SPATIAL STROOP INTERFERENCE AS A FUNCTION OF THE PROTOTYPICALITY OF SPATIAL POSITIONS. Brandi A. Klein. A Thesis

Transcription

1 SPATIAL STROOP INTERFERENCE AS A FUNCTION OF THE PROTOTYPICALITY OF SPATIAL POSITIONS Brandi A. Klein A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF ARTS May 2010 Committee: Dale Klopfer, Advisor Dara Musher-Eizenman Richard Anderson

2 ii ABSTRACT Dale Klopfer, Advisor This study examined whether changes in spatial Stroop interference and facilitation occurred in prototypical and non-prototypical spatial positions, as determined by distance from fixation. Participants were shown the spatial terms above, below, left, and right, and neutral words. These terms were either congruent or incongruent with their spatial position in reference to a fixation cross, and they occurred in three levels (1, 3, 6 ) above, below, left, and right of fixation. Results indicated that a spatial Stroop effect did occur, and that the effects of prototoypicality on interference and facilitation varied between participants. This suggests that spatial prototypes might not be the same for all people, and that some interference may occur at the semantic level.

3 iii TABLE OF CONTENTS Page CHAPTER 1: INTRODUCTION Prominent Accounts of Stroop Interference... 1 Semantic Effects and Stroop Interference... 4 Variants of the Stroop Task... 6 Spatial Categories... 8 The Current Study CHAPTER 2: METHOD Participants Apparatus Stimuli Design Procedure CHAPTER 3: RESULTS CHAPTER 4: DISCUSSION REFERENCES APPENDIX A: FIGURES

4 iv LIST OF FIGURES Figure Page 1 Figure from Logan and Sadler (1996) depicting reaction time as a function of absolute distance between reference and located objects from two versions of Logan and Sadler s (1996) experiment 4 in which subjects judged above and below. True versus False responses and long (dotted lines) versus short (solid lines) distances are the parameters The twelve possible locations for stimuli presentation. Words were centered on positions marked by asterisks Facilitation (ms) as a function of prototypicality (1, 3, 6 from fixation) and position (above, below, left, right) Interference (ms) as a function of prototypicality (1, 3, 6 from fixation) and position (above, below, left, right) Mean overall reaction times (ms) to neutral words, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right) Example of neutral word analyses for an individual person. This individual responded quickest in the 6 above position, the 1 below position, the 3 left position, and the 6 right position; therefore, his/her above data will be analyzed with other participants who responded quickest in the 6 above position, his/her below data will be analyzed with other participants who responded quickest in the 1 below position, etc Mean overall reaction times (ms) to neutral words in the 1 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right)

5 v 8 Mean overall reaction times (ms) to neutral words in the 3 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right) Mean overall reaction times (ms) to neutral words in the 6 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right) Mean facilitation (ms) to neutral words in the 1 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate no significant difference. Asterisks (*) indicate marginal significance (p <.06) Mean facilitation (ms) to neutral words in the 3 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate no significant difference. Asterisks (*) indicate marginal significance (p <.06) Mean facilitation (ms) to neutral words in the 6 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate no significant difference. Asterisks (*) indicate marginal significance (p <.06) Mean interference (ms) to neutral words in the 1 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate no significant difference. Asterisks (*) indicate marginal significance (p <.06) Mean interference (ms) to neutral words in the 3 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate

6 vi no significant difference. Asterisks (*) indicate marginal significance (p <.065) Mean interference (ms) to neutral words in the 6 condition, as a function of prototypicality (1, 3, 6 ) and position (above, below, left, right). Different superscripts indicate a significant difference and same superscripts indicate no significant difference. Asterisks (*) indicate marginal significance (p <.07)... 40

7 1 CHAPTER 1: INTRODUCTION In the years since Stroop s original study in 1935, cognitive scientists have conducted hundreds of studies on variants of the Stroop effect. In the typical Stroop task, a color word is displayed in a given color that is to be identified by the participant. Stroop interference occurs when a person identifies the color of an incongruent color word (e.g., the word red displayed in blue) slower than he/she can identify the color of a neutral word (e.g., the word desk displayed in green). This finding is robust and reliable (see MacLeod, 1991, for a review). It has also been found that color identification of congruent color words (e.g., the word yellow displayed in yellow) is faster than color identification of neutral words, showing facilitation effects; however, this finding is both less robust and reliable (see MacLeod, 1991). The current study examined these interference and facilitation effects using a spatial variation of the Stroop task in which spatial terms were presented at differing locations and distances from fixation, thereby placing the terms in more and less prototypical spatial positions. Prominent Accounts of Stroop Interference A classic interpretation of the Stroop effect is the relative speed of processing theory. This theory begins with the observation that words can be read faster than colors can be named. In the Stroop task, there are two potential responses (the word and the name of the color), and these responses are processed in parallel. The difference in speed between reading a word and naming a color causes response competition, meaning that the word and the color, as two potential responses, are in competition to be the first response produced. For this theory, the time that is elapsed to resolve this competition is the source of interference. This theory is sometimes referred to as the horse race model because the two responses are in competition, racing to the finish line speed determines which response is given priority (MacLeod, 1991).

8 2 Another prevalent explanation of the Stroop effect is the theory of automaticity. Again, two responses (the word and the color) are in competition; however, this competition has less to do with speed and more to do with the amount of attention that is focused on each dimension. It requires much more attention to name an ink color than it does to read a word; in fact, according to this theory, word reading is automatic. When naming an ink color, voluntary effort must be used to choose a color from many possible color names, but there is only one possible response to word reading. This theory makes the assumption that the difference in the automaticity of word reading and color naming is caused by our extensive practice with word reading in comparison to color naming. Accordingly, more automatic processes should interfere with less automatic processes (MacLeod, 1991). Dunbar and MacLeod (1984) conducted a study to test the speed of processing account of the Stroop effect, theorizing that if speed of processing determines the outcome of the response competition, then slowing down word reading should produce smaller amounts of interference. They manipulated color words, transforming them to make them backwards or upside-down and backwards. This manipulation made the words harder to read, and thus, word reading time was slowed. They found no reduction in interference when word reading time was slowed, even when reading time was slowed down dramatically. They interpreted these results as evidence against the speed of processing theory, and also as evidence against the automaticity theory. Although it was previously believed that automaticity was an all-or-none concept, meaning that a process was either automatic or it was not automatic, MacLeod and Dunbar (1988) have discovered that there is evidence for a continuum of automaticity. MacLeod and Dunbar s (1988) study was designed to test the whether automaticity was really an all-or-none concept. They believed that automaticity could be learned through giving participants differing

9 3 amounts of practice. Extensive practice of one dimension should lead to greater automatization of that particular dimension, and thus, this dimension should cause greater interference when it is the dimension that is to be ignored in the Stroop task. They distinguish this from the relative speed of processing account of the Stroop effect by stating that it is quite possible for the slower dimension to interfere with the quicker dimension, as long as the slower dimension has a sufficient degree of automaticity. Because word reading has been practiced much more than any amount of training during the course of a study could give, they used color naming and novel shape naming instead of word reading. The novel shapes were given names color names, specifically, the names of the same colors that are used in the experiment. Participants were given shape naming practice over a period of 20 days. When naming colors, there was virtually no interference on day 1. This is not surprising because color naming is much more practiced than naming novel shapes. They hypothesized that as shape naming became more automatic (with practice), shapes should begin to interfere more with color naming. Indeed, there was strong interference in color naming by Day 5, and this interference persisted through Day 20. With regard to shape naming, there was interference on Day 1, which again is not surprising because of the greater practice of color naming, this interference persisted through Day 5. On Day 20, interference dropped off sharply. This indicates that shape naming had become so automatic that incompatible colors no longer interfered. The results of this study suggest that automaticity can be acquired, and that it is not an all-or-none concept. These prominent theories of Stroop interference both maintain that the locus of interference is the response competition stage of processing. The present study found some evidence that in addition to the interference that occurs during response competition, some

10 4 interference may also occur at the semantic level, prior to response competition. Many other studies have found similar evidence that interference occurs at the semantic level. Semantic Effects and Stroop Interference An interesting finding using the typical color-word task is that the relationship between the word and the ink color in an incongruent color-word pair can cause semantic confusion, and this confusion can cause differences in color identification times. For example, Klein (1964) found that the amount of Stroop interference changed depending on a semantic gradient, meaning that interference increased as the semantic distance between the to-be-named ink color and the meaning of the color-word decreased. He found that color words that were part of the response set (words that named the ink colors that were displayed in the experiment) caused the most interference, followed by color words not in the response set (words that named ink colors that were not displayed in the experiment), words that implicated other colors (e.g., the word fire implicated the color red), and neutral English words, respectively. Sharma and McKenna (1998) characterized Klein s (1964) semantic gradient as the result of four different components of Stroop interference: lexical, semantic relatedness, semantic relevance, and response set membership. For the lexical component, they explain that any word that is in the lexicon will show greater interference than a non-lexical item, such as nonsense syllables or symbols. For the semantic relatedness component, any word that is semantically related to the target response will show greater interference than any word that is not semantically related to the target response (i.e., the word fire is associated with the color red, so it will show more interference with the color blue than the word club). The semantic relevance component takes into account the fact that color words that are not part of the response set are still relevant when the task is to name ink colors, and thus, irrelevant color words (e.g., the word

11 5 purple when the response set only includes the colors red, green, blue, and yellow) will show greater interference than semantically related non-color words (e.g., fire). The final component, response set membership, states that any word that is also part of the response set will show greater interference than any word that is not part of the response set. Risko, Schmidt, and Besner (2006) set out to assess whether a response set effect occurred for color-associates. It had previously been established the color words in the response set interfered more than color words not in the response set (see Klein, 1964; Sharma & McKenna, 1998). It was the goal of Risko, Schmidt, and Besner to determine if a colorassociated word (e.g., the word lake is associated with blue) would still cause interference if the associated color (e.g., the color blue) was not part of the response set. Their results indicated that color-associates that were related to colors in the response set interfered more than colorassociates that were unrelated to colors in the response set, and color-associates that were unrelated to colors in the response set interfered more than neutral words. These results indicate that a response set effect occurs for color-associated words, although it is less than for color words. Many other researchers have studied semantic components of the Stroop effect and found similar results as the relationship between the semantic meaning of the to-be-ignored color word and the ink color increases, interference increases. Rather than varying the semantic categories of the color words, one study conducted by Klopfer (1996) varied the degree to which the ink color of the word was similar to the color denoted by the word. For example, the color orange is more similar to the colors yellow and red than to blue because the color orange can be broken down into individual components of yellow and red. He found that performance (Stroop interference and errors) was worse when the ink color of the word and the color denoted by the

12 6 word were more similar (e.g., the word orange caused greater interference with the color yellow than with the color blue). He explained that Stroop interference increased as the similarity between the word and the ink color increased. Klein (1964), Sharma and McKenna (1998), and Risko, Schmidt, and Besner s (2006) findings of a semantic gradient can be accounted for by mediated priming. This implies that the word lake primes the color blue, so the word lake indirectly invokes the semantic meaning of the color blue, which is why it causes interference and facilitation. Klopfer (1996), on the other hand, proposes that the interference observed when color words are more similar to ink colors arises out of confusions in perceptual (color) space. Color words and ink colors that are closer together in perceptual (color) space cause greater interference than color words and ink colors that are farther apart in perceptual (color) space. The current study employed a spatial Stroop task (explained in the following section) using elements of Klopfer s (1996) study, and found evidence that interference varies with prototypicality of spatial location. Spatial words that were in an individual person s prototypical spatial position caused greater interference than the spatial words that were not in an individual s prototypical position (in general). This supports the theory that at least some of the Stroop interference occurs at the semantic level. Variants of the Stroop Task Although the typical Stroop task involves color-word interference, there are many variations of the Stroop task (see MacLeod, 1991). A spatial variant of the Stroop task has been used in several studies, most notably in an experiment by Seymour (1973) in which participants were presented with the words above, below, left, and right, and had to name the position of the word relative to a dot or the position of a dot relative to the word. He found both facilitation and

13 7 interference effects, with significant interference for all four incongruent conditions (e.g., the word above presented below a dot) when the task was to name the position of the word relative to the dot, but no significant interference when the task was to name the position of the dot relative to the word. Seymour argues that this effect results from 1) the word being the center of focused attention when it is the object whose position must be named and not when the word is the reference point of the dot whose position must be named, and 2) the word being assigned a semantic interpretation when it is the object whose position must be named and not when the word is the reference point of the dot whose position must be named. A more recent series of spatial Stroop studies conducted by Walley, McLeod, and Weiden (1994) found that increased attention to the irrelevant dimension of the Stroop task increased interference (e.g., the spatial term is the irrelevant dimension when the task is to name the spatial location). They first showed that interference occurred when subjects were required to name the incongruent position of the words above, below, left, and right. They also found interference when subjects were first required to name the position of the word, and then to later recall which word had been presented. They then employed both word reading and position naming to demonstrate that large interference and reverse Stroop effects were found when subjects did not know whether they would be asked to read the word or name the position, and thus they had to be prepared to do either. Because of this manipulation, interference increased when attention to the irrelevant dimension was increased. Using a picture-word variant of the Stroop task, Lupker and Katz (1982) set out to study the automatic semantic processing of pictures and the effects of this processing on word judgments. Across two experiments, they presented different background pictures (line drawings) with differing words superimposed onto the pictures. These pictures and words were

14 8 both either of animals or non-animals, and non-pictures were used as a baseline measurement. The non-pictures were enclosed scribbles that had no resemblance to any physical object. The task was to ignore the picture and respond to whether the word was an example of the category animal. They found that reaction times were longest when animal words were superimposed onto non-animal pictures and when non-animal words were superimposed onto animal pictures. Although they did not originally intend to study the effects of the category typicality of these words and pictures, they considered category typicality as an alternative interpretation of their results. They used the production norms of Battig and Montague (1969) to conduct a median split of both the pictures and words, splitting them into a highly typical group and an atypical group, and then comparing the interference scores between the two groups. In both analyses (words and pictures), they found that the highly typical group produced more interference than the atypical group. This suggests that pictures and words that were more typical examples of their category cause greater interference than less typical pictures and words. These spatial and picture-word variants of the Stroop task are important to the present study because a modified version of the spatial Stroop task was used in order to find effects similar to the category typicality effects that were found in the picture-word variant of the Stroop task. In the present study, the prototypicality of the spatial words was manipulated by using varying distances from a fixation cross. The study determined that these manipulations did indeed have an effect on Stroop interference in the different spatial categories under certain circumstances. Spatial Categories Several studies have used acceptability ratings to indicate that the most prototypical spatial positions for above, below, left, and right relative to some reference object are along the

15 9 vertical and horizontal axes thru the object (Hayward & Tarr, 1995; Logan & Sadler, 1996; Carlson-Radvansky & Logan, 1997). All of these studies found that acceptability ratings for spatial terms such as above and below decrease as the angular distance from these axes increases. Logan and Sadler (1996) theorize that spatial templates which contain good, acceptable, and bad spatial positions are applied in parallel to the whole visual field, so distance between reference and located objects should not matter. Accordingly, both Logan and Sadler (1996) and Hayward and Tarr (1995) did not find a significant effect of varying the distance between reference objects and located objects when using acceptability ratings. Even though these particular studies have not found differences in acceptability ratings with different distances, several experiments have found significant differences in acceptability ratings with different distances. Carlson and Logan (2001, Experiment 1B) found that when the located object was placed directly above the reference object (as in the present experiment), a larger distance between the reference and located object yielded significantly lower acceptability ratings. Thus, objects further above the reference object were less acceptable examples of the location above. They reconcile these findings with previous failures to find distance effect (e.g., Logan & Sadler, 1996) by explaining that in this experiment, located objects were only placed directly above the reference object; in previous studies, the located object was placed in many different locations around the reference object. They believe that their experiment did not provide the contrast of the previous experiments, and that perhaps the lack of contrast in this experiment made the distance dimension more salient because it was the only contrast available. Similarly, Carlson-Radvansky and Irwin (1993) found that decreasing the distance between a reference and located object increased the acceptability ratings for the spatial term above. Taken

16 10 together, these studies indicate that there are more prototypical or better examples of spatial categories than others, and that shorter distances are more acceptable than longer distances. Although this is true, acceptability ratings are not the only way to measure prototypicality; there are also production tasks and reaction time studies. Production tasks have been performed by Logan and Sadler (1996), where participants are given a square and asked to draw a dot above the square (or below, left, etc.). They found that participants do not draw the above dot extremely close to the square: there is a gap between the top of the square and the bottom-most dot. All participants dots clustered in this area. These data, taken with the data from acceptability ratings that closer distances are more acceptable, suggest that there is a prototypical area for each spatial direction (above, below, left, right) that is close to the reference object, but not too close. Participants produce dots in spatial positions that are not too close and not too far away from the reference object. [Although this is true, prototypical positions probably depend on the size and shape of the reference object, as Regier and Carlson (2001) found that acceptability ratings for the spatial location above increase with distance from the reference object when the reference object is a wide rectangle or L-shaped]. These production data are supported by the reaction time data of Logan and Sadler (1996). The spatial template approach of Logan and Sadler (1996) theorizes that when deciding whether something is above or below, spatial templates are applied in parallel to the whole visual field, so different distances between reference and located objects should not create any differences in reaction time. They manipulated the distance between a dash and a plus sign from 1.48 to 5.92 and reported a significant trend for reaction time to decrease with distance; however, upon examining the trend lines, it is found that reaction time was longest at the shortest distance (1.48 ), decreased to the shortest reaction times at the intermediate distances (2.96,

17 ), and increased to the second longest reaction time at the farthest distance (5.92 ) (see Figure 1). This suggests that it was easiest for participants to respond when the dash and the plus sign were not too close together or too far away an intermediate distance gave the shortest reaction times. These spatial category studies combined suggest that prototypical spatial positions for above, below, left, and right should exist at a distance that is not too close to the reference object (such as 1 ), but not too far away either (such as 6 ). The present study employs this knowledge in the experimental design. The Current Study The present spatial Stroop study examined the prototypicality of spatial positions and employed a vocal response to test the question of whether spatial Stroop stimuli that vary in the degree to which they are above, below, left, or right of fixation produce equivalent amounts of interference and facilitation. The stimuli were the words above, below, left, right, and neutral words (paper, sound, home, and world) which were presented in three levels above, below, left, and right (1, 3, 6 ) of a fixation cross. The 3 locations were considered to be the prototypical locations based on Logan and Sadler s (1996) reaction time data (again, see Figure 1), and because the location is an intermediate distance between distances that were too close (1 ) and too far away (6 ). Participants were required to name the location and ignore the word. The words were both congruent (e.g., the word above in a position above fixation) and incongruent (e.g., the word below in a position above fixation) with their spatial location in reference to the fixation cross, and neutral words were presented as a baseline measurement. The reaction time to these neutral words was used to determine if participants really did consider the 3 location to be the prototypical location.

18 12 Taken together, previous studies allow several predictions for the current study. Based on the data of Logan and Sadler (1996), reaction time for neutral words should be quickest at the prototypical positions. The facilitation of congruent spatial terms should also be greatest at the prototypical positions because of semantic activation of the spatial term. If the spatial term is already in its prototypical spatial position, then no semantic confusion will occur. Interference should also be greatest in the prototypical spatial positions because of the greater amount of semantic confusion that should occur. For example, there should be greater interference when the word above is in the prototypical below position than when it is in a non-prototypical below position. This is because the semantic meanings of above and below invoke a particular spatial position, and greater semantic confusion will occur if the wrong words are in these positions. Finding greater interference in prototypical spatial positions would thus lend support to the theory that not all Stroop interference occurs during the response competition stage of processing at least some interference must occur at the semantic level.

19 13 CHAPTER 2: METHOD Participants The participants were 35 psychology student volunteers who were tested individually in one 45 minute session and received partial course credit as compensation. All participants spoke English as their first language and had normal or corrected-to-normal vision. Apparatus The experiment was run using E-Prime (Schneider, Eschman, & Zuccolotto, 2002) software that controlled stimulus presentation and collected data on a Dell Dimension 8250 computer with a 16 inch color monitor. Stimulus presentation started a millisecond timer, and a vocal response into a microphone stopped the timer. The participant s vocal response was detected using a PST Serial Response Box that was connected to the microphone. Stimuli The stimuli were the words above, below, left, and right, and neutral words (paper, sound, home, and world) which were presented in three levels (1, 3, 6 ) above, below, left, and right of a fixation cross. The neutral stimuli were matched for length and frequency of usage (Carroll, Davies, & Richman, 1971). The words were presented in Arial font, in all capital letters, and were either congruent or incongruent with their position. The words subtended a visual angle of approximately 1 vertically and 2 horizontally when participants were seated at a viewing distance of approximately 60 cm. Design This experiment consisted of a 5 stimulus type (congruent, 3 different incongruent words, neutral) X 3 prototypicality (1, 3, 6 ) X 4 position (above, below, left, right) within-subjects

20 14 design. The visual angles of 1, 3, and 6 were chosen to be in the same range used by Logan and Sadler (1996). There were 8 trials in each condition for a total of 480 experimental trials. All conditions varied randomly throughout the experiment. Procedure Participants were instructed to name the position of the word on the screen as being above, below, left, or right of the fixation cross, and to do so as quickly and accurately as possible. Because the voice trigger was very sensitive, participants were asked to be careful to not make any response-unrelated noises during the experiment (such as coughing or saying umm or oops ). The experiment began with a microphone check, during which participants were asked to say 20 words into the microphone so that the sensitivity of the microphone could be tuned to the participant s voice. The microphone check was followed by a set of 20 practice trails, after which the 480 experimental trials began. Participants initiated each trial by pressing the space bar. Each trial began with a 500 ms presentation of a fixation cross, after which the word above, below, left, right, or a neutral word was presented in one of twelve locations in reference to the fixation cross (see Figure 2). The word remained on the screen until a vocal response was made. The screen then went blank until the experimenter keyed in the participant s response. If the participant gave two responses, the initial response was recorded. An error was recorded if the participant made any response-unrelated sound prior to their response. After this, the participant pressed the space bar to begin their next trial.

21 15 CHAPTER 3: RESULTS Consistent with previous Stroop studies, any trial during which a participant made an error (e.g., responds above to the location below) was removed from the analysis, resulting in 2.02% data removal. Participants made significantly more errors with incongruent spatial terms than with congruent or neutral terms, F(2,68) = 57.56, p <.001, η 2 p = 0.63, and there was no significant difference in errors between the congruent and neutral conditions (p =.94). Error 2 rates did significantly differ by position, F(3,102) = 4.02, p =.009, η p = 0.10, with the above position having significantly fewer errors than left (p =.012) and right (p =.018). There were no differences in error rates among the prototypicality conditions, and there were no significant interactions (all p s >.10). Error analyses revealed that 56.74% of the total errors were Stroop errors (replying with the spatial term instead of its location), and 64.18% were in the left and right locations (with 56% of these errors occurring on the left). Of the left errors, 73.3% were said to be on the right; and of the right errors, 63.8% were said to be on the left. Out of the total above/below errors (35.82%), 60.38% were in the below position. In order to remove outliers associated with microphone errors, any reaction time that was further than 2.5 standard deviations away from each participant s mean reaction time within each congruency condition was also removed from the analysis, resulting in 0.30% data removal. The remaining data was analyzed using a 3 Congruency (congruent, incongruent, neutral) X 3 Prototypicality (1, 3, 6 ) X 4 Position (above, below, left, right) within-subjects ANOVA to determine if reaction time differed significantly between conditions. There was a main effect of congruency [F(2, 68) = , p <.001, ηp 2 = 0.80], with incongruent spatial terms having the longest reaction times (M = , SD = 94.97), followed by neutral terms (M = , SD = ) and congruent terms (M = , SD = 89.47). Pair-wise comparisons indicated that

22 16 the reaction time to the incongruent words was significantly different from both the congruent words (p <.001) and the neutral words (p <.001). Difference scores were used to measure facilitation (congruent reaction time neutral reaction time) and interference (incongruent reaction time neutral reaction time). Facilitation scores were analyzed using a 3 Prototypicality (1, 3, 6 ) X 4 Position (above, below, left, right) within-subjects ANOVA. Mean facilitation scores as a function of prototypicality condition and position can be found in Figure 3. Results indicated that facilitation effects did not differ significantly between prototypicality conditions, F(2, 68) = 0.759, p =.472, and positions F(3, 102) = 1.408, p =.245. Interference scores were also analyzed using a 3 Prototypicality (1, 3, 6 ) X 4 Position (above, below, left, right) within-subjects ANOVA. It should be noted that the incongruent terms could be broken into two groups, incongruent terms that are 180 apart from each other (above/below and left/right) and incongruent terms that are 90 apart from each other (above/left, above/right, below/left, and below/right); however, analyses revealed that there was no significant difference when using 180 incongruent spatial terms compared to using 90 spatial terms. Because of this, all incongruent spatial terms were collapsed together. Mean interference scores as a function of prototypicality condition and position can be found in Figure 4. Results indicated that there was no significant difference between prototypicality positions, 2 F(2, 68) = 2.28, p =.110. There was a main effect of position [F(3, 102) = 3.91, p =.011, η p = 0.10], with the most interference occurring in the below position (M = 38.77, SD = 51.58), followed by above (M = 28.11, SD = 47.19), right (M = 18.20, SD = 57.55), and left (M = 14.58, SD = 53.71). The overall reaction times to neutral words were analyzed to determine in which location the participants were quickest to respond, which would indicate which spatial location was

23 17 prototypical. Neutral reaction times were analyzed using a 3 Prototypicality (1, 3, 6 ) X 4 Position (above, below, left, right) within-subjects ANOVA. Mean neutral reaction times as a function of prototypicality condition and position can be found in Figure 5. There was a marginally significant main effect of prototypicality [F(2, 68) = 2.86, p =.064, η 2 p = 0.08], with participants responding the quickest at the 3 location (M = , SD = 95.50), followed by the 6 location (M = , SD = ), and the 1 location (M = , SD = 98.40). Pair-wise comparisons indicated that the 3 location was significantly different from both the 6 (p =.047) and 1 (p =.023) locations. There was also a significant main effect of position [F(3, 102) = 9.72, p <.001, η 2 p = 0.22], with participants responding the quickest in the above position (M = , SD = ), followed by the below position (M = , SD = 84.07), the right position (M = , SD = ), and the left position (M = , SD = ). There was 2 also a significant prototypicality X position interaction [F(6, 204) = 2.56, p =.021, η p =0.07], which shows that the marginally significant main effect of prototypicality (reaction time was quickest at the 3 position) was entirely driven by the left position, as the right position shows a negligible trend in this direction and the above and below positions do not show this trend at all (again, see Figure 5 for means). This lack of a uniform prototypical spatial location can explain the surprising finding of a non-significant prototypicality effect with interference. To gain further insight into these findings, the neutral reaction time data were further analyzed to explore the possibility that prototypes might not be the same for everyone. For each individual participant, the overall reaction times to neutral words were analyzed to determine which locations were responded to the quickest. It was found that some participants responded to neutral words the quickest in the 1 position, others were quickest in the 3 position, and still others responded quickest in the 6 position. The responses were divided into

24 18 three conditions based on which prototypicality position was responded to the quickest, and the locations (above, below, left, right) per condition were analyzed separately from each other. For example, the number of participants who responded to the 1 above position the quickest were analyzed together, the number of participants who responded to the 1 below position the quickest were analyzed together, the number of participants who responded to the 1 left position the quickest were analyzed together, the number of participants who responded to the 1 right position were analyzed together, and all of these were considered to be in the 1 condition (the same example could be applied for the 3 and 6 conditions) (see Figure 6 for an example). Mean overall reaction times for these conditions can be found in Figure 7 (1 condition), Figure 8 (3 condition), and Figure 9 (6 condition). For those in the 1 condition, the 1 position was responded to significantly quicker than the 3 and 6 positions, for all four locations (above, below, left, right), all p s <.05. For those in the 3 condition, the 3 position was responded to significantly quicker than the 1 and 6 positions, for all four locations (above, below, left, right), all p s <.05. For those in the 6 condition, the 6 position was responded to significantly quicker than the 1 and 3 positions, for all four locations (above, below, left, right), all p s <.05. For these reasons, interference and facilitation were also analyzed in the three separate neutral word conditions. Mean facilitation scores for the 1, 3, and 6 quickest neutral word response conditions can be found in Figures 10, 11, and 12, respectively. These data illustrate a trend for participants in the three different neutral word conditions to actually experience less facilitation in their prototypical position. This could possibly be due to floor effects, as they are already responding the quickest to the neutral words in that condition, and facilitation is the difference

25 19 score between the neutral words and the congruent words. Perhaps due to these floor effects, they had less room for facilitation in that prototype position than in the other positions. Mean interference scores for the 1, 3, and 6 quickest neutral word response conditions can be found in Figures 13, 14, and 15, respectively. The results provide evidence that different participants have different ideas of which position is the prototypical position, as those who respond to a particular position the quickest with neutral words (e.g. the 3 position) often show significantly greater amounts of interference in that position with incongruent words. One reason for this trend could be that the semantic meaning of the spatial terms invokes a certain prototypical position for each participant; and therefore, the participants experience greater amounts of interference at the positions that they perceive to be prototypical.

26 20 CHAPTER 4: DISCUSSION As predicted, a spatial Stroop effect was found for the spatial terms above, below, left, and right; however, the interference in relation to prototypical spatial positions was not as clear cut as predicted. For some positions, the predicted prototypical position (3 ) yielded the greatest amounts of interference and facilitation; however, this was not true for all positions, and the difference was not significant. Upon examining the reaction times to neutral words (baseline), it was discovered that participants could be divided into three different categories based on which position (1, 3, 6 ) they responded to the neutral words the quickest. Because of this, each location (above, below, left, right) was analyzed separately based on the three conditions of the neutral words. After dividing the participants into the quickest response to neutral words conditions of 1, 3, and 6, results indicated that participants often experienced significantly greater amounts of interference in the position in which they responded to the neutral words the quickest. Perhaps participants responded quickest to the 1 position (for example) because that position was their prototypical position for the spatial terms. This would provide evidence that these participants are invoking a semantic representation of the spatial terms before making their response; and thus, some interference is occurring at the semantic level. The finding of significantly less facilitation at participants prototypical positions could be explained by either floor effects (responses were fastest in the prototypical location and thus where was not room for facilitation) as mentioned previously, or similarly, by acknowledging that congruency could help location naming more in the non-prototypical positions than in the prototypical positions. This result could also provide evidence that prototypes vary among participants (as facilitation varied between the neutral word conditions), and that spatial terms are activated at a semantic level.

27 21 The spatial locations above, below, left, and right were considered separately in the neutral word analysis because unlike color-word Stroop interference, spatial Stroop interference might not generalize to all spatial categories. The assumption at the outset of the study was that the spatial prototypes would be a uniform distance for all four spatial categories, but this assumption could be wrong. For example, what would be considered a prototypical distance for above might not be considered a prototypical distance for left. The data from this study suggests that this could indeed be the case. Even in past research, it has proven difficult to find prototypes for the left and right locations. Indeed, most of the studies cited in the introduction dealt solely with the above location. An additional thought to consider is that this study rests on an implicit assumption that the linguistic access of spatial categories is the same for above, below, left, and right. The higher error rates with the words at the left and right locations than at the above and below locations cast some doubt on this assumption. There was also an asymmetry in the left/right error rates, with more words on the left erroneously said to be on the right than vice-versa. This study supports Seymour s (1973) finding of a spatial Stroop effect for all four spatial positions (above, below, left, right). However, Logan and Sadler s (1996) spatial template theory (that uniform spatial templates are applied in parallel to the whole visual field, so distance between reference and located objects should not matter) was not supported. The current study found evidence that overall reaction time, interference, and facilitation all vary depending on the distance from the fixation cross. Evidence was also found that could be interpreted as each individual person having a separate prototypical or ideal position for each of the four locations above, below, left, and right. Moreover, there is also some evidence that these four locations operate independently, and thus, the same rules might not apply to all four locations.

28 22 The Stroop effect manifests as a failure of selective attention. In today s world, people are bombarded with so much different stimulation that selective attention becomes extremely important for survival. It seems that people generally take attention for granted, assuming that it always works properly, and that they can attend to whatever is necessary. The Stroop effect is just one example of how attention is imperfect. The color-word version of the Stroop task has been implemented in many hospitals and neurological studies help determine the nature of different neurodegenerative and psychological disorders, and to evaluate executive functions of patients (Howieson, Lezak, & Loring, 2004). The Stroop task is also used in brain imaging studies to study the nature of attention and processing speed (Howieson, Lezak, & Loring, 2004). The spatial Stroop task could also be useful in these regards, especially in patients with brain damage, as this task would presumably recruit spatial areas of the brain in addition to attention and memory areas. In addition, determining the nature of spatial categories will also be very helpful in assessing how the brain processes spatial information. Overall, the evidence from this study that the prototypical spatial positions for each participant varied, and that in those prototypical positions the interference was greater than in the non-prototypical positions, provides support for the theory that not all Stroop interference occurs during the response competition stage of processing. Finding that interference is greater in an individual s prototypical spatial location suggests that the semantic meaning of the spatial term is invoked, and that semantic confusion occurs prior to response competition.

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32 26 APPENDIX A: FIGURES FIGURE 1 Figure from Logan and Sadler (1996) depicting reaction time as a function of absolute distance between reference and located objects from two versions of Logan and Sadler s (1996) experiment 4 in which subjects judged above and below. True versus False responses and long (dotted lines) versus short (solid lines) distances are the parameters.

33 27 FIGURE 2 The twelve possible locations for stimuli presentation. Words were centered on positions marked by asterisks.