gold and Shadlen 2000
; Schall 2001
; Platt 2002
; Ratcliff et al. 2003
; Reddi et al. 2003
; Ratcliff and Smith 2004
). Computational and neural models of information processing assume that this accumulation is driven by both systematic and random influences that alter the speed and strength of representations in the brain, thereby determining the relative strength of each choice when the response system is activated.
The classic Stroop interference task (Stroop 1935
) has provided a fruitful platform by which to test models of forced-choice decision and response selection under situations where compatible or incompatible components of the stimulus facilitate or impair task performance. In the typical Stroop task, participants are instructed to report the physical color of a written color word (e.g., “RED”), while ignoring the semantic meaning of the word. In cases where the physical color of the presentation is congruent with the semantic meaning of the word, participants are both faster and more accurate at reporting the physical color. However, when the physical color differs from the semantic meaning of the word (i.e., is incongruent) participants are slower and more prone to error (see MacLeod 1991
for a review).
Numerous theoretical accounts of Stroop interference have been proposed over the more than 70-year history of this phenomenon. Although early speed-of-processing (“horse race”) models interpreted Stroop effects as resulting from the faster, more “automatic” processing of word information (Dyer 1973
; Posner and Snyder 1975
; Dunbar and MacLeod 1984
), more recent theoretical and computational explanations of Stroop-related interference have tended to model the effects as arising from response competition occurring in a parallel and hierarchical network (Cohen et al. 1990
; Cohen et al. 1992
; Phaf et al. 1990
; Stafford and Gurney 2007
). Under such ‘connectionist’ frameworks, processing is determined by activity spreading throughout pathways of differing strengths, with the response decision ultimately occurring when the output of these pathways crosses a certain threshold (Rumelhart et al. 1986
). According to these views, interference occurs when 2 simultaneously activated pathways produce conflicting activity at their processing intersection, whereas facilitation occurs when the 2 paths produce compatible activation. The intersection of conflicting activity can occur at any phase in the processing hierarchy (e.g., semantic evaluation or response selection) following sensory processing, where the pathways rely upon a common set of processing resources, a notion that has been called the “multiple-resource” view (Allport 1982
; Hirst and Kalmar 1987
; Cohen et al. 1990
One key piece of evidence that has argued against a simple speed-of-processing account came from behavioral experiments in which the color and word components of the Stroop stimulus were presented with varying stimulus onset asynchronies (SOAs) (Dyer 1971
; Glaser and Glaser 1982
; Glaser and Dungelhoff 1984
; Glaser and Glaser 1989
; Sugg and McDonald 1994
). In these experiments, the task-relevant stimulus component could be preceded or followed by presentation of the task-irrelevant component with SOAs typically ranging from −400 to +400 ms. If interference in the Stroop task was due to word meaning being processed faster than the physical color, then presenting the color information earlier should give its processing a sufficient head start to eliminate interference from the word-meaning information. Similarly, if the participant's task was to report the word name, and the physical-color information was presented early enough, it should be possible to elicit a robust “reverse Stroop” effect in which the processing of the color information would temporally coincide with the processing of the word, therefore creating interference in the naming of the word. Neither of these types of results are typically observed, however (MacLeod 1991
; MacLeod and MacDonald 2000
). Thus, although pre-exposure to a task-irrelevant color component did not have an effect on naming the word, pre-exposure of the task-irrelevant word did have a substantial effect on naming the physical color. Moreover, such interference was observed even if the word was presented up to 100 ms after the physical color. These SOA manipulations, and resultant data patterns, have called into question the idea that interference arises strictly because words are processed faster than color, suggesting rather that interference is due to interactions that alter the strength of activation patterns in a distributed and parallel network.
Temporal Relationships in the Stroop SOA Task—Priming and Backward Influences
As illustrated by the Stroop SOA manipulations, successful goal-oriented behavior involves the filtering of task-irrelevant information, especially when it is conflicting or distracting in some way. It is also well appreciated, however, that the temporal relationship between the components of visual stimuli greatly influences the processing and perception of those stimuli.
Priming reflects one such category of stimulus–stimulus temporal interactions in which processing of a target stimulus is altered when it is preceded by a meaningfully related “prime” stimulus. These automatic (or implicit) effects can occur either on the basis of perceptual features of the stimulus, such as color (Marcel 1983
) or motion (Jiang et al. 2002
), or on the basis of semantic aspects of the stimuli (reviewed in Neely (1991)
, even in the absence of conscious awareness (Marcel 1983
). Although priming is most typically associated with enhanced processing of a stimulus due to the occurrence of a previous stimulus, it has also been shown that there are types of priming that can exert negative, inhibitory influences (Tipper 2001
In contrast, when a target is followed in time by the subsequent presentation of an irrelevant distractor, backward influences may occur (reviewed in Enns and Di Lollo 2000
). Although these influences can also in theory act to facilitate or inhibit target processing, they are most commonly demonstrated as the relative reduction in perceptibility of a target when information is lost because of interference by a subsequently presented stimulus. Such ‘backward masking’ is generally believed to be a precategorical process that depends entirely on the sensory aspects of the 2 inputs and not on lexical or semantic factors. Because Stroop SOA variants, such as the one used in the present study, reflect processing interactions that may be either facilitory or inhibitory in nature, we refer more generally to instances in which the irrelevant stimulus component is presented prior to the relevant target as “priming influences” and instances when the irrelevant stimulus component comes after the target as “backward influences.”
ERPs as a Measure of Stimulus Conflict and Semantic Processing
Event-related potentials (ERPs) provide a measure of brain dynamics with high temporal resolution, allowing researchers to characterize the cascade of processes that behavioral measures such as reaction time cannot offer. ERPs therefore constitute a quantitative measure optimally suited for delineating the nature of cognitive interference effects, such as those elicited by the Stroop task.
Stroop-like and priming influences share similarities in that both relate to biasing in perceptual systems; therefore, they have sometimes been described in similar cognitive and mechanistic terms (MacLeod 1991
; MacLeod and MacDonald 2000
). For example, the N400 ERP, a broad negative ERP wave over central-parietal scalp locations, has been shown to be sensitive to semantic priming effects and accordingly has been often used as a marker of semantic processing (Kutas and Hillyard 1980
; Kutas and Federmeier 2000
). This component is larger for words that are semantically incongruent versus semantically congruent with a preceding priming word or sentence. Because the N400 depends substantially on the temporal separation of the prime and the target (Kiefer and Spitzer 2000
; Kiefer and Brendel 2006
), it is thought to reflect effects on the processing of the target word resulting from the preactivation of semantic representations of words associated with the prime. Although functional similarities between the semantic N400 component and the Stroop-evoked negativity have been noted, it is thought that the Stroop response reflects interference interactions amongst more general central-executive control processes rather than more specifically semantic incongruency effects (West 2003
; West et al. 2004
; Hanslmayr et al. 2008
). Nonetheless, the observation that the N400 is sensitive to the temporal relationship between stimulus components suggests the utility of similar SOA manipulations on the Stroop interference ERP effect.
The goal of the present study was to investigate the temporal sensitivity of brain processes that detect and resolve stimulus conflict using modified versions of the classic Stroop paradigm. In separate experimental sessions, reaction times and error rates were collected with and without concurrently recorded ERPs as subjects reported the physical color of the stimulus. In these tasks, stimuli were presented with 5 levels of SOA, in which the relative timing of the physical-color and color-word components of the stimuli were varied from trial-to-trial. This approach of presenting the task-relevant stimulus component first (“relevant-first”) or the task-irrelevant component first (“irrelevant-first”) allowed us to examine the influence of pre- and postexposure of congruent versus incongruent information on both behavior and brain activity.
In our main experimental session we assess the behavioral and neural responses elicited by stimulus incongruency by considering the ERP difference waves produced by subtracting congruent from incongruent trials, and we then relate these “incongruency difference waves” at the different SOAs to behavioral performance on this task. We explicitly focus our ERP analyses here on the incongruency difference waves, because the SOA manipulation utilized in these experiments introduces differential amounts of overlap in the ERP record depending on the temporal separation between stimulus components (Woldorff 1993). As this overlap is equivalent for the congruent and incongruent stimuli within each SOA condition, the difference wave isolates processes related to the Stroop stimulus incongruency and serves as a principled ERP marker for assessing interactions between the SOA and the neural processing related to the conflict processing interactions. In an additional behavioral control study, we evaluate the role of both facilitation and inhibition by comparing reaction times and error rates for compatible and incompatible color-word pairings in relation to a task-neutral control condition.
In theory, these SOA manipulations could have resulted in several outcomes relating to behavioral performance and/or the amplitude or latency of the Stroop ERP effects. For example, based on the common observation that mainly the amplitude, and not the latency, of the language-related N400 component is modulated by the strength of the prime-target semantic relationship (Kutas and Federmeier 2000
), our SOA manipulation might only manifest as amplitude changes in the ERP incongruency effects. Alternatively, the pretarget stimulus may serve to alter the temporal characteristics of the processing of the upcoming target, in which case the response latency of the ERP incongruency effects may also be influenced. Nonetheless, in line with the priming and backwards influences discussed above, we would expect that pretarget exposure of the irrelevant stimulus is likely to result in the largest and earliest incongruency effects in relation to simultaneous presentation, and that the effects of post-target exposure to the irrelevant stimulus is likely to be relatively diminished and delayed.