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 across the entire RT distribution in Experiment 1. Hommel (1996)
suggested that in some cases, such as simple RT tasks, response selection occurs before compatibility effects have enough time to develop to an observable level. This view is confirmed by the results of Experiment 2 - when discriminability of both the irrelevant and relevant response features were reduced, the Simon effect was restored to the same size as when there was no stimulus degradation at all. According to the temporal overlap account, degrading the quality of the task-relevant and irrelevant features at the same time delayed processing of both, thereby increasing overall RTs but not changing the amount of overlap between response selection and automatic response activation. Consequently, the size of the compatibility effect remained unchanged. This interpretation is applicable to previous studies that found no change in compatibility effects following the general degradation of task stimuli (Adam, 2002; Christensen et al., 1996
; Hasbroucq, Guiard, & Kornblum, 1989
Although the current data favor a temporal overlap account of compatibility effects, a level of activation account cannot be fully ruled out. There is some evidence that automatic response activation in the Stroop color-naming task, a task that is similar to the Simon task, is sensitive to attention. For example, Kahneman and Chajczyk (1983)
found that the influence of a flanker word is diminished when presented in conjunction with additional distractor words, suggesting that lexical processing is strongly related to attention demands (for an alternative account, see Brown, Roos-Gilbert, & Carr, 1995
). Furthermore, Lien et al. (2008)
found that a secondary task can attenuate the semantic processing of words. Degrading task-irrelevant spatial information in the Simon task may also minimize processing of the spatial information conveyed by a location word and reduce the overall level of automatic response activation, thereby lowering its interference with response selection and eliminating the compatibility effect in Experiment 1. Additionally, the compatibility effect in the low discriminability version of the Simon task remained small across the entire RT distribution, which would be expected if automatic response activation were minimized.
However, an activation account has more difficulty explaining the existence of a compatibility effect in Experiment 2, in which the discriminability of the task-relevant feature was reduced along with the task irrelevant feature. In terms of an overall level of activation account, the degradation of the task-irrelevant spatial information should always lead to a weaker automatic activation of the corresponding response. It is possible that degradation of the task-irrelevant feature in the Simon task may have indeed reduced the overall level of automatic response activation. However, the task-relevant feature now required additional processing in order to select the correct response, and this additional processing led to increased activation of the task-irrelevant feature, thereby restoring its influence on behavior. If this is the case, then the magnitude of response activation is determined by at least two factors: automatic activation of the response by the task-irrelevant stimulus feature and the amount of attention allocated to processing the feature.
Second, we showed that temporal overlap is a factor when spatial information is conferred by location words as well spatial locations. Temporal overlap predicts that compatibility effects will be reduced either when task-irrelevant stimulus information is delayed by reduced visual discriminability and thereby not fully established prior to response selection, or when the task-relevant information is delayed, allowing the activation of spatial information to decay. The latter of these predictions is well established in spatial location Simon tasks (for review, see Lu & Proctor, 1995
). Since word-based compatibility effects increase rather than decay at longer durations (Proctor, Yamaguchi, & Vu, 2007
), delaying response selection in the high discriminability versions of the Simon task does not reduce compatibility effects at the longest part of the RT distribution in the current experiments. However, we provided evidence that reducing the legibility of the word leads to a reduction in the compatibility effect, and this is likely due to the delay, and not the elimination, of automatic activation of the corresponding response. This finding is consistent with the view that when attention is allocated to a word’s location its processing is obligatory. Although task-irrelevant word processing may be delayed by reducing its visual quality (Experiment 1), it nonetheless occurs when there is sufficient time prior to response selection (Experiment 2).
Third, compatibility effects originating in both task-relevant and task-irrelevant stimulus dimensions are moderately correlated (r
’s between .40 and .50) with one another. To our knowledge, this is the first correlational analysis between compatibility effects in the SRC and Simon tasks. The relationship between SRC and Simon task compatibility effects has been contested over the years (see, e.g., Hasbroucq & Guiard, 1991
). However, as evidenced by the consistent correlation between SRC and Simon task compatibility effects in both of the current experiments, it appears that location word compatibility effects share some common underlying factor regardless of whether the spatial feature of the stimulus is relevant or irrelevant to response selection. The current research does not provide evidence concerning the nature of these shared factors. However, as mentioned in the introduction, since compatibility effects in the Simon task originate in task-irrelevant stimulus information they may be caused by automatic response activation rather than stimulus-response translation. This is less clear for compatibility effects in the SRC task, which are usually attributed to either more complicated stimulus-response translation in incompatible trials, automatic activation of the corresponding response, or a combination of both. If the compatibility effect in the Simon task cannot be attributed to stimulus-response translation, then the shared variance between the Simon and SRC task is likely due to a factor related to automatic response activation. Other than the current research, direct comparisons of the individual variability across multiple forms of compatibility tasks have yet to be performed and may be fruitful in better understanding the mechanisms that underlie this phenomenon. Furthermore, it is unknown whether different types of compatibility effects such as those found for spatial or symbolic locations are also due to the same underlying mechanisms as well.
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 corresponding response, but does not eliminate it. This finding is currently limited to a pure-block, discrete manipulation of individual feature discriminability; it is of interest to examine a more continuous manipulation (see e.g., Bosbach, Prinz, & Kerzel, 2004
). For example, our current conclusions would predict that the closer the discriminability between the task-relevant and irrelevant features, the larger the compatibility effect in the Simon task. This should hold true regardless of the overall level of discriminability, as long as these features remain identifiable.
Lastly, although various models presuppose some relation between various forms of compatibility effects, we provide a correlational analysis that shows a clear relationship between word-based compatibility effects when they originate in either task-relevant or task-irrelevant stimulus features. However, this relation accounts for only a portion of the variability in the current experiments, and a more comprehensive multivariate approach is needed to examine the unique cognitive components that distinguish between compatibility effects in the SRC and Simon tasks.