Visual Half-Field Stroop Effects with Spatial Separation of Words and Color Targets

63, 122–142 (1998) BL971940

BRAIN AND LANGUAGE ARTICLE NO.

Visual Half-Field Stroop Effects with Spatial Separation of Words and Color Targets Tracy L. Brown, Christopher L. Gore, and Tara Pearson University of North Carolina at Asheville Past inconsistencies in the occurrence of differential visual half-field Stroop effects were addressed in two experiments using a visual half-field presentation technique incorporating brief displays (100 ms) and a fixation task designed to ensure proper eye fixation at display onset. Experiment 1 used displays in which distractor words and color targets were presented in contralateral visual fields. Experiment 2 compared contralateral with ipsilateral displays where words and color targets appeared one above the other in the same visual field. Stroop effects were larger whenever a word occupied the right as opposed to the left visual field, regardless of whether the color target was left or right. Results are consistent with the idea that words are processed more efficiently or automatically in right visual field/left hemisphere presentations.  1998 Academic Press

Research and theory on the lateralization of function and the coordination of information between the cerebral hemispheres has accumulated to form an impressively large body of literature. Similarly, since J. Ridley Stroop’s (1935) classic article, work on the well-known Stroop effect has also developed into a large and complex area (see MacLeod, 1991). Interestingly, however, only a handful of studies have addressed the intersection of these two areas, dealing with the lateralization of Stroop effects as revealed by visual half-field presentation techniques. Superficially, the basic prediction appears straightforward. Given intact right-handers making vocal naming responses to color stimuli, one could expect greater Stroop interference when distractor words are presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/ right hemisphere (LVF/RH). This is because information presented to RVF/ This research was supported in part by a sabbatical award to the first author from the James McKeen Cattell Fund. We thank Sandra Hall for her assistance in programming and data collection. Address correspondence and reprint requests to Tracy L. Brown, Department of Psychology, University of North Carolina at Asheville, Asheville, NC 28804-3299. E-mail: tlbrown@ unca.edu or [email protected]. 122 0093-934X/98 $25.00

Copyright  1998 by Academic Press All rights of reproduction in any form reserved.

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LH is transmitted directly to the left hemisphere, which is widely believed to be more functionally specialized for language than the right hemisphere (Gazzaniga, 1970; Searleman, 1977). Given the commonly observed RVF/ LH advantage for visual word processing (e.g., Leehey & Cahn, 1979; Soares & Grosjean, 1981), the idea that words will have greater impact when presented to RVF/LH seems plausible (Dyer, 1973a,b; Tsao, Feustel, & Soseos, 1979). However, given the complexities of interhemispheric processing (see, e.g., Banich, 1995; Hellige, Taylor, and Eng, 1989) such a straightforward prediction must be approached with caution. With vocal color naming responses— the standard response modality in the Stroop task—the left hemisphere will control the vocal naming response. This is because the left hemisphere retains virtually exclusive control of speech in intact right-handers (Gazzaniga, 1970; Moscovitch, 1976; Searleman, 1977). Because information driving the vocal response must be present in the responding hemisphere, regardless of the hemisphere to which information was originally presented, there is ample opportunity for Stroop effects to occur independently of the visual half-field to which colors and words are presented. If, for example, Stroop conflict and its resolution are entirely localized in the left hemisphere, then the presentation of a Stroop color-word stimulus to LVF/RH might show slower responding overall due to callosal transmission effects, but the dynamics of Stroop interference once the stimulus information reached the left hemisphere would be unchanged. In that case, there might be an overall RVF/ LH advantage for color naming but no interaction of Stroop effects with visual half-field of presentation. Empirical investigations seem warranted in view of this and similar unknowns involving the level of representation at which information is transmitted across hemispheres and the lateralization of Stroop conflict and resolution processes. Such investigations must use a proper display technique to ensure that visual information is projected exclusively to one or the other cerebral hemisphere. Because eye movements during the Stroop display may alter the hemisphere to which visual information is projected, the display duration must be less than about 150 ms. Equally important, a fixation task should be used to ensure that the eyes are fixated on the center of the screen at display onset. Trials on which the eyes are fixated away from center could cause projection to the unintended hemisphere, or to both hemispheres, and would likely undermine the power of the experiment. In light of these complications, it is not surprising that studies on the lateralization of Stroop effects have produced mixed results. Of the ten relevant and available studies, five have reported larger Stroop effects with words presented to RVF/LH (Franzon & Hugdahl, 1986, 1987; Guiard, 1981; Schmit & Davis, 1974; Tsao et al., 1979). All of these experiments used appropriately brief displays. Some used a fixation task (e.g., Tsao et al., 1979), though others did not (Franzon & Hugdahl, 1986, 1987). Interestingly,

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all of the studies showing the effect used the traditional integrated colorword stimuli. Note that in this case color and word information are presented to the same hemisphere. Of the five remaining studies, four failed to find visual half-field differences in Stroop effects (David, 1992; Dyer, 1973a; Dyer & Harker, as cited in Dyer, 1973b; Warren & Marsh, 1977), and one found the reversed pattern with larger effects in LVF (Long & Lyman, 1987). This latter study used non-integrated stimuli with the color target at center and the distractor word arrayed either left or right. None of the four null-results studies used a fixation task to ensure that the eyes were properly fixated at display onset. Three of the studies (Dyer, 1973a; Dyer & Harker, as cited in Dyer, 1973b; David, 1992) used vertical letter arrangements in lieu of the normal left to right sequence. The use of vertical arrangements to gain tighter control of retinal eccentricity and to reduce the possibility of a left-to-right scanning process (Dyer, 1973a; Heron, 1957; White, 1969) may have been well-motivated, but disrupted the internal orthographic structure of the distractor word in the process. In light of neuroimaging data indicating that some visual areas of the left hemisphere respond more selectively to orthographic structure than the corresponding areas of the right hemisphere (Petersen, Fox, Snyder, & Raichle, 1990), it seems likely that vertical letter displays worked against the occurrence of the effect. The use of integrated as opposed to separated color and word stimuli seems to play a major role across these studies. All of the studies reporting larger RVF/LH Stroop effects used integrated colored-word stimuli (Franzon & Hugdahl, 1986, 1987; Guiard, 1981; Schmit & Davis, 1974; Tsao et al., 1979). None of the studies using separated color and word stimuli reported the effect (David, 1992; Dyer, 1973a, Long & Lyman, 1987). Two of the null-results studies used integrated stimuli, but one (Warren & Marsh, 1977) neglected to record accuracy data, which Franzon & Hugdahl (1986, 1987) have argued is the primary locus of the visual half-field difference. The other (Dyer & Harker, as cited in Dyer, 1973b) used the vertical arrangement and did not mention the use of a fixation task. The present experiments were designed to determine if differential visual half-field Stroop effects can be obtained with non-integrated color and word stimuli, and to gain a stronger grip on the presence and implications of such effects. In addition to the use of brief display durations of 100 ms, a fixation task was developed and incorporated into the display sequence to ensure that the eyes were properly fixated at onset of the Stroop display. Trials on which the participant could not accurately report the nature of the fixation stimulus were eliminated from the analyses. The design of the fixation task warrants careful consideration. Because the Stroop task is fundamentally a speeded response task, the response to the fixation task must occur after the response to the Stroop display. This

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is in contrast with many visual half-field tasks where recognition accuracy to near-threshold stimuli is the dependent measure (e.g., McKeever & Huling, 1971). In that case, responses to the primary task can be made following a quick response to the fixation stimulus. Because of the speeded nature of the Stroop task, the information that is to be reported about the fixation stimulus must be retained by the subject over the span of time in which the Stroop display is processed, resulting in what is essentially dual-task performance. It is therefore important to design the fixation task so that it is as structurally independent of Stroop processes as possible, in the same way that dualtask methods endeavor to separate encoding and response processes of the primary and secondary tasks so that attention demands can be measured independently from low-level structural interference between tasks (Brown & Carr, 1989; Kahneman, 1973; Logan, 1979, 1980). In the present case, the more the fixation task loads differentially on one or the other cerebral hemisphere, the more that conflict or interference between tasks in terms of access to response mechanisms or in the coordination of verbal codes will exacerbate difficulties in interpreting visual half-field differences. For this reason, we argue that the design of the fixation task in visual half-field studies must take into account not only the possibility that the fixation task may load differentially on one cerebral hemisphere—as is already widely recognized—but also that the fixation task should avoid specific interactions with the primary task under investigation. A single fixation task is probably not the best choice across all visual half-field laterality studies; the nature of the primary task must also be taken into account. In light of these considerations, we decided not to use digit or letter identification as the fixation task. Though often used in the past (Hines, 1972; McKeever & Huling, 1971; Tsao, Feustal, & Soseos, 1979), these tasks require subjects to use lexical or phonological codes. Instead, we developed the dot location task with the following properties in mind: (a) the involvement of a low-level sensory or perceptual code which does not require any type of verbal code and which does not easily suggest a name or label; (b) a task for which there are not obvious hemispheric asymmetries; and (c) an easy task with minimal attention demands and simple response alternatives so that any hemispheric asymmetries, if present, cause as small an effect as possible on the hemisphere involved. In the dot location task we developed, a dot subtending about 0.25 degrees appeared at screen center following the offset of a pre-trial fixation cross. A second and much smaller dot appeared in extremely close proximity to the first, so close in fact that perceptually the second dot appeared as a bulge or extension. Across trials, the second dot appeared randomly on the top, bottom, left, or right of the first. This display was brief—100 ms—and immediately followed by the Stroop display. After the color naming response was made, the subject made a manual response on the arrow keys of a computer keyboard to indicate where the dot had occurred (up arrow for above,

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down arrow for below, etc.). The mapping of response alternatives to the arrow keys was intended to establish a high degree of stimulus-response compatibility, and hence to minimize the task’s processing demands. Although possible, it is unlikely that subjects would use a verbal code to retain dot information across the Stroop trial. The span of time between onset of the dot display and the subject’s Stroop response would be substantially less than a second, and mostly taken up by processing of the Stroop stimuli. Because dot location does not involve a readily nameable object, deliberate verbal recoding of the fixation stimulus (e.g., ‘‘above’’, etc.) in such a brief time would seem unlikely. Similarly, the nature of the manual response does not require or encourage a verbally coded response. In addition, the color and word stimuli of the subsequent Stroop display are presented in the left or right visual fields, and hence do not mask the dot display in iconic memory. In light of these considerations, it would seem likely that the dot task did not involve the same kinds of verbal codes important in the Stroop task. Whether the dot task would load differentially on one hemisphere is a much more difficult question. Dot localization tasks have sometimes shown a right visual field advantage (e.g., Hellige & Michimata, 1989); and at other times a left field advantage (e.g., Kimura, 1969). In another study, no difference was found across visual hemi-fields (Bryden, 1973). Although the categorical version of the dot localization task used by Hellige and Michimata (1989) could arguably be viewed as most similar to ours, the previous tasks differ substantially in terms of the vertical and horizontal eccentricity of dot presentation and in the degree of response complexity. Thus, it would be difficult to say whether our dot task differentially taxes one cerebral hemisphere. Because of its high degree of stimulus-response compatibility, and because it would appear to depend on low-level visual codes that do not require higher order computations, the argument could be made that the processing demands of our dot task are negligible. Support for this assumption is indirect, however, and the possibility of differential loading cannot be completely dismissed. This possibility will be raised again in discussion. The use of non-integrated Stroop stimuli allow color and word location to be manipulated independently of each other. We opted to present color bar targets in either the left or right visual field rather than using the Long and Lyman (1987) technique in which the color bar target is kept at center and the distractor words arrayed left or right. In addition to allowing independent presentation of words and colors in left and/or right visual fields, our procedure generates display types that are conceptually comparable to the integrated displays that have shown differential Stroop effects in the past— in which word and color are presented to the same visual field. Experiment 1 was designed to test for the effect in non-integrated displays where the word and color target occupied contralateral visual fields. Dyer (1973a) failed to find a difference under these conditions but used vertical letter arrangements and no fixation task. In the present experiment, words

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were presented in their standard horizontal orientation, in a large font, beginning four degrees to the right of fixation in RVF/LH presentations and ending four degrees to the left of fixation in LVF/RH displays. A colored rectangle, subtending the same visual angle vertically as the word, and approximately the same angle horizontally, was presented in the visual field contralateral to the word. In Experiment 2, words and color targets of the same size and eccentricity were presented either in contralateral visual fields (as in Experiment 1), or in the same visual field with one arrayed above the other. Because previous studies showing heightened RVF/LH Stroop effects used integrated colorword stimuli—in which both word and color information are presented in the same visual field—and because previous studies that failed to show the effect used non-integrated contralateral color and word presentations, the present arrangement was chosen because it allows a test of whether the effect is due to presentation of word and color to the same visual hemi-field or to the integrated nature of the color-word stimulus. It may also reveal whether the presence of the word or of the color information in one or the other visual half-field is the more important determinant of the effect.

EXPERIMENT 1

Method Participants. Thirty right-handed participants were recruited from undergraduate Psychology courses. All were either native or fluent readers of English, and were free from apparent visual, motor, or speech impairments. Handedness was assessed by an informal interview including self-report of limb preference for writing, throwing, and kicking, and by an eye dominance test. Design and materials. Eight trial types were tested. Six were formed by the factorial combination of congruency (neutral, congruent, or conflicting) and the visual half-field in which the color target appeared (left or right, with the word always in the contralateral field). The remaining two trial types were color bar alone trials where the color target was presented to either LVF/RH or RVF/LH without any stimulus in the contralateral field. Excluding 30 warmup trials, each stimulus type was replicated 15 times in the experiment, resulting in a total of 120 test trials completed by each participant. Five target colors were used: Blue, green, red, gray, and white. These colors were used equally often across the eight trial types, and displayed on a black background. Distractor words for conflicting and congruent trials used these color names, and assignments of distractor words to target colors on conflicting word trials were held constant across visual half-fields. Neutral word distractors were frequent nouns, verbs, and adjectives equated in length and frequency to the color names. As noted earlier, a fixation task was used to ensure that participants’ eyes were correctly fixated at the center of the screen at onset of the Stroop display. The stimuli for the fixation task consisted of a small round dot that appeared at the center of a plus sign that was used as a pre-trial fixation stimulus, plus a smaller secondary dot that appeared in very close proximity either above, below, left, or right of the first dot. The second dot was very small and very close to the first. In fact, the boundary between the dots was not readily apparent, and both dots fit easily within the area subtended by the plus sign. Pilot testing revealed that the location

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of the secondary dot could not be discerned if the eyes were fixated more than about 0.75 degrees away from center. Displays were programmed such that the nearest letter of the distractor word was approximately four degrees from fixation. Words were presented in their standard horizontal orientation in a large upper case font subtending approximately 0.75 degrees vertically and 3–4 degrees horizontally, depending on the number of letters. Luminance of the words was 25 cd/ m2. Color bar targets were rectangular in shape and subtended approximately the same visual angle as the words, with the location of the edge nearest center matched to that of the words. Average luminance of the color targets was 15 cd/m2. Displays were symmetrical in the sense that words and color bars were equidistant from fixation and subtended roughly the same visual angle. A total of about 8 degrees separated words from color targets. Apparatus and procedure. Stimulus displays, counterbalancing, and data collection were programmed on an IBM-compatible microcomputer with VGA graphics, using MEL (Micro Experimental Lab) software from Psychology Software Tools. Vocal naming responses were timed from display onset to vocal response onset using a microphone connected to the MEL response box. Accuracy for the fixation task was recorded via participants’ manual response on the arrow keys of the computer keyboard, with up arrow for above, down arrow for below, and so on. Accuracy for color naming was recorded via manual keypress by the experimenter following the participants’ dot location response. Each session began with a set of verbal instructions and an informed consent procedure. A slow-motion version of the trial sequence was then presented to the subject, with accompanying text explaining each display. This was followed by 30 warm-up trials. The sequence of the remaining 120 test trials was independently randomized for each participant. Each trial began with a plus sign presented in the center of the screen for 500 ms. This was replaced, without delay, by the dots of the fixation task, which remained on the display for 100 ms. Offset of the dot display was followed immediately by the Stroop display, which was also presented for 100 ms. Following the color naming response, participants were prompted to enter their dot location response via the arrow keys of the computer keyboard. Subsequent to feedback on the dot location response, the experimenter entered an accuracy score for the color naming response, and then initiated the next trial.

Results For reaction time, cell medians for each subject were computed with exclusion of trials where color naming or dot location responses were incorrect. Color naming accuracy (percent correct) was also conditionalized on correct responding to the dot location task. Cell medians (for reaction time) and percent correct were submitted to separate 3 ⫻ 2 ANOVA’s, with three levels of congruency (neutral, congruent, and conflicting) and two levels of color bar location (LVF/RH or RVF/LH). The two color bar alone conditions (color bar left or color bar right) were tested separately via planned t-tests for reaction time and accuracy. Independent analysis of accuracy on the dot location task revealed an overall accuracy mean of 95.6%. Thus, less than 5% of the trials were excluded on this basis. A 3 ⫻ 2 ANOVA on dot location accuracy failed to produce reliable effects for congruency, color bar location, or their interaction. As a caution against ceiling effects and the threat of skew in the distributions of the accuracy data, all of the analyses of the accuracy data were repeated following an arc sin transformation of the raw scores (Myers, 1972). None of the effects were changed in the re-analysis.

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Tests of the color bar alone means revealed faster responding to color targets with RVF/LH presentation (M ⫽ 581 ms, SD ⫽ 98.2) than with LVF/ RH presentation (M ⫽ 614 ms, SD ⫽ 112.8), t(29) ⫽ 3.41, p ⬍ .05. However, accuracy scores were higher for color targets in LVF/RH (M ⫽ 99.3%, SD ⫽ .049) than in RVF/LH (M ⫽ 97.2%, SD ⫽ .027), t(39) ⫽ 2.8, p ⬍ .05). The reversal of visual half-field effects across reaction time and accuracy precludes any clear decision as to the presence of an overall half-field advantage, at least for these two display types where words were not present. However, field of presentation did affect speed-accuracy tradeoff criteria. The 3 ⫻ 2 ANOVA on reaction times for displays with words revealed main effects for both visual half-field, F(1, 29) ⫽ 12.08, p ⬍ .05, MSe ⫽ 3575.7, and congruency, F(2, 58) ⫽ 50.09, p ⬍ .05, MSe ⫽ 4753.3. The visual half-field effect indicated faster responding when color targets were presented to RVF/LH (M ⫽ 616 ms) than to LVF/RH (M ⫽ 647 ms). In contrast to the color bar alone data, the accuracy data produced a significant effect in the same direction, F(2, 58) ⫽ 8.84, p ⬍ .05, MSe ⫽ 0.0041, with means of 97.8% correct with color targets presented to RVF/LH and 95.0% correct in LVF/RH presentations. Thus, there was a clear RVF/LH advantage for color naming when a word occupied the contralateral visual field, though it should be noted that this effect is averaging across congruency. The main effect of congruency (whether a color-neutral, congruent, or conflicting word was present in the opposite visual field) revealed fastest responding with congruent words (M ⫽ 568 ms), intermediate responding with neutral words (M ⫽ 631 ms), and slowest responding with conflicting words (M ⫽ 694 ms). The corresponding main effect in the accuracy data was significant, F(2, 58) ⫽ 19.14, p ⬍ .05, MSe ⫽ 0.0036, corroborating this pattern. The overall Stroop effect (computed as the difference between conflicting and congruent means) was 126 ms. Before proceeding with the analysis of Stroop effects across visual halffields, it is important to determine whether the facilitation and interference components of the total Stroop effect do not vary substantially across conditions. This is done to simplify interpretation of the interaction of congruency and color bar location, which could otherwise result from variations in the relative magnitude of facilitation and interference components across visual half-fields in the absence of a net change in the total Stroop effect. Computing facilitation effects as the difference between neutral and congruent conditions, and interference effects as the difference between conflicting and neutral conditions, 2 ⫻ 2 ANOVA’s with visual half-field (color bar left or right) and type of effect (facilitation or interference) were conducted on both reaction time and accuracy measures. There were no reliable main effects or interactions in the reaction time data. In the accuracy data, there was a main effect revealing an overall difference in the size of interference (M ⫽ 5.0%) and facilitation (M ⫽ 1.5%) effects, F(1, 29) ⫽ 4. 31, p ⬍ .01, MSe ⫽ .0086. The fact that this difference was evident only in accuracy suggests

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TABLE 1 Reaction Time, Accuracy, and Total Stroop Effect (TSE) Means for the Effects of Congruency across Visual Half-Fields in Experiment 1 Color target left

Color target right

Congruency

RT

ACC

RT

ACC

Neutral Congruent Conflicting TSE

645 574 722 148

96.8 99.0 89.1 9.9

617 563 667 104

98.3 99.1 96.0 3.1

Note. RT, reaction time (ms); ACC, accuracy (% correct responses). Total Stroop Effect (TSE) is a change score computed as the difference between conflicting and congruent word means, with the Stroop effect expressed as a positive number for both RT and ACC.

that a ceiling effect may have compressed the facilitation component of the accuracy scores. More importantly, however, the interaction of effect type with visual field was not significant. Having established that the relative magnitudes of facilitation and interference effects did not change, the question of whether Stroop effects were larger when words occupied the right visual field can be addressed. The interaction of color bar location and congruency was marginal in the reaction time data, F(2, 58) ⫽ 2.51, p ⫽ .09, MSe ⫽ 2908.2, and significant in the accuracy data, F(2, 58) ⫽ 6.07, p ⬍ .05, MSe ⫽ 0.0033. Examination of the accuracy means (presented in Table 1) revealed that the difference between congruent word and conflicting word responses was 9.9% when words appeared in RVF/LH (and color targets were in LVF/RH), decreasing to 3.1% when words appeared in LVF/RH (and color targets were in RVF/LH). The trend in the reaction time data was in the same direction, with total Stroop effects of 148 ms with words in RVF/LH and 104 ms with words in LVF/ RH. Neutral word interference. Brown, Roos-Gilbert, and Carr (1995) reported that neutral words caused interference by themselves relative to displays where color targets appeared alone. The occurrence of this effect raises the question of whether interference from neutral words is lateralized in the same way as the Stroop effects arising from congruent or conflicting color names. Accordingly, a 2 ⫻ 2 ANOVA was conducted with the independent variables of color bar location (LVF/RH versus RVF/LH) and word presence (whether a neutral word was present or absent in the contralateral visual field). In reaction time, the presence of a neutral word slowed responding by 33 ms, from 631 ms with neutral words present to 598 ms with color bar alone, F(1, 29) ⫽ 27.71, p ⬍ .05, MSe ⫽ 1214.27. There was also a significant main effect of color bar location, F(1, 29) ⫽ 13.35, p ⬍ .05, MSe ⫽ 2109.74, but no interaction.

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The main effect of word presence was not reliable in the accuracy data, but interacted with color bar location, F(1, 29) ⫽ 8.40, p ⬍ .05, MSe ⫽ 0.0014. Neutral word interference (measured as the difference between color bar alone and neutral word conditions), was 2.5% when the word was in the right visual field and ⫺1.1% when the word appeared in the left visual field. The reversed interference effect of ⫺1.1% was not reliable when tested alone. Collectively, the reaction time and accuracy data replicate the neutral word interference effect reported by Brown et al. (1995), and suggest that the effect is heightened by RVF/LH presentation in the same way as Stroop effects from color names. Discussion Although data from the color bar alone condition failed to produce clear evidence of a visual half-field effect when color targets appeared without words, displays in which words were present produced a main effect RVF/ LH advantage for color naming, and showed heightened Stroop effects when the word was in RVF/LH and the color target in LVF/RH. Smaller total Stroop effects were observed when color targets were in RVF/LH and words were in LVF/RH. The presence of a general RVF/LH advantage for color naming in displays without words is not necessarily supported by these data. When color targets appeared alone, the RVF/LH advantage in reaction time was offset by a reliable reverse effect in accuracy. In displays containing words, the appearance of an RVF/LH advantage is compromised by the fact that a word occupied the contralateral visual field. The main effect of visual half-field could therefore reflect differences in word processing across visual half-fields, and cannot be taken as support for a general RVF/LH advantage for color naming. EXPERIMENT 2

In Experiment 1, color targets and words were always in contralateral visual fields. This design feature makes the occurrence of larger RVF/LH Stroop effects ambiguous: Are words advantaged by RVF/LH presentation, or are color targets disadvantaged by LVF/RH presentation? Experiment 2 was designed to address this ambiguity. In addition to the trials from Experiment 1 where words and color targets occupied contralateral visual fields, trials were included where words and color targets appeared in the same visual field, either both left or both right. If the primary source of the differential Stroop effect lies in the benefits accruing to word processing in LH, then the effect should persist whenever a word is presented to RVF/LH independently of whether the color target is also presented to RVF/LH or split to LVF/RH. If the location of the color target is the de-

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termining variable, then the effect should occur whenever the color target is presented to LVF/RH, independently of where the distractor word is located. Method Participants. Thirty-nine participants were recruited from the same source as those in Experiment 1. All were self-reported right-handers with native English reading fluency and no apparent visual, motor, or speech problems. Design and materials. Fourteen display types were tested. Twelve display types were formed by the factorial combination of color bar location (LVF/RH or RVF/LH), congruency (neutral, congruent, or conflicting), and display type (same-side displays, in which word and color target occupied the same visual field, or split displays, in which word and color target were contralateral). Ten trial replications were used for each of these display types, two with each of the five colors, yielding a total of 120 trials containing words. The remaining color bar alone trials had 10 replications each for RVF/LH and LVF/RH, with each color used twice. Excluding 22 warm-up trials, each participant completed a total of 140 test trials. Displays were programmed with words and color bar targets of the same size and eccentricity as in Experiment 1. When both appeared in the same visual field, they were arrayed one above the other, centered about the horizontal midline of the display, with about 0.75 degrees of separation measured from the nearest points of each. The relative positioning of word and color bar target (top or bottom) was equiprobable. Apparatus and procedure. With the exception of the number of trials and the display changes noted, the data collection and trial sequence were the same as in Experiment 1.

Results Overall accuracy on the dot location task was 94.8% and did not vary across congruency, display type, or color bar location. As in Experiment 1, accuracy scores were re-analyzed following arc sin transformation of the raw data. None of the effects were altered. Using subject-cell median reaction times conditionalized as before on naming and dot location accuracy, comparison of color bar alone trials revealed a RVF/LH advantage for color naming, t(38) ⫽ 2.42, p ⬍ .05. Mean reaction time was 577 ms (SD ⫽ 163.5) with color targets in RVF/LH and 607 ms (SD ⫽ 184.4) in LVF/RH. In contrast to Experiment 1, accuracy scores failed to produce a reliable effect in the opposite direction (t ⬍ 1). In the absence of a significant speed-accuracy tradeoff indication, it would appear that a modest RVF/LH advantage occurred for color naming. Data from trials with words were submitted to a 2 ⫻ 3 ⫻ 2 ANOVA reflecting the levels of visual half-field and congruency as in Experiment 1, plus the third variable of display type (same-side or split display). The reaction time data produced a main effect of congruency, F(2, 76) ⫽ 79.97, p ⬍ .05, MSe ⫽ 8571.67. Examination of the means revealed an overall Stroop effect of 132 ms, which compares favorably with the 126 ms Stroop effect of Experiment 1. The same effects were evident in the accuracy data, with a significant main effect of congruency, F(2, 76) ⫽ 33.2, p ⬍ .05, MSe ⫽

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TABLE 2 Accuracy (% Correct Responses), Reaction Time (RT), and Total Stroop Effect (TSE) Means for the Interaction of Congruency, Display Type, and Color Target Location in Experiment 2 Color target left Display type Display type Neutral Congruent Conflicting TSE

Same-Side RT 661 576 699 123

ACC 99.7 99.5 95.8 3.7

Color Target right

Split RT 650 565 714 149

ACC 97.7 99.5 87.9 11.6

Same-Side RT 655 578 719 141

ACC 98.9 99.1 92.4 6.7

Split RT 631 565 680 115

ACC 98.2 97.8 94.2 3.6

Note. ACC, Accuracy; RT, reaction time in milliseconds. In Same-Side displays, both word and color target were presented in the same visual half-fields. In Split displays, the word was contralateral to the color target. Total Stroop Effect (TSE) was computed as the difference between congruent and conflicting means, with a Stroop effect expressed as a positive number.

0.0060, and a 6.1% overall Stroop effect compared to the 6.5% overall Stroop effect from Experiment 1. As in Experiment 1, analyses with interference and facilitation effects broken out separately failed to produce any main effects or interactions in the reaction time data. In the accuracy data, interference effects (M ⫽ 6.05%) were once again larger than facilitation effects (M ⫽ 1.2%), F(1, 38) ⫽ 28.16, p ⬍ .05, MSe ⫽ 0.0092, but the interaction with visual half-field was not significant. The critical test in the present experiment is whether Stroop effects varied as a function of the visual half-field in which the word and color target were presented. This is tested in the three-way interaction of visual half-field, display type, and congruency, which was significant in the accuracy data, F(2, 76) ⫽ 7.53, p ⬍ .05, MSe ⫽ 0.0042, but not in reaction time. Examination of the means (presented in Table 2) is best approached by calculating the total Stroop effect across the cells of visual half-field and display type. As before, the total Stroop effect is calculated as the difference between congruent and conflicting word types. Using this analysis, the largest Stroop effect was observed in split displays when the color target was on the left and the word was on the right, with an average total Stroop effect of 11.6%, from 99.5% correct with congruent color names to 87.9% correct with conflicting color names. The next largest Stroop effect was observed in same-side displays, where both word and color target were presented to RVF/LH, with a total Stroop effect of 6.5%, from 99.1% with congruent color names to 92.4% with conflicting color names. The remaining two display types— same-side left and split target right—showed smaller and equivalent total Stroop effects: 3.7% in same-side left displays and 3.6% in split target-right displays.

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The reaction time data also revealed a marginal main effect of display type (same-side versus split), F(1, 38) ⫽ 2.88, p ⫽ .098, MSe ⫽ 7775.8. Examination of the means showed a trend toward faster responding overall to split displays than to same-side displays (where word and color target occupied the same side of the visual field). The difference of 14 ms (734 ms for split displays versus 748 ms for same-side displays) was offset by a reliable main effect in the opposite direction in the accuracy data, F(1, 38) ⫽ 6.03, p ⬍ .05, MSe ⫽ 0.0058, with 95.9% accuracy in split displays and 97.6% accuracy in same-side displays. It seems likely that the accuracy effect is due at least in part to the fact that Stroop effects were largest in split displays with the color target left (M ⫽ 11.6%), combined with the finding noted earlier that interference effects were larger than facilitation effects in accuracy but not in reaction time. This means that main effects and interactions in the accuracy data that are averaged across word congruency will show decreases in performance because interference effects are not offset by equally-sized facilitation effects. This is not so in the reaction time data. Thus, the decrease in accuracy in split displays is probably a reflection of the large interference effect in the split target-left displays. This phenomenon may also explain the interaction between visual halffield and display type. In reaction time, F(1, 38) ⫽ 4.63, p ⬍ .05, MSe ⫽ 3266.16, the visual half-field difference was negligible in same-side displays (745 ms with both word and color left versus 751 ms with both right). In split displays, faster responses were observed when the color target was on the right and the word was on the left (M ⫽ 725 ms) than when the color target was on the left and the word was on the right (M ⫽ 743 ms). The corresponding interaction in the accuracy data, F(1, 38) ⫽ 5.41, p ⬍ .05, MSe ⫽ 0.0055, was complicated by the fact that interference effects were larger than facilitation effects in the accuracy data. When averaging across word congruency, those conditions that showed the largest Stroop effects (split target left displays and same-side target right displays) showed decreased accuracy. As a result, same-side target left displays (M ⫽ 98.3%), which had small Stroop effects, showed higher accuracy than same-side target right displays (M ⫽ 96.8%), which had larger Stroop effects. In turn, same-side target right displays did not differ from split target-right displays (M ⫽ 96.7%). Split target-left displays showed the lowest accuracy (M ⫽ 95.0%), as would be expected. Thus, the shape of the interaction in the accuracy data is a by-product of averaging across levels of word congruency. Neutral word interference. Tests for neutral word interference in Experiment 2 were divided into two sets, one comparing the color bar alone conditions against the split-display versions of the neutral word trials (which replicates the conditions of Experiment 1), and the other comparing the color bar alone conditions to the same-side displays. In the split-display analysis, the 2 ⫻ 2 ANOVA with color bar location (RVF/LH, LVF/RH) and word pres-

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ence (a neutral word or nothing in the field contralateral to the color target) replicated the main effect of word presence in reaction time, F(1, 38) ⫽ 22.51, p ⬍ .05, MSe ⫽ 4054.80. Examination of the means revealed an overall neutral word interference effect of 49 ms. The effect was only marginal in the accuracy data, F(1, 38) ⫽ 3.76, p ⫽ .06, MSe ⫽ .0010. The interaction of word presence and color bar location that was reliable in Experiment 1 failed to replicate in either dependent measure. In the second analysis, the same-side displays were submitted to analogous ANOVA’s, this time using same-side neutral word displays in 2 ⫻ 2 ANOVA’s reflecting word presence and color bar location. As with the split displays, the neutral word interference effect was reliable in reaction time, F(1, 38) ⫽ 32.99, p ⬍ .05, MSe ⫽ 5204.51, but did not attain significance in accuracy. The interaction of word presence and color bar location was not reliable in reaction time or accuracy. Combined analysis of Experiments 1 and 2. Because the split displays of Experiment 2 were identical to the split word and color conditions in Experiment 1, split display data from both experiments were combined and submitted to 3 ⫻ 2 ⫻ 2 ANOVA’s, with Experiment (1 or 2) as a between-subjects variable. All of the effects that were reliable in the separate analyses of each experiment were also reliable in the combined analysis for both reaction time and accuracy. In addition, the interaction of color bar location (RVF/LH versus LVF/RH) with congruency (neutral, congruent, or conflicting word) was reliable in the reaction time data, F(2, 134) ⫽ 3.42, p ⬍ .05, MSe ⫽ 3685.45. The total Stroop effect was 149 ms with words in RVF/LH and 110 ms with words in LVF/RH. Because this effect was only marginal in the reaction time data of Experiment 1, its reliability in the combined analysis adds weight to the conclusion from the accuracy data that Stroop effects are larger when the word is presented to RVF/LH and the color bar to LVF/RH. The combined data was also analyzed with reference to facilitation and interference effects broken out separately. As in the individual analyses of each experiment, the magnitude of facilitation and interference effects did not differ in the reaction time data, and did not interact with color bar location. In the accuracy data, interference effects were larger than facilitation effects, F(1, 67) ⫽ 14.86, p ⬍ .05, MSe ⫽ 0.0109, as would be expected on the basis of the individual analyses, and there was no interaction. Neutral word interference was also examined in the combined analysis. The main effect of word presence, reflecting overall neutral word interference, was reliable in both reaction time, F(1, 68) ⫽ 42.67, p ⬍ .05, MSe ⫽ 2838.98, and accuracy, F(1, 68) ⫽ 4.51, p ⬍ .05, MSe ⫽ 0.0010. The interaction of word presence with visual half-field of presentation was reliable in the accuracy data, F(1, 68) ⫽ 4.33, p ⬍ .05, MSe ⫽ 0.0019, indicating that neutral word interference was larger with RVF/LH than with LVF/RH presentation. This interaction was not significant in the reaction time data.

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Discussion The results of Experiment 2 indicate that the heightened Stroop effects are primarily due to word location. Stroop effects were small when the word was presented to LVF/RH regardless of whether the color target was also on the left or split to the right. Stroop effects were larger whenever a word was presented to RVF/LH, though of intermediate size when the color target accompanied the word on that side. GENERAL DISCUSSION

The experiments have produced solid evidence that Stroop effects are larger when a word is presented to RVF/LH relative to LVF/RH. This was true in the accuracy data from Experiments 1 and 2 and in reaction time and accuracy data in the overall analysis of split displays from Experiments 1 and 2 combined. Comparison of same-side and split displays in Experiment 2 indicated in addition that heightened Stroop effects occurred whenever a word was presented to RVF/LH as opposed to LVF/RH—Stroop effects were larger whenever a word was on the right regardless of whether the color bar target was split to the left or accompanied the color word on the right. All of the previous studies that have found larger Stroop effects with RVF/LH presentation have used integrated color-word stimuli (Franzon & Hugdahl, 1986; 1987; Guiard, 1981; Schmit & Davis, 1974; Tsao et al., 1979), in which case, of course, both word and color information are presented in the same visual hemi-field. The present experiments indicate that physical integration of the word and color stimuli is not necessary to the effect, and moreover that words and colors need not be presented in the same visual field. In fact, the largest Stroop effect occurred when color targets were in the left visual field and color names were on the right. The failure of previous studies to find the effect with non-integrated stimuli can be attributed to vertical letter arrangements, the decision not to use a fixation task, or both (David, 1992; Dyer, 1973a; Dyer & Harker, as cited by Dyer, 1973b). As noted previously, vertical letter arrangements may disrupt orthographic information to which the left hemisphere is differentially sensitive (Petersen, Fox, Snyder, & Raichle, 1990). Only one of the six studies using integrated stimuli failed to find the effect (Warren & Marsh, 1977), and this can probably be attributed to their decision not to record accuracy data. Franzon and Hugdahl (1986, 1987) have argued that lateralized Stroop effects show up more in accuracy than in reaction time, and that was born out in the present experiments. The reversed visual half-field Stroop effect reported by Long and Lyman (1987) remains unexplained. Although a left-to-right scanning hypothesis has been used in the past to account for an LVF/RH advantage in bilateral

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word presentations (Heron, 1957; see also White, 1969), the displays used by Long and Lyman—with the color target at center and the color name left or right—contained only one word and were not really bilateral. It is therefore unclear why a left-to-right scanning process would occur in the Long and Lyman study, and why, if so, it did not also occur in the split displays of the present experiments. In any event, subjects in the Stroop task are trying to name colors, not words. Analyses with facilitation and interference effects measured separately failed to produce evidence that these components of the Stroop effect were differentially affected by the manipulations of visual half-field and display type. Although interference effects were larger overall than facilitation effects in the accuracy data, this difference did not vary reliably across conditions. The relatively small facilitation effects seen in the accuracy data are likely a result of a ceiling effect that compressed the difference between neutral and congruent word stimuli. This does not compromise the interaction of Stroop effects with visual half-field, however. If anything, the ceiling effect would cause visual half-field differences in Stroop effects to be underestimated. It is possible, however, that variations in the relative size of facilitation and interference effects could have gone undetected in the present experiments. Evidence from Brown (1996), and Brown, Roos-Gilbert, and Carr (1995), suggests that increases in the spatial separation of words and color bars affect interference more than facilitation. Thus, manipulations do not always affect these two components in the same way. The ceiling effect may have obscured such an effect in the present study. Comparisons of displays with color bars alone versus displays with neutral words indicated that neutral words themselves produced interference to color naming. This is consistent with data reported by Brown et al. (1995), who found that neutral words produced interference relative to displays where the color target appeared alone, and that various non-word stimuli generally failed to produce interference. This effect suggests that the presence of any word in the display causes some interference, perhaps because it generates a phonological code that competes with the color naming response. The additional effects of congruent and conflicting color names are overlaid on the basic neutral word interference effect, and would likely reflect the activation of lexical or semantic codes that are specifically relevant to or associated with the correct color naming response. Interestingly, there is evidence from the present experiments that neutral word interference is lateralized along the same lines as the Stroop effects caused by congruent and conflicting words. Neutral word interference effects in RVF/LH were larger than in LVF/RH in the accuracy data from the split displays of Experiment 1, and in the accuracy data of the combined analysis of split displays in Experiments 1 and 2. However, some caution is warranted in light of the fact that the effect failed to replicate in Experiment 2 when analyzed alone. Nevertheless,

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the tentative indication that interference effects are greater when neutral words are presented to RVF/LH is interesting, and adds to the idea that the effects are related to the linguistic nature of the competing stimulus. It is also important to note that the Stroop effects observed in the present experiments were quite large relative to other Stroop experiments using nonintegrated displays where words and color targets were spatially separated (Brown, 1996; Brown, Roos-Gilbert, & Carr, 1995; Gatti & Egeth, 1978; Kahneman & Chajczyk, 1983; Merikle & Gorewich, 1979; Yee & Hunt, 1991). Moreover, all of the studies in which the degree of separation between word and color target was manipulated have shown that Stroop effects drop off rather steeply with increasing spatial separation (Brown, 1996; Brown et al., 1995; Gatti & Egeth, 1978; Merikle & Gorewich, 1979). The contrasting results from the present experiments are due to the role of locational uncertainty. With the exception of Brown et al. (1995, Experiment 4) all of the studies using non-integrated displays have presented color targets at a fixated location known in advance to the subject and constant across trials. This type of presentation maximizes the powers of attentional selection on the color target and puts words at a distinct disadvantage in terms of retinal eccentricity and, hence, visual acuity. In Brown et al. (1995, Experiment 4) Stroop effects comparable in magnitude to those obtained in the present experiments were obtained when the color target varied in location above, below, left, or right of the subject’s fixation point. In another relevant experiment by Kahneman and Henik (1981, Experiment 2), integrated color-word stimuli were presented to both sides of fixation, one surrounded by a square and the other by a circle, and subjects were instructed to name the color of the word contained in the square (for half the subjects) or the circle (for the other half), with the location of the circle (or square) varying randomly across trials. Reliable interference was found when a colored neutral word was the target and a conflicting word was on the opposite side of fixation, with 8.8 degrees separating the two. These findings make it clear that when the location of the color target is uncertain, reliable Stroop effects can be obtained even when words and color targets are widely separated. The present results, taken in combination with these previous findings, indicate that locational uncertainty causes a dissociation between the degree of separation and the magnitude of Stroop effects. In this regard, it should be noted that in the present experiments the degree of separation is confounded with display type (same-side or split). In split displays, words and color targets were separated by about 8 degrees; in sameside displays, the separation (vertically from the nearest edges) was 0.75 degrees. This confound does not threaten the conclusion that laterality effects played a major role in our findings. Because of the dissociation between degree of separation between word and color target when target location is uncertain across trials, there is no particular reason to expect larger Stroop effects in same-side displays. Even if such a confounding effect persisted,

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Stroop effects were laterally asymmetrical even when spatial separation was controlled for (i.e., in the drop between split color-left and split color-right displays, and in the drop between same-side left and right displays). Therefore, the confounding of spatial separation with display type cannot be invoked to explain the laterality effects obtained. A final methodological caution must be raised in connection with the dot location task used to ensure proper fixation. In light of the fact that all of the prior studies that have failed to find differential half-field Stroop effects also did not use a fixation task (David, 1992; Dyer, 1973a; Dyer & Harker, as cited in Dyer 1973b; Warren & Marsh, 1977), the use of a fixation task in the present study seems warranted. Nevertheless, fixation tasks can create problems of their own if they interact with the task being performed or with hemispheric asymmetries. In the present case, it is possible that the dot location task differentially loaded on one or the other cerebral hemisphere. If dot processing loaded more on the right hemisphere, as might be expected on the basis of the dot localization task used by Kimura (1969), then processing of words presented to LVF/RH may have been degraded because of concurrent processing from the dot task, causing reduced Stroop effects in displays where words appeared on the left. On the other hand, nonspecific arousal effects could be invoked to explain superior processing of words on the right (Hellige & Cox, 1976), if it turned out that the dot task loaded more on the left hemisphere, as would be suggested by the findings of Hellige and Michimata (1989). The net result is that interactions between the fixation task and hemispheric asymmetry cannot be ruled out conclusively. This is true of other laterality experiments in which fixation tasks are used. In conclusion, the heightened Stroop effects observed when words were presented to RVF/LH are probably real, and, as such, are consistent with the general tendency for the left hemisphere of the cerebral cortex to be specialized for language processing (Gazzaniga, 1970; Searleman, 1977). They are also consistent with the commonly observed RVF/LH advantage for words (e.g., Leehey & Cahn, 1979; Soares & Grosjean, 1981), supporting the idea that more efficient processing of words in the left hemisphere will enhance their tendency to produce Stroop interference. In addition, the finding falls in line with the indication from neuroimaging data that the left hemisphere contains areas which are particularly sensitive to orthographic structure (Petersen, Fox, Snyder, & Raichle, 1990). To the degree that Stroop effects are mediated by automaticity in visual word processing (Brown, Roos-Gilbert, & Carr, 1995; Cohen, Dunbar, & McClelland, 1990; MacLeod, 1991), it is reasonable to surmise that such automaticity-based processing differences are associated with the presence of specialized neural structures. It is also possible, however, that callosal transmission dynamics can account for part or all of the present findings without reference to hemispheric specialization for language per se. Because the present experiments used vocal responses, controlled by the left hemisphere in our right-handed sub-

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jects, information presented in the right visual field gains quicker access to response mechanisms than information on the left. This fact alone could explain why split target-left displays showed the largest Stroop effects—in which case, words are projected directly to the responding hemisphere while information on color targets must be transmitted over the corpus callosum— and why interference decreased when the color target accompanied the word in the right visual field—in which case both stimuli are projected directly to the responding hemisphere. It is also possible to invoke explanations based on attentional allocation (e.g., Brown & Kosslyn, 1985; Kosslyn, Flynn, Amsterdam, & Wang, 1990), or on interhemispheric interactions (Banich, 1995; Hellige, Jonsson, & Michimata, 1988; Hellige, Taylor, & Eng, 1989), the latter of which raises the possibility that a given hemisphere may dominate processing independently of hemispheric specialization per se. In that case, left hemisphere dominance in the Stroop task, which is essentially a verbal task, could explain lateralized Stroop effects without actually measuring the degree to which words are processed more efficiently in the left hemisphere. The present experiments cannot distinguish among these theoretical alternatives for interpreting visual half-field effects. As such, our results must take their place among other empirical findings whose accumulation may ultimately narrow the possibilities. Additional neuroimaging work on the localization and lateralization of Stroop processes (e.g., Bench et al., 1993; Pardo, Pardo, Janer, & Raichle, 1990) would constitute an important converging operation. REFERENCES Banich, M. T. 1995. Interhemispheric processing: Theoretical considerations and empirical approaches. In R. J. Davidson & K. Hugdahl (Eds.), Brain asymmetry. Cambridge, MA: MIT Press. pp. 427–450. Bench, C. J., Frith, C. D., Grasby, P. M., Friston, K. J., Paulesu, E., Frackowiak, R. S. J., & Dolan, R. L. 1993. Investigations of the functional anatomy of attention using the Stroop task. Neuropsychologia, 31, 907–922. Brown, H. D., & Kosslyn, S. M. 1995. Hemispheric differences in visual object processing: Structural versus allocation theories. In R. J. Davidson & K. Hugdahl (Eds.), Brain asymmetry. Cambridge, MA: MIT Press. Pp. 77–97. Brown, T. L. 1996. Attentional selection and word processing in Stroop and word search tasks: The role of selection for action. American Journal of Psychology, 109, 265–286. Brown, T. L., & Carr, T. H. 1989. Automaticity in skill acquisition: Mechanisms for reducing interference in concurrent performance. Journal of Experimental Psychology: Human Perception and Performance, 13, 686–700. Brown, T. L., Roos-Gilbert, L., & Carr, T. H. 1995. Automaticity and word perception: Evidence from Stroop and Stroop dilution effects. Journal of Experimental Psychology: Learning, Memory, and Cognition, 21, 1395–1411. Bryden, M. P. 1973. Perceptual asymmetry in vision: Relation to handedness, eyedness, and speech lateralization. Cortex, 9, 418–435. Cohen, J. D., Dunbar, K., & McClelland, J. L. 1990. On the control of automatic processes:

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