C Wehrhahn

Visual course control in flies relies on neuronal computation of object and background motion

The spatial distribution of light intensity received by the eyes changes continually when an animal moves around in its environment. These retinal activity patterns contain a wealth of information on the structure of the environment, the direction and speed of self-motion, and on the independent motion of objects1,2. If evaluated properly by the nervous system this information can be used in visual orientation. In a combination of both behavioural and electrophysiological analysis and modelling, this article establishes the neural mechanisms by which the visual system of the fly evaluates two types of basic retinal motion patterns: coherent retinal large-field motion as induced by self-motion of the animal, and relative motion between objects and their background. Separate neuronal networks are specifically tuned to each of these motion patterns and make use of them in two different visual orientation tasks.

Tracking and chasing in houseflies (Musca)

The flight trajectories of free flying female and male houseflies have been analyzed in 3 dimensions. Both female and male flies track other flies. The turning velocity α (around the vertical axis) is linearly dependent upon the horizontal angle ψF (that is the angle between the trajectory of the tracking fly and the target) for small values of ψF in females and for the whole range of ψF in males. The 3-dimensional velocity υxyz of the chasing fly is linearly dependent upon the distance between leading and chasing fly in males but not in females. Male chasing thus appears to be more efficient than female tracking. It is shown that earlier assumptions on visual control of flight in female flies derived from experiments on fixed flying flies are justified.

Microsurgical lesion of horizontal cells changes optomotor yaw responses in the blowfly Calliphora erythrocephala

The horizontal cells of flies are giant output neurons of the otpic lobes that respond selectively to horizontal motion in the visual environment. The effect of microsurgical lesion of the cells on visually induced flight behaviour was investigated in blowflies (Calliphora erythrocephala). The results indicate that the horizontal cells are the parts of the neural circuitry that control the generation of optomotor yaw torque.

Sex-specific differences in the chasing behaviour of houseflies (Musca)

Flies were filmed simultaneously from above and from the side. Their flight tracks were analyzed frame by frame. Male and female flies were found to chase other flies. But female chases are brief and poorly controlled as compared to male chases. Female flies use the lower frontal part of their visual field for tracking other flies. Male flies use the upper frontal part of their visual field for that purpose. Male flies are capable of controlling their forward velocity roughly proportional to the distance to their target. Implications for the function of recently found sexdimorph visual interneurones are discussed.

The spatial precision of macaque ganglion cell responses in relation to vernier acuity of human observers

Responses of parafoveal macaque ganglion cells were measured as a function of the contrast and position of an edge flashed within their receptive fields. The goal was to determine the ability of different cell types to signal edge location. For comparison, parafoveal vernier thresholds of human observers were measured with pairs of flashed edges. Cells of the magnocellular (MC-) pathway gave larger responses than cells of the parvocellular (PC-) pathway. Neurometric analyses comparing a cells response at different edge positions were performed. The positional signal from single MC-pathway cells was more precise than from PC-pathway cells, especially at lower contrasts. In a second analysis, based on the neurophysiological results, responses from a matrix of ganglion cells were generated. Using a simple model, vernier performance expected from such a matrix was predicted as a function of edge length and contrast. Again, the MC-pathway gave a more precise positional signal than the PC-pathway despite the latters numerical advantage. At contrasts of 20% and below, only the MC-pathway would appear capable of supporting vernier performance with our stimuli. At higher contrasts either the MC- or PC-pathway could provide an adequate signal.

How is tracking and fixation accomplished in the nervous system of the fly

The optomotor yaw torque response of fixed flying female houseflies, Musca domestica to three different types of visual stimuli is analyzed. In contrast to most previous investigations, the stimuli were displayed for short time intervals only in order to approximate transiently occuring visual stimuli, which mainly govern the torque generation during free flight. Monocular stimulation with a periodic pattern moving in different positions in the equatorial plane of the compound eyes reveals that (1) flight torque responses are mainly induced by progressive (front to back) motion; regressively moving stimuli are significantly less effective. (2) the strength of the response to motion in the horizontal direction depends on the position of the stimulus and (3) vertical motions do not elicit flight torque responses. Correspondingly the response to a single vertical black stripe moving clockwise in a cylindrical panorama centered around the fly is small if the stripe is in the visual field of the left eye but becomes large and strongly depending on position if the stripe enters the visual field of the right eye. The response to counterclockwise motion of the stripe is small if the stripe is in the visual field of the right eye but becomes large and strongly depending on position if the stripe enters the visual field of the left eye. Torque responses to two adjacent stripes whose intensities are modulated in time with a rectangular function can be elicited if apparent motion is generated by means of a phase difference between the intensity modulations of the two stripes. Apparent progressive motion elicits strong torque responses, apparent regressive motion is less effective. Synchronous flicker of both stripes does not elicit torque responses. The extraction of positional information from the incoming visual signals has been considered to play an important role in the orientation behaviour, and especially in the tracking behaviour of flies. The results of the experiments indicate, that under transient stimulation the evaluation of positional information is in general not mediated by formerly postulated flicker detectors but is bound to the computation of motion. These findings are implemented in a model, describing the free flight tracking behaviour of a female fly on the horizontal plane. It is shown that tracking can be achieved by a mechanism whose sensitivity to motion is parametrized in the stimulus position as outlined above. The results of the behavioural experiments are interpreted in view of electrophysiological and anatomical data on giant interneurons in the third optic ganglion of the fly.

How vernier acuity depends on contrast.

SummaryVernier 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 exponentially with decreasing contrast (Fig. 1). 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.

Detection facilitation by collinear stimuli in humans : Dependence on strength and sign of contrast

We measured detection of a thin vertical line (target) in the presence of a slightly thicker collinear, adjacent line (inducer). Sign and strength of contrast of the inducer were varied. Test lines could be either bright or dark. Detection thresholds were obtained through a temporal two-alternative forced-choice (2AFC) procedure with the method of constant stimuli. When target and inducer had equal contrast polarity, low thresholds of target lines were observed for low inducer contrasts and increased with increasing inducer contrast. With opposite contrast polarity of target and inducer, thresholds were high for low inducer contrasts and decreased for increasing contrast thereof. Our results support the hypothesis that cortical mechanisms with different sensitivity to the sign and strength of contrast participate in the detection facilitation of line contours.

Visually induced height orientation of the fly Musca domestica

The visually controlled height orientation of fixed flying flies (Musca domestica) was investigated. The flight lift force measured by a transducer drives the vertical motion of a panorama. The dynamical conditions of the free flight are electronically simulated for the fly with respect to this degree of freedom of motion. In most of the experimentally investigated cases the panorama consists of a horizontally oriented narrow dark stripe on a bright background. The fly orientates with respect to the stripe, transporting it into a stable fixation position just below the equatorial plane of its compound eyes. It is experimentally demonstrated that the formalism of the linearized theory of the pattern induced flight orientation — Poggio and Reichardt (1973a) — can be applied to describe the height orientation of the fly. The experimental evidence concerning the simultaneous perception of stripes moving in a well defined manner in front of each of the two compound eyes is consistent with the hypothesis that the two halves of the visual system are perceptually additive.

Is the landing response of the housefly (Musca) driven by motion of a flow field

The landing response of tethered flying housefliesMusca domestica elicited by motion of periodic gratings is analysed. The field of view of the compound eyes of a fly can be subdivided into a region of binocular overlap and a monocular region. In the monocular region the landing response is elicited by motion from front to back and suppressed by motion from back to front. The sensitivity to front to back motion in monocular flies (one eye covered with black paint) has a maximum at an angle 60°–80° laterally from the direction of flight in the equatorial plane. The maximum of the landing response to front to back motion as a function of the contrast frequencyw/λ is observed at around 8 Hz. In the region of binocular overlap of monocular flies the landing response can be elicited by back to front motion around the equatorial plane if a laterally positioned pattern is simulataneously moved from front to back. 40° above the equatorial plane in the binocular region the landing response in binocular flies is elicited by upward motion, 40° below the equatorial plane in the binocular region it is elicited by downward motion. The results are interpreted as an adaptation of the visual system of the fly to the perception of a flow field having its pole in the direction of flight.