The physiology and psychophysics of vibrotactile sensation

Response characteristics and tactile coding capacities of single neurons of the dorsal column nuclei (DCN), and the dorsal horn, in particular, neurons of the spinocervical tract (SCT), were investigated in anaesthetized cats.Purely dynamically-sensitive tactile neurons of the DCN could be divided into two classes, one associated with hair follicle afferent (HFA) input, the other with Pacinian corpuscle (PC) input. The HFA-related class was most sensitive to low-frequency (&lt50 Hz) vibration, had phaselocked responses to vibration frequencies up to ~75 Hz and had a graded response output as a function ofvibrotactile intensity changes. PC-related neurons had broader vibrotactile sensitivity, extending to ~300 Hz with tightest phaselocking between 50 and 200 Hz. The SCT neurons in the lumbar dorsal horn had tactile receptive fields on the hairy skin of the hindlimb and a very limited capacity to signal, in a graded way, the intensity parameter of the vibrotactile stimulus. Furthermore, because of their inability to respond on a cycle-by-cycle pattern at vibration frequencies above 5-10 Hz, these neurons were unable to provide any useful signal ofvibration frequency beyond ~5-10 Hz, in contrast to DCN neurons. In the parallel human psychophysical study, the capacity for vibrotactile frequency detection and discrimination was examined in five subjects in glabrous and hairy skin. Thevibrotactile detection threshold values obtained at four standard frequencies of 20, 50, 100 and 200 Hz were markedly higher on the hairy skin than on the glabrous skin. The discrimination task was examined by means of a two-alternative, forced-choice psychophysical procedure. Measures of thediscriminable frequency increment (∆) and the Weber Fraction (∆ / ), revealed similar capacities for frequency discrimination at the two different skin sites at the standard frequencies of 20, 100 and 200 Hz, but an equivocal difference at 50 Hz. Cutaneous local anaesthesia in the dorsal forearm produced a marked impairment in vibrotactile detection and discrimination at the low frequencies of 20 and 50 Hz but little effect at higher frequencies, confirming that vibrotactile detection and discrimination in hairy skin depend upon superficial receptors at low vibrotactile frequencies, but depend on deep, probably Pacinian corpuscle receptors for high frequencies