The effects on locomotion of lesions to the visuo-motor system inOctopus

Abstract

The visuo-motor system in cephalopods comprises paired eyes, paired optic lobes, paired peduncle lobes and the basal lobes. The organization of this system was investigated by observing behavioural changes immediately after surgical interference to different parts of it. Lesions were made that removed the optic and peduncle lobes and sectioned the optic nerves bilaterally, unilaterally, separately and in all the possible combinations. In all, twenty types of lesion were made: they fell into four sets. The first set were unilateral, interfering with only one visual system (i.e. eye, optic lobe, peduncle lobe). After such lesions, locomotion was relatively unaffected. The second set of lesions bilaterally reduced the visuo-motor system. After bilateral optic nerve section locomotion is modified but not markedly impaired. After bilateral removal of the optic and peduncle lobes together there is marked loss of muscle tone and the preparation is unable to move. This does not occur when either the optic or peduncle lobes are removed alone. Following removal of both optic lobes the animal is 'blind' but locomotion and posture are normal. Removal of both peduncle lobes leads to locomotor dysfunction, locomotion being 'coarse' and 'uncontrolled'. There are oscillatory movements about one or more axes. This oscillation may turn into uni-directional 'spin' about one or more axes: this 'spin' is reversible in direction. Such dysfunction does not occur if the peduncle lobes are removed from an octopus with optic nerves sectioned. The third set of lesions were asymmetrical for the peduncle lobes. Some of these lesions produced 'forced circling' movements. This is a continual movement about the yaw and/or roll axes, whose sense is fixed: forward about the intact peduncle lobe in a tight circle, i.e. clockwise if the right peduncle lobe remains. The fourth set of lesions controlled for the possibility that asymmetries in the optic lobe system cause forced circling movement: they do not. Evidence from all four sets of data suggest that the optic lobes are the prime instigators of motor 'commands' following visual stimuli and that the peduncle lobes are a subordinate system that modifies these commands, on the basis of changing visual cues. A hypothesis is proposed that the peduncle lobes are important in co-ordinating commands from the two optic lobes: this ensures a unified response by the whole animal even when the visual information differs on the two sides of the animal.