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Thrust Can Operate Angular Motion
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Thrust
Can Operate Angular Motion
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In this paradigm, the fly can use either its yaw torque (as in the
Drosophila flight simulator)
or its thrust to control the horizontal angular motion of a vertical dark
stripe in its visual field. In open loop, the fly modulates its thrust
in response to the movements of the stripe: front-to-back motion causes
a decrease, back-to-front motion an increase of thrust. A closed loop couples
larger-than-average thrust to clockwise and lower than average to counter-clockwise
rotation of the arena with the stripe. The fly immediately adjusts its
thrust to a level that corresponds to zero net rotation of the stripe.
The fly stabilizes the stripe on its right where the translatory motion
component of the stripe is negatively coupled to thrust, i.e. below average
thrust causes back-to-front motion and larger than average thrust the opposite.
If the coupling between thrust and rotational direction is inverted, the
stripe is stabilized to the left of the fly. Simultaneously recording yaw
torque shows only occasionally parallel or antiparallel modulation; most
of the time there is no correlation between the motion of the stripe and
yaw torque. If the fly is allowed to use yaw torque to control the panorama,
thrust is 'floating' freely. After a switch from thrust to yaw torque control
the fly needs approximately 200ms to stabilize the stripe. It takes the
fly up to several seconds after the vice versa switch, suggesting that
the system more frequently (or more efficiently) activates yaw torque than
thrust.
Angular velocity and position of a target is alternatively
controlled by yaw torque and thrust. Yaw torque (T), thrust (Th) and stripe
position (Psi), i.e. the angle between the fly's longitudinal body axis
and a vertical dark stripe (width d = 5°, height delta = +-40°)
are recorded simultaneously in the flight simulator under 3 different feedback
conditions. Normal flight simulator conditions are an inverse proportionality
between angular velocity of the panorama (simulating self-rotation of the
fly) and yaw torque as shown in (a) on the right. In order to stabilize
the panorama the fly must keep yaw torque around T = '0' [Nm]. The stripe
preferentially is kept in the frontal part of the visual field (upper histogram
on right). In this situation thrust is modulated over a wide range. Feedback
control of angular velocity by thrust, on the other hand, requires that
an arbitrary thrust level is associated with zero angular velocity. With
this reference value positive (Th > '0') and negative (Th < '0') thrust
can be defined. The two shaded histograms of thrust in the left and middle
columns of (a) show that during feedback with thrust the latter is kept
around zero whereas now yaw torque floats freely. Feedback conditions differ
in the experiments shown in the left and middle columns. On the left increasing
positive thrust leads to increasing clockwise rotation of the stripe, increasing
negative thrust to increasing counterclockwise rotation. In the middle
column this relation is inverted (Th>'0'=>ccw; Th<'0'=>cw). For each
of the two coupling modes the stripe is stabilized only on one side. This
is due to the fact that front-to-back motion decreases and back-to front
motion increases thrust in open loop. Therefore, negative feedback (i.e.
stabilization of the target) is given only on that side where Th>'0' leads
to front-to-back motion. In (b) individual traces of yaw torque, thrust
and stripe position are shown for switches (arrowheads) from yaw torque
to thrust control and vice versa (shaded areas= periods of feedback control).
Note that after being coupled to the target thrust may take several seconds
to adjust to the 'zero' level whereas yaw torque takes only a fraction
of a second. |
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