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Stroop Experiment

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Introduction
Stimuli
Design
Procedure
Data Analysis
References

Introduction

This experiment employs divided visual field methodology to determine whether the Stroop phenomenon is equally strong in both brain hemispheres. There is a slight hint in the literature (Schmit & Davis, 1974; Warren & Marsh, 1979), that hemispheric effects might be present but the nature and direction of those effects are difficult to discern given methodological differences in the studies and gender differences in the subjects. On intuitive grounds alone, hemispheric effects might be expected based on the fact that the Stroop phenomenon is created through semantic interference in a color judgment task. In that semantics is a cognitive specialty of the left hemisphere, one might expect that left hemisphere processing of Stroop stimuli would be superior to right hemisphere processing. On the other hand one might predict the reverse, arguing that the not-so-semantically-aware right hemisphere should be less affected by semantic issues than the left.
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Stimuli

The stimuli used in this experiment are the color words RED, BLUE, GREEN, and YELLOW printed in color. The word and color are either congruent, as illustrated by the words RED, BLUE, YELLOW, GREEN, each of which is printed in the color identified by the word, or non-congruent, as illustrated by these words RED, BLUE, YELLOW, GREEN, each of which is printed in a color different from that identified by the word.
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Design

There are two independent variables, both of which are manipulated within subjects. First is word position. Words appear to the right, to the left, or in the middle of the field of vision established by a pulsating focal point used to initiate each trial. The second variable is nature of the stimulus word, which is either congruent in word and color or non-congruent. The dependent variable is response time, which is the interval between a word's appearance and a keypress that indicates the font color in which the word is printed. The keypress is executed on the numeric pad of the computer where a "2" represents the color red, "4" represents blue, "6" represents green, and "8" represents yellow. In addition to recording response time, the program also records whether a response was correct or incorrect. To assure that research participants master the number-color code before generating data, the program sets a performance criterion that must be achieved on practice trials. Once the criterion is met they can proceed to the experiment.
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Procedure

Research participants elect the number of trials they conduct but they must complete at least 72 trials for the data to be admissible for inclusion in the PsychExps database.

Each trial begins with a pulsating plus sign that serves to establish a focal point on the computer screen. Research participants press a key when they are ready for a word to be presented. Following the key press, there is a random delay from 1 to 3 seconds before the word appears.

When words appear, they are written vertically on the screen. The central axis of the word is designed to be approximately 3^o of visual angle to the right or left of the fixation point on lateral presentations, though the actual visual angle will vary depending on monitor size and the distance of research participants from the monitor. On central presentations, the word axis is at 0^o relative to the fixation point. Display times are fixed at 150 msec to assure that laterally presented words are experienced in a single visual hemifield.

With four color words (RED, BLUE, GREEN, and YELLOW) that are printed in a word-color congruent form or non-congruent form appearing at any of three positions (left, center, and right), there are 24 possible stimulus events. Which of these 24 events occur and the order in which they occur is determined by sampling randomly with replacement from the set of 24 events. This means that the number of congruent and non-congruent trials as well as the number of left, right, and central presentations will vary somewhat between participants.
« Return to Top»

Data Analysis

For analyses that use the response time measure, users will have to choose whether to use the response times from all the trials independent of correctness or whether to use only the data from correct trials. The macro we provide combines correct and incorrect trials. However, one could easily go through and delete trials on which participants were incorrect. Once these data were eliminated from the data file, the macro would provide an analysis of data from correct trials only.

Once the preferred data have been selected, they can be examined to determine whether word type (congruent or non-congruent) and word position had effects on response time and whether the effect for word type was conditional on word position. Tests for these effects can be obtained using a 2x3 repeated measures analysis of variance. The anticipated effect for word type is that non-congruent stimuli take longer to judge than congruent stimuli. Obtaining this effect replicates the well-known Stroop phenomenon. As indicated in the introduction, there is a possibility that the strength of the Stroop phenomenon might be conditional on where the stimulus appears, which would be evident in the word position X word type interaction term in the analysis of variance.
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References

MacLeod, C.M. (1991). Half a century of research on the stroop effect: An integrative review. Psychological Bulletin, 109, 523-553.

Schmit,V. & Davis, R. (1974). The role of hemispheric specialization in the analysis of Stroop stimuli. Acta Psychologica, 38, 149-158.

Warren, L.R. & Marsh, G.R. (1979). Hemispheric asymmetry in the processing of Stroop stimuli
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Last revised:June 03, 2003 11:46:58 AM
Copyright© 1995 The University of Mississippi. All rights reserved.
Questions about this page?PsychExps



Stroop Experiment

Back to Instructor's Page
Introduction Stimuli Design Procedure Data Analysis References
Introduction
This experiment employs divided visual field methodology to determine whether the Stroop phenomenon is equally strong in both brain hemispheres. There is a slight hint in the literature (Schmit & Davis, 1974; Warren & Marsh, 1979), that hemispheric effects might be present but the nature and direction of those effects are difficult to discern given methodological differences in the studies and gender differences in the subjects. On intuitive grounds alone, hemispheric effects might be expected based on the fact that the Stroop phenomenon is created through semantic interference in a color judgment task. In that semantics is a cognitive specialty of the left hemisphere, one might expect that left hemisphere processing of Stroop stimuli would be superior to right hemisphere processing. On the other hand one might predict the reverse, arguing that the not-so-semantically-aware right hemisphere should be less affected by semantic issues than the left. « Return to Top».
Stimuli
The stimuli used in this experiment are the color words RED, BLUE, GREEN, and YELLOW printed in color. The word and color are either congruent, as illustrated by the words RED, BLUE, YELLOW, GREEN, each of which is printed in the color identified by the word, or non-congruent, as illustrated by these words RED, BLUE, YELLOW, GREEN, each of which is printed in a color different from that identified by the word. « Return to Top»
Design
There are two independent variables, both of which are manipulated within subjects. First is word position. Words appear to the right, to the left, or in the middle of the field of vision established by a pulsating focal point used to initiate each trial. The second variable is nature of the stimulus word, which is either congruent in word and color or non-congruent. The dependent variable is response time, which is the interval between a word's appearance and a keypress that indicates the font color in which the word is printed. The keypress is executed on the numeric pad of the computer where a "2" represents the color red, "4" represents blue, "6" represents green, and "8" represents yellow. In addition to recording response time, the program also records whether a response was correct or incorrect. To assure that research participants master the number-color code before generating data, the program sets a performance criterion that must be achieved on practice trials. Once the criterion is met they can proceed to the experiment. « Return to Top»
Procedure
Research participants elect the number of trials they conduct but they must complete at least 72 trials for the data to be admissible for inclusion in the PsychExps database. Each trial begins with a pulsating plus sign that serves to establish a focal point on the computer screen. Research participants press a key when they are ready for a word to be presented. Following the key press, there is a random delay from 1 to 3 seconds before the word appears. When words appear, they are written vertically on the screen. The central axis of the word is designed to be approximately 3^o of visual angle to the right or left of the fixation point on lateral presentations, though the actual visual angle will vary depending on monitor size and the distance of research participants from the monitor. On central presentations, the word axis is at 0^o relative to the fixation point. Display times are fixed at 150 msec to assure that laterally presented words are experienced in a single visual hemifield. With four color words (RED, BLUE, GREEN, and YELLOW) that are printed in a word-color congruent form or non-congruent form appearing at any of three positions (left, center, and right), there are 24 possible stimulus events. Which of these 24 events occur and the order in which they occur is determined by sampling randomly with replacement from the set of 24 events. This means that the number of congruent and non-congruent trials as well as the number of left, right, and central presentations will vary somewhat between participants. « Return to Top»
Data Analysis
For analyses that use the response time measure, users will have to choose whether to use the response times from all the trials independent of correctness or whether to use only the data from correct trials. The macro we provide combines correct and incorrect trials. However, one could easily go through and delete trials on which participants were incorrect. Once these data were eliminated from the data file, the macro would provide an analysis of data from correct trials only. Once the preferred data have been selected, they can be examined to determine whether word type (congruent or non-congruent) and word position had effects on response time and whether the effect for word type was conditional on word position. Tests for these effects can be obtained using a 2x3 repeated measures analysis of variance. The anticipated effect for word type is that non-congruent stimuli take longer to judge than congruent stimuli. Obtaining this effect replicates the well-known Stroop phenomenon. As indicated in the introduction, there is a possibility that the strength of the Stroop phenomenon might be conditional on where the stimulus appears, which would be evident in the word position X word type interaction term in the analysis of variance. « Return to Top»
References
MacLeod, C.M. (1991). Half a century of research on the stroop effect: An integrative review. Psychological Bulletin, 109, 523-553. Schmit,V. & Davis, R. (1974). The role of hemispheric specialization in the analysis of Stroop stimuli. Acta Psychologica, 38, 149-158. Warren, L.R. & Marsh, G.R. (1979). Hemispheric asymmetry in the processing of Stroop stimuli « Return to Top»

Last revised:June 03, 2003 11:46:58 AM Copyright© 1995 The University of Mississippi. All rights reserved. Questions about this page?PsychExps