M. J. Rowe

Human tactile detection thresholds: modification by inputs from specific tactile receptor classes.

1. Human detection thresholds for a vibratory stimulus applied to the volar surface of the index finger were examined under conditions where afferents from specific tactile receptor classes were simultaneously activated from the thenar eminence. The experiments were designed to test whether stimuli which have been shown previously to induce afferent inhibition of ‘tactile’ neurones in the cuneate nucleus of the cat could modify human subjective performance in a tactile detection task. Conditioning stimuli to the thenar eminence were usually of three forms; steady indentation to engage slowly adapting tactile receptors; 300 Hz vibration to engage Pacinian corpuscles; and 30 Hz vibration to engage the intradermal, rapidly adapting tactile receptors which are thought to be Meissners corpuscles.

Mossy and climbing fiber inputs from cutaneous mechanoreceptors to cerebellar Purkyně cells in unanesthetized cats

Summary1.Mossy and climbing fiber inputs from cutaneous mechanoreceptors to Purkyně cells of vermis and pars intermedia of the cerebellar anterior lobe were studied in locally anesthetized, paralyzed cats prepared for painless recording sessions. In this preparation the mossy fiber and climbing fiber pathways remain fully functional. Simple spikes and climbing fiber discharges were recorded simultaneously through extracellular glass micro-electrodes and thereafter filtered off from each other for separate, computer-assisted analysis. Controlled mechanical stimulation (air jets, taps, pressure) was performed on the foot pads of all four limbs and on the hairy skin of the limbs and the body.2.Long term recording of the spontaneous activity of 110 Purkyně cells revealed a simple spike activity of 85 imp./s ± 49 imp./s (mean ± S.D.) and 1.00 ± 0.78 climbing fiber responses per second.3.Taps to foot pads and air jets to hairy skin revealed that most of the short latency responses via mossy fibers resulted from activation of the receptors of the ipsilateral forefoot. With the same stimuli climbing fiber discharges from the ipsilateral feet were more frequently evoked than from the contralateral feet. Both via mossy and climbing fibers the contralateral hindlimb gave the smallest contribution.4.Simple spike responses were evoked more commonly by pad stimulation (tap stimuli) than by hair stimulation (air jets). For both types of stimuli excitatory responses were more frequent (3 ∶ 1) than inhibitory ones. Similarly, pad stimulation was more effective than hair stimulation in inducing climbing fiber responses. Ipsilateral stimuli were much more effective than contralateral ones in evoking both simple spike and climbing fiber responses.5.Steady pressure stimuli modify the Purkyně cell discharges via mossy and climbing fiber pathways. Excitatory and inhibitory effects often of very long duration have been observed via both pathways. Again the ipsilateral forelimb was more effective than the other limbs. Mossy fiber responses were at least three times as common as climbing fiber responses and excitatory responses were more frequent than inhibitory ones.6.There is no apparent relation between the spontaneous discharge rates of the Purkyně cells and the response magnitudes of the mossy fiber and climbing fiber induced excitatory and inhibitory changes in the impulse pattern of Purkyně cells during steady pressure stimuli.

Respones of trigeminal ganglion and brain stem neurones in the cat to mechanical and thermal stimulation of the face

Abstract Microelectrode penetrations were made in the trigeminal ganglion and brain stem nuclei to characterize the responses of single neurones to mechanical and thermal stimulation of the hairy skin of the face of the unanaesthetized cat. The mechanoreceptive fields of primary neurones innervating the hairy skin of the face were associated with 3 hair follicle types. Receptive fields involving single vibrissae were the most common. Both slowly and rapidly adapting units innervated each follicle type and a preliminary survey of peripheral conduction velocity indicated that the neurones were associated with myelinated nerve fibres. The mechanical response properties of brain stem neurones in nuclei oralis and caudalis of the unanaesthetized cat were similar to those noted in previous investigations of decerebrate or anaesthetized animals. The receptive field areas of brain stem neurones were considerably larger than those of primary neurones. Moreover, receptive fields in the perioral region of the face were significantly smaller than receptive fields in posterior facial areas. Many of these mechanosensitive neurones were also responsive to thermal changes in their receptive field. There was no evidence of specific thermoreceptive units innervating facial hairy skin in either the trigeminal ganglion or brain stem nuclei of the unanaesthetized cat. This contrasts with previous findings of specific thermoreceptors in intraoral and peri-nasal regions. Most of the bimodal neurones showed marked sensitivity to even small temperature changes. The common response was an increased discharge to cooling and a depression of discharge on warming. A few units showed some sensitivity to constant temperatures. The response properties of bimodal neurones in nuclei oralis and caudalis could be completely accounted for by an input from primary neurones sensitive to mechanical as well as thermal stimulation.

Corticothalamic influences on transmission of tactile information in the ventroposterolateral thalamus of the cat: effect of reversible inactivation of somatosensory cortical areas I and II

The influence of the corticothalamic projections from somatosensory areas I and II (SI and SII) on the transmission of tactile information through the ventroposterolateral (VPL) thalamus was investigated by examining the effects of cooling-induced, reversible inactivation of SI and/or SII on the responsiveness of 32 VPL neurons to controlled tactile stimulation of the distal forelimb in anaesthetized cats. Both the response levels and spontaneous activity were unaffected in 21 (66%) of the VPL neurons as a result of inactivation of SI or SII singly, or both SI and SII simultaneously. In the remaining 11 neurons, 10 displayed a reduction in response level, an effect observed over the whole of the stimulus-response relations for the neurons studied at different stimulus amplitudes, and one neuron displayed an increase in response level in association with cortical inactivation. When responses in VPL neurons were affected by inactivation of one cortical somatosensory area, they were not necessarily affected by inactivation of the other. Of 14 neurons studied for the effects of the separate inactivation of SI alone and of SII alone, 7 were affected, one from both areas, but the remaining 6 were affected by inactivation of only one of these areas. Phaselocking, and therefore the precision of impulse patterning in the responses of VPL neurons to skin vibration, was unchanged by the cortical inactivation irrespective of whether the response level was affected. The results suggest that SI and SII may exert a facilitatory influence on at least a third of VPL neurons and in this way may modulate the gain of transmission of tactile signalling through the thalamus.

Perceived pitch of vibrotactile stimuli: effects of vibration amplitude, and implications for vibration frequency coding.

1. The effect of changes in amplitude on the perceived pitch of cutaneous vibratory stimuli was studied in psychophysical experiments designed to test whether the coding of information about the frequency of the vibration might be based on the ratio of recruitment of the PC (Pacinian corpuscle‐associated) and RA (rapidly adapting) classes of tactile sensory fibres. The study was based on previous data which show that at certain vibration frequencies (e.g. 150 Hz) the ratio of recruitment of the PC and RA classes should vary as a function of vibration amplitude. 2. Sinusoidal vibration at either 30 Hz or 150 Hz, and at an amplitude 10 dB above subjective detection thresholds was delivered in a 1 s train to the distal phalangeal pad of the index finger in eight human subjects. This standard vibration was followed after 0.5 s by a 1 s comparison train of vibration which (unknown to the subject) was at the same frequency as the standard but at a range of amplitudes from 2 to 50 dB above the detection threshold. A two‐alternative forced‐choice procedure was used in which the subject had to indicate whether the comparison stimulus was higher or lower in pitch (frequency) than the standard. 3. Marked differences were seen from subject to subject in the effect of amplitude on perceived pitch at both 30 Hz and 150 Hz. At 150 Hz, five out of the eight subjects reported an increase in pitch as the amplitude of the comparison vibration increased, one experienced no change, and only two experienced the fall in perceived pitch that is predicted if the proposed ratio code contributes to vibrotactile pitch judgements. At 30 Hz similar intersubject variability was seen in the pitch‐amplitude functions. 4. The results do not support the hypothesis that a ratio code contributes to vibrotactile pitch perception. We conclude that temporal patterning of impulse activity remains the major candidate code for pitch perception, at least over a substantial part of the vibrotactile frequency bandwidth.

Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive "tactile" neurones.

1. Responses were recorded in decereberate, unanaesthetized cats from individual cuneate neurones in order to determine firstly, the afferent sources of inhibition on cuneate neurones and secondly, the influence of afferent‐induced inhibition on those response features of dynamically sensitive tactile neurones which determine their capacity to code information about parameters of tactile stimuli.

Functional capacities of tactile afferent fibres in neonatal kittens

1. Responses were recorded from individual tactile afferent fibres isolated by microdissection from the median nerve of pentobarbitone‐anaesthetized neonatal kittens (1‐5 days post‐natal age). Experiments were also conducted on adult cats to permit precise comparisons between neonatal and adult fibres.

PARALLEL ORGANIZATION OF SOMATOSENSORY CORTICAL AREAS I AND II FOR TACTILE PROCESSING

1. The two principal tactile processing areas in the cerebral cortex, somatosensory areas I and II, receive direct projections from the thalamus and, as well, are linked through intracortical reciprocal connections. Tactile information may therefore be conveyed to SII, for example, over either a direct path from the thalamus or an indirect, or serial, path from the thalamus via SI.

Actions of single sensory fibres on cat dorsal column nuclei neurones: vibratory signalling in a one-to-one linkage.

1. The synaptic linkage between single, identified sensory fibres associated with Pacinian corpuscle (P.c.) receptors and central neurones of the dorsal column nuclei was examined in decerebrate or anaesthetized cats. Paired recordings were made from individual neurones in the gracile division of the dorsal column nuclei and from the hind‐limb interosseous nerve in which it is possible to identify and monitor the activity of each P.c. fibre activated when recording from the intact nerve with a platinum hook electrode. Individual P.c. fibres were activated by vibration delivered with an 0.2 mm diameter probe to the interosseous P.c. receptors. 2. Thirty‐five P.c. fibre‐gracile neurone pairs were isolated in which activity in the single, identified P.c. fibre evoked suprathreshold responses (mean latency +/‐ S.D., 10.3 +/‐ 1.5 ms) in the gracile neurone. A single impulse arriving over one P.c. fibre could generate pairs or triplets of output spikes from several target neurones thus revealing a potent synaptic organization within the dorsal column nuclei for the transmission and amplification of weak sensory signals. 3. The potency of the linkage for some pairs resulted in post‐synaptic response levels of up to 400 impulses s‐1 when a single input fibre was discharging one impulse on each vibration cycle at 200‐400 Hz. 4. Gracile neurones driven by single P.c. fibres had phase‐locked responses to vibration at frequencies of up to 400‐500 Hz. However, the responses displayed much greater phase dispersion than those of P.c. fibres, indicating that a major component of phase dispersion in the vibration‐induced responses of dorsal column nuclei neurones is attributable to the properties of the synaptic linkage between an individual fibre and the target neurone. 5. The potent actions of single, identified P.c. fibres on their target neurones are consistent with the hypothesis that phase‐locked responses in dorsal column nuclei neurones to vibration at 100‐400 Hz may reflect the functional domination of the target neurones output by one or a few of its converging fibres.

Synaptic transmission between single slowly adapting type I fibres and their cuneate target neurones in cat.

1. The synaptic linkage between single, identified slowly adapting type I (SAI) fibres and their central target neurones of the cuneate nucleus was examined in pentobarbitone‐anaesthetized cats. Simultaneous extracellular recordings were made from individual cuneate neurones and from fine, intact fascicles of the lateral branch of the superficial radial nerve in which it was possible to identify and monitor the activity of each group II fibre. Individual SAI fibres were activated by static displacement and by vibration delivered with a fine probe (0.25‐2 mm diameter) to their associated touch domes in the hairy skin of the forelimb. 2. Transmission properties across the synapse were analysed for nine SAI‐cuneate pairs in which the single SAI fibre of each pair provided a suprathreshold input to the cuneate neurone. Neither spatial nor temporal summation was required for effective impulse transmission, and often more than 80% of SAI impulses led to a response in the cuneate neurone. Responses of the cuneate neurones to single SAI impulses occurred at a short, fixed latency (S.D. often < 0.1 ms), and frequently consisted of a burst of two or three impulses, at low SAI input rates in particular. 3. The tight phase‐locking in the responses to vibration of single SAI fibres was preserved in the cuneate responses for frequencies up to approximately 400 Hz. However, as the impulse rates of the cuneate neurones were less than 150 impulses s‐1, their impulse patterns could not directly signal the vibration periodicity at frequencies > 100‐150 Hz despite 1:1 responses in their single SAI input fibres up to approximately 500 Hz. 4. The reliable transmission of touch dome‐associated SAI input across the cuneate nucleus indicates that transmission failure at this first relay is unlikely to be responsible for the reported failure of touch dome‐SAI inputs to contribute to tactile perception. 5. The transmission characteristics for the SAI fibres were very similar to those demonstrated previously for fibres associated with Pacinian corpuscles, which argues against any marked differential specialization in transmission characteristics for dorsal column nuclei neurones that receive input from different tactile fibre classes.