The representation of cutaneous sense in the thalamus of the cat and monkey.

Abstract

1. The electrical activity of single neural units in the thalamus of the cat and monkey has been investigated with unipolar electrodes of tip diameter 5–15 µ. The position of the electrode was always verified histologically; the great majority of the responses were recorded from the ventrolateral nuclear region.2. The order of contralateral representation of tactile sense described by others has been confirmed, but it has been found, in addition, that a substantial anatomical overlap occurs between the regions for face, fore limb and hind limb, both in cat and monkey.3. The thalamic representation of cutaneous sense in the hind limb has been studied in more detail, using a method by which the saphenous nerve could be stimulated and its compound action‐potential recorded, and the resulting thalamic activity recorded simultaneously. The nerve was not cut, so that the response of any thalamic unit to cutaneous stimulation could also be observed.4. Sixty‐three thalamic units from the cat were studied by the method just described. They may be described as falling into three main groups:(a) Those responding to electrical stimulation of α β or χ saphenous fibres, the response being but rarely affected by further increasing the stimulus (81 per cent of the total units). Twenty per cent of the group responded to stimulating at a fibre threshold. The mean latency for the group was 22·7 msec. (knee to thalamus). The majority responded to bending hairs, a few to light touch not involving hairs, and a few to strong mechanical stimuli.(b) Those responding to stimulating saphenous δ fibres (17 per cent of the total units). The majority responded only to strong mechanical stimuli, and only one to bending hairs. The mean latency was 47·3 msec.(c) Those responding to stimulating saphenous C fibres when the larger fibres had been blocked by cooling the nerve. The latency (630 msec.) was only measured satisfactorily for one such unit, and no skin stimulus appropriate for the unit was found.Our failure to excite some of the units in each group by stimulating the skin is discussed, particularly in relation to the use of general anaesthesia.5. In the cat's thalamus, no anatomical segregation could be detected amongst the different types of unit just described.6. The main types of unit described above were also found in experiments on three monkeys, but this series was too small to justify quantitative assessment.7. A substantial ipsilateral representation of the hind leg was found in the cat's thalamus, in a region anatomically indistinguishable from that for contralateral responses. The evidence suggested that units responding to hair movement were relatively more common in the contralateral than in the ipsilateral thalamus. Ipsilateral representation was also found to exist in the monkey's thalamus.