Intensity of sensation related to activity of slowly adapting mechanoreceptive units in the human hand

Publisher Website
Google Scholar
Abstract
Impulses were recorded from single afferent fibers in the median and ulnar nerves of human subjects. The response of slowly adapting mechanosensitive units with receptive fields in the glabrous skin of the hand were studied when rectangular indentations of varying amplitudes and invariant time duration were delivered. Simultaneously the subject was asked to estimate the magnitude of his sensation associated with the stimuli. Stimulus-response plots of the afferent units were constructed and compared with the psychophysical magnitude estimation plots. The stimulus-response of the afferent units fell along largely decelerating monotonous curves when stimuli above the static threshold were considered. When responses below the static threshold were considered, many plots were S-shaped. The psychophysical plots were monotonous and decelerating, linear or accelerating. Power functions were fitted to the 2 sets. The group average differed considerably in the exponent of the fitted functions 0.7 for the neural and 1.0 for the psychophysical function. Considerable variation existed between shapes of curves derived from individual test points. The range of exponents for the neural function was 0.26-1.92 and for the psychophysical function 0.36-2.09. The variation in psychophysical functions was partly due to relatively stable inter-subject differences. No such inter-subject difference was evident for the neural functions, which seemed to vary randomly. No indication of a correlation between the shapes of the neural functions and the shapes of the psychophysical functions exists when individual subjects or individual test points were compared. When an accelerating and decelerating psychophysical function was considered, the associated neural functions did not differ. Comparing the average group and individual subjects and individual target points reveals a close agreement between the stimulus-response functions of slowly adapting mechanoreceptors in the human hand and the psychophysical magnitude estimation functions is not tenable. Shapes of the psychophysical magnitude estimation functions apparently are highly dependent on central mechanisms and are not a direct function of the properties of the afferent units.