THE REPRESENTATION OF PROLONGED AND INTENSE, NOXIOUS SOMATIC AND VISCERAL STIMULI IN THE VENTROLATERAL ORBITAL CORTEX OF THE CAT

The responses of single neurones in the ventrolateral orbital (VLO) cortex to noxious pinch, heating of the skin, twisting of the joints and distension of the gall bladder were studied in cats anaesthetized with halothane. Of 60 neurones studied, 44 responded to prolonged (> 10 sec) stimuli that were well within the noxious range. Neurones were relatively unresponsive to innocuous stimuli or to the transient application of noxious stimuli. Many single neurones responded to a variety of modalities of noxious stimuli (e.g., skin heating and gall bladder distension). Many neurones studied showed a fluctuating level (5-15 Hz) of ongoing spontaneous activity. Neurones responded with either an increased frequency of spikes (excitation) or an inhibition of spontaneous discharge, irrespective of the source of noxious stimulation. Noxious stimuli delivered simultaneously to two different tissues (e.g., skin and visceral) sometimes produced excitation of the neurone under study, to levels above that produced by the application a noxious stimulus to only one of the tissues. Receptive fields were often large involving both contralateral and ipsilateral areas of the body, as well as both fore and hind limbs. No evidence of somatotopic organization was obtained. The responses of some neurones outlasted the application of the stimuli by many minutes. It is concluded that single neurones in the ventrolateral orbital cortex respond to the prolonged application of intensely noxious stimuli to a variety of body tissues, in a manner that is in keeping with the involvement of this cortical area in both the physiological, autonomic and experiential components of the affective-motivational aspect of pain. Furthermore, from the consequences of lesion studies in man and animals, it is proposed that the activation of cells in the orbital cortex by a variety of noxious stimuli reflects its more general role in the development and maintenance of behaviour in response to negative reinforcement of both social and physical origins.