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C Wehrhahn

Publications and source records attributed to C Wehrhahn.

13 recordsLinked to original sources
PubMed1992

ON- and OFF-pathways form separate neural substrates for motion perception: psychophysical evidence.

We have tested the hypothesis that in humans the signals carried by ON- and OFF-pathway respectively are processed for the perception of motion by two distinct physiological substrates. In vertebrates, onset of a bright visual stimulus is signaled to the CNS by ON-center retinal ganglion cells; onset of a dark stimulus is transmitted by OFF-center cells. We chose apparent motion generated by successive presentation of two adjacent lines as a stimulus. Lines presented on a bright background were either darker or brighter than this background. Delayed onset of a pair of bright or dark lines elicits apparent motion at the same time fulfilling the constraint of stimulating either ON- or OFF-center ganglion cells, respectively. We determined the threshold delay needed for subjects to perceive the temporal order of the onset of the two lines for various angular separations. The threshold delay for a pair of bright lines stayed low for separations from 2' to 7'. The threshold delay for a pair of dark lines was low only within a narrow range of separations centered around 3'. The variation of thresholds with line distance must reflect the existence of a limited processing zone for the perception of motion. The diameter of the processing zone for bright lines is about twice as large as that for dark lines. This suggests that in humans the separation of ON- and OFF-pathways extends to the early stages of motion perception. To test this hypothesis independently, thresholds were determined when a bright and a dark line were presented in succession. This was done for a separation of 3' where thresholds for a pair of lines with equal contrast are similarly low. Temporal order was perceived correctly only when the delay was at least two to four times as high as the threshold delays found for the equal contrast stimuli.

Humans↗
PubMed1991

Motion perception in the peripheral visual field.

Thresholds were determined for the perception of the motion of a single bar moving at different positions in the field of view. Performance in the temporal hemified was slightly superior to that in the nasal hemifield and depended on the orientation as well as on the direction of the motion. The perception of horizontal motion was better than that of vertical motion. In spite of large variations, centrifugal motion was significantly more readily perceived than centripetal motion.

Humans↗
PubMed1990

How vernier acuity depends on contrast.

Vernier acuity was measured by finding the just discriminable offset for an edge separating fields of different luminances. The contrast of this stimulus is easily specified by the formula c = (Lstim - Lsur)(Lstim + Lsur). Vernier thresholds are about 4-5 sec of arc for contrasts 0.22 and higher, but increase exponetially with decreasing contrast. By comparison, the presence of the stimulus could be detected at a contrast of 0.016. The possible role of the magnocellular and parvocellular pathways in carrying the input signals to the fine localization process is discussed.

Form Perception↗
PubMed1990

Binocular summation in temporal-order detection.

Thresholds for temporal-order detection of two successively displayed vertical stripes were determined for monocular and binocular stimulation. Thresholds are lower for binocular stimulation by roughly a factor of 1.4 compared with those found under monocular stimulation. This corresponds to quadratic summation. This is found for angular separations of the two stripes amounting to 5 and 40 min of arc.

Humans↗
PubMed1990

Neural circuits mediating visual flight control in flies. II. Separation of two control systems by microsurgical brain lesions.

The role of 2 sets of interneurons in the optic lobes of blowflies in visual course control was studied by means of brain lesions. The first set comprises the cells HS and H2, which respond to global horizontal motion. The second set are the FD-cells, which respond selectively to local horizontal motion. All these cells are output neurons of the third optic ganglion of flies and are thought to be coupled via descending neurons to the flight motor system. In 2 series of experiments specific cells of these 2 sets were inactivated by microsurgical brain lesions L1 and L2 respectively. The effects of the lesions on visual course control were tested by measuring the yaw torque responses of the animals in restrained flight before and after the operation. The flies were stimulated in these tests with monocular and binocular motion of periodic gratings moving in either the horizontal or the vertical direction. Lesion L1 in the right side of the brain inactivates the right HS-cells and the left H2- and FD-cells. This leads to a complete block of the response to binocular clockwise horizontal motion and a reduction of the response to monocular motion from front to back on the right side of the animal. Application of L1 also leads to a pronounced response to binocular motion from front to back not observed in normal animals. The response to monocular vertical motion is unaffected. Lesion L2 reduces all responses to monocular and binocular horizontal motion present in normal animals. The behavioral effects of the lesions are highly specific and consistent with predictions based on the well-known anatomical and physiological properties of the neural circuitry investigated. The results demonstrate directly that the HS-, H2-, and FD-cells control motion-induced steering maneuvers in flight.

Animals↗
PubMed1989

Neural circuits mediating visual flight control in flies. I. Quantitative comparison of neural and behavioral response characteristics.

The motion-sensitive horizontal cells in the lobula plate of the fly are assumed to play a key role in the sensory control of yaw torque generated by the flying animal during course-stabilization maneuvers and the fixation of objects. This inference results from comparisons of electrophysiological data obtained from blowflies (Calliphora erythrocephala) and behavioral data obtained mainly from houseflies (Musca domestica) and fruitflies (Drosophila melanogaster). Apart from few exceptions, the compatibility of these physiological and behavioral data has not been critically tested. In the present study, the responses of the equatorial horizontal cell HSE of Calliphora and the yaw torque responses of Calliphora and Musca were recorded under identical visual stimulation with moving periodic gratings. The goal of the experiments was to obtain electrophysiological and behavioral data on Calliphora, on the one hand, and behavioral data on Calliphora and Musca, on the other hand, that allow direct comparisons between the physiological properties of the HSE and the visually induced torque responses in both species. The dependence of the HSE responses and the yaw torque responses on the direction, contrast frequency, and brightness of a moving periodic grating were evaluated quantitatively. The results of the electrophysiological recordings and torque measurements are in close agreement and thus represent strong evidence that the horizontal cells are, in fact, involved in yaw torque control in both species. Measurements of the cellular and behavioral responses as function of the stimulus position in the visual field, however, reveal differences between the spatial sensitivity of the horizontal cells and the sensory input to the motor system.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Color constancy in the honeybee.

A multicolored display was illuminated by 3 bands of wavelengths corresponding to the maxima of the spectral sensitivities of the 3 types of photoreceptors found in the bee retina. The intensity of each band could be varied individually. The light fluxes emitted by the colored areas of the multicolored display were determined quantitatively. Free-flying honeybees were trained with sugar solution to choose one of the colored areas. The illumination was then changed in such a way that the light fluxes formerly emitted by the training area were now measured on another area. When the trained bees were tested under those conditions, they still chose the training area. The relative positions of the colored areas were changed in order to exclude learning of position. It is concluded that color vision in bees is, in a certain range, independent of the spectral content of the illumination. Model calculations show that the behavior observed in bees is consistent with the retinex theory (Land, 1977), i.e., an algorithm using long-range interactions is required to explain color constancy.

Animals↗
PubMed1987

Comparison of color sensation in dichoptic and in normal vision.

Color vision in humans is independent over a wide range of the spectral composition of the illuminating light (Young 1807; Hering 1879). The retinex theory accounts for this color constancy by assuming that for each of the three waveband channels determined by the retinal cones a global lightness record of the scene is first computed by the visual system. The three records then serve to generate color at every point (Land 1983). Where do these computations take place? In this report a scene consisting of fourteen colored fields was viewed while one band of wavelengths enters one eye and a different band enters the other (dichoptic case) or while both bands enter both eyes (normal case) under otherwise identical conditions. The perceived color of every field is very similar in both cases although the physical stimulation of the eyes differs. It is also found that color constancy is maintained under dichoptic conditions. The results show that the cortex is crucial for the computation of color.

Adult↗
PubMed1978

The angular orientation of the movement detectors acting on the flight lift response in flies.

The lift response of houseflies Musca domestica in fixed flight to periodic gratings movins in 12 different orientations has been measured. Two projectors were arranged symmetrically to the flies stimulating successively 18 circular patches of 50 degrees (25 degrees) diameter (9 for each eye) in their visual field. The shapes of the lift responses measured as a function of the orientation of the moving grating varied when different patches in the visual field were stimulated. A qualitative comparison of these response curves leads to the conclusion that the orientation of the movement detecting substrate acting on the flight lift response varies as a function of the stimulated area in the visual field. A straightforward correlation between the geometry of the ommatidial pattern and the orientation of the movement detecting substrate valid for all stimulated areas of the compound eyes does not seem very likely.

Animals↗