A MODEL SYSTEM FOR THE DETECTION OF MOTION 193 6 Use of the movement information

The mopaddler makes very limited use of the additional information it acquires through its motion system. There are no qualitatively different, new, navigation strategies that depend on image-motion. Rather the existing strategy of equalising the brightness responses in the two eyes is facilitated. It will be shown that this facilitation alone is enough to increase the paddler’s fitness.

6.3.6 a The paddle control centre The facilitation of eye-response equalisation is acquired through control of the paddle rota- tions. In the mopaddler the paddles can be extended (rotated outward) or retracted (rotated inward, towards the tailfin) relative to a resting position‚ . Extension of a paddle generally

causes the rotational component of the thrust generated by this paddle to increase; retraction generally causes a decrease of the rotational thrust. If both paddles extend or retract, the resultant translational thrust decreases respectively increases. (see figure 5.4 in chapter 5).

Paddle rotation is controlled by the paddle control centre (not to be mistaken for the paddle controllers...). The paddle control centre consists of an antagonistic pair of extensor and retractor command neurones with leaky integrator characteristics. These command neurones receive input from various neurones in the central motion processing centre, as described below. Paddle rotation can be specified by 1

X

the time constants of the command neurones; 2

X

two parameters describing the maximal angles of extension and retraction; and 3

X

two parameters describing the strength of the extensor and retractor muscles. The strength of both antagonistic muscles is always equal, and allows for maximal extension or retraction (i.e. it is equal to the largest of the maximal extension and maximal retraction parameters).

Image-motion too is used in the course control system (which uses the paddle control centre), and the fixation/tracking system (which in addition alters the swim commands).

6.3.6 b The course control system The course control centre modifies the direction of the generated thrust. This takes place through control of the paddle rotation. Objects generating outward motion, which indicates that they are disappearing from the visual field, cause an ipsilateral extension, combined with a contralateral retraction of the paddle. As a result, the (unaltered) thrust is redirected towards the disappearing target.

Note that this behaviour is similar to natural optokinetic course control in that it corrects for and minimises global image rotation. It will therefore be referred to as the course control component of the paddle control. It is different from natural course control systems in that its response to outward motion in one eye is independant of the image-motion perceived in the other eye, as long as the latter is not outward. Natural course control systems in e.g. the fly respond optimally to a combination of outward image motion in one eye, and similar inward image-motion in the other eye. In other words they use binocular image motion that indicates rotation of the animal.

The course control component also lacks any form of efference copy which would subdue adverse affects on voluntary rotations. An efference copy of the turn command could be used to tune the setpoint of the course control system: the control system is then capable of controlling voluntary rotation. This kind of actively controlled navigation is known to exist

194 CHAPTER6. THE MOTION DETECTION SYSTEM

in a number of insects (Wehner 1981). Other insects are known to suppress the optokinetic course control system while performing turns, which then become saccadic.

The mopaddler employs a similar tactic. Objects that fall within one of the binocular regions cause a retraction of the contralateral paddle. This response, which is mediated through the luminance channels, counteracts the course control component, and redirects the thrust in the direction of the object, regardless of the direction in which it moves. This paddle control response is part of the fixation/tracking system of the mopaddler.

6.3.6 c The fixation/tracking system

The job of the fixation/tracking system is to keep an object, once "caught" in binocular vision, in sight until it can be swallowed. As part of the solution to this problem, the response to targets in the binocular region is increased relative to the response in the monocular regions. In the mopaddler’s ancestors this binocular weight (g ) results in a stronger swim

command for both paddles, regardless of whether the object was straight ahead or not. In the mopaddler the binocular weight works only ipsilaterally, which does not result in "dead-ahead stampedes" when a glowballs enters a corner of one of the binocular regions.

In addition to being responsible for a higher interest in "binocular" objects, the fixa- tion/tracking system also mediates a response to objects that approach too fast, or threaten to escape.

Approaching objects are signalled by theZOneurone, and cause a bilateral extension of the paddles and bilateral inhibition of paddle retraction, combined with a bilateral decrease in thrust. The combined effect is that the mopaddler swims slower with a locomotion system that is relatively more capable of fine manoeuvering.

Escaping objects are detected by the ZI neurone: they cause a reverse reaction. The paddles are bilaterally retracted, extension is bilaterally inhibited, and the thrust is increased bilaterally. The combined effect of these actions is that the mopaddler will make a little sprint, using a locomotion system that is altered to generate higher speeds.

It is interesting to note that in wild-typeDrosophila melanogasterthe walking speed can be modulated in exactly the opposite way as described above for the mopaddler (G ¨otz 1975). When a bilateral inward moving striped pattern (=zoom out) is presented the walking speed increases, while bilateral outward motion (=zoom in) decreases the walking speed.

6.3.6 d Overview

The paddle control centre is shown as part of the motion processing centre in figure 6.10. The normalised monocular outward motion responses (RMO andLMO) have a weight of 1 on the ipsilateral extensor and contralateral retractor neurones. The binocular outward motion responses (RBOandLBO) only project onto the ipsilateral extensor neurones, with weight 1₄. TheZIandZOprojections have weight 1.

The rest of the visuomotor centre is shown in figure 6.15 (for reasons of clarity the motion processing centre is omitted, but for its ZI and ZO neurones). Apart from the ipsilateral additional binocular projection (g

05), and the acceleration and deceleration projections

from the fixation/tracking component, the visuomotor centre is identical to that of the diurnal paddler. The projections of theZOandZIneurones have weights of 1

8 and 1

6.3. AMODEL SYSTEM FOR THE DETECTION OF MOTION 195

binocular input extra weighted zero for constant inputs, Weber-adapted eye response,

Wander Pacemaker Motor neurones input normalisor 1 8 / /₄ 1 Z O 1

left motor right motor

Σ ? ? ? ω ω 1−ω Σ Σ τ τ >> Σ Σ >> Χ Χ G1p + G1n right G2 right Binocular G2 right Binocular G2 left G2 left G1p + G1n left Σ ϖ Ω Ω τ τ % % ϖ Z I Figure 6.15

Revised visuo-motor centre of the0j1¶3 that makes use of the available motion information. Convergence and expansion information in the binocular region of the visual field is interpreted as respectively approach and escape of a target. The

ZOandZIneurones from the central motion processing centre therefore project in such a way to the motor command neurones as to cause a decrease respectively increase of paddling thrust. The paddle control centre of figure 6.10 is omitted for reasons of clarity.

196 CHAPTER6. THE MOTION DETECTION SYSTEM

To summarise: the mopaddler’s paddle control system is used by two systems: a course control system, and a fixation/tracking system. The course control system uses the binocular and monocular, outward, image motion information to control the direction of thrust through independant rotation of the paddles. The paddle control system has nothing to do with the swim command: it just alters the effect of a given swim command.

The fixation/tracking system uses binocular image zoom out and zoom in to shift the emphasis from turnability to speed by coupled rotation of the left and right paddles. In addition, binocular luminance information is used to control the direction of thrust through the retraction of the contralateral paddle.

The fixation/tracking system also has athrust component. This component (which will be indicated by the symbolZ3) makes use of the same binocular image zoom out/in information to alter the swim command. This is done in such a way that escaping objects induce a (bilaterally) stronger swim command, and approaching objects a (bilaterally) weaker swim command.