SPATIAL ORIENTATION IN THE LAMPREY .2. VISUAL INFLUENCE ON ORIENTATION DURING LOCOMOTION AND IN THE ATTACHED STATE

The responses of attached lampreys to homogeneous visual stimulation a nd the role of visual stimuli in orientation during locomotion were in vestigated, Experiments were performed by video recording the response s of intact and lesioned animals to illumination, The following result s were obtained, 1, In lampreys attached with their sucker mouth to th e bottom of the aquarium, illumination of one eye evoked several possi ble motor responses (ordered after mean latency): (a) movement of the illuminated eye downwards, and the contralateral eye upwards; (b) rota tion of the body around the longitudinal axis, with the illuminated si de tilting downwards; (c) deviation of the caudal part of the anterior dorsal fin in the contralateral direction (away from the light); and (d) flexion of the neck and body towards the side of illumination, 2, Illumination of one eye in attached lampreys often resulted in detachm ent and subsequent movement in a direction away from the light source (negative phototaxis), This response was not related to the degree of roll tilt before detachment, so the negative phototaxis does not appea r to be a consequence of the vestibular stimulation, 3, Negative photo taxis was also seen during locomotion: lampreys turned through 180 deg rees when they approached a brightly illuminated area, Photostimulatio n also affected their orientation in the transverse plane during swimm ing, Illumination of one eye from the side induced a roll movement, so that the illuminated side tilted downwards and the dorsum of the lamp rey became turned towards the light, This is similar to the 'dorsal li ght response' of fish and shows that vision also plays a role in postu ral control in lampreys, 4, The behaviour of blinded animals differed strikingly from that of intact ones, Whereas intact animals preferenti ally swam close to the bottom, along horizontal trajectories, blinded animals showed episodes of continuous swimming upwards, near the water surface. During horizontal swimming, their orientation in the transve rse plane remained normal, with the dorsal side up.