Neural mechanisms in vibrotactile adaptation

1. Peripheral and central neural contributions to vibrotactile adaptation were investigated in decerebrate or anesthetized cats by recording from sensory nerve fibers associated with Pacinian corpuscle (PC) receptors and from central neurons of the dorsal column nuclei that receive their input from vibration-sensitive receptors of the forelimb footpads. Responsiveness of units was assessed using 1-s duration, test vibration stimuli delivered with 1- to 2-mm-diam probes at different times following adapting trains of vibration (usually 300 Hz) that lasted from less than 1 min up to 50 min. 2. Cuneate neuron responsiveness underwent marked depression following prior vibration. The extent of the depression and the time course of recovery in responsiveness were dependent on the intensity and duration of the adapting vibratory stimulus. The recovery time course (often several minutes) was approximately exponential and resembled the reported time course of subjective vibrotactile adaptation obtained in psychophysical experiments. 3. Response depression in PC fibers was only seen at low amplitudes of the test vibration and displayed a brief time course of recovery in comparison with that seen in cuneate neurons. It is therefore unlikely to account for the adaptation time course either in cuneate neurons or at a subjective level. Furthermore, as the adaptation seen in PC fiber responses had a similar time course in both cutaneous and mesenteric PC fibers it is unlikely that mechanical changes in the skin contribute significantly to the adaptation in PC fiber responses to vibration. 4. The time course of afferent-induced inhibition following long periods of prior vibration was too brief to account for the response adaptation in cuneate neurons. 5. As the long-term response depression in cuneate neurons following their prior activation was seen for inputs from unconditioned sites within the neuron's excitatory receptive field, as well as from the conditioned site, it appears that the response adaptation is attributable to changes in the central neuron or in synaptic processes associated with the central neuron. It is proposed that this adaptation may be due to an increase in extracellular potassium ion concentration that alters the responsiveness of the central neurons.