Patterns of activity in single cortical units following stimulation of the digits in monkeys

Facilitatory effect of jaw opening on somatosensory (SI) cortical neurones sensitive to tooth pressure in the cat

In anaesthetized cats, an upper canine tooth was stimulated mechanically at two different levels of jaw opening, the resting position and an open position with 20 +/- 2 mm between the upper and lower canines. The evoked field potentials and neuronal discharges were recorded from the caudal part of the contralateral coronal gyrus (SI cortex). The waveforms of the evoked potentials appeared in a positive-negative sequence. There were no significant changes in them when the jaws were open. Discharge patterns elicited in the cortical neurones by mechanical stimulation of the teeth consisted of initial 'burst' discharges, inhibitory pauses and/or large after-discharges. Jaw opening did not influence any phases of these responses to suprathreshold stimulation, spontaneous activities, or the sizes of the receptive fields. However, jaw opening did affect the initial 'burst' phase of the response to threshold stimulation, i.e. that which caused the neurones to fire with a probability of 30-50% with the jaw closed. Jaw opening enhanced this response probability in half (6/12) of the units that had very small receptive fields restricted to the canine tooth, but did not influence it in the majority (21/24) of the units that had larger receptive fields including the oral mucosa and the facial skin. There was no difference in distribution in the coronal gyrus between the two groups of neurones categorized by whether or not they were influenced by jaw position.

Interhemispheric transfer of plasticity in the cerebral cortex

Each half of the body surface is represented topographically in the contralateral cerebral hemisphere. Physiological data are presented showing that homotopic regions of primary somatosensory cortex are linked such that plasticity induced in one hemisphere, in the form of receptive field expansion brought about by a small peripheral denervation, is immediately mirrored in the other hemisphere. Neurons which display the plasticity show no responsiveness to stimulation of the ipsilateral body surface. This suggests that the pathways and mechanisms mediating this transfer are specific to the role of maintaining balance, or integration, between corresponding cortical fields.

Spatial features of vibrotactile masking effects on airpuff-elicited sensations in the human hand

It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. The masking effect was stronger in the glabrous than in the hairy skin, especially in the fingers. In the glabrous skin, the spread of masking effect produced by a high-frequency masker was more extensive than that produced by a low-frequency masker. The mechanical spread of high-frequency vibration was less extensive than that of low-frequency vibration in the skin. In the glabrous skin, a masker applied to the tip of the finger produced a stronger masking effect on sensations in the base of the finger than when the masker was located at the base and the test stimulus was located at the tip. It is concluded that mechanical spread of vibration in the skin is of minor importance in explaining the masking effects. Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.

Responses of somatosensory cortical neurones to tooth pressure and their modulation by transient mouth opening in the cat

One hundred and thirty two cortical neurones sensitive to mechanical tooth stimulation were detected from the caudal part of the coronal gyrus of the cat. They responded to transient mechanical stimulation of the tooth (TMTst) with initial burst discharges (latency; 14.9 +/- 8.7 ms) and sequential suppression of spontaneous discharges (duration; 218 +/- 142 ms). They also fired strongly at the onset and removal of sustained mechanical stimulation of the tooth (SMTst) with both on and off responses, but the responses for the duration of stimulation were indistinguishable from the spontaneous discharge level. Conditioning stimulation by transient mouth opening (TMOst) inhibited the on and off responses elicited by tooth pressure in 13 (56.5 per cent) of the 23 cortical neurones and in 16 (88.9 per cent) of the 18 neurones tested, respectively. On reversing the conditioning-test sequence, the response elicited by TMOst was inhibited by the onset and removal of SMTst in 12 (52.2 per cent) of the 23 neurones and in 8 (44.4 per cent) of the 18 neurones tested, respectively. The inhibitory period ranged from 50 to 250 ms.

Motor cortex reflexes associated with learned movement

In primates, sensory input can generate reflex motor cortex output in association with learned movement when the sensory input has a strong and direct connection to the motor cortex-for example, when a stimulus calling for repositioning of the hand consists of a perturbation of hand position. This finding supports the proposal that neurons of primate motor cortex may function in a transcortical servo-loop.

An attempt to demonstrate contralateral masking in pressure adaptation

Adaptation to punctate pressure Stimulation of the ring finget (test stimulus) was studied under conditions of more intense contralateral stimulation of homologous or non-homologous loci (masking stimuli). Contrary to expectation, the temporally contiguous masking stimulus did not affect adaptation time to the test stimulus. This finding was discussed in terms of other tactile masking studies and electrophysiological investigations of the somatosensory system.

Cortical responses to paired stimuli applied peripherally & at sites along the somato-sensory pathway

1. Experiments have been performed on animals anaesthetized with various anaesthetics to determine the responsiveness of the cortex to the second of a pair of identical stimuli applied at three sites along the sensory pathway, i.e. to the periphery, the medial lemniscus and to thalamocortical fibres.2. It has been found that in deeply anaesthetized animals the mass response recorded from the cerebral cortex to the second of a pair of peripheral or lemniscal stimuli became reduced in size if the interval between the stimuli was 30-500 msec. If the interval was less than 30 msec for peripheral stimuli or between 10 and 30 msec for lemniscal stimuli responses were not obtained to the second stimulus. This was found to be a basic pattern which could be modified in animals less deeply anaesthetized. In these animals, periods of relatively increased responsiveness were seen after peripheral stimulation.3. The post-synaptic responses recorded from the ventrobasal thalamus showed the same behaviour to the second of a pair of peripheral stimuli as did the cortex both as regards size and latency of the responses.4. The post-synaptic responses recorded from the cuneate nucleus rarely showed any reduction in size unless the separation between the stimuli was 10 msec or less; even at intervals as low as 3 msec there was no increase in the latency of the response.5. When a pair of stimuli were applied to thalamocortical fibres, a different pattern of cortical responsiveness was found. At the time the cortical response to stimulation at pre-thalamic sites was reduced or abolished, the response to stimulation at post-thalamic sites was unaltered or increased in size.6. Finally an attempt was made to correlate the mass response recorded from the cortical surface and the activity of single cortical cells. Two types of cell could be distinguished in the rat. Those lying from 0.35 to 1.2 mm deep in the cortex showed a response pattern, to paired stimuli, closely resembling that of the cortical mass response. Others situated deeper in the cortex were found which had a very long absolute unresponsive time, from 50 to 80 msec and a very long relative unresponsive time of 1 sec.