Vibrotactile sensitivity of slowly adapting type I sensory fibres associated with touch domes in cat hairy skin.

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1. Recordings were made from single slowly adapting type I (SAI) afferent fibres associated with touch domes in the cat hairy skin. Controlled vibratory stimuli were used first, to characterize the precision with which these SAI afferents reflect the temporal aspects of vibrotactile stimuli, and second, to determine whether earlier disparate reports of SAI responsiveness to vibration may be attributable to highly specific stimulus requirements. 2. Eighteen SAI fibres from femoral cutaneous nerve branches were examined; each was associated with one to three touch domes. SAI responses to both steps and sinusoidal vibration (1-1.5 s in duration) were affected profoundly by both probe size and position. Punctate stimulus probes (250 microns) produced much higher response levels and steeper stimulus-response relations than those elicited with large (2 mm) probes, probably on account of focal distortion created within the dome by the smaller probes. SAI sensitivity to vibration was also affected markedly by the amplitude of any pre-indentation on which the vibration was superimposed; sensitivity was much lower when the pre-indentation exceeded 100 microns, in particular with larger stimulus probes. 3. Measures of both vibration sensitivity and the precision of impulse patterning demonstrated that, if appropriate stimulus parameters are chosen, the SAI fibres can respond to 1 s trains of vibration (amplitude < or = 100 microns) in a tightly phase-locked, 1:1 manner for frequencies up to 500 Hz. At frequencies from approximately 100-500 Hz the SAI fibres displayed broad 1:1 plateaus, where their response rate remained constant over a range of amplitudes, and phased-locking was tightest. Responses remained phase-locked up to 1000 Hz, but could not follow the vibration with a 1:1 pattern above 500 Hz. 4. The results demonstrate that with appropriate stimulus parameters, touch dome-associated SAI fibres are capable of signalling vibrotactile information over a similar bandwidth of frequencies as do Pacinian sensory fibres. The variability in past reports of SAI vibration sensitivity may relate principally to differences in stimulus conditions. However, in view of the SAI capacity for responding to vibration with temporally precise, patterned activity, it appears that their reported failure to contribute to vibrotactile sensibility must be attributed to limitations imposed in the central processing of SAI signals.

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