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 unde r psychophysical scrutiny at the fingertip and control body sites. Acu ity is viewed as encompassing the discrimination of four features of s imple stimulus configurations: (1) discontinuity (gaps in lines or dis ks), (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 structur e of braille. For their measurement, psychophysical tests were develop ed and refined. These were aimed at freedom from bias, rapid estimatio n of acuity thresholds in hundreds of subjects, and eventual applicabi lity to the whole body surface. Some 14 versions of the tests were adm inistered in three experiments, yielding 1478 individual thresholds. E xperiment I (15 young and 15 elderly subjects) and Experiment II (131 subjects, ages 18 to 87 years) shed light on the nature of discriminat ion of discontinuity and orientation. These mainly concern pitfalls of measurement and influence of exact stimulus configuration. Experiment III (1 15 subjects, ages 8 to 86 years) examined refined versions of tests for all four dimensions of acuity. Four principal findings emerg ed: (1) At all ages, thresholds for the four dimensions of acuity diff er from one another in size-in order from smallest to largest: length, locus, orientation, and discontinuity. Exact sizes differ for transve rse and longitudinal stimulus alignment. (2) All four acuity dimension s deteriorate with age, to a first approximation manifesting a constan t increase in threshold of approximately 1% per annum between ages 20 and 80 years. That similar rates of deterioration characterize all fou r dimensions in the fingertip suggests a common mechanism, possibly th inning of the same mediating receptor network. (3) Acuity at more cent ral sites (forearm, lip) deteriorates more slowly than at the fingerti p. (4) Individual differences in acuity abound, even after the effects of aging are discounted.