Joseph C. Stevens - US grants

It has become abundantly clear that aging takes a toll on all three chemical senses: taste, smell, and common chemical sense (CCS). Of these, taste seems the most resilient; although old people, on the average, lose the ability to detect faint levels of tastes, the perception of supra-threshold taste magnitudes remains relatively intact. In contrast, losses of smell and CCS frequently pervade the entire sensory continua from low to high stimulus levels. These are accompanied by loss of ability to identify and discriminate odors. The present proposal concerns the relation of olfactory and CCS losses to other aspects of chemosensory function about which much is known from youthful persons, and concerning which there is reason to hypothesize that aging might matter. These include (a) Olfactory affect, or the perception of the pleasant-unpleasant dimension of odors, the main question being to what extent affective narrowing takes place in old age and to what extent it pertains to the loss of suprathreshold magnitude; (b) Olfactory memory, or the ability to recognize odors shortly after inspection and then at intervals of a day and a week. Questions of interest include: the relation of odor recognition to olfactory identification and discrimination, to visual recognition, and to gender. (c) Adaptation, or loss of sensitivity over time with ongoing stimulation, especially as regards its role in the detection of a gas warning agent, ethyl mercaptan; (d) Olfactory--CCS mutual inhibition, or the power of nasal irritants to mute smells, and vice versa; (e) Detection of danger-related substances, in the present study the ability to detect smoke and ethyl mercaptan; and (f) Age-related dementia (especially Alzheimer's disease) as it affects three basic functions of olfaction: detection, odor identification, and perception of the affective dimension, i.e., the capacity to discriminate between good and bad odors and tastes.

The goal here is to explore how three stimulus dimensions (thermal, mechanical, and chemical) can operate jointly on each of four different sense modalities to determine perceptual events. These modalities are (1) touch, (2) taste, (3) warmth and cold, and (4) common chemical sense (CSS). The CCS, often slighted by students of the senses, is mediated by receptors in the nose and mouth and plays and double role of (a) protecting against intake of caustics and (b) giving to food and beverages agreeable pungency or "spiciness" (e.g. peppers, mustards, carbonated and alcoholic beverages). Neuroanatomically these three senses are peripherally distinct, so that "true" interaction among them supposedly takes place in the CNS. In addition, however, all four receptor types can respond to multimodal stimulation, i.e. to mechanical, thermal, and/or chemical stimulation. For example, our research has revealed that warmth and cold can greatly enhance pressure sensations on the skin, with extensive psychophysical evidence that enhancement takes place through joint thermal and mechanical arousal of touch receptors. This led us to the novel hypothesis that spatial acuity of tough can be sharpened by simultaneous application of thermal and mechanical energy. Pilot work on two-point discrimination favored this hypothesis, and the proposal is to select a battery of tests to assess the role of the thermal component of stimulation in (a) various parts of the body, parametrically, especially glabrous sites, (b) various methods, including the classical two-point discrimination and point localization, but also newer methods, e.g. accuracy of length and area discrimination, and (c) the potential usefulness of thermal stimulation for tactile aids for the deaf and blind. Other previous work showed significant taste-temperature interactions in the mouth. However, there remain potentially important questions about interactions among taste, temperature, and CCS. We will obtain psychophysical scales relating degree of pungency to concentration of two agents: (1) capsaicin (active ingredient in hot red peppers) and (2) ethyl alcohol. We will then seek to quantify what pilot studies here (together with cues from the archival literature) suggest may be important interactions, e.g. possible effects on taste and temperature sensations by CSS and alteration of CSS by thermal input. We will also try to assess long-term exposure to capsaisin by alternation between usage and abstinence.

The goal is to enrich the understanding of the preception and psychophysics of texture (of fabrics of various densities) and of roughness (of grooved metallic surfaces) and to seek a closer integration of these two percepts with each other and with other measures of dermal perception. Variables of interest include the spacing of the fabric filaments and plate grooves (a known primary variable), the temperature of the skin and of stimulators touching the skin, the local site of stimulation over the body surface, and various physical properties of the skin such as the coefficient of friction, skin blood flow, hardness, and elasticity. The two psychological dimensions of interest are the perceived magnitude of texture and roughness, to be scaled by the method of magnitude estimation, and the hedonic tone (pleasantness- unpleasantness), to be scaled by visual analog rating. This proposal to extend our work on thermo-tactile interactions was spurred by a preliminary study in which the texture magnitude and hedonic tone of various common fabrics were found to vary regularly with the force required to drag the fabric across the skin, and that this force, together with perceived texture and unpleasantness, rose rather dramatically as skin wettedness increased in warm and humid air. This finding helps to account for the discomfort aroused by clothing in the heat, but it also leads to a number of interesting theoretical questions regarding tactile perception.

Psychophysical research on the human senses has tended to slight the cutaneous senses (tactile and temperature), when it comes to their capacities to function over the life span. The goal is to examine five fundamental (and largely mutually independent) measures of cutaneous sensitivity, over the whole body surface, and from childhood to old age. The five types, all implicated as age-related by background research and pilot data, are: (1) spatial acuity (alignment threshold), (2) error of point localization, (3) texture discrimination, (4) absolute thresholds to warming and cooling, and (5) absolute thresholds to high-frequency vibration. State-of-the-art, forced-choice, psychophysical procedures will be brought to bear throughout. The somatic profiles of tactile sensitivity therewith constructed will help to elucidate two basic issues: (a) Does aging influence these five types of sensitivity uniformly or differentially? (b) Does aging affect all body regions uniformly or differentially? Preliminary evidence favors the working hypotheses that (1) all five types may suffer impairment but in varying degrees, and that (2) the more peripheral the region, the greater may be the toll taken by aging. Measurement will address primarily the gradual changes characterizing the normal aging process. They may nonetheless have important secondary implications for other age-related human conditions, whose study in turn may also reciprocally help to characterize normal aging. Four of these will come under study: (1) spatial acuity and texture discrimination in blind persons of various ages, and the relation of age-related changes in cutaneous sensitivity to the speed of braille-reading; (2) the effects of chronic cigarette smoking on cutaneous sensitivity and the possible recovery after quitting; (3) the manifestation of long-term occupational exposure to vibration on cutaneous sensitivities of patients with Hand-Arm Vibration Syndrome (and a sub-set of these patients who are also diabetic); and (4) the relation of habitual physical activity and fitness to peripheral circulation and cutaneous sensitivity, especially in older persons.