Expectation Modulates Hedonic Experiences and Midbrain Responses to Sweet Flavor

Non-nutritive sweeteners are sugar substitutes that may promote weight management by reducing an individual's calorie intake. It is, however, unclear whether (1) sugar and non-nutritive sweetener elicit distinct orosensory responses in the human brain and (2) whether the neural responses to these flavors are modulated by expectancy. Addressing these questions has direct relevance to our understanding of food choice behavior and how it may be modified in dietary interventions. We screened <i>N</i> = 99 healthy adults of either sex to select a sample (<i>N</i> = 27; <i>M</i>[SD]_age= 24.25[2.94] years) who reported similar perceptual experiences of sugar and sweetener, thus removing a potential confound of sensory differences, for fMRI scanning. While scanning, they received sugar- and artificially sweetened beverages in two conditioning paradigms, which manipulated participants' expectation of flavor delivery: first in a probabilistic and second in a deterministic way. Participants' ability to accurately distinguish sugar from non-nutritive sweetener depended largely on their expectations, which also significantly affected the perceived pleasantness of each flavor. Expectation altered brain responses to flavor delivery during the deterministic task only, where the (mistaken) expectation of sugar significantly increased midbrain responses to sweetener compared with when sweetener was expected. Trial-wise confidence and pleasantness ratings differentially scaled with brain responses to sugar and sweetener delivery. These results highlight the importance of expectancy in both the behavioral and neural encoding of sweet flavor, particularly when sensory information is unreliable. The expectation of sugar appears to increase the subjective value of noncaloric sweetener, which may result from flavor-nutrient conditioning that preferentially reinforcers sugar.