Postnatal development of pretectal and NRTP neuron responses to optokinetic stimulation in the rat

The postnatal development of single unit responses in the pretectum (Pt) and the nucleus reticularis tegmenti pontis (NRTP) to large horizontal moving visual patterns at constant velocity (range 0.5-5 deg./s) has been studied in pigmented rats (DA/HAN). The earliest detectable response was recorded on postnatal day 16 in the Pt and day 19 in the NRTP. The number of responding units increased with age: 36% in group I (16-26 days) and 60% in group II (27-36 days) in Pt; 43% and 61% in group I (19-26 days) and II (27-36 days), respectively, in NRTP. Response magnitude developed gradually: Mean delta F (impulses/s) = 4.9 and 7.6 in group I and II, respectively, in Pt and 6.1 and 7.9 in group I and II, respectively, in NRTP. Different types of response patterns were identified relative to their ocular origin and directionality of the stimulation. In addition to the adult-like patterns, other units were either excited or inhibited by stimulation of the ipsilateral eye. These findings indicate that movement-evoked responses of Pt and NRTP neurons appear earlier than vestibular nucleus (VN) responses to such visual stimuli, and that the day of response appearance progresses along the visual-vestibular pathways from the primary central relay. The maturation is characterized by a relative decrease in time in the role of ipsilateral afferents and a relative increase in time in the role of contralateral afferents. As these contralateral afferents to the Pt are well known to be involved in generating optokinetic nystagmus, the postnatal increase in the sensitivity of their responses may parallel the postnatal development of optokinetic behavior in the rat.