Effects Of Bilateral Caloric Habituation On Nystagmus Responses Of The Cat

Habituation of nystagmus elicited by repetitive caloric stimulation was studied in seven decorticated cats. The histopathologic studies revealed total ablation of neocortex, excepting small remnants in two cases, and extensive damage to basal ganglia and diencephalon. All animals exhibited response decline of nystagmus, demonstrating that neocortex and probably basal ganglia and diencephalon are not essential for inducing habituation. There was no conclusive data regarding transfer and long-lasting retention of the response decline. The locus and neurophysiologic basis of the phenomenon are discussed. (Author).

Ten cats were exposed to a series of abovethreshold accelerations and sub-threshold deceleration. Unilateral caloric irrigations, provoking nystagmus in the same direction as the above-threshold rotational stimuli preceded and followed the set of accelerations. A marked adaptation of nystagmus, characterized by specific changes in early trials, resulted from repeated rotation. Although neither duration nor total slow-phase eye displacement to caloric stimulation was affected, the intervening rotational experience produced some reduction in the frequency of the nystagmic beats. Two factors were proposed in explanation for the minimal transfer of adaptation from the rotational to the caloric situation. Secondary nystagmus activity was also examined and appeared closely related to preceding primary reactions. The data indicate that clinical (caloric) responses to vestibular stimulation may not give an accurate indication of a subject's state of adaptation to practiced levels of angular accelaration. Such findings provide cautions in the establishment of appropriate testing techniques for the analysis of vestibular function in air-or spacevehicle crew members. (Author).