Oral and Oropharyngeal Perceptions of Fluid Viscosity Across the Age Span

Abstract

Research demonstrates that varying sensory input, including the characteristics of a bolus, changes swallow physiology. Altering the consistency of fluids is a common compensatory technique used in dysphagia management to facilitate change. However, it is not known what variations in viscosity can be perceived in the oral cavity or oropharynx or if age affects oral and oropharyngeal perceptions of fluid viscosity. This study aims to establish the ability of normal adults to perceive fluid viscosity in the oral cavity and oropharynx and to determine if, within this population, there are age-related changes in oral and oropharyngeal perceptions. Sensitivity was established by deriving the exponent for the psychophysical law for fluid viscosity in both the oral cavity and the oropharynx, using modulus-free magnitude estimation with Newtonian fluids of corn syrup and water. Sixty normal volunteers, aged 21–84 years, participated. Results indicate that the exponent for oral perception of fluid viscosity was 0.3298, while for oropharyngeal perception it was 0.3148. Viscosity perception deteriorates with increasing age. Men exhibited a more marked deterioration in sensitivity than women. This study contributes to the literature on oral and oropharyngeal perceptions and on aging. The results provide a basis for work with individuals with dysphagia.

Appendix: Description of the Exponent of the Psychophysical Law

When the psychophysical power law is described mathematically, it can be quantified and the power component is termed the exponent of the relation. This relation can be defined thus:

$$ {{\rm{\psi }} = k\phi ^{{\rm{\beta }}} } $$

where ψ is the psychological magnitude, φ is the intensity of the physical stimulus, k is a constant determined by the choice of unit selected for measurement, and β is the exponent. The exponent is the only part of this equation that changes depending on the sensory system being examined. The value of the exponent (β) serves as a kind of signature that may differ from one sensory continuum to another. (e.g., taste in the form of salt has an exponent of 1.4). When we plot the results of magnitude estimation experiments, the exponent of the psychophysical law is graphically displayed reflecting the rate at which a stimulus response grows in relation to the stimulus. The psychophysical law for electric shock, with an exponent of more than one (3.5), when displayed graphically on ordinary graph paper is a line that is concave upward, ascending in an ever-steepening slope. Length has an exponent of 1.1 and is almost a straight line, while brightness, with an exponent of less than one (0.33), has a curvature downward, with the line becoming ever more horizontal (Stevens 1986). Thus, the exponent of the psychophysical law determines, and reflects, the curvature of this type of line plot. An interesting feature of the power function is that, if it is plotted with logarithmic scales on both axes, all the lines become straight lines. The slope of the power function for electrical shock is steep, while that for brightness is shallow, and length is approximately 45° to the horizontal and vertical axes, but all are straight lines. In terms of logarithms, the power law equation becomes

$$ {{\rm{log\psi }} = {\rm{\beta }}\log \phi + \log k} $$

This formula describes a straight line in log-log coordinates, and the exponent (β) becomes the slope of the line. The exponent is directly displayed in the slope of the power function. Alternatively one can say that the slope of the line is a direct measure of the exponent (Fig. 1).