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The Stroop Task

The Stroop Task is a choice reaction time task where there is dimensional overlap between all three dimensions of the task: the relevant stimulus, the irrelevant stimulus, and the response.  In the dimensional overlap taxonomy, it is considered a Type 8 task.

Stroop (1935; see also Dyer, 1973; Lu & Proctor, 1995; MacLeod, 1991) studied the effects of an irrelevant color word in a color-naming task. In the simplest and most well-known version of the Stroop task, subjects are told to name the color of a stimulus (e.g. say the word “blue” when they see a blue stimulus). Along with the color, however, a color word (e.g. “blue” or “green”) is also presented. The color word can be either presented in the colors, or super-imposed on a colored background (see MacLeod, 1998). The color word is irrelevant, but it can be either consistent with the stimulus color and the response name (e.g. the word “blue” written in blue ink, requiring a response of “blue”) or inconsistent with the stimulus color and the response name (e.g. the word “green” written in blue ink, requiring a response of “blue”).

 

A simple Stroop task

 

Performance is slower and less accurate when the irrelevant stimulus word does not match the relevant stimulus and response, and faster and more accurate when it matches the relevant stimulus and response. The difference in reaction time is usually called the Stroop effect.

Stroop tasks permit a great deal of variation while still yielding the same basic consistency effect. This basic Stroop effect has been found using shapes and shape names (e.g. Irwin, 1978; Redding & Gerjets, 1977; Shor, 1971), pictures and picture names (e.g. Babbit, 1982; Dunbar, 1986; Rayner & Springer, 1986; Toma & Tsao, 1985) and spatial locations and their labels (e.g. Baldo, Shimamura, & Prinzmetal, 1998; Lu & Proctor, 1995; Palef, 1978; Seymour, 1973; Virzi & Egeth, 1985). Auditory Stroop tasks have also been studied, with typical stimuli being the words “left” and “right” spoken in the left and right ear (e.g. Green & Barber, 1981; McClain, 1983; Proctor & Pick, 1998; Ragot & Piori, 1994; Simon & Rudell, 1967). Although Stroop tasks have most commonly used conceptual dimensional overlap between dimensions (e.g. the learned associations between colors and their names) they have also used perceptual overlap (e.g. flanker stimuli) (e.g. Glaser & Glaser, 1982, 1989; Zhang & Kornblum, 1998).

In this standard version of the task, however, there is a problem: there is no way to determine whether this overall difference in reaction time is due to the S-S consistency or the irrelevant S-R consistency, or a combination of both.

Because there are three kinds of dimensional overlap in a Type 8 task, there are in principle three different compatibility effects that could appear: a mapping (relevant S-R) effect, an irrelevant S-R consistency effect, and an S-S consistency effect.

Zhang and Kornblum (1998) were able to measure each of these effects independently by constructing a four-choice Stroop task that included both a congruent and incongruent mapping. In this task, subjects responded to one of four words (“red”, “green”, “blue”, “yellow”) by saying one of four words from the same list. Above and below the target word appeared another irrelevant word from the same list. In the incongruent mapping condition, the words that appeared above and below the target could be either S-S inconsistent and S-R consistent (e.g. “blue” flanked by “blue” requiring a response of “green”), S-S inconsistent and S-R consistent (e.g. “blue” flanked by “green” requiring a response of “green”), or both S-S and S-R inconsistent (e.g. “blue” flanked by “red” requiring a response of “green”).

 

complete Stroop task and effects

 

When the mapping was incongruent, the reaction times were fastest for S-S consistent and S-R inconsistent flankers, intermediate for S-S inconsistent and S-R consistent flankers, and slowest for S-S and S-R inconsistent flankers. This demonstrates that both S-S consistency and S-R consistency contribute independent and separately measurable effects in the Stroop task, as predicted by the dimensional overlap model.

 

 

 
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The SS x SR Task

The SS x SR Task, or SS x SR Factorial-Combination Task, is a choice reaction time task with two different irrelevant stimulus dimensions, where one has dimensional overlap with the relevant stimulus and the other has dimensional overlap with the response. This allows an examination of both an S-S consistency effect and an irrelevant S-R consistency effect in the same task. In the dimensional overlap taxonomy, it is considered a Type 7 task.

Stoffels and van der Molen (1988) were the first to explore a factorial combination task, by integrating an auditory Simon task with a Flanker task in the same experimental design. Subjects responded to the letter “H” or the letter “S” with a left or right key-press. These targets were presented with either “H” or “S” flankers on both sides, and an auditory tone presented in either the left or the right ear. The flankers could be S-S consistent or S-S inconsistent, while the tone could be either S-R consistent or S-R inconsistent. The S-S and S-R consistency had additive effects: the S-S consistency effect was the same regardless of S-R consistency, and the S-R consistency effect was the same regardless of S-S consistency.

A number of studies have also combined a visual Simon task with the Stroop-like task (Simon & Berbaum, 1990; Kornblum, 1994; Hommel, 1998; Kornblum et al., 1999). In these studies, subjects were told to respond to a stimulus color with a left or right key-press. The stimulus color was presented on the left or the right side of the display, either with a color word super-imposed on a colored rectangle or with the color itself spelling out the color word. Both the position of the color and the color word are irrelevant. Several of these experiments showed additive effects of S-S and S-R consistency.

Factorial Combination Task

These tasks have played an important role in the debate over the locus of the S-S consistency effect in cognitive processing. Specifically, Kornblum et al. (1999) used the computational dimensional overlap model to demonstrate that both the additivity and the different time-courses of S-S and S-R effects can be explained by assuming that stimulus and response processing occur in discrete stages and that S-S and S-R dimensional overlap impact different processing stages.

 

 
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The Hedge and Marsh Task

The Hedge and Marsh Task, also sometimes called the Reverse-Simon Task, is a choice reaction time task where there is dimensional overlap between the irrelevant stimulus and the response and between the relevant stimulus and the response, but there is no dimensional overlap between the two stimulus dimensions. This is accomplished by having two separate but correlated response dimensions. In the dimensional overlap taxonomy, it is considered a Type 5 task.

Hedge and Marsh (1975) examined a variation of the standard Simon task in which subject pressed a left key or a right key in response to the color of a stimulus (red or green), while the stimulus color appeared on the left or right side of the screen; in addition, however, they color-coded the response keys so that the response set not only had dimensional overlap with the irrelevant stimulus set (due to left-right position) but also with the relevant stimulus set (due to color). In addition to S-R consistency, therefore, the instructions given to subjects could be either congruent (e.g. “press the key with the same color as the stimulus”) or incongruent (“press the key with the opposite color from the stimulus”). There is no dimensional overlap between the relevant stimulus set (color) and the irrelevant stimulus set (location), and therefore there is no S-S consistency.

Hedge and Marsh Task

Because the Hedge and Marsh task has two types of overlap, there are two compatibility effects that can be obsereved.  First, the experiment showed a standard mapping effect: overall reaction times were much faster wth compatible (i.e congruent) mapping, i.e. when  each color stimulus was mapped to a correspondingly colored response (e.g. “press the key with the same color as the stimulus”) than when the mapping was incompatible, that is incongruent  (e.g. “press the key with the opposite color from the stimulus”).

The effect of irrelevant stimulus-response consistency, however, was unusual. Hedge and Marsh found the surprising result that the irrelevant S-R consistency effect reverses when the mapping instructions are incompatible. That is, when the instruction were to press the key with the same color as the stimulus, the subjects responded faster when the stimulus position was consistent with the response position (the normal S-R consistency effect); however, when the instructions were to press the key with the opposite color from the stimulus, subjects responded faster when the stimulus position was inconsistent with the response key position (a reversed S-R consistency effect). This effect of the irrelevant stimulus in a Hedge and Marsh task is called a Hedge and Marsh effect or a reverse-Simon effect.

The reverse-Simon effect was met with some initial skepticism. Simon, Sly and Vilapakkam (1981) suggested that the result might just be an artifact of the way that the display and the response keys were arranged: if the left key is green and the right key is red, then a display that shows a green patch on the left side or a red patch on the right side matches the arrangement of the keys. On the other hand, a display that shows a red patch on the left and a green patch on the right does not. They called this display-control correspondence, suggesting  that people reach faster when the displays and controls correspond than when they do not. This would give the appearance of a reverse irrelevant S-R consistency effect.

However, Kornblum and Stevens (1999) replicated the conditions for the Hedge and Marsh (1975) task with auditory stimuli and verbal responses. Stimuli were the words “boot” and “gate” spoken at either a high pitch or a low pitch. Subjects had to respond to either the pitch (making the word irrelevant) or the word (making the pitch irrelevant), by making one of two responses: saying the word “boot” at a high pitch or the word “gate” at a low pitch. This task has the same structure and dimensional overlap as the standard Hedge and Marsh task, making it a Type 5 task in the Dimensional Overlap Taxonomy. However, there was naturally no display and there were no controls in the experiment, and therefore there could be no display-control correspondence. Despite the fact that this task had no display or controls, both the regular S-R consistency effect (for compatible mappings) and the reversed S-R consistency effect (for incompatible mappings) were obtained. (See also DeJong, Liang, & Lauber, 1994, for another attempt to remove display-control correspondence from the task.)

Despite initial suspicions, the reverse S-R consistency effect has also been replicated under a wide variety of conditions and manipulations (e.g. Berbner, 1979; DeJong, Liang & Lauber, 1994; Hasbroucq & Guiard, 1991; Lu & Proctor, 1994; Smith & Brebner, 1983; Zhang, 1999). A number of explanations have been suggested to account for this reversal of the S-R consistency effect in the Hedge & Marsh task, although it is still a matter of much debate.

It should be pointed out that a number of studies have incorrectly claimed that they replicate the Hedge and Marsh task in the auditory domain (e.g. Arend & Wandmacher, 1987; Proctor & Pick, 1999;Ragot & Guiard, 1992). In these tasks, the relevant stimuli, irrelevant stimuli, and responses are all characterized by spatial location. For example, the word “left” or “right” might be spoken into the left or the right ear, requiring a response of a left or right key press. In these tasks, all three dimensions overlap with one another, including the two stimulus dimensions, which are both spatial. As a result, these are Type 8 tasks in the dimensional overlap taxonomy and should be considered variants of the Stroop task, not variants of the Hedge and Marsh task.

 


 

NOTE: Sometimes you will see researchers use the term “Simon task” to refer to this task. However, in the Dimensional Overlap taxonomy, the standard Simon task is not a Type 5 task: it is a Type 3 task. Although the standard Simon task does have an irrelevant stimulus dimension (usually location) that overlaps with the response, it does not have any overlap between the relevant stimulus dimension and the response. In a traditional Simon task, the mapping between stimulus and response is completely arbitrary. The dimensional overlap model therefore contends that the cognitive processing in the Simon task and the Hedge and Marsh task are fundamentally different, because of this difference in relevant stimulus-response overlap.

 
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Parallel identification processes

Most models of reaction time based on the formation of mental codes assume not only that separate mental codes are formed for relevant and irrelevant stimulus features, but that the codes are formed in parallel by separate processes. Whenever a stimulus is characterized by more than one dimension (i.e. whenever there is more than one stimulus set), each stimulus dimension can be understood as a separate functional stimulus (Miller, 1988), from the point of view of the perceptual system.

Thus, instead of there being a single stimulus identification process, each dimension of the stimulus is identified by its own separate process, or separate “channel.” These processes can operate completely concurrently, and they do not depend on one another for information. (There is debate, however, as to whether there is “cross-talk” between different stimulus identification processes that are going on at the same time; see, e.g., Egeth, 1977; Estes, 1972, 1982; Mordkoff, 1991; Morton, 1969).

The assumption of multiple identification processes was developed by Eriksen and colleagues (Eriksen, 1966; Eriksen & Lappin, 1965, 1967; Eriksen & Spencer, 1969) for displays with multiple elements, such as flanker stimuli. They suggested that display items that are presented in different spatial locations are identified through separate and independent processes that act in parallel. The idea that the stimulus dimensions of color and word are processed by separate “perceptual analyzers” has also been assumed in even the earliest accounts of performance in the Stroop task (e.g. Morton & Chambers, 1973; Posner & Snyder, 1975). These assumptions were brought together to form the general assumption of separate stimulus identification processes or “channels” (see Egeth, 1977; Miller, 1988): when a stimulus consists of multiple dimensions, they form separate functional stimuli, and are processed by separate stimulus identification processes.

 

 
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Stimulus codes form automatically

One of the assumptions shared by all coding models of reaction time is that mental codes for irrelevant stimuli are formed automatically, even though they are not necessary to carry out a task. Coding models explain consistency effects in terms of a match or a mismatch between this irrelevant stimulus code and one of the mental codes required to perform the task.

This kind of explanation is very general, because mental codes can be about anything. As a result, any kind of irrelevant stimulus can give rise to a consistency effect: letters, words, locations, colors, and so on.  What these models must specify is exactly when and how irrelevant stimulus codes influence the formation of one or more of the mental codes that are required to carry out a task.

Wallace (1971, 1972) first suggested that the S-R consistency effect in a Simon task appears because people automatically, involuntarily form a spatial code, even though the stimulus position is irrelevant. Eriksen and Eriksen (1974; see also Eriksen & Schultz, 1979) similarly suggested that flanker letters in a Flanker task are identified (forming their own letter codes) even though they are known to be irrelevant to the task. This view has been generalized since then to apply to any irrelevant stimulus characteristic, and is an assumption made by all coding models of consistency effects.

Irrelevant stimulus codes form automatically, and influence the formation of other mental codes.  Many coding models assume that mental codes form gradually, and that selective attention will eventually suppress the formation of the irrelevant stimulus code once it is identified as irrelevant. This mechanism of attention produces a rising-then-falling, or inverted U-shaped activation of the irrelevant stimulus code.

 
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Inverted-U activation of irrelevant stimuli

In most connectionist models of reaction time that try to account for the effects of irrelevant stimuli, they model the influence of attentional focus by having the activation of the irrelevant stimulus unit increase at first, and then decrease again, producing a non-monotonic, inverted U-shaped activation function over time (Kornblum, et al., 1999; Lu, 1997).  This activation curve can be implemented in a computational model in a number of ways.  For example, if the input to the irrelevant stimulus is turned off or decreases shortly after it is initially turned on, activation would follow a rising and falling course over time (e.g. Kornblum et al., 1999).  Alternatively, an actual “attentional inhibition” mechanism could be made explicit, where other units become activated in response to irrelevant stimulus unit activation, and these units subsequently inhibit activation in the irrelevant stimulus units (e.g. Houghton & Tipper, 1994).  Regardless of the mechanism, the basic assumption of these models is that irrelevant stimulus unit activation increases and then decreases again over time.

This characteristic of irrelevant stimulus unit activation has been inferred primarily from empirical results that have used stimulus onset asynchrony (SOA), or the relative timing of the relevant and irrelevant stimulus characteristics, to measure how the size of consistency effects changes over time.  All consistency effects seem to follow a non-monotonic, inverted U-shaped time-course, with different effects differing only in the shape and peak of this curve (see Kornblum et al., 1999; Lu, 1997).  Moreover, in connectionist network models, the size of a consistency effect reflects the amount of activation in the irrelevant stimulus unit during processing.

It should be noted that most early models actually do not include this assumption (Cohen, Dunbar, & McClelland, 1990; Cohen & Huston, 1994; Phaf, et al., 1990).  However, these models also do a poor job of accounting for time-course, and are unable to account for the decrease in the size of consistency effects for long SOA values (Cohen & Huston, 1994).  More recent models have either included this assumption from the outset (Kornblum, et al., 1999; Zorzi & Umilta, 1995), or incorporated the assumption at some point later on (Barber & O’Leary, 1997; Zhang, Zhang, & Kornblum, 1999).

 
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