Angular tracking and the optomotor response an analysis of visual reflex interaction in a hoverfly
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MaleSyritta pipiens have been filmed tracking other flies within a rotating drum in order to analyse the interaction between the optomotor and smooth tracking systems. Males have a forwardly directed region of enlarged facets with enhanced spatial resolution, and smooth angular tracking operates to keep targets on this fovea. The male's angular velocity (φ) is driven by the angular position of the target fly on its retina (θe 10 to 20 ms earlier (i.e. φ =k= κθe, where κ is about 30–40s−1). The optomotor system is used for controlling a fly's course. The rotational component of this reflex causes a fly to turn in the same direction as externally generated retinal image motion. When placed within a rotating drum, the angular velocity of freely flyingSyritta follows that of the drum (i.e. φ =Gφdrum, whereG = 0.8 for φdrum<200 °.s−1). Thus, whenSyritta turns to pursue another fly, the image of its static surroundings will sweep across its retina in the other direction, tending to generate optomotor torque in opposition to that caused by the tracking reflex. Unless arrangements are made to cope with this predictable optomotor input, tracking will be slowed.
The angular velocity of a male tracking in a rotating drum has two components: one caused by the drum velocity; the other by the retinal position of the target image (i.e. φ =Gφdrum +kκθe). During tracking, then, the optomotor system is fully active and will minimiseunwanted image motion caused by a disturbance. There are at least three forms of interaction between the two reflexes which in stationary surroundings will allow tracking to operate without interference from an active optomotor system. It is shown that all of them are compatible with the above result.
One of these schemes is the follow-up servo in which the tracking system works by injecting a command into the optomotor loop, so changing the latter's set point, thereby inducing an angular velocity greater than zero. This scheme in its simplest form requires that tracking has the same delays and frequency behaviour as the optomotor response. Since the two reflexes probably have different frequency responses, the follow-up servo can be rejected. The gain of the optomotor response (G) measured in an oscillating drum falls steeply between 0.5 Hz and 5 Hz, as it should, if the optomotor response is to be stable. However, there are indications that the constant κ, which relates ϕ to φe during tracking has the same value, if θe oscillates at more than 6 Hz, as it has for very low input frequencies.
That the value ofk seems to be limited by stability requirements and is to a first approximation independent of input frequency suggests that the tracking system has indeed been tailored to cope with the optomotor input that must oppose tracking at low frequencies. A simple additive model in which the tracking and optomotor commands first come together at a final common pathway will give the required performance provided that the tracking gain (x in Fig. 9) is enhanced atlow frequencies to allow for the opposing optomotor input. However, a simpler way of eliminating the unwanted optomotor torque is to use the efference copy scheme of von Holst and Mittelstaedt (1950). In this scheme a copy of the tracking command signal is sent to the optomotor input in order to cancel the expected visual consequences of tracking. In general, efference copy is a useful way of mixing two reflexes with different temporal properties.
FemaleMusca exhibit behaviour that is somewhat analogous to the smooth tracking of male flies (Reichardt and Poggio, 1976), but is probably concerned, not with chasing small targets, but with maintaining the fixation of large stationary objects. When maintaining fixation of stationary objects, the tracking and optomotor systems will complement each other, rather than acting in opposition, so that the form of interaction between tracking and the optomotor system may be different in the two sexes. Thus the female tracking system need make no provision for the optomotor response, even though the latter is active during tracking (Virsik and Reichardt, 1976).
KeywordsAngular Velocity Tracking System Input Frequency Image Motion Efference Copy
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