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The role of sensitivity to external food cues in attentional allocation to food words on dot probe and Stroop tasks
Eating Behaviors 5 (2004) 261 – 271
The role of sensitivity to external food cues in attentional allocation to food words on dot probe and Stroop tasks Linda Johansson a,b,*, Ata Ghaderi b, Gerhard Andersson b a
Department of Psychology, Stockholm University, Sweden Department of Psychology, Uppsala University, Sweden
b
Accepted 15 January 2004
Abstract The role of sensitivity of external food cues in producing attentional bias toward food-, body-weight-, and shape-related words on the Stroop and the dot probe tasks was examined. Contrary to expectations, individuals high in responsiveness to external food cues directed attention away from food words, whereas individuals low in responsiveness to external food cues directed their attention toward food words on the dot probe task. No significant differences were found between the groups high and low in sensitivity to external food cues for body words on the dot probe task or for food or body words on the Stroop task. Results are discussed with reference to theoretical views of differences between the Stroop and the dot probe tasks. D 2004 Elsevier Ltd. All rights reserved. Keywords: External eating; Attentional bias; Cognitive avoidance
1. Introduction According to the externality theory of overeating, obese individuals are characterised as being unresponsive to internal physiological signals of hunger and satiety but overresponsive to external food cues such as the sight and smell of palatable food (Schachter & Rodin, 1974). This tendency to use the external food environment as a determinant of eating behaviour thus supposedly leads to overeating and obesity. However, high degrees of sensitivity to external food stimuli have also been found in normalweight individuals, suggesting that other factors are involved in determining body weight (e.g., Rodin,
* Corresponding author. Department of Psychology, Uppsala University, Tra¨dga˚rdsgatan 20, Box 1225, 751 42 Uppsala, Sweden. Tel.: +46-18-471-57-51; fax: +46-18-471-21-23. E-mail address: [email protected]. (L. Johansson). 1471-0153/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eatbeh.2004.01.005
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1981). Nonetheless, external responsiveness has been found to predict sensitivity to environmental food cues (Rodin & Slochower, 1976). The modified version of the Stroop colour-naming task (Stroop, 1935) has been used as a measure of attentional bias in a variety of research areas such as eating disorders (e.g., Cooper & Fairburn, 1992; Cooper & Todd, 1997). Delayed colour naming of stimuli specifically related to participants’ concerns are proposed to reflect preferential allocation toward the emotionally salient stimuli. Several studies have, for example, found colour-naming retardation for food- and body weight-related words in eatingdisordered individuals as compared with nonclinical control participants (e.g., Cooper & Todd, 1997). Such attentional bias for food- and body-related words has also been demonstrated for non-eatingdisordered but restrained eaters (e.g., Green & Rogers, 1993; Perpina, Hemsley, Treasure, & de Silva, 1993). However, some investigators (e.g., de Ruiter & Brosschot, 1994) have argued that the Stroop procedure does not provide an adequate measure of attentional allocation. Colour-naming delays can arise either as a result of attention being directed toward or away from emotionally relevant material. Due to the limitations of the Stroop task, some investigators (e.g., Rieger et al., 1998) have instead used a modified dot probe detection task that provides a more specific measure of attentional allocation (MacLeod, Mathews, & Tata, 1986). In the dot probe task, participants are presented with a series of word pairs on a computer screen, one above the other, followed by a visual probe shown in the same location as one of the words (MacLeod et al., 1986). Participants are required to indicate, as quickly as possible, whether the probe appeared in the upper or lower location. It is hypothesised that responses will be made faster to attended, rather than unattended, spatial positions. If attention is directed toward emotionally salient stimuli, probe detection will be faster when the probe replaces words related to these concerns, compared with when the probe replaces neutral words (MacLeod et al., 1986). With a visual probe detection task, it is thus possible to differentiate between attention directed toward versus away from a particular class of stimulus words due to faster or slower probe detection, respectively. Using this procedure, Rieger et al. (1998) found that in comparison with controls, women with eating disorders preferentially directed attention toward words related to a large physique, while avoiding words related to a thin physique, thus suggesting attentional bias toward information of specific concerns for these individuals. However, Boon, Vogelzang, and Jansen (2000) did not find attentional bias for words related to food, and body weight and shape on this task in restrained eaters. Attentional biases on the dot probe task have, however, been demonstrated for food words in fasted individuals, both high and low in preoccupation with eating, weight, and shape (Mogg, Bradley, Hyare, & Lee, 1998; Placanica, Faunce, & Job, 2002). The latter investigators proposed that their findings could not be explained simply in terms of hunger because both weight-concerned and normal individuals directed attention toward high-calorie foods when fasted, whereas only weight-concerned individuals showed attentional bias to low-calorie food words when nonfasted, indicating concern with food-related information both when fasted and nonfasted. In summary, previous research suggests that attentional bias toward food words may not be limited to emotional concerns but may extend to normal motivational states such as hunger. The aim of the present study was to investigate the role of sensitivity to external food cues in determining attentional bias toward food words in a normal sample. The external eating behaviour scale of the Dutch Eating Behaviour Questionnaire (DEBQ; Van Strien, Frijters, Bergers, & Defares, 1986) was used to measure the levels of external eating behaviour. All participants were tested on both the modified Stroop and the dot probe tasks. The specific hypotheses in the present study were that individuals scoring high on the external eating scale of the DEBQ would show greater attentional bias toward food words on both the
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Stroop and the dot probe tasks, compared with individuals scoring low on this measure. The main focus of the present study was to investigate attentional bias for food words, but body weight and shape words were also included because attentional bias has previously also been identified for this type of stimuli.
2. Method 2.1. Participants Fifty-eight women aged between 19 and 28, studying for a psychology degree at Goldsmiths College, University of London, participated in the present study for course credits. Fifteen of these participants had to be excluded from the analyses; eleven participants experienced technical problems on the Stroop task, whereas one participant had an error rate above 10% on the dot probe task and another three participants had scores more than three standard deviations above their own mean on the dot probe and/ or the Stroop tasks. The remaining sample thus consisted of 43 participants. None of these individuals had an error rate above 10% on either the Stroop or dot probe tasks. All participants had a body weight within the normal range; body mass index (BMI) ranged between 19.90 and 24.30. More detailed information with regard to the age and BMI for the two participant groups, formed on the basis of the median split on the external eating scale of DEBQ, and for the whole sample can be found in Table 1. All women had normal or corrected to normal visual acuity. 2.2. Materials and measures 2.2.1. Measures of cognitive bias Both the Stroop and the dot probe tasks were run on a personal computer. To record the participants’ vocal responses on the Stroop task, a microphone and a voice onset relay device were connected to the computer. The stimulus words used consisted of words drawn from previous research on eating-disorderrelated attentional biases (e.g., Cooper & Todd, 1997; Green, Elliman, Rogers & Welch, 1996). There were two types of eating-disorder-related words; 10 words related to high-calorie food (e.g., chocolate) and 10 words related to body parts or negative body shape, assumed to be of concern for many women (e.g., stomach and chubby). Each word list was divided into two equivalent lists that were matched for word length and usage, based on English language norms established by Carroll, Davies, and Richman Table 1 Means and standard deviations on self-report measures, age, and BMI for the two groups formed on the basis of the median split on the external eating scale of DEBQ, separately, and for the whole sample Characteristics RSE BSQ EAT Age BMI
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
18.77 78.59 10.27 22.23 21.76
(4.90) (32.45) (12.59) (2.11) (1.16)
17.38 81.24 9.62 22.24 22.19
(4.73) (30.34) (9.66) (2.21) (1.12)
18.09 79.88 9.95 22.23 21.97
(4.81) (31.09) (11.12) (2.14) (1.15)
RSE = Rosenberg self esteem scale; BSQ = Body Shape Questionnaire; EAT = eating attitudes test; BMI = body mass index.
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(1971). Half of the list of food words and half of the list of body words were used as the stimuli in the Stroop task, and the other halves were used in the dot probe task. The food- and body-related words used in the Stroop and dot probe tasks were matched with neutral control words for word length and frequency. This resulted in four neutral word categories. However, the analyses performed after the data collection revealed that the response latency for one of the neutral word categories used in the Stroop task was affected differently by different colours. None of the other word categories showed this effect of colour. Since all words were matched with each other for word length and word frequency, the decision was therefore made to exclude this neutral word category. In the dot probe task, each food and body word was paired with the corresponding control word. Furthermore, in the dot probe task, an additional 10 neutral word pairs, all matched for word length and frequency, were used as filler material. These 20 words were divided up into four separate semantic categories with 5 words in each. These words were also matched for word length and frequency with the other words. A complete list of the stimulus words can be found in Appendix A. 2.3. Self-report questionnaires The external, restraint, and emotional scales of the DEBQ (Van Strien et al., 1986), comprising 33 questions, were used to assess sensitivity to external food cues (10 items), tendencies to consciously restrict food intake due to concerns about weight (10 items), and sensitivity to emotional eating (13 items). Chronbach’s alpha for various samples has reportedly ranged between .79 and .81 for the external scale and between .92 and .95 for the emotional and the restraint scales (Van Strien et al., 1986). The Eating Attitudes Test (EAT-26; Garner, Olmsted, Bohr, & Garfinkel, 1982), comprising 26 questions, was used to assess the levels of eating disturbance and weight preoccupation, and has a Chronbach’s alpha value of .85. The Body Shape Questionnaire (BSQ; Cooper, Taylor, Cooper, & Fairburn, 1987) was used to measure the concerns about body shape. This measure consists of 34 questions and has a Chronbach’s alpha value of .97. The Rosenberg Self-esteem Scale (RSE; Rosenberg, 1989), consisting of 10 questions, was used to assess global self-esteem. Chronbach’s alpha for various samples has been found to range between .77 and .88 (Blascovich & Tomaka, 1993; Rosenberg, 1986). High scores on this measure indicate low self-esteem. 2.4. Procedure Each participant was tested individually. To avoid order effects, half the women were given the Stroop task first followed by the dot probe task, and the other half were given the dot probe task first followed by the Stroop task. The participants were randomly allocated to the order of task. In the Stroop task, participants were instructed to name out loud the colours, in which words were displayed as quickly, but as accurately, as possible. To become familiar with the procedure, the participants were first given 10 practice trials. Each word was presented individually in the centre of the computer screen in one of four colours: red, green, blue, or yellow. Each word was randomly assigned to one of these colours each time it was displayed. There were 20 stimulus words randomly mixed within a block. The block was repeated four times, resulting in a total of 80 trials for each participant, with a new randomised order of presentation every time. Once a verbal response had been made, the computer
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recorded the time duration between the onset of the stimulus and the response. Errors were recorded manually by the experimenter. After a 2-s delay, the next word was displayed. Participants were given a break halfway through the main trials. In the dot probe task, participants similarly received 10 practice trials before the main trials. There were 80 experimental trials, in which word pairs from each category (food/neutral, body/neutral, neutral/ neutral, neutral/neutral) were repeated four times. The order of presentation of word pairs was randomised for each participant, with a restriction of immediate repetition of the preceding word pair. Word pairs were presented separately in the centre of the computer screen for 500 ms, with one word above and one below the central point. Immediately after the offset of the word pair, a small dot appeared in the location of one of these two words, in line with previous research using the dot probe procedure (e.g., MacLeod & Mathews, 1988). Within each word pair, each word was equally likely to fall in either the upper or lower position on the computer screen, and the dot was equally likely to replace either of the two words. The participants were instructed to indicate where the dot appeared as quickly, but as accurately, as possible by pressing either an upper or lower response key. The computer recorded the response latency, to the nearest millisecond, and the number of errors. After 500 ms, the next trial began. Halfway through the main trials, the participants were given a break. After completing both the Stroop and the dot probe tasks, the participants were asked to fill in the self-report measures. The order of self-report measures given was randomised for each participant.
3. Results 3.1. Self-report measures On the basis of responses on the external scale of the DEBQ, the participants were categorised as either high (n = 22) or low (n = 21) on external eating behaviour, using the median split, the cut-off point being 3.60. The whole sample had a mean value of 3.41 (S.D. = 0.62) on the external scale of DEBQ. The participants categorised as low in terms of external eating behaviour had a mean value of 2.86 (S.D. = 0.30) on this measure, whereas participants categorised as high had a mean value of 3.93 (S.D. = 0.30). The sample mean on the emotional scale of the DEBQ was 2.87 (S.D. = 0.80). The mean values for high and low external eaters on this measure were 3.10 (S.D. = 0.71) and 2.63 (S.D. = 0.83), respectively. The sample mean on the restraint scale of the DEBQ was 2.77 (S.D. = 0.93), where individuals low in external eating had a mean value of 2.93 (S.D. = 0.89) and individuals high in external eating had a mean value of 2.61 (S.D. = 0.95) on this measure. Table 1 presents the means and standard deviations for individuals high and low in terms of external eating behaviour, separately, as well as for the whole sample on the other self-report measures, age, and BMI. Independent sample t tests revealed no significant differences between groups high and low in external eating with respect to age, BMI, or self-report measures. 3.2. Attentional bias indices Bias scores were calculated for each type of eating-disorder-related word type, for both the Stroop and the dot probe tasks. For the Stroop procedure, the interference indices were computed by subtracting the median response latency time for neutral words from the median response latency time for body and
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Table 2 Mean and standard deviation response latencies for each word type and bias indices on the Stroop task for high and low external eaters separately and for the whole sample Stroop Food Body Neutral Food index Body index
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
620.27 610.14 619.09 1.18 8.95
(69.11) (68.03) (62.57) (29.55) (32.85)
(75.18) (78.33) (69.35) (28.25) (36.09)
615.29 613.31 619.95 4.66 6.64
(71.46) (72.43) (65.18) (29.21) (34.14)
610.07 616.64 620.86 10.79 4.21
food words (e.g., Overduin, Jansen, & Louwerse, 1995). Consequently, two attentional bias scores were produced for each participant: one for food words and one for body words. These indices reflect the relative degree of interference in colour naming, where positive scores indicate a greater interference effect of eating-disorder-related words relative to neutral words. Table 2 shows the mean and standard deviation response latencies, in milliseconds, for each word type and for the bias indices on the Stroop task for participants characterised as high and low on external eating, separately, and for the whole sample, calculated from the participants’ median response latencies. In the dot probe procedure, bias scores were calculated for food and body words by using the following formula of MacLeod and Mathews (1988): (upper probe/lower target upper probe/upper target)+(lower probe/upper target lower probe/lower target)/2. In line with Rieger et al. (1998), the bias indices were calculated from the participants’ median detection latencies. For each participant, one bias score was calculated for trials with food-neutral word pairs and one bias score for trials with bodyneutral word pairs. These bias scores provide an index of the degree to which attention was directed toward or away from eating-disorder-relevant stimuli. A positive score indicates faster reaction times for the probes replacing eating-disorder-relevant stimuli rather than neutral stimuli, hence, an attentional bias toward eating-disorder-relevant stimuli. In contrast, a negative score reflects attention being directed away from such stimuli. Table 3 presents the mean and standard deviation response latencies, in Table 3 Sample mean and standard deviation response latencies for each condition involved in computing the food and body dot probe indices and for bias indices shown for high and low external eaters, separately, and for the whole sample Word type Food
Body
Food index Body index
Word position
Probe position
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
Lower Upper Upper Lower Lower Upper Upper Lower
Upper Upper Lower Lower Upper Upper Lower Lower
409.68 422.77 395.59 401.95 427.36 424.32 410.82 416.55 9.73 1.34
(111.94) (117.35) (86.37) (64.61) (108.00) (107.78) (123.89) (147.92) (26.99) (40.26)
439.05 432.52 426.86 404.95 413.38 410.62 416.05 405.95 10.19 6.43
(118.53) (106.29) (119.36) (89.04) (90.04) (85.03) (77.10) (74.76) (23.98) (28.22)
424.02 427.53 410.86 403.42 420.53 417.63 413.37 411.37 0.00 2.45
(114.79) (110.86) (103.75) (76.58) (98.70) (96.43) (102.53) (116.75) (27.20) (34.71)
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milliseconds, for each condition in the dot probe index formula for food and body words, separately, and for complete indices for the high and low external eaters and the whole sample, calculated from the participants’ median response latencies. The analyses were first carried out to test whether there were overall attentional bias effects on the Stroop and the dot probe tasks. Paired sample t tests were carried out to compare word types in the Stroop task, where results revealed no significant differences in colour-naming latencies between either food and neutral words or between body and neutral words (both P>.2). One-way within-subject ANOVAs were carried out on the various conditions involved in the formula for producing dot probe indices. For food words, results showed a significant difference between the conditions, F(2.52,106.03) = 3.60, P=.022, with Huynh-Feldt correction. Paired sample t tests revealed that there was a significant difference between the condition where both food words and dots appeared in the lower location and the condition where the food words appeared in the lower location, whereas dots appeared in the upper location, t(42) = 2.12, P=.040. Participants responded more quickly when the food words and dots both appeared in the lower location. There was also a significant difference between the condition where both food words and dots were presented in the upper location and the condition where food words were presented in the upper location, whereas dots were presented in the lower location, t(42) = 2.29, P=.027. The participants responded faster in the condition where the food words and dots appeared in different locations. There were no significant differences between the conditions involved in the formula for dot probe body words ( P>.5). To test the hypotheses that individuals high in external eating behaviour would show greater attentional bias toward food and body words on the Stroop and dot probe tasks than individuals low in external eating will, a mixed 2 4 ANOVA was carried out, with bias indices as the within-subjects factor and participant groups as the between-subjects factor. The analysis was carried out on the standardised scores (Z scores) of the bias indices. Results revealed a significant interaction between
Fig. 1. Interaction between Stroop and dot probe attentional bias indices, performed on standardised scores, and participants high (n = 22) and low (n = 21) on external eating behaviour.
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indices and groups, F(2.89,118.34) = 2.89, P=.040, with Huynh-Feldt correction. Independent t tests were carried out to analyse the effects of participant groups at each level of attentional bias indices. There was a significant difference between the two participant groups on the dot probe food index, t(41) = 2.55, P=.014, where participants low in external eating showed greater attentional bias toward food words on this task than participants high in external eating. There were no significant differences between the participant groups on any of the other bias indices (all P>.1). However, the P value, in combination with effect size, indicated a trend toward greater Stroop interference for individuals high in external eating compared with individuals low in external eating behaviour for food words, t(41) = 1.36, P=.182, Cohen’s d=.41. The interaction is displayed in Fig. 1.
4. Discussion The main finding of the present study was that, in contrast to expectations, individuals scoring high on the external eating behaviour scale of the DEBQ directed attention away from food words, whereas individuals scoring low on this scale directed attention toward food words on the dot probe task. No significant differences were found between these participant groups on either the responses to body words on the dot probe task or the responses to food or body words on the Stroop task. Furthermore, an overall attentional bias effect was found on the dot probe task for food words when replaced by dots in the lower position of the computer screen. In contrast, when the dots appeared in the upper location, participants were faster to detect the dots replacing the neutral words. This pattern of results might seem to indicate a potential position bias on the dot probe task. However, looking at previous studies using the dot probe task (e.g., Brosschot, de Ruiter, & Kindt, 1999; Mogg, Bradley, & Hallowell, 1994; Mogg et al., 1998), it is clear from the mean response latencies to the probes in these studies that similar trends of consistent lower response latencies for a certain probe position are evident, especially for the upper probe position. No overall attentional bias effect was found on this task for body words and no overall attentional bias effect was found on the Stroop task for either food or body words. The cognitive avoidance found for food words on the dot probe task in individuals highly responsive to external food cues may suggest that these individuals adopted the strategic processing to counteract tendencies to direct attention toward such concern-relevant material. It is interesting to note that on the Stroop task, these individuals instead showed a trend toward attentional bias for food words, although the difference between high and low external eaters did not reach significance. These conflicting findings produced from the Stroop and the dot probe tasks are in line with previous research testing participants on both of these attentional tasks (Brosschot et al., 1999; Mogg et al., 2000). Brosschot et al. (1999) proposed that the Stroop and the dot probe tasks tap different stages of information processing. Decisions with regard to the concern-relevance of stimuli in the Stroop task must be very rapid, tapping bias at an early automatic stage of processing. The gap between the probe and the emotional information on the dot probe task would instead allow more time for assessing significance of concern, thus tapping bias at a later stage in the cognitive processing. Based on these suggestions, it might be plausible to propose that the dot probe task could be more sensitive to strategic processing than the Stroop task is. Considering that individuals high in responsiveness to external food stimuli have been characterised as prone to overeat when faced with palatable foods, attempts may be made by these individuals to avoid such external food cues to avoid overeating. Such avoidance would
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thus be expected on the dot probe task, more amenable to strategic control, but not on the Stroop task assessing earlier automatic processing priorities. However, as mentioned previously, the Stroop task has been criticised for not providing an adequate measure of attentional allocation, where retarded colour-naming could arise either as a result of attention being directed toward or away from emotionally relevant material (de Ruiter & Brosschot, 1994). Considering that the Stroop and the dot probe tasks, presumably providing a more specific measure of attentional allocation, produced opposite results with regard to food words in the same individuals, the current pattern of results may indicate that delayed colour naming on the Stroop task could indeed reflect cognitive avoidance. With regards to the attentional bias found for the food words in individuals low in external eating behaviour, previous research has, as mentioned previously, demonstrated similar results on the dot probe task for high-calorie food words in fasted normal individuals. A limitation of the present study was that the levels of hunger were not assessed. Furthermore, the food words used in the present study only comprised high-calorie foods, while research by Placanica et al. (2002) revealed that processing bias toward high- or low-calorie food depends on state of hunger and levels of concerns with dieting and weight. Moreover, since the aim of the study was to investigate the role of sensitivity to external food cues in producing attentional bias, it might have been more optimal to use pictures of food rather than word stimuli related to food. It is further possible that the use of a self-report measure of external eating behaviour did not provide accurate answers where it might have been more informative to use behavioural assessments of external eating behaviour. In summary, the results of the present study suggest that demonstrations of cognitive avoidance versus attentional bias may depend on the extent to which the attentional task used to assess information bias is susceptible to early automatic processing versus strategic control. Considering that performance on different measures of information bias may tap different underlying mechanisms, it might be advisable to use more than one measure when investigating avoidant versus vigilant biases. Future research is needed to clarify the precise mechanisms involved in the Stoop and the dot probe tasks. Acknowledgements Lars-Gunnar Lundh and Colin Latham for help and advice. Ian Hannent and Rob Davis for technical support. Goldsmith’s College for providing the facilities.
Appendix A Stimulus words used in the Stroop task Food
Body
Neutral
Chocolate Pastry Chips Sweets Biscuit
Stomach Chubby Thigh Flabby Breasts
Monkeys Chick Owls Rodent Dolphin
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Stimulus word pairs used in the dot probe task Food/Neutral
Body/Neutral
Sugar/Clean Cake/Soap Bacon/Dusty Pizza/Washer Fudge/Scrub
Bottom/Notes Fat/Desk Waist/File Plump/Folder Bulky/Diary
Neutral/Neutral
Neutral/Neutral
Songs/Clouds Drum/Storm Tunes/Sunny Stereo/Warming Dancer/Windy
Sheep/Coat Lion/Boots Tiger/Hats Pandas/Necktie Snail/Glove
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Mogg, K., Bradley, B. P., & Hallowell, N. (1994). Attentional bias to threat: Roles of trait anxiety, stressful events, and awareness. Quarterly Journal of Experimental Psychology, 47 A, 841 – 864. Mogg, K., Bradley, B. P., Hyare, H., & Lee, S. (1998). Selective attention to food-related stimuli in hunger: Are attentional biases specific to emotional and psychopathological states, or are they also found in normal drive states? Behaviour Research and Therapy, 36, 227 – 237. Overduin, J., Jansen, A., & Louwerse, E. (1995). Stroop interference and food intake. International Journal of Eating Disorders, 18, 277 – 285. Perpina, C., Hemsley, D., Treasure, J., & de Silva, P. (1993). Is selective information processing of food and body words specific to patients with eating disorders? International Journal of Eating Disorders, 14, 359 – 366. Placanica, J. L., Faunce, G. J., & Job, R. F. S. (2002). The effect of fasting on attentional biases for food and body shape/weight words in high and low eating disorder inventory scorers. International Journal of Eating Disorders, 32, 79 – 90. Rieger, E., Schotte, D. E., Touyz, S. W., Beumont, P. J. V., Griffiths, R., & Russell, J. (1998). Attentional biases in eating disorders: A visual probe detection procedure. International Journal of Eating Disorders, 23, 199 – 205. Rosenberg, M. (1986). Conceiving the self. Malabar, FL: Krieger. Rosenberg, M. (1989). Society and the adolescent self-image. Middletown, CT: Wesleyan University Press. Rodin, J. (1981). Current status of the internal-external hypothesis for obesity: What went wrong? American Psychologist, 36, 361 – 372. Rodin, J., & Slochower, J. (1976). Externality in the non-obese: The effects of environmental responsiveness on weight. Journal of Personality and Social Psychology, 29, 557 – 565. Schachter, S., & Rodin, J. (1974). Obese humans and rats. . Washingon, DC: Erlbaum/Halsted. Stroop, J. R. (1935). Studies of interference in serial verbal interactions. Journal of Experimental Psychology, 18, 643 – 662. Van Strien, T., Frijters, J. E. R., Bergers, G. P. A., & Defares, P. B. (1986). The Dutch Eating Behaviour Questionnaire (DEBQ) for assessment of restrained, emotional and external eating behaviour. International Journal of Eating Disorders, 5, 295 – 315.
The role of sensitivity to external food cues in attentional allocation to food words on dot probe and Stroop tasks Linda Johansson a,b,*, Ata Ghaderi b, Gerhard Andersson b a
Department of Psychology, Stockholm University, Sweden Department of Psychology, Uppsala University, Sweden
b
Accepted 15 January 2004
Abstract The role of sensitivity of external food cues in producing attentional bias toward food-, body-weight-, and shape-related words on the Stroop and the dot probe tasks was examined. Contrary to expectations, individuals high in responsiveness to external food cues directed attention away from food words, whereas individuals low in responsiveness to external food cues directed their attention toward food words on the dot probe task. No significant differences were found between the groups high and low in sensitivity to external food cues for body words on the dot probe task or for food or body words on the Stroop task. Results are discussed with reference to theoretical views of differences between the Stroop and the dot probe tasks. D 2004 Elsevier Ltd. All rights reserved. Keywords: External eating; Attentional bias; Cognitive avoidance
1. Introduction According to the externality theory of overeating, obese individuals are characterised as being unresponsive to internal physiological signals of hunger and satiety but overresponsive to external food cues such as the sight and smell of palatable food (Schachter & Rodin, 1974). This tendency to use the external food environment as a determinant of eating behaviour thus supposedly leads to overeating and obesity. However, high degrees of sensitivity to external food stimuli have also been found in normalweight individuals, suggesting that other factors are involved in determining body weight (e.g., Rodin,
* Corresponding author. Department of Psychology, Uppsala University, Tra¨dga˚rdsgatan 20, Box 1225, 751 42 Uppsala, Sweden. Tel.: +46-18-471-57-51; fax: +46-18-471-21-23. E-mail address: [email protected]. (L. Johansson). 1471-0153/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eatbeh.2004.01.005
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1981). Nonetheless, external responsiveness has been found to predict sensitivity to environmental food cues (Rodin & Slochower, 1976). The modified version of the Stroop colour-naming task (Stroop, 1935) has been used as a measure of attentional bias in a variety of research areas such as eating disorders (e.g., Cooper & Fairburn, 1992; Cooper & Todd, 1997). Delayed colour naming of stimuli specifically related to participants’ concerns are proposed to reflect preferential allocation toward the emotionally salient stimuli. Several studies have, for example, found colour-naming retardation for food- and body weight-related words in eatingdisordered individuals as compared with nonclinical control participants (e.g., Cooper & Todd, 1997). Such attentional bias for food- and body-related words has also been demonstrated for non-eatingdisordered but restrained eaters (e.g., Green & Rogers, 1993; Perpina, Hemsley, Treasure, & de Silva, 1993). However, some investigators (e.g., de Ruiter & Brosschot, 1994) have argued that the Stroop procedure does not provide an adequate measure of attentional allocation. Colour-naming delays can arise either as a result of attention being directed toward or away from emotionally relevant material. Due to the limitations of the Stroop task, some investigators (e.g., Rieger et al., 1998) have instead used a modified dot probe detection task that provides a more specific measure of attentional allocation (MacLeod, Mathews, & Tata, 1986). In the dot probe task, participants are presented with a series of word pairs on a computer screen, one above the other, followed by a visual probe shown in the same location as one of the words (MacLeod et al., 1986). Participants are required to indicate, as quickly as possible, whether the probe appeared in the upper or lower location. It is hypothesised that responses will be made faster to attended, rather than unattended, spatial positions. If attention is directed toward emotionally salient stimuli, probe detection will be faster when the probe replaces words related to these concerns, compared with when the probe replaces neutral words (MacLeod et al., 1986). With a visual probe detection task, it is thus possible to differentiate between attention directed toward versus away from a particular class of stimulus words due to faster or slower probe detection, respectively. Using this procedure, Rieger et al. (1998) found that in comparison with controls, women with eating disorders preferentially directed attention toward words related to a large physique, while avoiding words related to a thin physique, thus suggesting attentional bias toward information of specific concerns for these individuals. However, Boon, Vogelzang, and Jansen (2000) did not find attentional bias for words related to food, and body weight and shape on this task in restrained eaters. Attentional biases on the dot probe task have, however, been demonstrated for food words in fasted individuals, both high and low in preoccupation with eating, weight, and shape (Mogg, Bradley, Hyare, & Lee, 1998; Placanica, Faunce, & Job, 2002). The latter investigators proposed that their findings could not be explained simply in terms of hunger because both weight-concerned and normal individuals directed attention toward high-calorie foods when fasted, whereas only weight-concerned individuals showed attentional bias to low-calorie food words when nonfasted, indicating concern with food-related information both when fasted and nonfasted. In summary, previous research suggests that attentional bias toward food words may not be limited to emotional concerns but may extend to normal motivational states such as hunger. The aim of the present study was to investigate the role of sensitivity to external food cues in determining attentional bias toward food words in a normal sample. The external eating behaviour scale of the Dutch Eating Behaviour Questionnaire (DEBQ; Van Strien, Frijters, Bergers, & Defares, 1986) was used to measure the levels of external eating behaviour. All participants were tested on both the modified Stroop and the dot probe tasks. The specific hypotheses in the present study were that individuals scoring high on the external eating scale of the DEBQ would show greater attentional bias toward food words on both the
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Stroop and the dot probe tasks, compared with individuals scoring low on this measure. The main focus of the present study was to investigate attentional bias for food words, but body weight and shape words were also included because attentional bias has previously also been identified for this type of stimuli.
2. Method 2.1. Participants Fifty-eight women aged between 19 and 28, studying for a psychology degree at Goldsmiths College, University of London, participated in the present study for course credits. Fifteen of these participants had to be excluded from the analyses; eleven participants experienced technical problems on the Stroop task, whereas one participant had an error rate above 10% on the dot probe task and another three participants had scores more than three standard deviations above their own mean on the dot probe and/ or the Stroop tasks. The remaining sample thus consisted of 43 participants. None of these individuals had an error rate above 10% on either the Stroop or dot probe tasks. All participants had a body weight within the normal range; body mass index (BMI) ranged between 19.90 and 24.30. More detailed information with regard to the age and BMI for the two participant groups, formed on the basis of the median split on the external eating scale of DEBQ, and for the whole sample can be found in Table 1. All women had normal or corrected to normal visual acuity. 2.2. Materials and measures 2.2.1. Measures of cognitive bias Both the Stroop and the dot probe tasks were run on a personal computer. To record the participants’ vocal responses on the Stroop task, a microphone and a voice onset relay device were connected to the computer. The stimulus words used consisted of words drawn from previous research on eating-disorderrelated attentional biases (e.g., Cooper & Todd, 1997; Green, Elliman, Rogers & Welch, 1996). There were two types of eating-disorder-related words; 10 words related to high-calorie food (e.g., chocolate) and 10 words related to body parts or negative body shape, assumed to be of concern for many women (e.g., stomach and chubby). Each word list was divided into two equivalent lists that were matched for word length and usage, based on English language norms established by Carroll, Davies, and Richman Table 1 Means and standard deviations on self-report measures, age, and BMI for the two groups formed on the basis of the median split on the external eating scale of DEBQ, separately, and for the whole sample Characteristics RSE BSQ EAT Age BMI
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
18.77 78.59 10.27 22.23 21.76
(4.90) (32.45) (12.59) (2.11) (1.16)
17.38 81.24 9.62 22.24 22.19
(4.73) (30.34) (9.66) (2.21) (1.12)
18.09 79.88 9.95 22.23 21.97
(4.81) (31.09) (11.12) (2.14) (1.15)
RSE = Rosenberg self esteem scale; BSQ = Body Shape Questionnaire; EAT = eating attitudes test; BMI = body mass index.
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(1971). Half of the list of food words and half of the list of body words were used as the stimuli in the Stroop task, and the other halves were used in the dot probe task. The food- and body-related words used in the Stroop and dot probe tasks were matched with neutral control words for word length and frequency. This resulted in four neutral word categories. However, the analyses performed after the data collection revealed that the response latency for one of the neutral word categories used in the Stroop task was affected differently by different colours. None of the other word categories showed this effect of colour. Since all words were matched with each other for word length and word frequency, the decision was therefore made to exclude this neutral word category. In the dot probe task, each food and body word was paired with the corresponding control word. Furthermore, in the dot probe task, an additional 10 neutral word pairs, all matched for word length and frequency, were used as filler material. These 20 words were divided up into four separate semantic categories with 5 words in each. These words were also matched for word length and frequency with the other words. A complete list of the stimulus words can be found in Appendix A. 2.3. Self-report questionnaires The external, restraint, and emotional scales of the DEBQ (Van Strien et al., 1986), comprising 33 questions, were used to assess sensitivity to external food cues (10 items), tendencies to consciously restrict food intake due to concerns about weight (10 items), and sensitivity to emotional eating (13 items). Chronbach’s alpha for various samples has reportedly ranged between .79 and .81 for the external scale and between .92 and .95 for the emotional and the restraint scales (Van Strien et al., 1986). The Eating Attitudes Test (EAT-26; Garner, Olmsted, Bohr, & Garfinkel, 1982), comprising 26 questions, was used to assess the levels of eating disturbance and weight preoccupation, and has a Chronbach’s alpha value of .85. The Body Shape Questionnaire (BSQ; Cooper, Taylor, Cooper, & Fairburn, 1987) was used to measure the concerns about body shape. This measure consists of 34 questions and has a Chronbach’s alpha value of .97. The Rosenberg Self-esteem Scale (RSE; Rosenberg, 1989), consisting of 10 questions, was used to assess global self-esteem. Chronbach’s alpha for various samples has been found to range between .77 and .88 (Blascovich & Tomaka, 1993; Rosenberg, 1986). High scores on this measure indicate low self-esteem. 2.4. Procedure Each participant was tested individually. To avoid order effects, half the women were given the Stroop task first followed by the dot probe task, and the other half were given the dot probe task first followed by the Stroop task. The participants were randomly allocated to the order of task. In the Stroop task, participants were instructed to name out loud the colours, in which words were displayed as quickly, but as accurately, as possible. To become familiar with the procedure, the participants were first given 10 practice trials. Each word was presented individually in the centre of the computer screen in one of four colours: red, green, blue, or yellow. Each word was randomly assigned to one of these colours each time it was displayed. There were 20 stimulus words randomly mixed within a block. The block was repeated four times, resulting in a total of 80 trials for each participant, with a new randomised order of presentation every time. Once a verbal response had been made, the computer
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recorded the time duration between the onset of the stimulus and the response. Errors were recorded manually by the experimenter. After a 2-s delay, the next word was displayed. Participants were given a break halfway through the main trials. In the dot probe task, participants similarly received 10 practice trials before the main trials. There were 80 experimental trials, in which word pairs from each category (food/neutral, body/neutral, neutral/ neutral, neutral/neutral) were repeated four times. The order of presentation of word pairs was randomised for each participant, with a restriction of immediate repetition of the preceding word pair. Word pairs were presented separately in the centre of the computer screen for 500 ms, with one word above and one below the central point. Immediately after the offset of the word pair, a small dot appeared in the location of one of these two words, in line with previous research using the dot probe procedure (e.g., MacLeod & Mathews, 1988). Within each word pair, each word was equally likely to fall in either the upper or lower position on the computer screen, and the dot was equally likely to replace either of the two words. The participants were instructed to indicate where the dot appeared as quickly, but as accurately, as possible by pressing either an upper or lower response key. The computer recorded the response latency, to the nearest millisecond, and the number of errors. After 500 ms, the next trial began. Halfway through the main trials, the participants were given a break. After completing both the Stroop and the dot probe tasks, the participants were asked to fill in the self-report measures. The order of self-report measures given was randomised for each participant.
3. Results 3.1. Self-report measures On the basis of responses on the external scale of the DEBQ, the participants were categorised as either high (n = 22) or low (n = 21) on external eating behaviour, using the median split, the cut-off point being 3.60. The whole sample had a mean value of 3.41 (S.D. = 0.62) on the external scale of DEBQ. The participants categorised as low in terms of external eating behaviour had a mean value of 2.86 (S.D. = 0.30) on this measure, whereas participants categorised as high had a mean value of 3.93 (S.D. = 0.30). The sample mean on the emotional scale of the DEBQ was 2.87 (S.D. = 0.80). The mean values for high and low external eaters on this measure were 3.10 (S.D. = 0.71) and 2.63 (S.D. = 0.83), respectively. The sample mean on the restraint scale of the DEBQ was 2.77 (S.D. = 0.93), where individuals low in external eating had a mean value of 2.93 (S.D. = 0.89) and individuals high in external eating had a mean value of 2.61 (S.D. = 0.95) on this measure. Table 1 presents the means and standard deviations for individuals high and low in terms of external eating behaviour, separately, as well as for the whole sample on the other self-report measures, age, and BMI. Independent sample t tests revealed no significant differences between groups high and low in external eating with respect to age, BMI, or self-report measures. 3.2. Attentional bias indices Bias scores were calculated for each type of eating-disorder-related word type, for both the Stroop and the dot probe tasks. For the Stroop procedure, the interference indices were computed by subtracting the median response latency time for neutral words from the median response latency time for body and
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Table 2 Mean and standard deviation response latencies for each word type and bias indices on the Stroop task for high and low external eaters separately and for the whole sample Stroop Food Body Neutral Food index Body index
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
620.27 610.14 619.09 1.18 8.95
(69.11) (68.03) (62.57) (29.55) (32.85)
(75.18) (78.33) (69.35) (28.25) (36.09)
615.29 613.31 619.95 4.66 6.64
(71.46) (72.43) (65.18) (29.21) (34.14)
610.07 616.64 620.86 10.79 4.21
food words (e.g., Overduin, Jansen, & Louwerse, 1995). Consequently, two attentional bias scores were produced for each participant: one for food words and one for body words. These indices reflect the relative degree of interference in colour naming, where positive scores indicate a greater interference effect of eating-disorder-related words relative to neutral words. Table 2 shows the mean and standard deviation response latencies, in milliseconds, for each word type and for the bias indices on the Stroop task for participants characterised as high and low on external eating, separately, and for the whole sample, calculated from the participants’ median response latencies. In the dot probe procedure, bias scores were calculated for food and body words by using the following formula of MacLeod and Mathews (1988): (upper probe/lower target upper probe/upper target)+(lower probe/upper target lower probe/lower target)/2. In line with Rieger et al. (1998), the bias indices were calculated from the participants’ median detection latencies. For each participant, one bias score was calculated for trials with food-neutral word pairs and one bias score for trials with bodyneutral word pairs. These bias scores provide an index of the degree to which attention was directed toward or away from eating-disorder-relevant stimuli. A positive score indicates faster reaction times for the probes replacing eating-disorder-relevant stimuli rather than neutral stimuli, hence, an attentional bias toward eating-disorder-relevant stimuli. In contrast, a negative score reflects attention being directed away from such stimuli. Table 3 presents the mean and standard deviation response latencies, in Table 3 Sample mean and standard deviation response latencies for each condition involved in computing the food and body dot probe indices and for bias indices shown for high and low external eaters, separately, and for the whole sample Word type Food
Body
Food index Body index
Word position
Probe position
High external (n = 22)
Low external (n = 21)
Total sample (N = 43)
Mean
(S.D.)
Mean
(S.D.)
Mean
(S.D.)
Lower Upper Upper Lower Lower Upper Upper Lower
Upper Upper Lower Lower Upper Upper Lower Lower
409.68 422.77 395.59 401.95 427.36 424.32 410.82 416.55 9.73 1.34
(111.94) (117.35) (86.37) (64.61) (108.00) (107.78) (123.89) (147.92) (26.99) (40.26)
439.05 432.52 426.86 404.95 413.38 410.62 416.05 405.95 10.19 6.43
(118.53) (106.29) (119.36) (89.04) (90.04) (85.03) (77.10) (74.76) (23.98) (28.22)
424.02 427.53 410.86 403.42 420.53 417.63 413.37 411.37 0.00 2.45
(114.79) (110.86) (103.75) (76.58) (98.70) (96.43) (102.53) (116.75) (27.20) (34.71)
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milliseconds, for each condition in the dot probe index formula for food and body words, separately, and for complete indices for the high and low external eaters and the whole sample, calculated from the participants’ median response latencies. The analyses were first carried out to test whether there were overall attentional bias effects on the Stroop and the dot probe tasks. Paired sample t tests were carried out to compare word types in the Stroop task, where results revealed no significant differences in colour-naming latencies between either food and neutral words or between body and neutral words (both P>.2). One-way within-subject ANOVAs were carried out on the various conditions involved in the formula for producing dot probe indices. For food words, results showed a significant difference between the conditions, F(2.52,106.03) = 3.60, P=.022, with Huynh-Feldt correction. Paired sample t tests revealed that there was a significant difference between the condition where both food words and dots appeared in the lower location and the condition where the food words appeared in the lower location, whereas dots appeared in the upper location, t(42) = 2.12, P=.040. Participants responded more quickly when the food words and dots both appeared in the lower location. There was also a significant difference between the condition where both food words and dots were presented in the upper location and the condition where food words were presented in the upper location, whereas dots were presented in the lower location, t(42) = 2.29, P=.027. The participants responded faster in the condition where the food words and dots appeared in different locations. There were no significant differences between the conditions involved in the formula for dot probe body words ( P>.5). To test the hypotheses that individuals high in external eating behaviour would show greater attentional bias toward food and body words on the Stroop and dot probe tasks than individuals low in external eating will, a mixed 2 4 ANOVA was carried out, with bias indices as the within-subjects factor and participant groups as the between-subjects factor. The analysis was carried out on the standardised scores (Z scores) of the bias indices. Results revealed a significant interaction between
Fig. 1. Interaction between Stroop and dot probe attentional bias indices, performed on standardised scores, and participants high (n = 22) and low (n = 21) on external eating behaviour.
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indices and groups, F(2.89,118.34) = 2.89, P=.040, with Huynh-Feldt correction. Independent t tests were carried out to analyse the effects of participant groups at each level of attentional bias indices. There was a significant difference between the two participant groups on the dot probe food index, t(41) = 2.55, P=.014, where participants low in external eating showed greater attentional bias toward food words on this task than participants high in external eating. There were no significant differences between the participant groups on any of the other bias indices (all P>.1). However, the P value, in combination with effect size, indicated a trend toward greater Stroop interference for individuals high in external eating compared with individuals low in external eating behaviour for food words, t(41) = 1.36, P=.182, Cohen’s d=.41. The interaction is displayed in Fig. 1.
4. Discussion The main finding of the present study was that, in contrast to expectations, individuals scoring high on the external eating behaviour scale of the DEBQ directed attention away from food words, whereas individuals scoring low on this scale directed attention toward food words on the dot probe task. No significant differences were found between these participant groups on either the responses to body words on the dot probe task or the responses to food or body words on the Stroop task. Furthermore, an overall attentional bias effect was found on the dot probe task for food words when replaced by dots in the lower position of the computer screen. In contrast, when the dots appeared in the upper location, participants were faster to detect the dots replacing the neutral words. This pattern of results might seem to indicate a potential position bias on the dot probe task. However, looking at previous studies using the dot probe task (e.g., Brosschot, de Ruiter, & Kindt, 1999; Mogg, Bradley, & Hallowell, 1994; Mogg et al., 1998), it is clear from the mean response latencies to the probes in these studies that similar trends of consistent lower response latencies for a certain probe position are evident, especially for the upper probe position. No overall attentional bias effect was found on this task for body words and no overall attentional bias effect was found on the Stroop task for either food or body words. The cognitive avoidance found for food words on the dot probe task in individuals highly responsive to external food cues may suggest that these individuals adopted the strategic processing to counteract tendencies to direct attention toward such concern-relevant material. It is interesting to note that on the Stroop task, these individuals instead showed a trend toward attentional bias for food words, although the difference between high and low external eaters did not reach significance. These conflicting findings produced from the Stroop and the dot probe tasks are in line with previous research testing participants on both of these attentional tasks (Brosschot et al., 1999; Mogg et al., 2000). Brosschot et al. (1999) proposed that the Stroop and the dot probe tasks tap different stages of information processing. Decisions with regard to the concern-relevance of stimuli in the Stroop task must be very rapid, tapping bias at an early automatic stage of processing. The gap between the probe and the emotional information on the dot probe task would instead allow more time for assessing significance of concern, thus tapping bias at a later stage in the cognitive processing. Based on these suggestions, it might be plausible to propose that the dot probe task could be more sensitive to strategic processing than the Stroop task is. Considering that individuals high in responsiveness to external food stimuli have been characterised as prone to overeat when faced with palatable foods, attempts may be made by these individuals to avoid such external food cues to avoid overeating. Such avoidance would
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thus be expected on the dot probe task, more amenable to strategic control, but not on the Stroop task assessing earlier automatic processing priorities. However, as mentioned previously, the Stroop task has been criticised for not providing an adequate measure of attentional allocation, where retarded colour-naming could arise either as a result of attention being directed toward or away from emotionally relevant material (de Ruiter & Brosschot, 1994). Considering that the Stroop and the dot probe tasks, presumably providing a more specific measure of attentional allocation, produced opposite results with regard to food words in the same individuals, the current pattern of results may indicate that delayed colour naming on the Stroop task could indeed reflect cognitive avoidance. With regards to the attentional bias found for the food words in individuals low in external eating behaviour, previous research has, as mentioned previously, demonstrated similar results on the dot probe task for high-calorie food words in fasted normal individuals. A limitation of the present study was that the levels of hunger were not assessed. Furthermore, the food words used in the present study only comprised high-calorie foods, while research by Placanica et al. (2002) revealed that processing bias toward high- or low-calorie food depends on state of hunger and levels of concerns with dieting and weight. Moreover, since the aim of the study was to investigate the role of sensitivity to external food cues in producing attentional bias, it might have been more optimal to use pictures of food rather than word stimuli related to food. It is further possible that the use of a self-report measure of external eating behaviour did not provide accurate answers where it might have been more informative to use behavioural assessments of external eating behaviour. In summary, the results of the present study suggest that demonstrations of cognitive avoidance versus attentional bias may depend on the extent to which the attentional task used to assess information bias is susceptible to early automatic processing versus strategic control. Considering that performance on different measures of information bias may tap different underlying mechanisms, it might be advisable to use more than one measure when investigating avoidant versus vigilant biases. Future research is needed to clarify the precise mechanisms involved in the Stoop and the dot probe tasks. Acknowledgements Lars-Gunnar Lundh and Colin Latham for help and advice. Ian Hannent and Rob Davis for technical support. Goldsmith’s College for providing the facilities.
Appendix A Stimulus words used in the Stroop task Food
Body
Neutral
Chocolate Pastry Chips Sweets Biscuit
Stomach Chubby Thigh Flabby Breasts
Monkeys Chick Owls Rodent Dolphin
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Stimulus word pairs used in the dot probe task Food/Neutral
Body/Neutral
Sugar/Clean Cake/Soap Bacon/Dusty Pizza/Washer Fudge/Scrub
Bottom/Notes Fat/Desk Waist/File Plump/Folder Bulky/Diary
Neutral/Neutral
Neutral/Neutral
Songs/Clouds Drum/Storm Tunes/Sunny Stereo/Warming Dancer/Windy
Sheep/Coat Lion/Boots Tiger/Hats Pandas/Necktie Snail/Glove
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