Johannes M. Zanker

Theta motion: a paradoxical stimulus to explore higher order motion extraction

Apparent motion stimuli of increasing complexity have been applied to analyse the mechanisms underlying visual motion perception. In the present paper it is investigated how motion detectors respond to three classes of stimuli which are realized as random-dot kinematograms. (i) In the most conventional stimuli, Fourier motion, a group of dots is displaced coherently in a random-dot pattern. (ii) In drift-balanced motion stimuli a bar made of static random dots is shifted in front of another random-dot pattern. (iii) In the novel class of stimuli, theta motion, an object which is exclusively defined by dot motion into one direction, is moving itself into the opposite direction. It is shown in psychophysical experiments that human observers perceive the direction of object motion in all three classes of stimuli. Simple motion detectors, however, only extract the motion direction of the object in the case of Fourier stimuli, and in the case of drift-balanced stimuli, if a nonlinear preprocessing is assumed. Any of the model alternatives discussed so far just detects the moving dots but not the object in a theta-stimulus, as is illustrated by a combinatorial analysis using a simplified version of a motion detector of the correlation type, which operates on a discrete time scale and takes only discrete values. In order to account for the detection of theta-motion, a model consisting of two hierarchical layers of motion detectors is developed, and simulated for conditions as used in the psychophysical experiments. The perception of theta-motion and the two-layer model is discussed in relation to psychophysical data and theoretical considerations from the literature, to try to incorporate the proposed two-layer model into a general scheme of visual motion processing.

SPEED TUNING IN ELEMENTARY MOTION DETECTORS OF THE CORRELATION TYPE

Abstract. A prominent model of visual motion detection is the so-called correlation or Reichardt detector. Whereas this model can account for many properties of motion vision, from humans to insects (review, Borst and Egelhaaf 1989), it has been commonly assumed that this scheme of motion detection is not well suited to the measurement of image velocity. This is because the commonly used version of the model, which incorporates two unidirectional motion detectors with opposite preferred directions, produces a response which varies not only with the velocity of the image, but also with its spatial structure and contrast. On the other hand, information on image velocity can be crucial in various contexts, and a number of recent behavioural experiments suggest that insects do extract velocity for navigational purposes (review, Srinivasan et al. 1996). Here we show that other versions of the correlation model, which consists of a single unidirectional motion detector or incorporates two oppositely directed detectors with unequal sensitivities, produce responses which vary with image speed and display tuning curves that are substantially independent of the spatial structure of the image. This surprising feature suggests simple strategies of reducing ambiguities in the estimation of speed by using components of neural hardware that are already known to exist in the visual system.

A glimpse into crabworld

Almost all known arthropod compound eyes exhibit regional variations of resolving power, absolute light, spectral and polarisation sensitivity which are likely to be matched to the probability of significant events and the availability of cues in the visual world. To understand the signal processing requirements that have led to the evolution of matched sensory and neural filters, we thus need a detailed description of the input signals to a visual system and of the tasks to be performed under natural operating conditions. We report here on the first steps we took in an attempt to reconstruct an animals specific visual world with emphasis on the motion domain. Fiddler crabs (genus Uca) live in burrows on sand- and mudflats and are active during low tide. They carry their eyes on long, vertically oriented stalks and use vision to detect predators and conspecific signals generated by males waving one massively enlarged claw. The crabs sit on the ground plane of a flat world, where significant events are most likely to occur in a narrow band around the horizon. We recorded scenes in a crab colony with a video camera at crab eye height. The salience of relevant features in the spatial, spectral and polarisation domains was analysed in digitised video images and short sequences of film were processed by a two-dimensional network of motion detectors at various spatial scales. The output of the network provides us with histograms of the direction and strength of motion signals in various spatio-temporal frequency bands. We discuss our results in terms of detection problems, predictability of events, global vs local information content and higher level motion processing involved in intraspecific communication.

The development of vernier acuity in human infants

Vernier acuity, i.e. the detection of a small misalignment between lines, is about one order of magnitude finer than the resolution of periodic gratings in adult humans. This hyperacuity is generally attributed to cortical mechanisms, and the time-course of its development seems to differ from the development of grating resolution that probably is limited by retinal factors. We investigated 271 human infants and children between 2 months and 8 yr of age with essentially identical stimuli and experimental procedures. Vernier thresholds for Vernier targets were compared to grating resolution. The preferential looking experiments led to the following results: (i) Vernier acuity starts below grating resolution. (ii) Like grating resolution, Vernier acuity develops gradually, but more rapidly and longer; at the age of 5 yr performance becomes comparable to that of adults. (iii) Flanking borders without offset, added to the Vernier targets at various distances, did not affect thresholds consistently across distances and age groups.

How does lateral abdomen deflection contribute to flight control ofDrosophila melanogaster

SummaryTethered flyingDrosophila melanogaster change the posture of their caudal body appendages in response to visual stimuli. In the present paper the relevance of lateral abdomen deflections for flight control is analysed. During abdomen deflections the line of action of the gravitational force is shifted with the flys centre of mass. The line of action of aerodynamic drag forces is displaced accordingly, because friction is increased on the side of the body to which the abdomen is deflected. These two passive forces, together with the average flight forces generated actively by the wings, induce a yaw moment. In still air, the axis of this torque is tilted about 30° backwards relative to the vertical body axis. It will be called “yaw axis of the flight mechanics”. Two sets of observations support the notion of a combined yaw motor output. (a) The elementary motion detectors mediating the lateral abdomen deflection and the dynamics of the response resemble that of the optomotor response measured as yaw torque or as variation of wing beat amplitudes. (b) The asymmetric directional selectivity of the motion detecting system mediating the abdomen deflection corresponds to the orientation of the yaw axis of the flight mechanics. To explain the asymmetry, a nonlinear transfer characteristic is assumed in the motion detecting system.

The cortical representation of object motion in man is interindividually variable.

Corticalareas processing visual motion have been well investigated in monkeys, but comparatively little is known about these areas in man. In order to define such cortical areas in the brains of individuals, the magnetic field was recorded while subjects were watching motion-defined static and moving objects. The magnetic response showed a transient component with a clear dipolar magnetic field followed by a sustained component which exhibited some variation in magnetic field structure over time. For the transient component, the single equivalent current dipoles superimposed upon magnetic resonance images for individual subjects were clearly localized outside the primary visual areas. In most cases the neural generator was found in the region of the temporo-parieto-occipital junction of the lateral cortex. The results also suggest that the activated cortical areas show interindividual variations in location.

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.

On the mechanism of speed and altitude control in Drosophila melanogaster

ABSTRACT The total power output of tethered flying Drosophila melanogaster in still air depends on translational velocity components of image flow on the eye, whereas the orientation of the average flight force in the midsagittal plane of the fly is widely independent of visual input (Götz, 1968). The fly does not seem to control the vertical and the horizontal force component independently. Freely flying flies nevertheless generate different ratios between lift and thrust, simply by changing the inclination of their body. By the combined adjustment of the body angle and the total power output a fly appears to be able to stabilize height and speed (David, 1985). Here a possible mechanism is proposed by which the appropriate torque about the transverse body axis could be generated. Translational pattern motion influences the posture of the abdomen and the plane of wing oscillation. Thus the position of the centre of gravity relative to the flight force vector is changed. When abdomen and stroke plane deviate from an equilibrium state, a lever is generated by which the force vector will rotate the fly about its transverse axis.

Mechanisms of human motion perception: combining evidence from evoked potentials, behavioural performance, and computational modelling.

Based on single cell recordings in monkey, it has been suggested that neural activity can be related directly to psychophysically measured threshold behaviour. Here, we investigated in humans whether evoked potentials correlate with behavioural measurements like discrimination thresholds and reaction time. Subjects were asked to report the perceived direction of object motion stimuli which contained variable amounts of coherent motion. Simultaneously, we recorded evoked potentials with a multielectrode array, or measured the reaction time. We show here that motion coherence had a strong influence on both amplitude and latency of the evoked potential. Stronger motion signals evoked stronger and faster cortical responses. The latency reduction of the motion onset response with increasing coherence correlated very well with the concurrent decrease in reaction time. Taken together, these results suggest that temporal integration is an important step in analysing motion signals to generate a reliable behavioural response. We stimulated a two‐dimensional array of correlation‐type motion detectors with the same motion sequences, and analysed the distribution of local motion signals according to signal detection theory. Performance resembled that of human subjects when the decision strategy was optimized so as to exclude small signals and, in particular, when the ideal observer had some knowledge about a region of interest in which the object was to be expected.

Smooth-pursuit eye movements elicited by first-order and second-order motion

Abstract The perception of the displacement of luminance-defined contours (i.e., first-order motion) is an important and well-examined function of the visual system. It can be explained, for example, by the operation of elementary motion detectors (EMDs), which cross-correlate the spatiotemporal luminance distribution. More recent studies using second-order motion stimuli, i.e., shifts of the distribution of features such as contrast, texture, flicker, or motion, extended classic concepts of motion perception by including nonlinear or hierarchical processing in the EMD. Smooth-pursuit eye movements can be used as a direct behavioral probe for motion processing. The ability of the visual system to extract motion signals from the spatiotemporal changes of the retinal image can be addressed by analyzing the elicited eye movements. We measured the eye movement response to moving objects defined by two different types of first-order motion and two different types of second-order motion. Our results clearly showed that the direction of smooth-pursuit eye movements was always determined by the direction of object motion. In particular, in the case of second-order motion stimuli, smooth-pursuit did not follow the retinal image motion. The latency of the initial saccades during pursuit of second-order stimuli was slightly but significantly increased, compared with the latency of saccades elicited by first-order motion. The processing of second-order motion in the peripheral visual field was less exact than the processing of first-order motion in the peripheral field. Steady state smooth-pursuit eye speed did not reflect the velocity of second-order motion as precisely as that of first-order motion, and the resulting retinal error was compensated by saccades. Interestingly, for slow second-order stimuli we observed that the eye could move faster than the target, leading to small, corrective saccades in the opposite direction to the ongoing smooth-pursuit eye movement. We conclude from our results that both visual perception and the control of smooth-pursuit eye movements have access to processing mechanisms extracting first- and second-order motion.