Spontaneous and evoked activity of neurones in the somatosensory thalamus of the waking cat.

1. Adult cats were prepared for chronic micro-electrode recording in the nucleus ventralis posterolateralis (VPL) of the ventrobasal complex (VB) of the thalamus. Spontaneous and evoked activity of single neurones was recorded extracellularly while the animal stood in a hammock. The behavioural state of the animal and the electroencephalogram (e.e.g.) were observed and recorded.2. Three hundred and twenty-five single neurones sensitive to movement of hairs, depression of skin or movement of claws were isolated and studied. Peripheral receptive fields were on the contralateral surface of the body. They remained constant in size and location over long periods of time and despite changes in the behavioural state of the animal. Most of the receptive fields covered a cutaneous area at least as large as an entire paw and many covered an entire limb and extended on to the trunk. These fields are larger than those described by other investigators using natural stimulation in anaesthetized or paralysed animals.3. Moving hairs or touching skin in the peripheral receptive field elicited a burst of spikes at frequencies between 100 and 150/sec. Repeated stimulation did not result in obvious alteration or habituation of the response. There was no apparent change in responsiveness of neurones to peripheral stimulation when the animal was asleep.4. Except for claw-sensitive units, all neurones were active in the absence of intentional stimulation of the receptive field. When the animals were awake, VPL neurones fired more regularly than during sleeping. In slow sleep, with e.e.g. slow waves and spindles, firing patterns were characterized by the presence of more high-frequency bursts of spikes and more long interspike intervals than in waking. Claw-sensitive units, silent during waking, fired in bursts during sleep. When small doses of barbiturate were injected intravenously while recording the activity of neurones, the firing pattern changed to one in which all of the activity occurred in the form of high-frequency bursts. Interburst intervals were variable.5. The level of spontaneous activity in VB in conscious cats is greater than in sensory and sensorimotor cortex to which it projects. The pronounced effect of behavioural state on spontaneous activity of these neurones suggests that the background activity in somatosensory systems may play some role in the processing of afferent input.