Effect of Incisions in the Brainstem Commissural Network on the Short-Term Vestibulo-Ocular Adaptation of the CAT

This study was intended to test the adaptive plasticity of the vestibulo-ocular reflex before and after either a midsagittal or parasagittal incision in the brainstem. Eye movements were measured with the electromagnetic search coil technique during the vestibulo-ocular reflex (VOR_D) in the dark, the optokinetic reflex (OKN), and the visuo-vestibular adaptive training procedure. Two types of visual-vestibular combined stimulation were applied by means of low frequency stimuli (0.05 to 0.10 Hz). In order to increase or decrease the VOR_D gain, the optokinetic drum was oscillated either 180^∘ out-of-phase or in-phase with the vestibular stimulus turntable. This “training” procedure was applied for 4 hours. Initial measurements of the VOR_D were normal with a mean gain value of 0.92 ± 0.08. After 4 hours of “training” with the out-of-phase condition (180^∘), VOR_D gain reached mean values of 1.33 ± 0.11 (n = 6 cats). In the in-phase combination, the mean VOR_D gain decreased from 1.0 to 0.63 ± 0.02 (n = 2 cats). No significant change of VOR_D phase was found in any of the cats. Midsagittal or parasagittal pontomedullary brainstem incisions were performed in 4 cats. Recovery of the VOR was tested on the 2nd, 7th, and 30th day after operation. After the 30th day, recovery of the VOR_D gain stabilized at about 66% of the initial preoperative value. At this stage of the recovery, the optokinetic response (OKN) of the midsagittal-Iesioned cats was practically normal: in the parasagittal-Jesioned cats, the postoperative OKN responses were asymmetric. After stabilization of recovery, lesioned cats were trained with the same adaptation procedure. Although the direct effect of the visuo-vestibular combined stimulation during the training was still operative in all lesioned cats, the adaptive plasticity was completely abolished by the lesions. These results suggest that the commissural brainstem network may play a crucial role in the acquisition of the forced VOR adaptation.