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Cross-Modal Tasks

Cross-modal Tasks are a type of choice reaction time task that use structural overlap between stimulus dimensions of different sensory modalities. As a result, there is dimensional overlap between the irrelevant stimulus and the relevant stimulus, but no dimensional overlap with the responses. In the dimensional overlap taxonomy, these are considered to be Type 4 tasks. Other Type 4 tasks include the Flanker task and the Stroop-like task.

Robert Melara and Lawrence Marks (1990; Marks, 1987; Melara, 1989) are two of the first to study stimulus-stimulus consistency effects using cross-modal tasks. In a typical cross-modal task, subjects are told to press a left key or a right key depending on the brightness of a stimulus light. In conjunction with the stimulus light, subjects hear a tone of either a high or a low pitch, or a loud or a soft volume. The stimulus dimensions of brightness, pitch, and volume are not perceptually similar, and no specific level of brightness has an associative link with any particular pitch or loudness of sound. However, all of these dimensions can be rank-ordered from “low” to “high” along a continuum. As a result, the relationship between the elements of the relevant stimulus set (high and low brightness) and the relationship between the elements of the irrelevant stimulus set (high or low pitch or volume) allows them to be paired. Thus, although the tone is irrelevant to the task, it can be either consistent (e.g. a bright light paired with a loud sound) or inconsistent (e.g. a bright light paired with a soft sound) with the light. Responses are faster and more accurate for consistent stimuli than inconsistent stimuli.

Cross-Modal Task

This type of task validates the generality of the dimensional overlap theory. If the stimulus-stimulus consistency effect only appeared in Flanker tasks, a theory could be proposed that the effect was caused directly by a perceptual mechanism. If the stimulus-stimulus consistency effect only appeared in Stroop-like tasks, a theory could be put forth that was based on individual learned associations (such as the learned association between a color and its color word). The fact that the effect appears in cross-modal tasks, however, demonstrates that it is truly the dimensional overlap–in the most abstract sense–between the two stimulus dimensions that is key to the appearance of the consistency effect.

 
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The Stroop-like Task

The Stroop-like Task is illustrated by a choice reaction time task that uses color-word stimuli written in colored ink, the ink  is usually the relevant stimulus, but assigns them to key-press responses. As a result, there is dimensional overlap between the irrelevant and the relevant stimulus, but no dimensional overlap with the responses. In the dimensional overlap taxonomy, it is considered a Type 4 task. Other Type 4 tasks include the Flanker task and Cross-Modal tasks.

A number of researchers have explored performance in Stroop-like tasks (e.g. Hommel, 1998, exp. 1; Kahneman & Henick, 1981; Keele, 1972; Kornblum, 1994; Kornblum et al., 1999; Simon & Berbaum, 1990; Simon, Paullin, Overmyer & Berbaum, 1985, exp. 2). Typically, subjects are told to press a left key or a right key depending on the color of the stimulus. The color word is irrelevant, but can be either consistent (e.g. “blue” written in blue ink) or inconsistent (e.g. “green” written in blue ink) with the color. Responses are faster and more accurate for consistent stimuli than for inconsistent stimuli.

Stroop-like task

Unlike in the Flanker task, where the relevant and irrelevant stimuli are perceptually similar, this task only contains conceptual overlap between the two stimulus sets: color and color words are only linked through a learned symbolic association.

This task is called “Stroop-like” because the stimuli used in the task are the same as the stimuli first used by Stroop (1935) for the Stroop task. However, these tasks are different from actual Stroop tasks, because in Stroop tasks there is also overlap between the stimulus dimensions and the response dimension. You will sometimes see papers describing these tasks as “Stroop tasks”, disregarding the fact that the responses in these tasks have no overlap with color. According to Dimensional Overlap model, this difference is critical: cognitive processing in Stroop-like tasks and Stroop tasks is fundamentally different, based on the presence or absence of dimensional overlap with the response dimension.

 
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The Flanker Task

The Flanker Task, also sometimes called a Eriksen Task, is a choice reaction time task where there is dimensional overlap between the irrelevant stimulus and the relevant stimulus. In the dimensional overlap taxonomy, it is considered a Type 4 task. Other Type 4 tasks include the Stroop-like task and Cross-Modal tasks.

Charles Eriksen (Eriksen & Eriksen, 1974; see also Eriksen, 1995; Eriksen & Schultz, 1979; Cohen & Shoup, 1993, 1997; Miller, 1982, 1991) studied the effects of distracting “flanker” stimuli that appear around or near a relevant target stimulus. In a typical Eriksen task, subjects are shown a string of letters on a screen, and are told to press a left key or a right key depending on what letter appears in the center of the screen (the target letter). The surrounding flanker letters are irrelevant, but can be either consistent (“HHH”) or inconsistent (“SHS”) with the target. Responses are faster and more accurate for consistent stimuli than for inconsistent stimuli. The difference in reaction time is called the Eriksen effect or the flanker effect.

Flanker Task

Targets and flankers are both defined as values along the same dimension: in the example above, it is letter. As a result, the relevant and irrelevant stimulus have perceptual and conceptual overlap. Moreover, because the overlap is between stimulus properties, consistency is a property of the stimuli themselves, and is independent of the mapping instructions. However, because different letters are usually assigned to different responses, a confound arises in most tasks: when the stimulus is S-S consistent, the response assigned to the flankers is different from the response assigned to the target. This confound is central to the debate about different explanations of the S-S consistency effect.

Eriksen tasks permit a large number of variations, while still producing the same effect. For example, although flankers and targets are generally letters, they can also be words or shapes or symbols (e.g. Hommel, 1995; Shaffer & LaBerge, 1979; Zhang & Kornblum, 1998). Also,  although flankers are usually presented to the left and right of the target, they can also be presented above or below the target, or in other patterns around it (e.g. Eriksen & Hoffman, 1973; Eriksen & St. James, 1986; Eriksen & Murphy, 1987; Yantis & Johnston, 1990; Zhang & Kornblum, 1998).

The key factor that all flanker tasks have in common is the perceptual dimensional overlap between the irrelevant stimulus and the relevant stimulus.


NOTE: There are some times when you will see people use the term “Flanker task” to refer to tasks where left and right arrows are assigned to left and right key-presses, and each target arrow is flanked by irrelevant flanker arrows that are either consistent or inconsistent with the target arrow. In the Dimensional Overlap taxonomy, this is not a Type 4 task: it is a Type 8 task, like the Stroop task. Even though this task has flankers, the dimensional overlap model contends that cognitive processing in this task is fundamentally different from processing in the standard Eriksen task, because it adds overlap between the stimulus dimensions and the response dimension.

 
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Debate: What is the locus of the S-S Overlap Effect?

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