Joseph C. Stevens

Temperature sensitivity of the body surface over the life span

Detection thresholds to warming and cooling were measured in 13 regions of the body in 60 adults aged between 18 and 88 years. From these thresholds were constructed maps of thermal sensitivity homologous to body maps of spatial acuity (in the older literature two-point discrimination), long known to the somatosensory scientist. Maps of cold and warm sensitivity for young, middle-aged and elderly adults, show how sensitivity changes with age in the various body regions. Three characteristics emerge, irrespective of age: (1) sensitivity varies approximately 100-fold over the body surface. The face, especially near the mouth, is exquisitely sensitive, the extremities, by comparison, poor, other regions, intermediate. (2) All body regions are more sensitive to cold than to warm. (3) The better a region is at detecting cold, the better it is at detecting warm. With age, thermal sensitivity declines. The greatest changes take place in the extremities, especially the foot, where thresholds often become too large to measure. Central regions give up their sensitivity with age more slowly, and even (as in the lips) inconsequentially. Similar age-related changes have also previously been shown to characterize spatial acuity.

Spatial Acuity of the Body Surface over the Life Span

Spatial acuity over 13 regions of the body was assessed cross-sectionally in 122 male and female subjects between 8 and 87 years of age. Of two measures, the primary one was a threshold for detecting a gap between two points (a refinement of the conventional two-point threshold). The secondary one was a threshold of point localization in 7 of these 13 body regions. The two measures yielded similar pictures of body acuity and age-related changes in acuity, and they agreed in essentials with an early acuity map dating back to Weber in 1835, as cited and confirmed experimentally by Weinstein (1968). To this acuity map, the present study added the dimension of age. The main finding was that aging is much harder on some body regions than on others. Declining acuity with age was found to characterize all regions to one degree or another, but the hands and feet turned out to be far more vulnerable than the more central regions, including the very acute lip and tongue. Deterioration of acuity in the great toe (averaging 400% between youth and advanced age) and fingertip (averaging 130%) may adversely affect such diverse activities as braille reading, grasping, and maintaining balance. The acuity map determined by gap discrimination was essentially the same for males and females; however, males gave significantly smaller localization thresholds than females. In two body regions tested (fingertip and upper lip), children significantly outperformed young adults at gap discrimination.

Cross-modality matching functions generated by magnitude estimation.

It is possible to generate cross-modality matching functions by having subjects make magnitude estimates of sets of stimuli appropriate to different modalities. The sets are interspersed among each other in the same test session and judged on a common absolute scale of sensory magnitude. An appropriate statistical device locates stimulus levels that appear, on the average, to match. The method is fast, efficient, circumvents the need for continuous stimulus adjustment, and holds promise for the study of the individual as well as the average psychophysical function. To illustrate its potential uses, advantages, and limitations, we used the method to generate cross-modality matching functions relating loudness and brightness. Compared to the scales of loudness and brightness generated by the magnitude estimations of the same stimuli, the matching functions (1) conform better to power functions, (2) may show less variation in slope (exponent), and (3) show far less variation in absolute magnitude (position).

Spatial summation and the dynamics of warmth sensation

Various areal extents of the forehead and back were thermally irradiated at various levels of intensity. For any areal extent, the degree of apparent warmth grows approximately as a power function of intensity level; the larger the area, the smaller the exponent of the power function. Two families of psychophysical functions, one for the forehead, the other for the back, both show that the power functions extrapolate to a point of convergence in the neighborhood of the threshold for pain and tissue impairment. The rules that govern spatial summation of warmth reveal themselvesin the two families. Intensity and area trade one for the other to preserve the same level of warmth. At faint sensation levels, reciprocity is the rule of trading; but with increasing sensation level, area makes a weaker and weaker relative contribution to warmth, and, as a result, it takes a larger and larger percentage change in area to offset a given percentage change in intensity.

Regional sensitivity and spatial summation in the warmth sense

Abstract The magnitude of warmth sensation aroused by heat irradiation and assessed by the method of magnitude estimation depends on the level of the irradiation, its areal extent, and the particular region of the body stimulated. Within a given body region, area and irradiation level both contribute to the magnitude of the warmth sensation (signifying generous spatial summation of neural signals), except that the proportional contribution of area diminishes gradually with increasing level of warmth and finally becomes negligible as the pain threshold is approached. Some regions of the body are far more responsive than others to low-level heating, but all regions respond more or less uniformly when the level of heating is high enough.

Dimensions of Spatial Acuity in the Touch Sense: Changes over the Life Span

Spatial acuity of the touch sense and its variation in aging came under psychophysical scrutiny at the fingertip and control body sites. Acuity is viewed as encompassing the discrimination of four features of simple stimulus configurations: (11) discontinuity (gaps in lines or disks), (2) locus on the skin, (3) length (or area), and (4) orientation (e.g., along or across the finger). Each of these dimensions of acuity serves uniquely in tactile perception, as illustrated in the structure of braille. For their measurement, psychophysical tests were developed and refined. These were aimed at freedom from bias, rapid estimation of acuity thresholds in hundreds of subjects, and eventual applicability to the whole body surface. Some 14 versions of the tests were administered in three experiments, yielding 1478 individual thresholds. Experiment I (15 young and 15 elderly subjects) and Experiment II (131 subjects, ages 18 to 87 years) shed light on the nature of discrimination of discontiniuty and orientation. These mainly concern pitfalls of measurement and influence of exact stimulus configuration. Experiment III (115 subjects, ages 8 to 86 years) examined refined versions of tests for all four dimensions of acuity. Four principal findings emerged: (1) At all ages, thresholds for the four dimensions of acuity differ from one another in size--in order from smallest to largest: length, locus, orientation, and discontinuity. Exact sizes differ for transverse and longitudinal stimulus alignment. (2) All four acuity dimensions deteriorate with age, to a first approximation manifesting a constant increase in threshold of approximately 1% per annum between ages 20 and 80 years. That similar rates of deterioration characterize all four dimensions in the fingertip suggests a common mechanism, possibly thinning of the same mediating receptor network. (3) Acuity at more central sites (forearm, lip) deteriorates more slowly than at the fingertip. (4) Individual differences in acuity abound, even after the effects of aging are discounted.

Psychophysical scales of apparent heaviness and the size-weight illusion.

The apparent heaviness of a set of 40 cylindrical objects was scaled by the method of magnitude estimation. The objects varied in weight, volume. and density. There were three main conclusions: (1) For any constant volume, heaviness grows as a power function of weight; the larger the volume. the larger the exponent of the power function. The family of such power functions converge at a common point in the vicinity of the heaviest weight that can be lifted. (2) For any constant density (i:e., weight proportional to volume), heaviness does not grow as a power function of weight. (3) For any constant weight, heaviness decreases approximately as a logarithmic function of volume; the constants of the log function depend systematically on the weight of the object. The outcome furnishes a broad quantitative picture of apparent heaviness and of the size-weight illusion (Charpentier’s illusion).

Effects of Temperature on the Perceived Sweetness of Sucrose

The sweetness of sucrose depends on the temperature as well as the concentration of a solution. The main effect is that relatively low concentrations gain sweetness as temperature increases. This effect diminishes with progressively higher concentration and finally becomes negligible at about 0.5 M. At this concentration the various functions that relate perceived sweetness to concentration for various temperatures converge. The mechanism of the taste-temperature interaction is speculative, but the interaction is large enough to be of practical interest in the perception of common foods and beverages as well as a variable to be strictly controlled in taste experiments. An examination of method of tasting showed that swallowing stimuli did not substantially increase perceived sweetness.

Uniformity of Olfactory Loss in Aging

Most studies of how human olfaction changes with age have compared young and old. Essentially all such studies imply that aging takes a toll. The elderly have higher thresholds, perceive suprathreshold odors as being weaker, discriminate quality less well, recognize and identify common odors less well, and remember episodic presentations of odors poorly. To a first approximation, it appears that all odor qualities and functions undergo a general blunting. The few studies of persons between the young and the elderly suggest that the process of deterioration sets in early and progresses gradually. Such gradual deterioration would presumably allow the easiest accommodation to any loss and may account for why many elderly people seem oblivious to it. In some respects, ignorance may be bliss. For example, the diminished flavor of food may go unnoticed. In other respects, the loss of olfactory information may pose some nutritional and safety risks of which the elderly and perhaps even the middle-aged should be apprised. Longitudinal studies would seem to offer the only chance to decide the rate and magnitude of individual losses in olfaction. Such studies might also offer enlightenment regarding ways to forestall loss.

Reduction of odor and nasal pungency associated with aging

Aging can seriously blunt suprathreshold sensations mediated by the olfactory receptor system and by common chemical receptors. Despite large individual differences, on the average any given stimulus seemed only about half as intense to the elderly (20 subjects, 65-83 yrs) as to the young (20 subjects, 18-25 yrs). The nature of the loss was a constant percentage reduction of perceived magnitude at stimulus levels from weak to strong. The stimuli were iso-amyl butyrate (a nonirritating fruity odor) and CO2 (which is practically odorless but triggers common chemical sensations effectively). The method used was magnitude matching, by which subjects made numerical estimates of the perceived magnitude of various levels of the two chemical stimuli and of the loudness of low-pitched noises. The loudness estimates served to adjust each subjects chemical estimates to help compensate for individual idiosyncrasies in the use of numbers and potential biases associated with age. Common chemical and olfactory losses seem to be unrelated; aging can dull one sense and leave the other acute.