Application of Multidimensional Scaling to Ratings of ... - Science Direct

Physiology & Behavior, Vol. 21, pp. 417--422. Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A.

Application of Multidimensional Scaling to Ratings of Foods for Obese and Normal Weight Individuals S U S A N S. S C H I F F M A N , 1 G E R A R D M U S A N T E 2 A N D J U D I T H C O N G E R 3

Department of Psychiatry, Duke Medical Center, Durham, N C 27710 ( R e c e i v e d 13 April 1978) SCHIFFMAN, S. S., G. MUSANTE AND J. CONGER. Application of multidimensional scaling to ratings of foods for obese and normal weightindividuals. PHYSIOL. BEHAV. 21(3)417-422, 1978.--Sixteen obese patients, aged 19-31, and

27 normal weight individuals, aged 18-22, tasted and smelled fresh, unseasoned blended foods while blindfolded. Then they rated the foods on 51 semantic differential adjective scales which related to stimulation of the gustatory, olfactory, and trigeminal systems. The stimuli included fruits, vegetables, meat, fish, nuts, dairy products, grains, and a set of four standards: sucrose (sweet), NaCI (salty), lemon (sour), and coffee (bitter) in a thin cornstarch base. Proximity measures among stimuli for each subject were developed from the ratings on the adjective scales. Two multidimensional scaling (MDS) procedures, SINDSCAL (a variation of INDSCAL) and ALSCAL, were applied to the proximity measures yielding two-dimensional food maps which were very similar. Both MDS procedures provided weights for each subject on the two dimensions of the space common for all subjects. The weights indicated that the obese subjects found the f'wstdimension, which was related to the hedonic and flavorous aspects of the stimuli, relatively more important than normal weight individuals. Also, obese subjects were significantly better at identifying the blended foods when compared to normal weight subjects. Obesity

Taste

Smell

Multidimensional scaling

MUCH of the existing data related to eating behavior in obesity [20, 27, 28, 30, 31, 32] suggest that obese humans are more responsive than normal weight persons to what are termed "external" cues and characteristics of food (e.g., taste, smell, availability, and time of the day) than to "internal" physiological cues (e.g., blood sugar levels, gastric motility). Striking behavioral parallels between obese humans and animals that become obese after lesions of the ventromedial hypothalamus (VMH) have been reported [20,21]. VMH lesioned animals are hyperresponsive to both good and bad tasting food and unresponsive to physiological signals concerning nutritional state indicating satiety or deprivation. Adulteration of food with bitter-tasting quinine greatly suppresses food intake in the VMH lesioned rat [18,45]. Similar results have been found in VMH lesioned rat when food was adulterated with unpleasant-tasting kaolin [17]. Hyperresponsiveness to good tastes has been shown in VMH animals as well. Addition of sugar and fats led to greatly increased food consumption [18,45]. Obese humans also appear to be more responsive to taste than those of normal weight. Hospitalized obese patients and normal controls were fed a nutritionally sound but bland, unappetizing liquid diet [13]. Obese patients reduced their intake to an average of 500 calories per day while normal weight subjects maintained normal caloric intake. Overweight subjects were found to be much more responsive to

Hedonics

expensive French vanilla ice cream than cheap ice cream adulterated with quinine sulphate [19]. When subject's hedonic ratings were compared with the amount of the two kinds of ice cream eaten, it was concluded that overweight subjects tended to make a "gross 'edibility' decision," i.e., they maximally ate ice cream which they rated 'fairly good' or better and minimally ate ice cream which they rated less than 'fairly good'. Similar results were found for cake samples which ranged from excellent to unpleasant [23]. The results from the cake study also indicated that obese eating behavior had an "on-off'' character, with overweight subjects eating about the same amount of disliked cake as norreal and underweight subjects but much more of the cake which they liked. This same trend has been found for milkshakes and crackers as well. Obese subjects drank more ounces of a good-tasting milkshake than normals and less ounces when the milkshake was adulterated with quinine [8]. Obese subjects ate more crackers which they preferred most when compared to normals [24]. It has been suggested that the hedonic or pleasurable aspect of food stimuli is important in regulation of body weight [1-6]. According to this theory, stimuli are perceived as either pleasant or unpleasant depending on the internal state of the organism. If body weight is below a physiologically predetermined set-point (as in weight loss), the perceived pleasantness of food stimuli increases. In addition, obese

~This paper was supported in part by a grant to the senior author NIA AG00443 and a grant to Dr. R. P. Erickson, NSF-GB33464. 2present address is: Structure House, 707 Morehead Avenue, Durham, North Carolina. 3present address is: Department of Psychological Sciences, Purdue University, Lafayette, Indiana.

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subjects, unlike normal weight subjects, may show decreased sensitivity to internal signals because their taste ratings of sucrose did not change from pleasant to unpleasant as in normal subjects following ingestion of a glucose load [3]. Aspects of these findings have not been confirmed in other studies [10, 11, 46, 48]. The purpose of this study was to further investigate how food tastes and smells to both obese and normal weight subjects. A spatial map which represents how similar foods are perceived by obese and normal weight subjects was derived to determine whether differences exist between the two groups. A spatial map was achieved using multidimensional scaling (MDS) techniques which utilize measures of similarity to arrange stimuli which are rated similar to one another so that they fall near one another in the resultant space or map. Stimuli rated dissimilar are located distant from one another. The MDS techniques utilized here also provide individual difference options which revealed that obese and normal weight subjects rated the stimuli in idiosyncratic ways. Multidimensional scaling techniques have previously been applied to data in gustation and olfaction [34-36, 38--43, 49, 50].

METHOD Subjects

The subjects were 27 normal weight Duke University students, aged 18--22 years (mean=20.1) and 16 obese patients, aged 19-31 (mean=27.8). The student group consisted of 18 females and 9 males. None of the students was overweight according to the Metropolitan Life Insurance tables. All the obese subjects were female patients at the Dietary Rehabilitation Clinic in Community Health Sciences at Duke Medical Center who were participating in a 700 calorie diet. The obese patients were at least 45% overweight (mean=56.3% overweight) according to the Metropolitan Life Insurance tables. The two groups were approximately equal with regard to intelligence and socioeconomic status. All subjects were Caucasian, unmarried, and nonsmokers. Stimuli

The stimuli were fresh, unseasoned foods which were thoroughly blended to eliminate as many textural cues as possible. The stimuli included fruits, vegetables, meat, fish, nuts, dairy products, grains and a set of four standards: sucrose (sweet), NaCI (salty), lemon (sour), and coffee (bitter) in a thin cornstarch base. The four standards were determined to be of equal moderate intensity by pretesting with 5 college students. The foods were prepared as follows. The fruits, vegetables, and grains were steamed just enough to yield a soft texture after blending. The meats and fish were roasted in aluminum foil after the fat and bones had been removed. The eggs were fried without butter in a Teflon skillet. Slight amounts of water were added to some of the blended foods in order to equalize the consistencies as much as possible. The temperature of the foods was maintained at 160*F throughout the experiment. The stimuli have been used previously by Schiffman [36]. Procedure

The experiment took place in well-ventilated rooms with fans. The subjects first filled out biographical forms, providing information on height, weight, age, health, and favorite

foods. Then the subjects were blindfolded by the experimenter and given a container of blended food to smell at the level of the nostrils. Next, they were given one spoonful to taste, allowing it to come in contact with all parts of the mouth. After the subjects tasted the food, they ejected it into a paper napkin which was placed into a paper bag. This procedure was used to insure that the subjects did not see the color of the food substance. The blindfolds were removed, and the subjects rated the food substance on fifty-one semantic differential scales which related to stimulation of the gustatory, olfactory, and trigeminal systems. The judgments were made by marking an " X " along a five-inch line, e.g., GOOD __BAD The adjective scales have been employed previously by Schiffman and Erickson [41]; Schiffman and Dackis [381; Schiffman, Moroch, and Dunbar [42]; Schiffman and Engelhard [40]; and Schiffman [36]. After rating a substance, the subjects tried to identify it, making additional comments about its taste and smell. Ten to twelve subjects rated a maximum of six stimuli during one hour sessions. All testing sessions took place at 3:30 p.m. and the subjects were asked to refrain from eating and drinking for two hours prior to testing. Measures of height and weight were taken after the experiment was complete to corroborate biographical information. Analysis o f Data

The ratings on the semantic differential scales were transcribed from " 0 " to "100", such that a rating at " g o o d , " for example, was transcribed as " 0 , " and a rating at bad was transcribed as "100". These transcriptions were used to develop proximity measures among all pairs of the food stimuli. The proximity measures were computed in the following way: dij~ =

51

\ ~ (ai.~ - ajk~)2

k-|

)~/2

where: dij,=distance between stimulus i and stimulus j for subject s aik~=numerical transcription for subject s on adjective scale k for stimulus i ajk~=numerical transcription for subject s on adjective scale k for stimulus j The proximity measures were utilized to develop a flavor map for the food stimuli. Two MDS procedures were applied to the proximity measures: (1) SINDSCAL (a symmetric version of I N D S C A L developed by Pruzansky [25] which utilizes the Carroll and Chang [7] algorithm and (2) A L S C A L (Takane, et al. [44]). Both procedures arranged foods which were rated similar to one another (as indicated by the proximity measures) near one other in the resultant space or map. Foods judged to be dissimilar were positioned distant from one another. SINDSCAL is a metric procedure which assumes that the proximity measures obey the restrictions of the interval or ratio level of measurement. A L S C A L has a nonmetric option which treats the proximities at the ordinal level of measurement so that the distances between stimuli correspond to the rank order of the experimental proximities. There is no reason to assume a priori that the computed proximity measures obey the restrictions of interval or ratio measurement. Both SINDSCAL and A L S C A L yield measures of stress (or error) which describe how well a solution fits the experi-

MULTIDIMENSIONAL SCALING AND OBESITY mental data. The stress is generally lower for solutions with high dimensionality. However, solutions with three dimensions or less are more useful conceptually because they can be represented geometrically. Both SINDSCAL and ALSCAL have options which provide information on the individual differences among subjects. This is possible because both SINDSCAL and ALSCAL utilize matrices of proximities between all possible stimulus pairs for individual subjects as input. They yield as output a series of weights for each subject on each of the dimensions of a common multidimensional space which indicate the idiosyncratic importance on each of the dimensions for individual subjects. RESULTS

The percentage of subjects correctly identifying the blended foods is shown in Table 1. A stimulus was assumed to be correctly identified if it was named directly (e.g., "strawberries") or if it was named along with another food (e.g., "strawberries possibly mixed with rhubarb"). For the 26 stimuli tested with both groups, obese subjects were significantly better at identifying the foods (×2=5.05, p