Whole-field integration, not detailed analysis, is used by the crab optokinetic system to separate rotation and translation in optic flow

For optimal visual control of compensatory eye movements during locomotion it is necessary to distinguish the rotational and translational components of the optic flow field. Optokinetic eye movements can reduce the rotational component only, making the information contained in the translational flow readily available to the animal. We investigated optokinetic eye rotation in the marble rock crab, Pachygrapsus marmoratus, during translational movement, either by displacing the animal or its visual surroundings. Any eye movement in response to such stimuli is taken as an indication that the system is unable to separate the translational and the rotational components in the optic flow in a mathematically perfect way. When the crabs are translated within a pseudo-natural environment, eye movements are negligible, especially during sideways translation. When, however, crabs were placed in a gangway between two elongated rectangular sidewalls carrying dotted patterns which were translated back and forth, marked eye movements were elicited, depending on the translational velocity. To resolve this discrepancy, we tested several hypotheses about mechanisms using detailed analysis of the optic flow or whole-field integration. We found that the latter are sufficient to explain the efficient separation of translation and rotation of crabs in quasi-natural situations. Accepted: 6 May 1997     

The halteres of the blowfly Calliphora

We quantitatively analysed compensatory head reactions of flies to imposed body rotations in yaw, pitch and roll and characterized the haltere as a sense organ for maintaining equilibrium. During constant velocity rotation, the head first moves to compensate retinal slip and then attains a plateau excursion (Fig. 3). Below 500°/s, initial head velocity as well as final excursion depend linearily on stimulus velocities for all three axes. Head saccades occur rarely and are synchronous to wing beat saccades (Fig. 5). They are interpreted as spontaneous actions superposed to the compensatory reaction and are thus not resetting movements like the fast phase of vestibulo-ocular nystagmus in vertebrates. In addition to subjecting the flies to actual body rotations we developed a method to mimick rotational stimuli by subjecting the body of a flying fly to vibrations (1 to 200 m, 130 to 150 Hz), which were coupled on line to the fly's haltere beat. The reactions to simulated Coriolis forces, mimicking a rotation with constant velocity, are qualitatively and to a large extent also quantitatively identical to the reactions to real rotations (Figs. 3, 7–9). Responses to roll- and pitch stimuli are co-axial. During yaw stimulation (halteres and visual) the head performs both a yaw and a roll reaction (Fig. 3e,f), thus reacting not co-axial. This is not due to mechanical constraints of the neck articulation, but rather it is interpreted as an advance compensation of a banked body position during free flight yaw turns (Fig. 10).     

The halteres of the blowfly Calliphora

The movement of the halteres during fixed flight was video recorded under stroboscopic illumination phase coupled to the wing beat. The halteres swing in a rounded triangular manner through an angle of almost 80° in vertical planes tilted backwards from the transverse plane by ca. 30° (Figs. 1, 2).The physics of the halteres are described in terms of a general formula for the force acting onto the endknob of the moving haltere during rotations and linear accelerations of the fly (Eq. 1). On the basis of the experimentally determined kinematics of the haltere, the primary forces and the forces dependent on angular velocity and on angular acceleration are calculated (Figs. 3, 4).Three distinct types of angular velocity dependent (Coriolis) forces are generated by rotations about 3 orthogonal axes. Thus, in principle one haltere could detect all rotations in space (Fig. 6).The angular acceleration dependent forces have the same direction and frequency as the Coriolis forces, but they are shifted in phase by 90°. Thus, they could be evaluated in parallel and independently from the Coriolis forces. They are, however, much smaller than the Coriolis forces for oscillation frequencies of the fly up to 20 Hz (Fig. 5). From these considerations it is concluded that Coriolis forces play the major role in detecting body rotations.     

Intramolecular vibrations enhance the quantum efficiency of excitonic energy transfer

Photosynthesis Research - We study the impact of underdamped intramolecular vibrational modes on the efficiency of the excitation energy transfer in a dimer in which each state is coupled to its...