Cortical influences on the vestibular nuclei of the cat

Our goal was to study potential substrates for cortical modulation of vesti bular reflexes in the cat. In initial experiments, injections of wheat-germ -agglutinate-horseradish-peroxidase into Deiters' nucleus and the rostral d escending nucleus revealed bilateral colonies of retrogradely filled neuron s in cortical areas 6, 2, and 3a (about 60 cells per colony). In cats anest hetized with chloralose-urethane, we stimulated areas 2 and 3a with trains of pulses while recording from ipsilateral vestibular-nucleus neurons, whic h were characterized by their responses to sinusoidal tilts and tested for the presence of antidromic responses to stimulation of the upper cervical c ord. A majority of the neurons was affected by cortical stimulation, showin g either facilitation, inhibition, or a mixture of the two. Stimulation in area 2 was more effective than stimulation in area 3a. Despite the anatomic presence of direct cortico-vestibular projections, properties of facilitat ion and inhibition suggest that both were evoked by polysynaptic pathways. Cortical effects were broadly distributed to vestibular neurons without reg ard to responses of these neurons to sinusoidal tilts. There was no signifi cant difference between effects on lateral and medial vestibulospinal tract neurons, but, as a group, vestibulospinal neurons were much more likely to be affected by cortical stimulation than neurons not antidromically activa ted from the C2 segment. We conclude that, by their influence on vestibulos pinal neurons, neurons in cortical areas 2 and 3a should be able to modulat e, in behaving animals, vestibular reflexes acting on the neck and limbs.