Twenty-seven adolescents with excess weight (11 F and 16 M; aged 13–16) were selected according to their BMI (weight (kg)/height (m)2) and classified as overweight or obese according to the International Obesity Task Force criteria defined by Cole et al. (21). At the same time, 34 healthy adolescents (13 F and 21 M; aged 13–16) with normal weight, who had similar sex and IQ distributions, were also enrolled in the study forming the comparison group. Adolescents with excess weight were recruited as they enrolled in a research-based multidisciplinary therapeutic program aimed to decrease weight and to change eating-related lifestyles (EVASYON study) carried out at the San Cecilio Hospital in Granada, Spain. In all cases, the evaluations reported in this study were conducted before treatment onset. Nineteen participants within the excess-weight group met criteria for obesity according to age- and sex-corrected BMI cutoffs proposed by Cole et al. (21); the BMI range in these subjects was 28–51. The remaining eight participants within this group met criteria for overweight according to the same cutoffs; the BMI range in these subjects was 24–28. Individuals with excess weight were evaluated by a physician for exclusion criteria, which included significant medical or psychiatric illness, and current treatment with medication; their characteristics are shown in Table 1. Normal-weight adolescents (BMI range 17–24) were recruited through schools located in the same geographical area, from families with similar sociodemographic background to the ones forming the clinical group.
Table 1. Descriptive scores for sociodemographic (age, IQ) and physical characteristics (weight, height, and BMI)
Questionnaire measures of impulsivity and sensitivity to reward/punishment
UPPS-P impulsive behavior scale (22): This scale is a 59-item inventory designed to measure five distinct personality pathways to impulsive behavior: negative urgency, lack of perseverance, lack of premeditation, sensation seeking, and positive urgency. The total scores of each of these five dimensions were obtained for analyses.
Delay-discounting questionnaire, now or later (23): This is a monetary-choice questionnaire asking for individual preferences between smaller, immediate rewards and larger, delayed rewards varying on their value and time to be delivered. The dependent measure was the discounting parameter k, indexing the rate at which the individual depreciate rewards as a function of time, according to the formula: V = A/(1 + kD), where V is the present value of the delayed reward A at delay D, and k is a free parameter that determines the discount rate. As k increases, the person discounts the future more steeply.
SPSRQ (24): The SPSRQ (sensitivity to punishment and reward questionnaire) is a 48 yes–no response item questionnaire aimed to measure two neuropsychological systems driving motivated behavior: the behavioral activation system (SR) and the behavioral inhibition system (SP). The total scores from each scale (SP and SR) were obtained for analyses.
IQ–Kaufman brief intelligence test (25): It consists of two subtests, vocabulary and matrices. The vocabulary subtest provides an estimated verbal IQ, the matrices subtest provides an estimated nonverbal IQ, and the scores from both measures provide a composite IQ, which we used as the main dependent measure from this test.
Working memory–letter-number sequencing (26): Participants are read a sequence in which letters and numbers are combined, and are asked to reproduce the sequence heard, first placing the numbers in ascending order and then the letters in alphabetical order. The dependent variable from this test was the number of correct answers.
Analogical reasoning–similarities (26): Pairs of words are read that represent common objects or concepts, and participants have to indicate how these objects/concepts are similar or what they have in common. The dependent variable from this test was the number of correct answers.
Planning–zoo map (27): It provides information about the ability of the participant to plan a route that allows him to visit 6 of 12 locations in a section of the zoo. The main dependent measure from this test was the total raw score, based on the efficacy of the plan designed minus the number of errors committed.
Interference/response inhibition–Stroop test (28): This test consists of three forms, each containing 100 elements. The first form is made up of the words “RED”, “GREEN,” and “BLUE” ordered randomly and printed in black ink. In this condition, participants are asked to read aloud the words written. The second form consists of strings of “XXXX” printed in red, blue, or green ink. In this condition, participants are asked to name the color. The third form introduces the condition of interference, and it consists of the words from the first sheet printed in the colors of the second. In this condition, participants have to name the color of the ink and ignore the word. The dependent variable used in this test was the interference score.
Inhibition and shifting–five-digit test (FDT) (29): This test consists of four parts of increasing complexity. Each of these parts presents a series of 50 boxes that contain 1–5 digits (parts 1, 3, and 4) or stars (part 2), organized in similar patterns to those on domino pieces or playing cards. In part 1 (reading), participants are asked to read as quickly as possible the digit each box contains. In part 2 (counting), participants are asked to count how many stars each box contains. In part 3 (interference), participants are asked to count the number of digits each box contains, producing an effect of interference, as the boxes present groups of digits that do not correspond to their arithmetic value (e.g., in a box with five twos, the correct response would be five and not two). Finally, in part 4 (switching), participants are asked to count, just as in part 3, or read, as in part 1, depending on whether the outline of the box is normal (count, 80% of the stimuli) or of double thickness (read, 20% of the stimuli). Parts 1 and 2 constitute basic measures of attention and processing speed. In contrast, parts 3 and 4 are sensitive to executive processes of inhibition and switching. Therefore, the main dependent variables from this test were the difference between performance time on part 3 and the mean of parts 1 and 2 (FDT interference score), and the difference between performance time on part 4, and the mean of parts 1 and 2 (FDT switching score).
Set-shifting–trail-making test A and B (TMT): This test consists of two parts. Part A is a page with 25 numbered circles randomly arranged. Individuals are instructed to draw lines between the circles in increasing sequential order until they reach the circle labeled “End.” Part B is a page with circles containing the letters A–L and 13 numbered circles intermixed and randomly arranged. Individuals are instructed to connect the circles by drawing lines alternating between numbers and letters in sequential order, until they reach the circle labeled “End.” The main dependent measure from this test is the subtraction of time on part B minus time on part A (B−A), which taxes set-shifting abilities after subtraction of visuomotor speed.
Self-regulation–Revised Strategy Application Test (R-SAT) (30): This is a multitasking test that consists of three types of activities: figure tracing, sentence copying, and object numbering. The activities are presented on two different stacks (A and B), each containing 120 items. The main goal of the task is to win as many points as possible. However, the items can be of different difficulty: some of them are easy and quick to complete (i.e., they take a couple of seconds and are defined as “brief items”), whereas others are very laborious and time-consuming (i.e., they can take longer than 1 min and are defined as “lengthy items”). Given the limitation of time (10 min), the most efficient strategy (to be discovered) is to complete only the brief items to the exclusion of lengthy items, which the subjects must learn to skip as they are introduced in the latter pages of the test; this way subjects can optimize long-term profit by completing more items during the time limit. The main dependent variable from the R-SAT is the proportion of brief items completed in relation to the total number of items attempted.
Effective decision-making–IGT (31): This is a computer task that factors several aspects of decision-making, including uncertainty, risk, and evaluation of reward and punishing events. The IGT involves four decks or cards, decks A′, B′, C′, and D′. Each time a participant selects a card, a specified amount of play money is awarded. However, interspersed among these rewards, there are probabilistic punishments (monetary losses with different amounts). Two of the decks of cards (A′ and B′) produce high immediate gains; however, in the long run, these two decks will take more money than they give and are therefore considered to be the disadvantageous decks. The other two decks (C′ and D′) are considered advantageous, as they result in small, immediate gains, but will yield more money than they take in the long run. The main dependent variable from this task was the net score for each block of the task (5 blocks of 20 trials). We calculated net scores by subtracting the number of disadvantageous choices (decks A and B) from the number of advantageous choices (decks C and D) for each block. We also calculated the global IGT net score applying the same formula to the 100 trials of the task. In addition, we calculated the number of individuals that scored below a cutoff of zero, which represents clinically significant impairment in the task.
Prior to inclusion in the study, all participants and their parents signed an informed consent form. All assessments were conducted in accordance with ethical rules for research in human subjects following the Declaration of Helsinki (Edinburgh, 2000), World Medicine Association (http:www.wma.net). Moreover, the ethical approvals were obtained from the Bioethical Committee of the Clinical University Hospital San Cecilio of Granada and the Bioethical Committee of the University of Granada. Subjects were assessed on two different sessions, one for neuropsychological assessment, and one for questionnaire measures in order to avoid potential effects of fatigue. Both sessions were conducted on a comfortable adequately illuminated room at the hospital facilities. Each session had an approximate duration of 1 h. All tests were administered by a research assistant with master's degree in clinical psychology. Neuropsychological test administration was arranged to alternate between verbal and nonverbal tasks, and between more and less demanding tasks; these tests were administered in a fixed order to all participants. Questionnaire measures were counterbalanced for administration across participants.
Statistical analyses were implemented on SPSS v.17 (SPSS, Chicago, IL). We first explored dependent variables to examine missing data points, normality of distributions (tested by Kolmogorov–Smirnov tests), and presence of outliers (defined by the Explore command of SPSS v.17). Data from questionnaire measures of 10 controls were missed due to nonattendance to the questionnaire measures session (thus, n = 24 for the normal-weight group on these measures). One outlier was detected in the IGT distribution of the excess-weight group, and this subject was removed from further analyses of this task. Preliminary group comparisons for demographic variables showed that, despite having the same age range, the two groups were not significantly matched for age. Age was not significantly associated with any of the dependent variables (with the exception of the Stroop test, r = 0.21, P < 0.05, and the SR subscale from the SPSRQ questionnaire, r = 0.40, P < 0.01). However, we chose to apply a conservative approach to control for possible effects of this variable on neuropsychological performance. Therefore, we regressed age on the dependent variables using standard regression models, and then we saved the standardized residuals from these models for further analyses. Therefore, group comparisons for neuropsychological and personality measures were conducted on the standardized residual scores, after removing any effect of age. On those variables where residuals followed a normal distribution (UPPS-P subscales—with the exception of lack of perseverance, SR subscale, letter-number sequencing, Stroop interference, FDT interference, FDT switching, TMT B-A, and R-SAT proportion of brief items), we conducted independent sample t-tests to examine differences between participants with excess weight and participants with normal weight. On those variables where residuals failed to meet normality assumptions (k delay-discount parameter, UPPS-P lack of perseverance. SP subscale, similarities, zoo map, and IGT net score), we used nonparametric Z Kolmogorov–Smirnov tests. To facilitate reading, in the tables we reported raw descriptive scores from questionnaire and neuropsychological measures (instead of descriptive scores from standardized residuals); on the other hand, we reported P values obtained from statistical tests performed on the standardized residuals (age-corrected). In addition, to further explore the association between BMI and neuropsychological performance (while controlling for other relevant variables), we performed hierarchical multiple regression analyses. We included two blocks of predictor variables: (i) sex, age, and IQ (in a first block) and (ii) standardized zBMI scores (in a second block). For each new block of variables entered in the regression model, we estimated the R2 of the prediction change associated with that block and its statistical significance, with the aim of determining the differential contribution of each of the blocks to the prediction output. The dependent variables were the scores from personality and neuropsychological measures.