Cue combination in the motion correspondence problem

Image motion is a primary source of visual information about the world. However, before this information can be used the visual system must determine the spatio-temporal displacements of the features in the dynamic retinal image, which originate from objects moving in space. This is known as the motion correspondence problem. We investigated whether cross-cue matching constraints contribute to the solution of this problem, which would be consistent with physiological reports that many directionally selective cells in the visual cortex also respond to additional visual cues. We measured the maximum displacement limit (Dmax) for two-frame apparent motion sequences. Dmax increases as the number of elements in such sequences decreases. However, in our displays the total number of elements was kept constant while the number of a subset of elements, defined by a difference in contrast polarity, binocular disparity or colour, was varied. Dmax increased as the number of elements distinguished by a particular cue was decreased. Dmax was affected by contrast polarity for all observers, but only some observers were influenced by binocular disparity and others by colour information. These results demonstrate that the human visual system exploits local, cross-cue matching constraints in the solution of the motion correspondence problem.     

Characteristics of smooth pursuit in children and adults in apparent motion tests

The efficiency of smooth pursuit was estimated in healthy young subjects (college and school students) by a contactless method based on stroboscopic stimulation causing an illusion of smooth motion of an object. Stable individual differences in the smooth pursuit efficiency were found in both children and adults. Eleven-to 12-year-old children exhibited, on average, a less smooth pursuit of stimuli moving horizontally at velocities of 6–17 deg/s than young adults did.     

The computational measurement of apparent motion: A recurrent pattern recognition strategy as an approach to solve the correspondence problem

In short, the model consists of a two-dimensional set of edge detecting units, modelled according to the zero-crossing detectors introduced first by Marr and Ullman (1981). These detectors are located peripherally in our synthetic vision system and are the input elements for an intelligent recurrent network. The purpose of that network is to recognize and categorize the previously detected contrast changes in a multi-resolution representation of the original image in such a manner that the original information will be decomposed into a relatively small numberN of well-defined edge primitives. The advantage of such a construction is that time-consuming pattern recognition has no longer to be done on the originally complex motion-blurred images of moving objects, but on a limited number of categorized forms. Based on a numberM of elementary feature attributes for each individual edge primitive, the model is then able to decompose each edge pattern into certain features. In this way anM-dimensional vector can be constructed for each edge. For each sequence of two successive frames a tensor can be calculated containing the distances (measured inM-dimensional feature space) between all features in both images. This procedure yields a set ofK—1 tensors for a sequence ofK images. After cross-correlation of allN ×M feature attributes from image (i) with those from image (i+1), wherei = 1, ...,K - 1, probability distributions can be computed. The final step is to search for maxima in these probability functions and then to construct from these extremes an optimal motion field. A number of simulation examples will be presented.     

Low-level and high-level processes in apparent motion

When a group of dots within a random-dot array is discontinuously displaced, it appears as a moving region perceptually segregated from its stationary surround. The spastial, temporal and other constraints governing this effect are markedly different from those classically found for the apparent motion of isolated stimulus elements. The random-dot display appears to tap a low-level motion-detecting process, distinct from the more interpretive process elicited by the classical displays. The distinct contributions of these processes can be identified in 'multi-stable' displays which yield alternative percepts of apparent motion depending on which one or both of the processes is activated. Such experiments illustrate the interaction of relatively stimulus-constrained and relatively autonomous processes invisual perception.     

Auditory event-related potentials to the apparent auditory image motion

Auditory event-related potentials (ERP) were registered to the dichotically presented white noise stimuli (duration 1500 ms, band 150-1200 Hz). Abrupt or gradual change ofinteraural time difference in the middle of stimuli (750 ms after sound offset) was perceived as an apparent auditory image (AI) instant relocation or motion from the midline to one of the ears. In responses these stimuli two ERPs were observed: one to the sound onset, and second--to the onset of motion or AI relocation. ERPs to AI relocation differed from those to sound onset in longer components latencies (123 ms versus 105 ms for N 1,227 ms versus 190 ms for P2). In responses to AI motion component latencies were even longer (N1: 137 ms, P2: 240 ms); N1 amplitude was greater at sites contralateral to the AI motion direction.     

Massively parallel implementations of theories for apparent motion

Two solutions for the correspondence problem for long-range motion are investigated. The first is a modification of the Minimal Mapping Theory (S. Ullman: The Interpretation of Visual Motion, MIT Press, Cambridge, 1979) that is implemented by a massively parallel network. In this network, every two units are interconnected, and thus, its convergence is fast and relatively independent of the number of image features. Computer simulations show that our method accounts as well as the Minimal Mapping Theory for apparent-motion phenomena, although some differences exist. Mathematical proofs provide conditions for the convergence of the network. The second 'solution' for the correspondence problem is called the Structural Theory. This theory assumes that the three-dimensional structure of viewed objects does not change fast in time. Then, the theory looks for the correspondence and three-dimensional structure that best fulfill this assumption. A massively parallel network implementation of this theory is also possible. However, its performance is poor due to the high complexity of its solution space. This supports Ullman's (1979) suggestion that the visual system separates the structure-from-motion process into two stages. First, a stage for motion measurement, and then a stage for structure recovery.     

The apparent permeability coefficient for proton flux through phosphatidylcholine vesicles is dependent on the direction of flux

A dioleoylphosphatidylcholine unilamellar vesicle model system was used to determine proton permeability. The fluorescence of the pH reporter group, pyranine, trapped within vesicles with a difference in pH across the bilayer, was digitized and analyzed with numerical integration. When H+ flux was initiated by the acidification of the external buffer (acid jump), the apparent H+ permeability was found to be a linear function of the reciprocal of the internal H+ concentration with the slope inversely proportional to the initial size of the H+ gradient. When flux was initiated by the alkalinization of the external buffer (base jump), the apparent permeability coefficient was constant for each external H+ concentration. However, the value of the apparent permeability was linearly dependent on the reciprocal of the external H+. The possibility that carbonates (carbon dioxide, carbonic acid, bicarbonate and carbonate) could be acting as proton carriers was tested by adding millimolar concentrations of bicarbonate to solutions greatly reduced in carbonates. The slopes of the graphs of apparent permeability coefficient vs. reciprocal H+ were linear functions of added bicarbonate concentration for both acid and base jump conditions. These observations were interpreted in terms of a model suggesting that carbonic acid or carbon dioxide together with bicarbonate was an efficient proton carrier across phospholipid bilayers.     

Effects of adaptation to apparent movement on recovery of structure from motion.

Past work on the recovery of three-dimensional structure from dynamic two-dimensional images has led to inconsistent conclusions regarding the contributions of the short-range and long-range motion processes. In the present experiments, subjects adapted to displays (either four lines or 50 randomly positioned pixels) whose spatiotemporal parameters were chosen to favor either the short-range or long-range process. Adaptation periods were followed by test displays that simulated the rotation of a four-pixel random object about the vertical gamma-axis. The dependent measure was the angle of rotation between successive frames of the rotation display at which percepts of three-dimensional structure broke down. Both the original data and derived measures based on best-fitting polynomials showed small but consistent effects: Compared to control conditions, adaptation to short-range motion reduced the angle at which percepts of structure broke down; adaptation to long-range motion increased them. It is suggested that both low-level (i.e. short-range) and high-level (long-range) processes contribute to the recovery of structure from motion.     

The detection of real and apparent motion by the crabLeptograpsus variegatus

Summary Optokinetic eye movements can be evoked in rock crabs either by real or apparent motion of the visual field. For wide-field striped stimuli, the apparent motion stimulus is less effective than the real motion but for narrow field light-spot stimuli, intensity is an important factor: at low light intensities real motion is still a more effective stimulus but at high light intensities the opposite is true. The crab's complex response to apparent motion appears to be dominated by the combination of two central mechanisms, one of which decays exponentially during the Dark period, and one which becomes resensitized, after a short delay, in the dark.Supported by the Deutsche Forschungsgemeinschaft ER 79/1     

A statistical correlation technique and a neural network for the motion correspondence problem

A statistical correlation technique (SCT) and two variants of a neural network are presented to solve the motion correspondence problem. Solutions of the motion correspondence problem aim to maintain the identities of individuated elements as they move. In a pre-processing stage, two snapshots of a moving scene are convoluted with two-dimensional Gabor functions, which yields orientations and spatial frequencies of the snapshots at every position. In this paper these properties are used to extract, respectively, the attributes orientation, size and position of line segments. The SCT uses cross-correlations to find the correct translation components, angle of rotation and scaling factor. These parameters are then used in combination with the positions of the line segments to calculate the centre of motion. When all of these parameters are known, the new positions of the line segments from the first snapshot can be calculated and compared to the features in the second snapshot. This yields the solution of the motion correspondence problem. Since the SCT is an indirect way of solving the problem, the principles of the technique are implemented in interactive activation and competition neural networks. With boundary problems and noise these networks perform better than the SCT. They also have the advantage that at every stage of the calculations the best candidates for corresponding pairs of line segments are known.     

Non-uniform stomatal closure and the apparent convexity of the photosynthetic photon flux density response curve

The photosynthetic response of leaves to photosynthetic photon flux density (PPFD) may be described by parameters for the rate of dark respiration (R), the initial slope (Ф), the PPFD-saturated rate of net photosynthesis (A_mQ) and the apparent convexity (θ_a). We tested the hypothesis that non-uniformity in stomatal aperture across a leaf results in a clearly lower θ_a because PPFD saturation will occur at different irradiances in different regions of the leaf. A computer model was constructed to simulate the effects of bell-shaped and other distributions of stomatal conductance for CO₂ (g_s) across a model leaf. In the model, Ф and A_mQ decreased by up to 50% whereas θ_a decreased by at most 8%, essentially negating the hypothesis. The relationship between photosynthetic rate and g_s determined the size of the responses of θ_a and Ф to non-uniform g_s. In support of the model, experiments on sunflower leaves showed that Ф and A_mQ decreased by 32 and 52%, respectively, while no measurable change in θ_a occurred, when abscisic acid was used to induce patchiness in g_s. Although not all possible patterns of non-uniform stomatal conductance have been tested, it appears that if large variations in θ_a occur in nature they do not result from non-uniform g_s.     

Blade motion and nutrient flux to the kelp,Eisenia arborea

Marine algae rely on currents and waves to replenish the nutrients required for photosynthesis. The interaction of algal blades with flow often involves dynamic reorientations of the blade surface (pitching and flapping) that may in turn affect nutrient flux. As a first step toward understanding the consequences of blade motion, we explore the effect of oscillatory pitching on the flux to a flat plate and to two morphologies of the kelp Eisenia arborea. In slow flow (equivalent to a water velocity of 2.7 cm s(-1)), pitching increases the time-averaged flux to both kelp morphologies, but not to the plate. In fast flow (equivalent to 20 cm s(-1) in water), pitching has negligible effect on flux regardless of shape. For many aspects of flux, the flat plate is a reliable model for the flow-protected algal blade, but predictions made from the plate would substantially underestimate the flux to the flow-exposed blade. These measurements highlight the complexities of flow-related nutrient transport and the need to understand better the dynamic interactions among nutrient flux, blade motion, blade morphology, and water flow.     

Visual apparent motion and some preferred paths in the rotation group SO(3)

The sequential presentation of two distinct stimulus objects to the visual system will, under certain conditions, induce an apparentmotion effect, called beta motion, in which the first object appears to smoothly transform into the second. This study is concerned with the kinds of paths selected by the visual system in effecting beta motion in which the metric structure of the object is preserved throughout. Two schemes are advanced according to which the visual system might operate. The first concentrates upon the manifold M in which the object appears to transform and the second upon the group G of transformations of M onto itself in which the particular transformation describing this motion lie. Under the identification of M with the 2-sphere S ², each scheme specifies the action-minimizing curves in the rotation group SO(3) for a particular “natural” Riemannian metric. An experiment is described in which a determination is made of the actual paths taken by an object under-going various rigid-motion beta motions. The results obtained indicate that the visual system behaves more in accordance with the second scheme than with the first. A generalization of this result is briefly discussed.     

Initial predation and parasitism by muricid whelks demonstrated by the correspondence between drilled holes and their apparent enveloper

The morphologies of drilled holes enveloped by three coexisting muricid whelks, Morula musiva, Cronia margariticola and Ergalatax contractus, were compared. The results were used to demonstrate the replacement of feeders using drilled holes, and to estimate the frequency of this occurrence in the field. In the laboratory, M. musiva made circular internal holes, whereas C. margariticola and E. contractus made oval holes, when they preyed upon the mussel Hormomya mutabilis. The maximum inner diameter of the hole correlated with the driller's shell height in all three muricid species. In the field, mussels enveloped by M. musiva had circular drilled holes, as was found in the laboratory, and the correlation between maximum diameter and enveloper's shell height was not significantly different from that found in the laboratory. In contrast, holes enveloped by C. margariticola in the field were more circular than those observed in the laboratory, and there was no correlation between maximum diameter and enveloper's shell height. Statistical analyses of these results indicated that more than half of the mussels being ingested by C. margariticola had been drilled by M. musiva, i.e. C. margariticola would often kleptoparasitize or scavenge M. musiva's prey by taking over the drilled hole. Few M. musiva would take over the holes drilled by C. margariticola or E. contractus. Furthermore, probably few M. musiva would take over the hole made by a conspecific initial drilling predator. It appears that E. contractus rarely initiates predation by drilling.     

An audit of the BMJ's correspondence columns.

OBJECTIVE--To see whether some sections of the BMJ attract more comment than others, whether letters submitted in response to different sections of the journal are rejected at different rates, and whether the balance between letters that agree and disagree with articles in the published correspondence reflects that in submitted letters. DESIGN--Retrospective audit of letters submitted for publication in the correspondence columns of the BMJ in response to articles published between 1 January and 21 May 1989. SUBJECTS--A total of 1319 letters received by the journal, 974 submitted in response to the 1501 published articles and a further 345 raising new issues. MAIN OUTCOME MEASURES--The total numbers of letters submitted in response to the four main sections of the journal--editorials, news, papers, and middles--and the numbers published. Submitted and published letters were analysed according to whether they agreed or disagreed with articles. RESULTS--The overall rejection rate was 63% (831/1319), but among letters relating to articles it was 56% (543/974). Editorials and middles attracted proportionately more letters than papers, but letters relating to papers had a lower rejection rate (43% v 57% for editorials and 43% v 66% for middles). For all sections more letters disagreed than agreed, but a higher proportion of letters in response to editorials and middles disagreed than those submitted in response to papers (64% and 72% v 53%). Among the published letters, however, broadly equal numbers of letters agreed and disagreed with articles, irrespective of section. CONCLUSION--Those sections of the journal that aim at stimulating debate succeeded in attracting the most comment. The relative importance of original scientific research papers was reflected by the priority given to letters submitted in response to papers, and the final correspondence column was a balanced platform of debate despite an unequal submitted response in terms of letters that agreed and disagreed with different sections of the journal.     

Direction of visual apparent motion driven solely by timing of a static sound

In temporal ventriloquism, auditory events can illusorily attract perceived timing of a visual onset [1-3]. We investigated whether timing of a static sound can also influence spatio-temporal processing of visual apparent motion, induced here by visual bars alternating between opposite hemifields. Perceived direction typically depends on the relative interval in timing between visual left-right and right-left flashes (e.g., rightwards motion dominating when left-to-right interflash intervals are shortest [4]). In our new multisensory condition, interflash intervals were equal, but auditory beeps could slightly lag the right flash, yet slightly lead the left flash, or vice versa. This auditory timing strongly influenced perceived visual motion direction, despite providing no spatial auditory motion signal whatsoever. Moreover, prolonged adaptation to such auditorily driven apparent motion produced a robust visual motion aftereffect in the opposite direction, when measured in subsequent silence. Control experiments argued against accounts in terms of possible auditory grouping, or possible attention capture. We suggest that the motion arises because the sounds change perceived visual timing, as we separately confirmed. Our results provide a new demonstration of multisensory influences on sensory-specific perception [5], with timing of a static sound influencing spatio-temporal processing of visual motion direction.