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Biomedical subjects

D Varjú

Publications and source records attributed to D Varjú.

17 recordsLinked to original sources
PubMed1999

Visual system of the European hummingbird hawkmoth Macroglossum stellatarum (Sphingidae, Lepidoptera): motion-sensitive interneurons of the lobula plate.

The European hummingbird hawkmoth Macroglossum stellatarum is a member of the sphingid family, denoted by their ability to hover in place in front of flowers. The fast and acrobatic flight behavior is thought to be under the control of the optomotor system both during normal, forward flight and during stationary, hovering flight. Electrophysiologic and neuroanatomic studies presented here reveal large-field, motion-sensitive neurons in the lobula plate. Anatomic and physiologic features characterize these nerve cells as horizontal and vertical motion-sensitive neurons. Horizontal cells react best to horizontal movement of vertical gratings moving in either direction and show no response to vertically moving horizontal gratings. The dendritic arborizations of horizontal cells are found characteristically in the two outer layers of the lobula plate. In contrast, vertical cells react best to upward or downward motion of horizontal gratings. They are not sensitive to horizontal-moving gratings. The dendritic arborizations of the vertical cells are restricted to the two innermost lobula plate layers. These anatomic and physiologic features, as well as a number of other characteristics, such as tonic responses and cosine-shaped tuning curves, are most reminiscent of neurons that comprise the horizontal and vertical systems described from the fly, thus suggesting comparable roles in the control of optomotor behavior. The present study also identifies a small group of neurons that exhibit phasic responses and reactions to specific orientations of the pattern. These cells are discussed with regard to their possible roles in control tasks, such as edge-orientation detection or the detection of sudden displacements of the visual surround.

Animals↗
PubMed1998

Visual position stabilization in the hummingbird hawk moth, Macroglossum stellatarum L. I. Behavioural analysis.

Optomotor responses of freely flying hawk moths, Macroglossum stellatarum, were characterized while the animals were hovering in front of and feeding on a dummy flower. Compensatory translational and rotational movements of the hawk moth were elicited by vertical grating patterns moving horizontally, mimicking imposed rotational and translational displacements of the animal in the horizontal plane. Oscillatory translational and rotational pattern motion leads to compensatory responses that peak in the frequency range between 2 Hz and 4 Hz. The control systems mediating the translational and rotational components of the optomotor response do not seem to influence each other. The system mediating translational responses is more sensitive in the fronto-lateral part of the visual field than in the lateral part; the opposite is true for the rotational system. The sensitivity of the translational system does not change along the vertical, whereas the rotational system is much more sensitive to motion in the dorsal than in the ventral part of the visual field. These sensitivity gradients may reflect an adaptation to the specific requirements of position stabilization in front of flowers during feeding.

Animals↗
PubMed1995

Underwater refraction-polarization patterns of skylight perceived by aquatic animals through Snell's window of the flat water surface.

The grass shrimp (Palaemonetes vulgaris) orients itself by means of the polarization pattern of the sky visible through Snell's window of the water surface. The celestial polarization pattern viewed from water is distorted and modified because of refraction and repolarization of skylight at the air-water interface. This work provides a quantitative account of the repolarization of skylight transmitted through a flat water surface. The degree and direction of linear polarization, the transmissivity and the shape of the refraction-polarization oval are calculated at the air-water interface as functions of the polarization characteristics and the incident angle of partially linearly polarized incoming light. Two-dimensional patterns of linear polarization ellipses and of the degree and direction of polarization of skylight are presented for different zenith distances of the sun. The corresponding underwater refraction-polarization patterns are computed. Transmissivity patterns of a flat water surface are calculated for unpolarized light of an overcast sky and for partially polarized light of clear skies as a function of the zenith distance of the sun. The role of these refraction-polarization patterns in orientation and polarization vision of the grass shrimp (P. vulgaris) and rainbow trout (Oncorhyncus mykiss) is reviewed. The effects of cloud cover, surface waves and water turbidity on the refraction-polarization patterns are briefly discussed.

Animals↗
PubMed1990

Geometric optical investigation of the underwater visual field of aerial animals.

The underwater visual field distorted by refraction for aerial animals living near the water surface is investigated by means of geometric optics. The imaging of underwater objects by one and two aerial eyes is studied. The underwater binocular image field is determined for pairs of aerial eyes placed in horizontal and vertical planes. Some possible biooptical consequences of the visual detection of underwater prey and predator by aerial animals are discussed on the basis of the structure of their distorted visual field.

Animals↗
PubMed1989

Looking into the water with a facet eye.

The apparent velocity and apparent size of objects approaching an animal strongly depend on their size, their position within the visual field and on the path along which they move. It also makes a considerable difference whether both the animal and the objects are in air, or whether the animal looks from one medium into the other, as is the case in animals that live at the water surface. To systematically investigate this situation we calculated apparent velocity and size in the midsagittal plane of approaching objects of flat horizontal, rodlike vertical and spherical shape. We confine our investigation to object movements along straight horizontal and vertical paths and consider the situation for eyes with and without acute zones. The apparent velocity of an approaching object is low in the far field and increases rapidly close to the eye in both air and water. Along horizontal paths and close to the animal it is higher in water than in air. Along vertical paths and close to the animal it is higher in air than in water. Both relationships are exaggerated when there is an acute zone for vertical resolution along the animal's horizon. Boundary curves are calculated along which an approaching object is seen by a linearly increasing number of receptors. The change of apparent size is characterized by the density of these lines. Below the water surface the change of apparent size is similar to that of the apparent velocity.(ABSTRACT TRUNCATED AT 250 WORDS)

Models, Neurological↗
PubMedDate not supplied

Interactions of local movement detectors enhance the detection of rotation. Optokinetic experiments with the rock crab, Pachygrapsus marmoratus.

Walking crabs move their eyes to compensate for retinal image motion only during rotation and not during translation, even when both components are superimposed. We tested in the rock crab, Pachygrapsus marmoratus, whether this ability to decompose optic flow may arise from topographical interactions of local movement detectors. We recorded the optokinetic eye movements of the rock crab in a sinusoidally oscillating drum which carried two 10-deg wide black vertical stripes. Their azimuthal separation varied from 20 to 180 deg, and each two-stripe configuration was presented at different azimuthal positions around the crab. In general, the responses are the stronger the more widely the stripes are separated. Furthermore, the response amplitude depends also strongly on the azimuthal positions of the stripes. We propose a model with excitatory interactions between pairs of movement detectors that quantitatively accounts for the enhanced optokinetic responses to widely separated textured patches in the visual field that move in phase. The interactions take place both within one eye and, predominantly, between both eyes. We conclude that these interactions aid in the detection of rotation.

Animals↗
PubMedDate not supplied

Binocular interaction in the optokinetic system of the crab Carcinus maenas (L.): optokinetic gain modified by bilateral image flow.

We recorded optokinetic eye movements of the crab, Carcinus maenas, in split-drum experiments. The patterns were either oscillated in antiphase on both sides mimicking translational image flow or they were oscillated in phase producing rotational image flow. Eye movements elicited by the rotational stimulus were larger than those produced by the pseudotranslational pattern movements. The smaller response to the latter is mainly a consequence of binocular interaction, the strength of which depends on both the phase-shift and amplitude of pattern oscillation. We develop two hypotheses to explain our results: either (1) signals from each eye modify the gain of the linkage signals coming from the other eye, or (2) the signals coming from the other eye modify the gain of the control loop itself. Quantitative evaluation of the data favors the second of these two hypotheses, which comprises the models of Barnes and Horridge (1969) and Nalbach et al. (1985). In addition, we found that it is the signals from the two slow channels of the crab's movement-detecting system that are transferred from one eye to the other, while signals of the fastest channel act almost exclusively ipsilaterally. We discuss our results as an adaptation by which an animal with panoramic vision compensates exclusively the rotational component of image flow during locomotion. The fact that freely walking crabs distinguish the two components of image flow better than restrained crabs indicates that further visual and nonvisual signals help to disentangle image flow.

Adaptation, Physiological↗