Suppression of vibrotactile discrimination by transcranial magnetic stimulation of primary somatosensory cortex.

A number of human and animal studies have reported a differential representation of the frequency of vibrotactile stimuli in the somatosensory cortices: neurons in the primary somatosensory cortex (SI) are predominantly responsive to lower frequencies of tactile vibration, and those in the secondary somatosensory cortex (SII) are predominantly responsive to higher frequencies. We employed transcranial magnetic stimulation (TMS) over SI in human subjects to investigate the extent to which the inactivation of SI disrupted the discrimination of vibrotactile stimulation at frequencies that give rise to the tactile sensations of flutter (30 Hz) and vibration (200 Hz). Frequency discrimination around the 30-Hz standard following application of TMS to SI was reduced in seven of the eight subjects, and around the 200-Hz standard was reduced in all eight subjects. The average change in discrimination following TMS was about 20% for both low and high frequencies of vibrotactile stimulation. These data suggest that disruption of SI: (1) has a direct effect on the discrimination of both low and high frequencies of vibrotactile stimuli, consistent with a serial model of processing, or (2) has a direct effect on low-frequency vibrotactile stimuli and an indirect effect on the processing of high-frequency vibrotactile stimuli by SII via cortico-cortical connections between the two regions.

Functional characteristics of the parallel SI- and SII-projecting neurons of the thalamic ventral posterior nucleus in the marmoset.

The functional organization of the primate somatosensory system at thalamocortical levels has been a matter of controversy, in particular, over the extent to which the primary and secondary somatosensory cortical areas, SI and SII, are organized in parallel or serial neural networks for the processing of tactile information. This issue was investigated for the marmoset monkey by recording from 55 single tactile-sensitive neurons in the lateral division of the ventral posterior nucleus of the thalamus (VPL) with a projection to either SI or SII, identified with the use of the antidromic collision technique. Neurons activated from the hand and distal forearm were classified according to their peripheral source of input and characterized in terms of their functional capacities to determine whether the direct thalamic input can account for tactile processing in both SI and SII. Both the SI- and SII-projecting samples contained a slowly adapting (SA) class of neurons, sensitive to static skin displacement, and purely dynamically sensitive tactile neurons that could be subdivided into two classes. One was most sensitive to high-frequency (> or =100 Hz) cutaneous vibration whose input appeared to be derived from Pacinian sources, while the other was sensitive to lower frequency vibration (< or =100 Hz) or trains of rectangular mechanical pulse stimuli, that appeared to receive its input from rapidly adapting (RA) afferent fibers presumed to be associated with intradermal tactile receptors. There appeared to be no systematic differences in functional capacities between SI- and SII-projecting neurons of each of these three classes, based on receptive field characteristics, on the form of stimulus-response relations, and on measures derived from these relations. These measures included threshold and responsiveness values, bandwidths of vibrational sensitivity, and the capacity for responding to cutaneous vibrotactile stimuli with phase-locked, temporally patterned impulse activity. The analysis indicates that low-threshold, high-acuity tactile information is conveyed directly to both SI and SII from overlapping regions within the thalamic VP nucleus. This direct confirmation of a parallel functional projection to both SI and SII in the marmoset is consistent with our separate studies at the cortical level that demonstrate first, that tactile responsiveness in SII largely survives the SI inactivation and second, that SI responsiveness is largely independent of SII. It therefore reinforces the evidence that SI and SII occupy a hierarchically equivalent network for tactile processing.

Voluntary and reflex influences on the initiation of swallowing reflex in man.

The electrophysiological features of voluntarily induced and reflexive/spontaneous swallows were investigated. In normal subjects, swallows were elicited by infusing water either into the mouth (1-3 ml) or directly into the oropharyngeal region through a nasopharyngeal cannula (0.3-1 ml). For water infused orally, subjects were either requested to swallow voluntarily or instructed to resist swallowing and maintain the horizontal head position until swallowing occurred reflexively. Spontaneous saliva swallowing was investigated in patients with severe dysphagia who had a prominent clinical picture of suprabulbar palsy. Comparisons between different swallowing types were made by measuring the time interval between the onset of submental electromyographic activity (SM-EMG) and the onset of the upward movement of the larynx recorded by a movement sensor. This interval was less than 100 ms, even frequently less than 50 ms, in reflexive/spontaneous swallows, while in voluntarily induced swallows it was substantially longer. The rising time of submental muscle's excitation was also shorter in reflexive/spontaneous swallows. It was suggested that the triggering of voluntarily induced swallows commences more than 100 ms before the onset of swallowing reflex and that this mechanism is under the control of corticobulbar-pyramidal pathways. If the swallowing reflex is triggered within such a short period of time following the onset of SM-EMG, the central control by the bulbar swallowing center should be effective until the end of oropharyngeal swallowing.

Proximal and distal motor nerve conduction in obturator and femoral nerves.

OBJECTIVE: To study the proximal and distal motor conduction properties of obturator and femoral nerves. DESIGN: For evaluation of distal motor conduction properties, obturator and femoral nerves were stimulated at the inguinal ligament, and M responses were recorded with needle electrodes from gracilis and rectus femoris muscles. Upper lumbar roots were stimulated with needle electrodes inserted between L1-L2 vertebral laminae. PARTICIPANTS: Sixteen healthy adults, eight of each gender, age 22 to 52 years (mean 37.5). MAIN OUTCOME MEASURES: Description of a method for assessing motor conduction along the obturator nerve and evaluating proximal motor conduction measurements obtained with stimulation of obturator and femoral nerves. RESULTS: Distal motor conduction latencies were 3.9 +/- 0.7 msec for gracilis and 4.6 +/- 0.5msec for rectus femoris after stimulation of obturator and femoral nerves, respectively. Proximal conduction times from lumbar vertebral level were 10.4 +/- 0.3msec for the obturator nerve and 10.8 +/- 0.4msec for the femoral nerve. Conduction velocities of proximal segments of both nerves were similar, 62m/sec for the obturator nerve and 65m/sec for the femoral nerve. Submaximal stimulation of both nerves evoked H-reflex responses from their associated muscles. CONCLUSIONS: Motor conduction properties of the obturator nerve can be assessed by the method described, particularly to differentiate between peripheral, plexus, or radicular lesions that involve the obturator nerve.

Mechanisms of dysphagia in suprabulbar palsy with lacunar infarct.

BACKGROUND AND PURPOSE: The objective of the present study was to investigate the neural mechanisms of dysphagia in suprabulbar palsy (SBP) with multiple lacunar infarct. METHODS: We evaluated the swallowing disorders of patients with SBP (n=34) and age-matched healthy control subjects (n=35) by means of an electrophysiological method that recorded the oropharyngeal swallowing patterns. With this method, dysphagia limit, the triggering of voluntarily initiated swallows, duration of laryngeal relocation time, and total duration of oropharyngeal swallowing were recorded and measured. In addition, the EMG behavior of the cricopharyngeal (CP) muscle of the upper esophageal sphincter was also assessed. RESULTS: In patients with SBP, the dysphagia limit in all except 1 patient was pathological with limits of <20-mL bolus volume, which is contrary to normal subjects, in whom the dysphagia limit exceeds the 20-mL bolus volume. Either triggering of swallowing reflex was delayed (P<0.04), or the swallow could hardly be triggered in 7 patients on the voluntary attempts for 3 mL water. Whenever the reflex swallowing could be triggered, it was slow and prolonged (P<0.01). The CP muscle of the upper esophageal sphincter appeared to have become hyperreflexic and incoordinated with laryngeal movements during swallowing. CONCLUSIONS: It was proposed that the progressive involvement of the excitatory and inhibitory corticobulbar fiber systems linked with the bulbar swallowing center is mainly responsible for the triggering difficulties of the swallowing reflex and for the hyperreflexic/incoordinated nature of the CP sphincter. In addition, the dysfunction of the extrapyramidal system has a specific role in the slowing of oropharyngeal swallowing and the accumulation of saliva in the mouth.

Roles of periaqueductal gray and nucleus tractus solitarius in cardiorespiratory function in the rat brainstem.

Periaqueductal gray (PAG) and nucleus tractus solitarius (NTS) are important centres for regulation of cardiorespiratory function in cats. We aimed to study the effects of specific PAG stimulation on cardiorespiratory parameters in the rat. Microinjection of D, L-homocysteic acid (DLH) into dorsolateral PAG of anaesthetised rats, led to: marked increases in respiratory frequency (RF) and amplitude of diaphragmatic electromyogram, decreases in inspiratory and expiratory durations, and increased blood pressure and heart rate. Following injection of propranolol (150 pmol, 30 nl), a beta-adrenergic antagonist, into the commissural subnucleus of NTS, the DLH-induced increase in RF was markedly attenuated. Inspiratory neurones (late I cells) in NTS were excited upon stimulation of PAG and their increased activity was accompanied by increased RF. The changes in activity of the late I cells in response to stimulation of dorsolateral PAG provide physiological evidence of a link, possibly noradrenergic, between the two nuclei and involvement of the NTS in control of respiratory functions orchestrated by the PAG.

Parallel processing in cerebral cortex of the marmoset monkey: effect of reversible SI inactivation on tactile responses in SII.

1. Responsiveness within the hand region of the second somatosensory area of cortex (SII) was investigated in the marmoset monkey (Callithrix jacchus) in association with cooling-induced, reversible inactivation of the primary somatosensory area, SI. The aims were to determine whether thalamocortical systems in this primate species are organized according to a serial scheme in which tactile information is conveyed from the thalamus to SI and thence to SII as the next hierarchical level of processing and to establish whether primates are fundamentally different, in this respect, from mammals in which tactile information is conveyed in parallel from the thalamus to both SI and SII. 2. Inactivation of the SI had area was achieved when the temperature at the face of the silver cooling block over this SI region was lowered to < or = 13 degrees C. Inactivation was confirmed by abolition of the SI surface potential evoked by a brief tap stimulus to the hand and by the abolition of responsiveness in single SI neurons located beneath and around the edge of the block. 3. The effect of SI inactivation on SII-evoked potentials was investigated in 20 experiments by simultaneous recording of the SI- and SII-evoked potentials. The SII response was never abolished and was unchanged in the majority (12/20) of experiments. In the remainder, the SII-evoked potentials underwent a reduction in amplitude that was usually < 30% but never > 50%. 4. Tactile responsiveness was examined quantitatively in 47 individual SII neurons of different functional classes before, during, and after the inactivation of SI. Controlled tactile stimuli consisted of trains of sinusoidal vibration or rectangular pulses delivered to the glabrous or hairy skin of the hand. 5. Thirteen of the 47 SII neurons (28%) were unaffected in their response levels in association with SI inactivation. The remaining 34 SII neurons underwent some reduction in responsiveness, but in only 6% (3/47) was responsiveness abolished by SI inactivation. As the same range of functional classes of tactile neurons were represented among the affected and unaffected SII neurons, there was no evidence for a differential susceptibility among SII tactile neurons to the effect of SI inactivation. 6. Where reductions in amplitude of the SII-evoked potential or in response levels of SII neurons were observed, the effects were not attributable to direct spread of cooling from SI to the SII hand area as there was no cooling-induced prolongation of either the evoked potential or spike waveform in SII, an effect that is known to precede cooling-induced reductions in responsiveness. 7. These lines of evidence indicate that reductions in SII responsiveness in association with SI inactivation may be attributable to a loss of a background facilitatory influence rather than to a blockage of a component of peripheral input that comes over a putative serial path to SII via SI. First, as SI was cooled, there was a progressive increase in latency and time course of the SI responses before their disappearance, but no comparable delay in the SII responses as might be expected if SI were placed earlier than SII in a strict hierarchical scheme of thalamocortical processing. Second, SI inactivation failed to bring about a tightening in the phase-locking of SII responses to vibrotactile stimuli as might have been expected if the inputs to the SII neurons come via both a direct path from the thalamus and an indirect intracortical path via SI. Blockage of the indirect intracortical pathway through SI might be expected to reduce temporal dispersion in the input to SII neurons and result in an improvement in phase-locking in the SII responses to skin vibration. Third, the background activity of some SII neurons was reduced during SI inactivation along with the reduction in their responses to tactile stimulation.

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. 2. Reports in recent years that tactile responsiveness within the hand area of SII was abolished by surgical ablation of the hand area of the postcentral, or SI area of the cortex in the macaque and marmoset monkeys indicated that a serial processing scheme may operate at least in primates. However, as the surgical ablation is clearly irreversible and precludes examination of individual SII neurons in both the control and test circumstances, that is, when SI is intact and when it is inactivated, we have examined in the cat, the rabbit and the marmoset monkey the behaviour of SII neurons before, during and after the selective, rapidly-reversible inactivation of SI by means of localized cooling. 3. The results demonstrate that in the cat and rabbit, SII responsiveness is never abolished and infrequently affected by SI inactivation and that tactile inputs to SII therefore traverse a direct path from thalamus, organized in parallel with that to SI. In the marmoset (Callithrix jacchus), in contrast to earlier studies based on ablation of SI we found that with reversible inactivation of SI, SII responsiveness was unaffected in 25% of neurons and, although reduced in the remainder, was rarely abolished (< 10% of SII neurons). 4. The results indicate that there is substantial direct thalamic input to SII, even in this simian primate, and therefore necessitate revision of the hypothesis that tactile processing at the thalamocortical level in simian primates is based on a strict serial scheme in which tactile information is conveyed from the thalamus to SI and thence to SII.

Parallel processing of tactile information in cat cerebral cortex: effect of reversible inactivation of SII on SI responses.

1. Responsiveness of neurons in the distal forelimb region of primary somatosensory cortex (SI) was examined in cat in association with the cooling-induced, reversible inactivation of the corresponding region of the second somatosensory area (SII). The aim was to test whether a component of the stimulus-generated tactile input to SI came via an indirect, intracortical path from the thalamus through SII, or whether, when SI responsiveness fell in association with SII inactivation, the effect could be explained by a disfacilitation of the SI neuron; that is, a removal of a tonic facilitatory influence on the SI neuron that arises from within SII. 2. The responses of 33 SI neurons to controlled tactile stimuli, usually 1-s long trains of vibration or rectangular pulses delivered to the skin of the distal forelimb, were examined quantitatively before, during, and after the rapid, reversible inactivation of the SII area. 3. Nineteen of the 33 neurons (approximately 60%) were unaffected in their response level by SII inactivation. These included neurons of several functional classes whose input came from different classes of tactile afferent fibers, including the Pacinian corpuscle (PC) associated fibers, other rapidly adapting (RA) afferents from glabrous skin, and presumed hair follicle afferent (HFA) fibers. The remaining 14 neurons (approximately 40%), which also included different functional classes, displayed a reduction in response level with SII inactivation. Because this was not accompanied by significant prolongation of the SI spike waveforms, it is not attributable to direct spread of cooling from SII to SI. Construction of stimulus-response relations demonstrated that any effect of SII inactivation on individual SI neurons was consistent over the whole response range. 4. The fall in responsiveness for some SI neurons in association with SII inactivation may be attributable to disfacilitation, that is, a loss of tonic facilitation arising in SII, rather than to a block of peripherally generated inputs that traverse an indirect path from the skin to SI, via SII. There are three reasons for suggesting this. First, in the course of SII cooling, the latency and time course of SI evoked potentials were not delayed in a way that might be expected if part of the SI response had come via SII. Second, the SII inactivation could reduce the SI spontaneous activity (as well as the stimulus-related responsiveness). The facilitation from SII is therefore not necessarily dependent on overt tactile stimulation, and its source may therefore be endogenous to SII.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Parallel processing of tactile information in the cerebral cortex of the cat: effect of reversible inactivation of SI on responsiveness of SII neurons.

1. Localized cortical cooling was employed in anesthetized cats for the rapid reversible inactivation of the distal forelimb region within the primary somatosensory cortex (SI). The aim was to examine the responsiveness of individual neurons in the second somatosensory area (SII) in association with SI inactivation to evaluate the relative importance for tactile processing of the direct thalamocortical projection to SII and the indirect projection from the thalamus to SII via an intracortical path through SI. 2. Response features were examined quantitatively before, during, and after SI inactivation for 29 SII neurons, the tactile receptive fields of which were on the glabrous or hairy skin of the distal forelimb. Controlled mechanical stimuli that consisted of l-s trains of either sinusoidal vibration or rectangular pulses were delivered to the skin by means of small circular probes (4- to 8-mm diam). 3. Twenty-three of the 29 SII neurons (80%) showed no change in response level (in impulses per second) as a result of SI inactivation. These included seven neurons activated exclusively or predominantly by Pacinian corpuscle (PC) receptors, six that received hair follicle input, four activated by convergent input from hairy and glabrous skin, and six driven by dynamically sensitive but non-PC inputs from the glabrous skin. 4. Six SII neurons (20%), also made up of different functional classes, displayed a reduction in response to cutaneous stimuli when SI was inactivated. 5. Stimulus-response relations, constructed by plotting response level in impulses per second against the amplitude of the mechanical stimulus, showed that the effect of SI inactivation on individual neurons was consistent over the whole response range. 6. The reduced response level seen in 20% of SII neurons in association with SI inactivation cannot be attributed to direct spread of cooling from SI to the forelimb area of SII, as there was no evidence for a cooling-induced prolongation in SII spike waveforms, an effect that is known to precede any cooling-induced reduction in responsiveness. 7. As SI inactivation produced a fall in spontaneous activity in the affected SII neurons, we suggest that the inactivation removes a source of background facilitatory influence that arises in SI and affects a small proportion of SII neurons. 8. Phase-locking and therefore the precision of impulse patterning were unchanged in the responses of SII neurons to vibration during SI inactivation. This was the case whether response levels of neurons were reduced or unchanged by SI inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of cat ventroposterolateral thalamic neurons to vibrotactile stimulation of forelimb footpads.

Responses of neurons in the ventroposterolateral nucleus of the thalamus to vibration applied to the forelimb footpads were analyzed in anesthetized cats in order to describe the signalling properties of thalamic neurons that received input from the different classes of tactile afferents innervating the glabrous skin of the distal forelimb. Seventy-six thalamic neurons, the majority of which (60 of 76) were positively identified as thalamocortical projection neurons, were classified into two broad groups according to their responses to 1-s step indentations of the skin. A minority (24%) comprised neurons that had slowly adapting (SA) responses, whereas the remainder (76%), the dynamically sensitive neurons, had transient responses to the onset and offset phase of the step and were further classified according to their sensitivity to cutaneous vibrotactile stimuli into those activated by low-frequency vibration (rapidly adapting, RA, neurons) and those activated by high frequencies (Pacinian afferent, PC, neurons). Thalamic RA neurons displayed phaselocked responses to vibration at frequencies up to approximately 100 Hz, while PC neurons displayed phaselocked responses to vibration up to 400-500 Hz. Thalamic SA neurons varied in their responses to vibrotactile stimuli; half were most sensitive to vibration frequencies of 50 Hz or less, while the others responded over a broader range of frequencies. Although three major classes of footpad-related thalamic neurons were identified, there was evidence of convergent input to a small proportion of them. The study demonstrates that thalamic neurons have the capacity for responding to cutaneous vibration with phaselocked, patterned impulse trains, which would enable them to encode information about vibrotactile frequencies up to approximately 300 Hz.

A method for the generation of complex vibrotactile stimuli.

A method is described utilizing computer-generated sine wave data and purpose-built hardware to generate a complex vibrotactile stimulus. Two sine waves of different frequency were summed to produce a complex waveform with two temporal components, a high frequency component and a low frequency beat component. The computer-generated data points for each of the two component sine waves were downloaded to two banks of static memory in a dual synchronous arbitrary function generator. The data points in memory were fed to two 12-bit digital-to-analogue converters which sent the two analogue sine wave signals to a summing amplifier where the two sine waves were added. This method provides a complex waveform that can be gated on and off, has a fixed frequency ratio of the component sine waves and no phase drift between the component waves. Addition of the separate sine waves in a summing amplifier allows for easy alteration of the amplitude ratio of the sine waves. The output of the summing amplifier is sent to a feedback controlled mechanical stimulator, thereby allowing the stimulus to be presented to the skin of human subjects and experimental animals.