Optokinetic nystagmus in the pigeon (Columba livia). II. Role of the pretectal nucleus of the accessory optic system (AOS).

In birds, the accessory optic system (AOS) includes two nuclei: the nucleus ectomamillaris (nEM) and the pretectal nucleus superficialis synencephali (nSS). The role of the nSS in the production of a horizontal optokinetic nystagmus (OKN) was studied in the pigeon, by comparing the OKN before and after a unilateral lesion of this nucleus. The lesions were performed either by electrolysis or by local application of kainic acid (KA); the KA lesions gave more stable modifications of the OKN than the electrolytic lesions. A quantitative analysis of the slow-phase velocity (V) of the OKN was carried out on the animals receiving KA lesions. Lesion of the nSS provokes the almost total disappearance of the OKN for stimulation of the contralateral eye in the temporo-nasal direction, and a reduction of the OKN for stimulation in the naso-temporal direction. Thus, the nSS is essential for the production of the OKN in the temporo-nasal direction, but it also participates in the production of the OKN in the naso-temporal direction (slow-phase direction). The same lesion produces a large increase of the OKN (V) when the ipsilateral eye is stimulated in the temporo-nasal direction, and a smaller increase following stimulation in the naso-temporal direction. These increases suggest some kind of inhibitory (or disfacilitatory) interactions between the nSS (or the associated system) on one side, and the contralateral optokinetic centers. The lesion of one nSS does not provoke a deficit when the stimulation is binocular. This result probably reflects the combined effect of both monocular inputs. After a pretectal KA injection, a spontaneous nystagmus of the contralateral eye, in the naso-temporal direction, can be seen for several hours. The mechanism is still unknown, but it might be related to a reverse optokinetic after nystagmus (R-OKAN). The anatomical and physiological data so far available consistently support the hypothesis of a functional equivalence between the nSS in birds and the nucleus of the optic tract in mammals.