Abstract
Loss of visual suppression (VS) of caloric nystagmus was produced after creating flocculus destruction. The flocculus receives visual signals through a climbing fiber pathway and also through a mossy fiber pathway. In the previous report, we stated that the inferior olive as a source of the climbing fiber might be unrelated to the immediate modification of the vestibuloocular reflex by visual stimuli. A mossy fiber pathway presumably via the superior colliculus is a likely candidate for visual suppression of caloric nystagmus. A very recent anatomical and physiological experiment has revealed that the nucleus pontis (NP) and the nucleus reticularis tegmenti pontis (NRT) project mossy fibers to the flocculus.
In the present experiment, major concerns were to ascertain whether NP or NRT is the prefloccular relay nucleus responsible for the VS of caloric nystagmus. To achieve this goal, the VS of caloric nystagmus and optokinetic nystagmus (OKN) were investigated after making lesions in NP or NRT in 27 cats. After making the NP lesions, the VS of caloric nystagmus was revealed and OKN was normal in all the cats with the NP lesion. After making the NRT lesion, loss of VS was observed toward the side opposite to the lesion and OKN showed directional preponderence to the lesion site. These experimental findings are quite compatible with the fact that the early component of the field potentials in the flocculus induced by strobe flashes decreased markedly after making a lesion in the contralateral NRT. Together with the anatomical evidence that the superior colliculus sends its fibers to the NRT, the mossy fiber pathway via NRT is believed to be the final relay nuclei responsible for the VS of caloric nystagmus.
