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

1. Impulses were recorded from single afferent fibres 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.2. Stimulus-response plots of the afferent units were constructed and compared with the psychophysical magnitude estimation plots.3. The stimulus-response data of the afferent units fell along monotonous curves which were largely decelerating when stimuli above the static threshold were taken into account. When responses below the static threshold were taken into account many plots were S-shaped.4. The psychophysical plots were monotonous and either decelerating, linear or accelerating.5. Power functions were fitted to the two sets of data. The group average differed considerably with regard to the exponent of the fitted functions which was 0.7 for the neural and 1.0 for the psychophysical function.6. There was considerable variation 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 accounted for by relatively stable inter-subject differences, whereas no such inter-subject difference was evident for the neural functions, which seemed to vary randomly.7. There was no indication of a correlation between the shapes of the neural functions and the shapes of the psychophysical functions when data from individual subjects or individual test points were compared. Moreover, when two groups of data were considered, one with accelerating and one with decelerating psychophysical functions, the associated neural functions did not differ between the two groups.8. It was concluded that the hypothesis of 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. This was evident when average group data were compared as well as when data from individual subjects and individual target points were compared. The findings suggest that the shapes of the psychophysical magnitude estimation functions are highly dependent on central mechanisms and are not a direct function of the properties of the afferent units as has been claimed in previous investigations.