VISUAL PATHWAYS FOR POSTURAL CONTROL AND NEGATIVE PHOTOTAXIS IN LAMPREY

The functional roles of the major visuo-motor pathways were studied in lamprey. Responses to eye illumination were video-recorded in intact and chronically lesioned animals. Postural deficits during spontaneous swimming were analyzed to elucidate the roles of the lesioned structu res for steering and postural control. Eye illumination in intact lamp reys evoked the dorsal light response, that is, a roll tilt toward the Light, and negative phototaxis, that is a lateral turn away from ligh t, and locomotion. Complete tectum-ablation enhanced both responses. D uring swimming, a tendency for roll tilts and episodes of vertical upw ard swimming were seen. The neuronal circuitries for dorsal light resp onse and negative phototaxis are thus essentially extratectal. Respons es to eye illumination were abolished by contralateral pretectum-ablat ion but normal after the corresponding lesion on the ipsilateral side. Contralateral pre tectum thus plays an important role for dorsal ligh t response and negative phototaxis. To determine the roles of pretecta l efferent pathways for the responses, animals with a midmesencephalic hemisection were tested. Noncrossed pretecto-reticular fibers from th e ipsilateral pretectum and crossed fibers from the contralateral side were transected. Eye illumination on the lesioned side evoked negativ e phototaxis but no dorsal light response. Eye illumination on the int act side evoked an enhanced dorsal light response, whereas negative ph ototaxis was replaced with straight locomotion or positive phototaxis. The crossed pretecto-reticular projection is thus most important for the dorsal light response, whereas the noncrossed projection presumabl y plays the major role for negative phototaxis. Transection of the ven tral rhombencephalic commis sure enhanced dorsal light response; negat ive phototaxis was retained with smaller turning angles than normal. S pontaneous locomotion showed episodes of backward swimming and deficie nt roll control (tilting tendency). Transections of different spinal p athways were performed immediately caudal to the brain stem. All spina l lesions left dorsal light response in attached state unaffected; thi s response presumably is mediated by the brain stem. Spinal hemisectio n impaired all ipsiversive yaw turns; the animals spontaneously rolled to the intact side. Bilateral transection of the lateral columns impa ired all yaw turns, whereas roll control and dorsal light response wer e normal. After transection of the medial spinal cord, yaw turns still could be performed whereas dorsal light response was suppressed or ab olished, and a roll tilting tendency during spontaneous locomotion was seen. We conclude that the contralateral optic nerve projection to th e pretectal region is necessary and sufficient for negative phototaxis and dorsal light response. The crossed descending pretectal projectio n is most important for dorsal light response, whereas the noncrossed one is most important for negative phototaxis. In the most rostral spi nal cord, fibers for lateral yaw turns travel mainly in the lateral co lumns, whereas fibers for roll turns travel mainly in the medial spina l cord.