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| Guest Article | Psycho-physiological aspects of foam | Wolf Eisfeld, Peter Busch, Wolfram Boucsein |
1. Explanation of the word psycho-physiology
The discipline of psycho-physiology investigates the physiological consequences of psychic phenomena in connection with the central nerve system (brain, cerebral cortex) and the senses. It concerns human needs, our memory, generation of impetus and perception. Psychophysical correlations, i.e. connections between quality and quantity of stimuli and their perception play an important role.
2. Consumer expectations
There is actually only one reason which stimulates the producer of goods or services: to get the consumer interested in his skills. Thus he urgently needs the consumer's feedback after having used his products or services. He can obtain information in different ways. Most obvious and desirable: The consumer buys his products. Less trivial, he gains information concerning his conscious and unconscious perception. The conscious perception is expressed e. g. by a sensory assessment, unconscious reactions can be understood by physiological measurements. Since they are in connection with inner associations they are summed up as psycho-physiological results.
3. Foam as a parameter to buy or not to buy certain cosmetics
Foams play an important role for the consumer's experience of cosmetic products. The rich lather of a shampoo which promises cleansing power, the markedly dense shaving foam or the comforting foam of a bubble bath are illustrative examples. It is not too farfetched to claim "personalities" for different foams. This almost anthropomorphic view of foams requires more than their characterization by mere technical parameters like foam height or foam stability. In many cases, a subjective assessment is much more closely related to the consumer's needs and expectations.
Subjective assessments make use of linguistic terms and therefore are apt to the inadequacy and vagueness of our daily language, i.e., very often the consumer lacks the words to describe a certain experience or stumbles of the ambiguity of linguistic terms especially when they are embedded in idiomatic or cultural contexts.
Immediate emotional responses to stimuli can be assessed by means of psycho-physiological recordings in connection with the autonomous nervous system visible e.g. as facial expressions. The detection of these prior-to conscious responses is of great interest for an understanding of the acceptance or refusal of products by the consumers. In many cases, decisions to buy or not to buy a product are not rationally based but stem from instinctive depths of our personality. Since those physiological responses cannot be easily faked they give valuable insights of acceptance or non-acceptance decisions of consumers.
It is true that human perception is based on all senses i.e. the consumer perceives objects or his environment in a holistic way. It is further true that our perception is very dependent on inner associations, i.e. inner images or imaginations can considerably change our behavior, judgement, acceptance or refusal of something. Our studies are more or less based only on tactile and visual sensations. Nevertheless they furnish the researcher with new insights concerning cosmetics which produce foam.
4. Method of psycho-physiological foam measurement
4.1 Survey of four physiological measures (EDA, EKG, PVA, EMG)
The classical form of inquiry into the psycho-physiology of emotions is using measure of the autonomic nervous system. Psycho-physiological profiles of different emotions can be obtained by using measures of electrodermal activity (EDA) together with cardiovascular measures such as electrocardiogram (EKG) and finger pulse volume (PVA = pulse volume amplitude). Because of its particular sensitivity to emotions the electromyographic activity (EMG) of certain facial muscles forms an important somatomotor supplement to the autonomic measures.
4.2 Foam formulations
The formulations for the test foams 1-4 and the reference foam are given in Table 1.
All foams can be assigned to the category of body cleansing, typically shower gels or foam baths. They were designed in a way that the reference foam should show typical average properties whereas two foams should show inferior, the remaining two foams superior qualities in comparison to the reference. To avoid intersensory effects, the scent of all formulations was standardized by use of the same perfume.
4.3 Physiological testing by panelists
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The above described foams 1 - 4 were subjected to an objective emotional assessment (OEA) as defined by Boucsein et al. (4). A fifth foam (laurylether sulfate) was used as standard reference. The study was performed with 12 highly trained female panelists in a temperature and humidity controlled laboratory (23° C, 40 % r.h.). The tests were repeated 14 days later in order to check reproducibility and habituation effects.
First the standard reference foam was presented, followed by two foams to be tested. The subjects were seated in front of a camouflage preventing them from visual contact with the foam. Their left arm was put through an opening in the camouflage to allow for contact and manipulation with the foam. The foam was generated at the bottom of a bowl by pressing air through a sieve underneath the surfactant solution. After the bowl was filled, it was lifted by a pneumatic mechanism until the surface of the foam got into contact with the palm of the subject's hand. Each foam presentation started with projecting the identity of the foam on a screen to the right of the camouflage. Five to 10 s after this announcement the contact with the foam was made, the time point of which was automatically registered by means of a trigger switch. For the rest of a 30 s period, the subjects were asked not to move their hand but just stay into contact with the foam. Afterwards they were instructed to handle the foam for another 30 s by kneading or rubbing it between the fingers. Finally the subjects rated the subjective qualities of the foam on a Likert-type adjective checklist (compare Figure 1 ).
Electrodermal activity (EDA) was recorded from thenar/hypothenar sites at the right palm as skin conductance, using a constant voltage of 0.5 V, applied via 6 mm diameter sintered silver/silver chloride electrodes filled with an isotonic jelly made from Unibase. (1)
| Figure 2: Sensory profile sheet for assessment of foams 1 - 4, compared to the reference foam (zero line) |
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Finger pulse volume (PVA) was recorded by means of an infrared photoplethysmographic coupler from the tip of the right middle finger.
The electrocardiogram (EKG) was recorded via two electrodes between the sternum and the bottom of the left rib cage, with a ground electrode on the right forearm.
The electromyographic activity (EMG) of the three facial muscles was recorded by means of 3 mm silver/silver chloride electrodes filled with a highly conductive jelly. The facial muscles were the zygomaticus major (pulls the lip corner up and back), the levator labii superioris (raises the upper lip, widens nostrils) and the corrugator supercilii (knits the brow).
The subjective ratings were obtained by showing the item in question to the subjects in front of the camouflage.
5. Sensory assessment and hedonistic description
Prior to the application of the sensory assessment words and procedures were created which describe different sensations that occur while checking foam (2). Particularly interesting were flash foam, amount of foam, bubble size, creaminess, foam stability, foam cushion, moisture, consistency, pleasant feeling.
The panelists were asked to rank the respective foam parameters on a five-point scale, where 0 means that there is no sensory difference between test and reference foam; 0.5/1.0 and -0.5/-1,0 indicate that test foam and reference differ somewhat or significantly in one or the other direction. (see Figure 2)
Compared with the reference the foams 1 and 2 show only slight differences, in other words, they produce good average values. Foam 3, however, shows bad flash foam and a far too small volume. It is soapy but not creamy, and feels as if it was water with a very low surfactant content. It reminds of fine sandpaper and leaves a dry and porous skin feel. As a whole, it is quite inappropriate as a foam.
Exactly the opposite is true for foam 4. It produces good flash foam, the cushion is very pleasant. It is rather firm like a shaving foam or whipped cream. It is judged very cosmetic.
The different subjective ratings of like and refusal are listed in an adjective check list of the Likert scale type (see Figure 1).
6. Evaluation and interpretation of the psycho-physiological measures
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The foams were measured after different periods of evaluation. Thus, it turned out that there is a good correspondence between the results of the sensory assessment and the psycho-physiological reactions upon initial and extended contact. The psycho-physiological results however are more fine-grained. They impart insight into the generation of emotions and habituation effects. A good indicator for emotional excitement is the so called orienting response (OR), which signals that the testing person is not quite sure what will happen and stays positively or negatively interested in what will happen. This is especially true in the initial state and will change after a longer exposure to the test substrate. E.g. the two positive foams 1 and 2 are experienced with a certain emotional excitement which can be detected by high EDA-values (see Figure 3), although foam 1 elicits a subliminal unpleasant smile detectable by the comparatively high value of the levator labii value. During handling foam 2 appears less striking which indicates it becomes more dull, "having less energy, being less interesting". Very different from the "normal" foams 1 and 2 are foams 3 and 4. The increases of the electrodermal values are only little which can be interpreted as: The panelist is (unconsciously) able to judge the foams as different from the placebo right away. High finger pulse values found for foams 3 and 4 can be interpreted as indicating striking properties of the foam (Figure 4). In contrary to foam 3, which becomes more unfavorable after a longer period of time, the emotional interest for foam 4, observable by the zygomaticus change (one of the EMG-parameters), increases even after a longer period of handling time (compare Figures 5 and 6). It corresponds with the favorite ratings at a sensory assessment.
| Figure 4: Amount of PVA modulation in the 30 s during the contact with the foam and during the 30 s handling period before home use |
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The different orienting responses concerning the four foams are visible from the electrocardiograms (EKG's). The mean heart rate decelarations which refer to the interbeat intervals (IBI) indicate different expectances (Figure 7). (3)
7. Advantages of the psychophysiological studies
Subjective reports alone like sensory assessment or hedonistic investigations using a Likert scale rating show only limited aspects of foam experiences. The limitation is mainly caused by the usage of language which requires a conscious and therefore filtering process. Subjects deliberately react with their autonomic and somatomotor nervous system to the change in the foam's properties, such as becoming of more or less interest for the use, or simply fading away because e.g. of the foam's instability. Those fine-tuned reactions can only be detected by means of psycho-physiolgical recording. They are not easily susceptible to faking.
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| Figure 6: EMG power (mV) of the three facial muscles during the 30 s handling period before home use |
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It should not be forgotten, however, that emotional reactions in a real environment are much more liquid and for that reason are less predictable, as a series of laboratory experiments suggests. The continuous change of life conditions, expectances, hopes and fears, associations which run through our mind make it more or less impossible to predict precisely the emotional acceptance or non-acceptance of any product. Nevertheless psycho-physiological studies provide insights into product qualities which enable the producer to optimize his efforts (4).
| Figure 7: HR deceleration (increase of IBI in ms) during the initial contact with the foam (0 - 5 s after touching) before home use |
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Table 1: Formulations for test foams and reference foam
| Ingredients | Trade name | Foam 1 | Foam 2 | Reference | Foam 3 | Foam 4 |
| Sodium Laureth Sulfate 28% | Texapon® NSO | 43,0 | 43,0 | 43,0 | -- | 43,0 |
| -
MIPA-Laureth Sulfate - Laureth-3 - Cocoamide DEA |
Texapon® WW99 | -- | 5,0 | -- | 44,75 | -- |
| -
Coco-Glucoside - Glyceryl Oleate |
Lamesoft® PO65 | -- | -- | 10,0 | -- | -- |
| -
Cetyl Palmitate - Beheneth-10 - Hydrogenated Castor Oil - Glyceryl Stearate |
Lamesoft® PW45 | 5,0 | -- | -- | -- | -- |
| Coco Glucoside | Plantacare® 818UP | -- | 15,0 | -- | -- | -- |
| Guar Hydroxypropyl Trimonium Chloride | Cosmedia® Guar C261 | -- | -- | -- | -- | 2,0 |
| Paraffin | -- | -- | -- | 20,0 | -- | |
| Hexyldecanol | Eutanol® G16 | -- | -- | -- | 15,0 | -- |
| Octyl Stearate | Cetiol® 868 | -- | -- | -- | 20,0 | -- |
| Perfume | 0,25 | 0,25 | 0,25 | 0,25 | 0,25 | |
| Preservative | Phenonip | 1,0 | 1,0 | 1,0 | -- | 1,0 |
| Sodium Chloride | 4,0 | 2,75 | 1,8 | -- | -- | |
| Water | ad 100 | ad 100 | ad 100 | -- | ad 100 |
References:
1. Boucsein, W. (1992). Electrodermal Activity. New York: Plenum Press.
2. Busch, P. & Gassenmeier, T. (1997). Sensory assessment in the cosmetic field. Parfümerie und Kosmetik, 78, 16-21.
3. Graham, F. K. & Clifton, R. K. (1966). Heart rate changes as a component of the orienting response. Psychological Bulletin, 65, 305-320.
4. Boucsein, W., F. Schaefer, A. Schwerdtfeger, P. Busch, W. Eisfeld (1999). Objective Emotional Assessment of Foam. SÖFW-Journal 125 (11), 2 -17.
Author:
Dr. Wolf Eisfeld
Dr. Wolf Eisfeld studied physics at Freiburg and Göttingen universities and obtained his doctorate at Max-Planck-Institute for Biophysical Chemistry. In 1996 he assumed the position of laboratory head in the Hair Physics/ Sensorics Department within Henkel Düsseldorf's Chemical Research. Since the end of 2000 he has been responsible for building up a biophysical-sensory research team at Cognis Research&Technology, emphasizing hair performance properties as well as consumer perception of raw materials and formulations.
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