Two-stimulus versus one-stimulus procedure in the framework of functional measurement: a comparative investigation using quinine HCl/NaCl mixtures

Abstract

When subjects are requested to judge single stimuli the observable responses are the result of both sensory and judgmental processes. De Graaf et al. (1987) employed functional measurement in combination with a two-stimulus procedure in order to separate these two types of processes. This paper discusses the results of two experiments investigating taste interactions in quinine HCl/NaCl mixtures. The first experiment employed a one-stimulus procedure, the second experiment employed De Graaf et al.'s two-stimulus procedure. Comparing the two procedures, the main advantage of the one-stimulus procedure seems to lie in its simplicity. In addition, it enables the determination of a scale value for water. However, the obtained ratings are context-dependent and are affected by end effects of the response scale. The most important benefit of the two-stimulus procedure is that it allows for a post-experimental verification of the linearity of the response-output function. This check ensures that all scale values are assessed on an interval scale. If water can be assumed to be tasteless, ratio scale values can be obtained. It was shown that quinine bitterness is largely suppressed by NaCl, whilst the saltiness intensity elicited by NaCl remains virtually unaffected. In both experiments, the perceived total taste intensity of a mixture could be well predicted by the weighted sum of the saltiness and bitterness sensations within the mixture percept.