Optokinetic nystagmus in the pigeon (Columba livia). III. Role of the nucleus ectomamillaris (nEM): interactions in the accessory optic system (AOS).

The accessory optic system (AOS) in birds is composed of two structures: the nucleus Superficialis Synencephali (nSS), essential for the production of an optokinetic nystagmus (OKN) in the temporo-nasal direction (slow phase) for the eye contralateral to the nucleus, and the nucleus Ectomamillaris (nEM), or nucleus of the basal optic root (nBOR). The objectives of the present work were: (1) to study the importance of the nEM for the horizontal OKN, (2) to study the interactions between the nSS and the nEM. Experiments were realized by combining different kinds of lesion. (1) Results show that the nEM is essential for the production of an OKN in the naso-temporal direction (direction of the slow-phase), but it also participates in the temporo-nasal response. (2) After bilateral lesion of the nEM or the nSS, only a residual nystagmic response remains. (3) Synergic effects exist between (I) the homolateral nEM and nSS, or between the systems related to them; we call this relation "homolateral synergy", (II) a nEM and the contralateral nSS (or systems related to these nuclei). The synergic effect exerted by one nEM upon the contralateral nSS appears to be stronger than the reciprocal effect. (4) The effects obtained after combined lesions, either homolateral (nEM and nSS on the same side) or heterolateral (nEM and nSS on opposite sides) confirm the previous results and show that heterolateral interactions are stronger than homolateral interactions. (5) For all the lesions studied, the results obtained from binocular stimulation are compatible with a model of convergence of monocular inputs. The role of the nuclei of the AOS in birds is discussed in terms of existing anatomical and physiological data.