Jan was intrigued by the question of how we could measure the sensations that people perceive, these internal representations that are private and cannot be observed from the outside. He built his research on classical psychophysical methods, starting with Thurstonian modeling and Signal Detection Theory. In one of his classic papers with the elusive title “The Paradox of Discriminatory Nondiscriminators Resolved”, he describes how the differences in the number of correct responses between the triangular test and the 3-alternative forced choice (3-AFC) test can be explained by task differences. In both cases, the participants taste three samples, but while the triangular test asks to pick the deviant sample, the 3-AFC procedure asks to pick the least or the most intense of the three samples. In either case, a statistical argument would suggest that the probability of picking the correct one out is 0.33. However, the 3-AFC task is easier because the participant already knows whether the target sample is more or less intense than the others, while in the triangular test both a low or high-intensity sample may be selected (Frijters, 1979a). The descriptions of the cognitive decision rules underlying simple psychophysical tasks, calculating the expected response distributions of these tasks, and their use in interpreting the outcomes of sensory tests (Frijters, 1979b; Frijters, Kooistra, & Vereijken, 1980) have found application in sensory testing labs around the world and have formed the basis for standardized ISO testing protocols. Daniel Ennis later extended Jan’s work by using Thurstonian modeling to explain response distributions in many different tasks (e.g., Ennis, Mullen, & Frijters, 1988; Mullen, Ennis, De Doncker, & Kapenga, 1988).

 

In 1978 Jan Frijters took up an appointment at the Agricultural University in Wageningen, where he started as a staff member at the Department of Human Nutrition and in 1987 became professor in the Psychological and Sensory Aspects of Food and Nutrition at the Department of Food Science. In Wageningen Jan was initially responsible for all psychological research relevant to human nutrition, including determinants of unhealthy behavior and eating disorders. His most cited papers involve the development of a questionnaire to measure physical activity for epidemiological studies (Baecke, Burema, & Frijters, 1982) and a questionnaire for the assessment of restrained, emotional, and external eating behavior (van Strien, Frijters, Bergers, & Defares, 1986), but Jan was especially passionate about psychophysics.

 

I became acquainted with Jan Frijters at the Agricultural University, first as my lecturer, then as my graduation chair and as supervisor of my PhD thesis. For Jan, the question of how to measure sensory intensity was central to his work. He thereby built on classical psychophysics, starting from the determination of absolute and differential thresholds, using Fechner’s law to create a cumulative JND scale, the Weber fraction, and Stevens’s power law. He argued against the use of odor units as a measure of olfactory intensity, because dividing a suprathreshold concentration by the absolute threshold concentration would not provide a reliable estimate of perceived intensity according to these basic psychophysical principles (Frijters, 1978). The confusion here lies in the fact that physical concentrations rather than perceived intensities of sensations are used to infer psychological relations (Frijters, 1987a). Despite these objections, the odor unit can serve as a guideline in particular investigations and is still widely used in applied flavor research.

 

Although many researchers were using direct scaling methods to measure the perceived intensity of substance concentrations, Jan doubted whether these methods really provided unbiased measurements of intensity. In particular, the fact that category rating scales and line scales provided differently shaped psychophysical functions than magnitude estimation led to a debate about which scaling method was the correct one. By using the same stimulus sets in several studies with different task instructions Norman Anderson’s Information Integration Theory could be used to separate psychophysical from judgmental processes and to test hypotheses about underlying cognitive operations. This approach enabled Jan’s group to elucidate the cognitive operations underlying different direct scaling methods (e.g., De Graaf & Frijters, 1988) and obtain a validated scale of perceived intensity using intensity differences between two samples (De Graaf, Frijters, & van Trijp, 1987).

 

The phenomenon that Jan’s group studied primarily concerned the sensory interactions in mixtures of tastants and mixtures of odorants. After assessing the intensities of the different tastes or smells in mixed and unmixed samples, we tried to characterize and quantify the nature of the interactions, and we tested the predictions of existing mixture models (Frijters, 1987b). Jan has made a special contribution in this area by proposing a new mixture model, the equiratio mixture model. This model provides an original perspective on mixture interactions by regarding the intensities of a mixture series with a specific mixing ratio at different concentration levels as a psychophysical function for that mixture type (Frijters & Oude Ophuis, 1983). The characteristics of the model were investigated both for mixtures of tastants (Frijters & De Graaf, 1987) and for odorants (Schiet & Frijters, 1988).

 

The rigor that Jan applied in designing and conducting experiments was also applied in his writing of papers: every word had to be in the right place, and every superfluous word had to be deleted. Almost every time I gave him a revised manuscript, he started over on page 1 to improve and sharpen the argument. While his tendency to pursue the highest quality in his work earned him great respect, his inclination to make no compromises often hindered him in his personal relationships, both at work and in his personal life. Jan had a very strong sense of justice and went to great lengths to achieve this, but unfortunately the result was often to his disadvantage. A series of conflicts at work and a lengthy and painful divorce process made him decide to leave the academic world and move to the USA. Although he withdrew from the public eye, he continued to work in the field of sensory perception. Being a heavy smoker himself, he went to work for the Brown and Williamson Tobacco company. After a few years he came back to the Netherlands, where he worked as a consultant. During this period, he collaborated with International Flavors & Fragrances (IFF) to set up their perfumer training program and worked on the IFF flavor language. His divorce process made him so closely involved in legal processes that he applied his expertise in this field too, critically evaluating and proposing improvements for the use of dog scent lineups in criminal cases (Frijters, 2006; Frijters & Boksem, 2004). This work had a major local impact: nearly 2000 lawsuits had to be reopened and in 144 cases this led to an acquittal. In addition, nine police officers were prosecuted for malpractice during odor identification tests.

 

With the passing of Jan on 16 May 2022, we lose a high-quality, passionate researcher and teacher in the field of chemosensory psychophysics. While a focus on research methodology may not be the most exciting or popular topic, scientific methods are the foundation on which we build science. By solving some of the apparent paradoxes in the field, Jan provided fundamental knowledge about the mechanisms at work when participants use particular response methods. We build on his legacy as we study phenomena of interest in the field of chemosensory perception.

 

 

References

 

Baecke, J. A. H., Burema, J., & Frijters, J. E. R. (1982). A short questionnaire for the measurement of habitual physical activity in epidemiological studies. American Journal of Clinical Nutrition, 36(5), 936-942.

 

De Graaf, C., & Frijters, J. E. R. (1988). "Ratios" and "differences" in perceived sweetness intensity. Perception & Psychophysics, 44, 357-362.

 

De Graaf, C., Frijters, J. E. R., & van Trijp, H. C. M. (1987). Taste interaction between glucose and fructose assessed by functional measurement. Perception & Psychophysics, 41, 383-392.

 

Ennis, D. M., Mullen, K., & Frijters, J. E. R. (1988). Variants of the method of triads: Unidimensional Thurstonian models. British Journal of Mathematical and Statistical Psychology, 41(1), 25-36.

 

Frijters, J. E. R. (1978). A critical analysis of the odour unit number and its use. Chemical Senses and Flavour, 3, 227-233.

 

Frijters, J. E. R. (1979a). The paradox of discriminatory nondiscriminators resolved. Chemical Senses, 5, 355-358.

 

Frijters, J. E. R. (1979b). Variations of the triangular method and the relationship of its unidimensional probabilistic models to three‐alternative forced‐choice signal detection theory models. British Journal of Mathematical and Statistical Psychology, 32(2), 229-241.

 

Frijters, J. E. R. (1987a). Psychophysica in het kader van voedselacceptatieonderzoek [Psychophysics in relation to food acceptance research]. Inaugural address. Agricultural University. Wageningen.

 

Frijters, J. E. R. (1987b). Psychophysical models for mixtures of tastants and mixtures of odorants. In S. D. Roper & J. Atema (Eds.), Olfaction and Tast IX (pp. 67-78). New York: New York Academy of Sciences.

 

Frijters, J. E. R. (2006). De Geuridentificatieproef in het Licht van het Falsificatiebeginsel [The Dog Scent Lineup in the Light of the Falsification Principle]. Nederlands Juristenblad, 81, 945-948.

 

Frijters, J. E. R., & Boksem, J. (2004). Een Positieve Geuridentificatieproef dient vrijwel altijd in een tegenonderzoek te worden herhaald! [A Positive Dog Scent Lineup Nearly Always Has to Be Repeated Through a Countercheck]. Nederlands Juristenblad, 79, 729-734.

 

Frijters, J. E. R., & De Graaf, C. (1987). The equiratio taste mixture model successfully predicts the sensory response to the sweetness intensity of complex mixtures of sugars and sugar alcohols. Perception, 16, 615-628.

 

Frijters, J. E. R., Kooistra, A., & Vereijken, P. F. G. (1980). Tables of d′ for the triangular method and the 3-AFC signal detection procedure. Perception & Psychophysics, 27(2), 176-178.

 

Frijters, J. E. R., & Oude Ophuis, P. A. M. (1983). The construction and prediction of psychophysical power functions for the sweetness of equiratio sugar mixtures. Perception, 12, 753-767.

 

Mullen, K., Ennis, D. M., De Doncker, E., & Kapenga, J. A. (1988). Models for the duo-trio and triangular methods. Biometrics, 44(4), 1169-1175.

 

Schiet, F. T., & Frijters, J. E. R. (1988). An investigation of the equiratio-mixture model in olfactory psychophysics: a case study. Perception & Psychophysics, 44, 304-308.

 

van Strien, T., Frijters, J. E. R., Bergers, G. P. A., & Defares, P. B. (1986). The Dutch Eating Behavior Questionnaire (DEBQ) for assessment of restrained, emotional, and external eating behavior. International Journal of Eating Disorders, 5(2), 295-315.