Elsevier

Acta Psychologica

Volume 130, Issue 1, January 2009, Pages 95-102
Acta Psychologica

Reducing and restoring stimulus–response compatibility effects by decreasing the discriminability of location words

https://doi.org/10.1016/j.actpsy.2008.10.005Get rights and content

Abstract

In two experiments, we compared level of activation and temporal overlap accounts of compatibility effects in the Simon task by reducing the discriminability of spatial and non-spatial features of a target location word. Participants made keypress responses to the non-spatial or spatial feature of centrally presented location words. The discriminability of the spatial feature of the word (Experiment 1), or of both the spatial and non-spatial feature (Experiment 2), was manipulated. When the spatial feature of the word was task-irrelevant, lowering the discriminability of this feature reduced the compatibility effect. The compatibility effect was restored when the discriminability of both the task-relevant and task-irrelevant features were reduced together. Results provide further evidence for the temporal overlap account of compatibility effects. Furthermore, compatibility effects when the spatial information was task-relevant and those when the spatial information was task-irrelevant were moderately correlated with each other, suggesting a common underlying mechanism in both versions.

Introduction

Responses to objects in the environment are faster and more accurate when the responses match features of those objects. For example, subjects are faster at responding to a left stimulus with a left response and a right stimulus with a right response than the reverse (Simon & Rudell, 1967). This preference for the corresponding stimulus–response (S–R) mappings is broadly called the spatial S–R compatibility effect (Proctor & Vu, 2006). The compatibility effect is commonly viewed in terms of a dual-process model in which voluntary, goal-oriented response activation competes with an environmentally, or exogenously, activated response that corresponds to the features of the task stimulus (e.g., Kornblum, Hasbroucq, & Osman, 1990). Accordingly, to at least some degree, compatibility effects represent how much a feature of the environment automatically activates a response that conflicts with the production of the correct, task-relevant response. Responses are slower and less accurate when these two activated responses are in conflict than when they both lead to the same response. A variety of stimulus features convey information that may lead to an automatic response activation. In addition to the location of a stimulus, responses may be activated by conceptual information such as the spatial words “LEFT” and “RIGHT” or left- and right-pointing arrows (Lu and Proctor, 1994, Proctor and Wang, 1997; Weeks & Proctor, 1990). Therefore, it is of great interest to understand the determinants of the timing and magnitude of automatic response activation, and how this response activation may be modulated.

Fitts and Deininger (1954) were the first to formally describe the compatibility effect, in which participants are faster at responding to a spatial feature of a stimulus when this feature and the response closely correspond to each other. Deininger and Fitts (1955) subsequently explained this result as an issue of S–R translation: responses are slower when they do not match stimulus features than when they match because additional processing steps are required to translate the presented stimulus into a response. Within Kornblum et al.’s (1990) taxonomy of compatibility effects, this type of compatibility effect, based on the mapping of a task-relevant stimulus dimension to responses, is classified as a type 2 Ensemble. We will call tasks that involve type 2 Ensembles S–R Compatibility (SRC) tasks. Kornblum et al. emphasized the role of automatic activation in addition to S–R translation as a primary cause of SRC effects. The extent to which the SRC effect reflects intentional translation versus automatic activation remains a subject of some debate (Proctor and Wang, 1997, Wascher et al., 2001).

A compatibility effect also occurs when there is a dimensional overlap between a task-irrelevant stimulus feature and the task-relevant response (Ensemble 3 tasks in Kornblum et al.’s, 1990, taxonomy). The most commonly described Ensemble 3 task is the Simon task, in which the task-relevant stimulus dimension is unrelated to the response, but some irrelevant feature of the stimulus shares features with the response. For example, when participants must make a spatial response based on the color of a light that is presented on the left or right side of the screen, response times (RTs) are shorter when the light’s location matches that of the response. Since the Simon effect originates in a task-irrelevant stimulus feature that can be ignored during task performance, the processing of this feature is often described as occurring automatically.

It is likely that to some degree compatibility effects in the SRC and Simon tasks share a common cause. In both tasks, the compatibility effect is always contingent on the similarity between stimulus and response features, regardless of the relevance of the overlapping stimulus feature to task performance (Kornblum, 1992, Kornblum et al., 1990). Furthermore, transfer effects have been found when subjects practice an SRC task prior to performing a Simon task (Proctor and Lu, 1999, Proctor et al., 2007, Tagliabue et al., 2000). For example, Proctor and Lu (1999) found that the predisposition to respond in a compatible manner to the irrelevant spatial information in the visual Simon task was reversed following 912 trials of responding in a non-corresponding way to the same spatial information in the SRC task.

There are also differences in both tasks. Attending to the spatial feature of the task stimulus is compulsory in the SRC task and is not required in the Simon task. Furthermore, when using the same task stimuli and responses, there is a difference in the magnitude of compatibility effects in the SRC and Simon tasks, namely compatibility effects are larger in the SRC task (50–70 ms or higher) than in the Simon task (around 25 ms; see Umiltà and Nicoletti (1990)). Finally, although Proctor and Lu (1999) found a transfer effect from the SRC task to the Simon task, no transfer was found in the reverse direction – practice on the Simon task did not influence compatibility effects in a subsequently performed SRC task. Thus, it is unclear as to whether compatibility effects in the SRC and Simon tasks reflect the same cognitive mechanism or are at least partially the result of separate mechanisms.

Since the magnitude of the compatibility effect changes depending on the similarity between the stimulus and response sets in a task, it is likely that the automatic response activation elicited by a stimulus feature also varies in its level of activation. Indeed, the observation that automatic response activation dissipates over time in the standard spatial location Simon task (Hommel, 1993, Hommel, 1994) implies that this activation may vary in magnitude. As proposed by Kornblum (1992), the amount of response activation caused by a stimulus is positively related to the degree of feature overlap between the stimulus and response. Kornblum, 1992, Kornblum and Lee, 1995 described three ways that stimulus and response features may overlap. Conceptual overlap occurs when stimulus and response sets share overlapping conceptual information, such as a left or right response to the words “left” and “right.” Physical or “mode” overlap (Lu & Proctor, 2001) takes place when stimuli and responses are related to the same modality. For example, when task stimuli are auditory words, mode overlap is greater when responses are spoken words than when they are button presses. Structural overlap occurs, for example, when a saliently ordered set of stimuli such as the numbers “1, 2, 3, 4…” is mapped onto an ordered set of responses such as speaking the letters “A, B, C, D.” Responses are fastest when these two groups match (“A” with “1”, “B” with “2”, etc.) than when they mismatch. In each of these cases, a relationship between features of the task stimuli and responses exists that leads to the activation of the response that best matches the presented stimulus; the stronger the relationship between the stimulus and response, the larger the predicted compatibility effect.

The processing of irrelevant information within the Simon task thus represents both the amount of overall stimulus-based response activation and the temporal proximity of this activation to the point of response selection. In the left–right location version of the Simon task, the compatibility effect will be largest when stimulus-based response activation is strong and peaks immediately prior to response selection. If automatic activation occurs too early, it will dissipate before the critical interference stage prior to response selection (De Jong et al., 1994, Hommel, 1993, Umiltà and Liotti, 1987). If onset of the automatic activation occurs after response selection but before response execution, then this activation may not be sufficiently developed to lead to interference for the fastest responses (Hommel, 1995, Hommel, 1996, Lu and Proctor, 1994). The latter case is found in the Stroop task, a paradigm similar to the Simon task in which the goal relevant and irrelevant features of a stimulus promote different response tendencies. Palef and Olson (1975) proposed that the automatic activation of an irrelevant stimulus dimension only occurs if a response has not yet been selected based on the relevant dimension. Therefore, if presentation of the irrelevant stimulus feature is sufficiently delayed, it no longer affects response behavior.

The degree of similarity between stimulus and response features and the timing of stimulus processing are not the only determinants of the size of compatibility effects. Although compatibility effects are generally considered the result of automatic activation of the response that best corresponds to the features of the task stimulus, the magnitude of this automatic response activation may also be related to the ease with which task stimulus features can be identified, which we will refer to as their discriminability. Numerous variations of the SRC and Simon tasks have been used over the last 50 years, but only a handful of studies have manipulated the quality of the stimulus within the task, and the effect that this has on compatibility effects remains unclear. One possibility is that reducing the discriminability of a stimulus may harm overall RT and accuracy, but not influence compatibility effects, since stimulus degradation influences the perceptual processing stage and compatibility effects originate in the later response-selection stage of task performance (Adam, 2000, Christensen et al., 1996, Frowein and Sanders, 1978, Simon, 1982, Stoffels et al., 1985). This research shows that stimulus quality and compatibility effects are largely independent from each other, commonly leading to additive effects on RTs. For example, Adam (2000) presented participants with an SRC task in which participants made left or right responses based on whether an “X” target appeared on either the left or right side of the screen. The discriminability of the target’s location was manipulated by varying the distance of the targets from the center of the screen; the greater this distance, the more distinguishable the target location. RTs were shorter overall when the target location was easier to discriminate, but this did not influence the magnitude of the compatibility effect.

Christensen et al. (1996) found a similar result when they examined the influence of stimulus discriminability on compatibility effects elicited by centrally presented location words by adding a white noise overlay over target word stimuli. Left or right responses were made to the words “LEFT” or “RIGHT” written in either upper- or lower-case letters. The response criterion was the word-meaning, the case of the word, or both. In the framework of the current study, the word-meaning response and case response tasks were the versions of SRC and Simon tasks, respectively. Christensen et al.’s results showed that reducing the general visual quality of the target did not significantly influence the compatibility effect in the SRC version of the task. The compatibility effect was slightly reduced by lowering stimulus discriminability in the Simon task (from 9 ms to −3 ms), but the effect was very small and was not significantly different from zero in either condition.

However, there is also an evidence that automatic response activation is influenced when specific stimulus features are made less discriminable. There are two ways in which this may influence compatibility effects. First, lowering the discriminability of stimulus information may reduce the overall strength of automatic response activation and therefore lead to a diminished influence on response behavior. Second, reducing discriminability may not change the overall amount of automatic response activation but rather increase the duration of time required for this activation to occur. Evidence for this delay is apparent in both increasing RTs and neural activation associated with stimulus identification (P300 latencies) when the quality of a target stimulus is degraded (Pfefferbaum, Christensen, Ford, & Kopell, 1986). This delay likely reflects an increase in the duration of stimulus processing, which postpones the onset of response selection (Christensen et al., 1996, McCarthy and Donchin, 1981). Since automatic activation of the response corresponding to the target stimulus occurs quickly and then decays, a delay in the selection of the task-relevant response leads to a decrease in the amount of temporal overlap of the selected response and the automatically activated response, and therefore less response interference and smaller compatibility effects (Hommel, 1993). Conversely, if automatic activation of the corresponding response is delayed, then this activation may not be fully developed when the task-relevant response is selected, and once again there is a less overlap between responses and smaller compatibility effects. It is important to note that the overall level of automatic response activation and the amount of overlap between this activated response and activation of the task-relevant response may occur separately and both contribute to the magnitude of compatibility effects. A goal of the current article is to distinguish which of these factors, if either, is at play when the visual discriminability of a task stimulus is reduced.

Previous studies have utilized various forms of degradation of the relevant feature (Hommel, 1993) or both the relevant and irrelevant features of the task stimulus together in order to reduce their discriminability (Christensen et al., 1996). To form a clearer picture of the role of visual stimulus quality in compatibility effects, it is necessary to independently vary the discriminability of stimulus features along with their task relevance. Additionally, since the current experiments examine compatibility effects when the spatial feature of the stimuli is both task-relevant in the SRC task and task-irrelevant in the Simon task, it is possible to directly compare the magnitude of the compatibility effect in both cases for each participant. Although Kornblum (1992) suggested that the mechanism underlying SRC and Simon task compatibility effects is the same, with-participant correlations between the two have yet to be examined. If the sizes of the compatibility effects in the SRC and Simon tasks are strongly correlated with one another, it is likely that they share a common underlying cause.

In the current experiments, participants performed a choice response task in which spatial information conferred by the task stimulus was either relevant (SRC task) or irrelevant (Simon task) to selecting the appropriate response. Additionally, we changed the amount of stimulus processing that occurs by varying the visual quality of the spatial feature of the stimulus (Experiment 1) or both the spatial and non-spatial stimulus features (Experiment 2). The use of both the SRC and Simon tasks allows for a check of this visual discriminability manipulation. When the spatial feature has low discriminability, overall RTs should increase for the SRC task but not for the Simon task since identifying this feature is a response requirement in the SRC task but not in the Simon task. When both the spatial and non-spatial features have low discriminability, overall RTs should increase for both the SRC and Simon tasks. Additionally, the error rate in the SRC task serves as an indicator of the legibility of the word. The goal of the discriminability manipulation is to reduce word legibility but not to make the word illegible. Therefore, low discriminability of the location word should increase the time required to read the word but not the success in identifying the word, as measured by the percentage of errors.

There are several reasons why spatial information in the SRC and Simon tasks in the current experiments is conferred by centrally presented location words rather than the location of a target stimulus. First, information about the side of a stimulus is difficult to make less distinct, since categorizing the location (left versus right) occurs in reference to other stimulus locations. Thus, relative stimulus locations that are close together are as easy to categorize as locations that are further apart. Furthermore, increasing the distance between two response locations also increases the retinal eccentricity of the stimulus, leading to an increased difficulty in processing both the task-relevant and irrelevant stimulus features (Hommel, 1993). Since retinal eccentricity and spatial location are confounded with each other, it is difficult to make the location of a stimulus more discriminable without also reducing the visual quality of other features. Location words avoid this problem since spatial information is conferred by the meaning of the word without the need to vary the location of the target stimulus.

Section snippets

Experiment 1

The primary purpose of Experiment 1 was to determine whether lowering the discriminability, or legibility, of a location word by including a black-and-white noise overlay reduces the compatibility effect in the Simon task. In the SRC task, the word direction is required to perform the task, whereas in the Simon task, the word direction is irrelevant for selecting the correct response. Consequently, in the SRC task, reducing the discriminability of the spatial feature should cause an increase in

Experiment 2

Results so far found a reduction of the compatibility effect in the verbal Simon task when the discriminability of the spatial feature was reduced. In Experiment 2, both the spatial and non-spatial features of the task stimulus were reduced in discriminability. There are two likely outcomes in this case. The compatibility effect may be reduced whenever the discriminability of the spatial feature is lowered. In this scenario, lowering the discriminability of the spatial feature also reduces the

General discussion

The goal of the current research was to examine the influence of discriminability of the task-relevant and irrelevant stimulus features on the compatibility effect in location word versions of the Simon task. Three main conclusions can be drawn from the current results. First, reducing the discriminability of the irrelevant stimulus feature delayed automatic response activation until after response selection had taken place, as evidenced by the non-significant Simon task compatibility effect

Conclusion

The current experiments provide a detailed depiction of the influence of visual stimulus discriminability on compatibility effects in the SRC and Simon tasks. Together, the results of each of the current experiments support the temporal overlap account of compatibility effects and extend it to include irrelevant spatial information conveyed by location words. We show that reducing the visual discriminability of irrelevant spatial information in word stimuli delays the automatic activation of a

Acknowledgements

The research described in the present paper was supported in part by NIH Grant #0503001759.

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