Subjective intensity and pleasantness in taste

Abstract

This thesis contains studies on intensity and pleasantness in taste perception. There is a formal relationship between intensity and hedonic value of stimuli, which can be expressed in an inverted U. The fact that pleasantness depends partially on stimulus intensity poses a problem when one wants to study them independently. We attempted to dissociate intensity and pleasantness by manipulation of stimulus dimensions. Mixtures of two taste substances were used, one pleasant and the other unpleasant. Based on preceding subjective intensity matching, different proportional combinations of both substances could be composed in such a way as to keep stimulus intensity constant. Depending on the proportions of the components, the iso-intense stimuli had a different hedonic value. Manipulating stimulus dimensions may be characterized as a bottom-up approach. We also used a top-down approach to dissociate intensity and pleasantness. This part of the thesis involves several experiments dealing with timing differences between attention to and judgment of either pleasantness or intensity of taste stimuli. In one paper it is shown that the response time required to judge intensity is considerably shorter than the time needed to respond to pleasantness. In a related paper changes in intensity and pleasantness over time were studied. The results from these papers suggest that intensity can be judged much faster than pleasantness, since the latter is conceptually more fuzzy and complex. In another paper, subjects were required to switch their judgment of a taste stimulus from intensity to pleasantness or vice versa. The switch from pleasantness to intensity was much faster than from intensity to pleasantness. However, our observations show that pleasantness judgements become faster as pleasantness becomes a more clearly defined concept, for example in very unpleasant taste stimuli. Still another top-down way to affect taste judgments is by providing information on the stimulus and then studying the judgment of that stimulus. This approach was chosen in two experiments. When a preceding verbal prime is negative and hedonically congruent with the following stimulus (a bitter unpleasant taste), this stimulus is detected faster than when prime and stimulus are congruent and both positive. A finding like this supports the idea of preferential facilitation of the processing of negative stimuli under time constraints. A well-known effect is the synergism caused by adding the widely used taste-enhancer monosodium glutamate to food. We studied this phenomenon in a model bouillon solution and found that it is due primarily to pleasantness change rather than intensity change. The results of some of the tasks mentioned above were interpreted in the framework of attention. How people attend to a taste in terms of brain mechanisms is as yet unclear. So a study was carried out to find out whether increased attention to a taste stimulus is accompanied by enhanced activity in the gustatory cortex. This turned out to be true; however, even in the case of stimulus absence (tasteless solution) the mere instruction to attend to a taste already led to increased activity in the gustatory cortex as evidenced by fMRI measurements.