Smooth-pursuit eye-movement deficits with chemical lesions in macaque nucleus reticularis tegmenti pontis

Anatomic and neuronal recordings suggest that the nucleus reticularis tegme nti pontis (NRTP) of macaques may be a major pontine component of a cortico -ponto-cerebellar pathway that subserves the control of smooth-pursuit eye movements. The existence of such a pathway was implicated by the lack of pe rmanent pursuit impairment after bilateral lesions in the dorsolateral pont ine nucleus. To provide more direct evidence that NRTP is involved with reg ulating smooth-pursuit eye movements, chemical lesions were made in macaque NRTP by injecting either lidocaine or ibotenic acid. Injection sites first were identified by the recording of smooth-pursuit-related modulations in neuronal activity. The resulting lesions caused significant deficits in bot h the maintenance and the initiation of smooth-pursuit eye movements. After lesion formation, the gain of constant-velocity, maintained smooth-pursuit eye movements decreased, on the average, by 44%. Recovery of the ability t o maintain smooth-pursuit eye movements occurred over similar to 3 days whe n maintained pursuit gains attained normal values. The step-ramp, "Rashbass " task was used to investigate the effects of the lesions on the initiation of smooth-pursuit eye movements. Eye accelerations averaged over the initi al 80 ms of pursuit initiation were determined and found to be decremented, on the average, by 48% after the administration of ibotenic acid. Impairme nts in the initiation and maintenance of smooth-pursuit eye movements were directional in nature. Upward pursuit seemed to be the most vulnerable and was impaired in all cases independent of lesioning agent and type of pursui t investigated. Downward smooth pursuit seemed more resistant to the effect s of chemical lesions in NRTP. Impairments in horizontal tracking were obse rved with examples of deficits in ipsilaterally and contralaterally directe d pursuit. The results provide behavioral support for the physiologically a nd anatomic-based conclusion that NRTP is a component of a cortico-ponto-ce rebellar circuit that presumably involves the pursuit area of the frontal e ye field (FEF) and projects to ocular motor-related areas of the cerebellum . This FEF-NRTP-cerebellum path would parallel a middle and medial superior temporal cerebral cortical area-dorsolateral pontine nucleus-cerebellum pa thway also known to be involved with regulating smooth-pursuit eye movement s.