Response characteristics of peripheral mechanoreceptive units in man: relation to the sensation magnitude and to the subject's task.

The characteristics of responses of 23 peripheral mechanoreceptive units to tactile pulses of varying amplitude (50-950 micron) were studied by means of human microneurography during two different tasks. The simultaneously recorded unit responses were related to estimates of sensation magnitude in order to study the coding properties of units. The possible effects of the tasks were studied by comparing the response to identical stimulus pulses during the two tasks. The S-R functions of single units had smaller exponents than the magnitude estimation functions, but the relation between the number of impulses/frequency of impulses and the simultaneously given magnitude estimate was well described by linear functions for 74% of units. Thus both of these parameters may serve as codes for stimulus amplitude. For 18 units differences were obtained between the two task situations when thresholds, latency of the first impulse, mean frequency of impulses, or number of impulses in responses were compared. The differences were partly due to a continuous decrease of sensitivity of units with time. However, the results indicate also that the sensitivity of the units was higher during the magnitude estimation than during the counting task. The effect of the task of the subject on the sensitivity of the peripheral mechanoreceptive units may be mediated through direct neural connections. The results indicate that the reorganization of the neural processes according to the task of the subject is not limited to the central nervous system, but involves also the periphery.