Detection of binocular disparities

A stereo correspondence algorithm designed to perform matching on figurally similar images (arising in normal human binocular vision) is described. It is based on the observation that the operational principles underlying biological stereo disparity detection seem to be extremely general and few in number instead of an extended set of specific constraints. We identify one general characteristic of objects in the three dimensional world and use it to formulate a simple noniterative, parallel and local algorithm that successfully detects disparities generated by opaque as well as transparent surfaces.     

Interaction between luminance gratings and disparity gratings

It was shown from geometry and photographic measurement that the shading pattern for a sinusoidal corrugated surface of frequency f approximates to a luminance-defined grating of frequency f, 2f or f + 2f in specific relative phase. It was confirmed that a luminance grating modifies the appearance of a suprathreshold stereoscopic corrugated surface, suggesting an interaction between shading and binocular disparity. Disparity thresholds for detecting random-dot, disparity-defined gratings of spatial frequency 0.2 or 0.4 c/deg were measured in the presence of luminance gratings of spatial frequency 0.4 c/deg with the same orientation. Phase-specific facilitation of disparity thresholds was greatest for a phase relationship inconsistent with shading of a corrugated surface, and was disrupted by positional uncertainty. The presence of texture-defined lines (which served to mark explicitly the successive spatial locations of salient depth features in the image) produced a similar pattern of facilitation, in the absence of shape-from-shading cues. The pattern of results indicates direct local interactions, including spatial cueing, rather than interaction of depth cues.     

Evidence for surface-based processing of binocular disparity

It is convenient to think of an object's location as a point within a Cartesian framework; the x axis corresponds to right and left, the y axis to up and down, and the z axis to forward or backward. When an observer is looking straight ahead, binocular disparities provide information about distance along the z axis from the fixation plane. In this coordinate system, changes in disparity are treated as independent of changes in location along the orthogonal x and y axes. Does the human visual system use this three-dimensional coordinate system, or does it specify feature location in a coordinate frame determined by other nearby visible features? Here we show that the sensitivity of the human stereo system is determined by the distance of points with respect to a local reference plane, rather than by the distance along the z axis with respect to the fixation plane. There is a distinct advantage to using a local frame of reference for specifying location. It obviates the need to construct a complex three-dimensional space in either eye-centered or head-centered coordinates that must be updated with every shift of the eyes and head.     

Brain activation difference evoked by different binocular disparities of stereograms: An fMRI study

The binocular disparity of two retina images is a main cue of stereoscopic vision. However, the global dependency between brain response and binocular disparity still remains unclear. Here, we used functional Magnetic Resonance Imaging (fMRI) to identify stereopsis-related brain regions with a modified Random Dot Stereogram (RDS) and plotted the activation variation curves under different disparity size. In order to eliminate the confounding shape difference between the stereogram and the plane, commonly seen in RDS, we modified the RDS to a checkerboard version. We found that V3A, V7 and MT+/V5 in dorsal visual stream were activated in stereoscopic experiment, while little activation was found in ventral visual regions. According to the activation trends, 13 subjects were divided into three groups: 5 subjects with turning points (a shift from increased to decreased activation), 5 subjects without turning points and 3 subjects with activation unrelated to disparity. We inferred that the dorsal visual stream primarily processes spatial depth information, rather than shape information.     

Cooperation between different spatial frequencies in binocular rivalry

In binocular rivalry, stimuli made up of any limited spatial frequency (sf-) range are perceived for a shorter time than patterns consisting of the whole sf-spectrum. This finding indicates a non-linear summation of primarily independent sf-channels in the human visual system.     

Parallel processing of binocular disparity in the cat's retinogeniculocortical pathways

In the cat, parallel streams of information processing have been traced from X-, Y- and W-type retinal ganglion cells to visual cortical areas 17 (X-, Y- and W-type), 18 (Y-type) and 19 (W-type). In the present study we have examined, in the anaesthetized and paralysed adult cat, the role played by X-, Y- and W-subsystems, projecting to areas 17 and 19, in the processing of binocular retinal disparity. The tapetal reflection technique was used to monitor residual eye movements and to provide a map, for each eye, of the retinal blood vessels which could later be compared with retinal wholemounts stained with cresyl violet to reveal the area centralis. The receptive-field disparities of cells recorded from areas 17 and 19 were compared with each other and with reference to the visual axes defined by the area centralis of each eye. Cells of area 19 (receiving W-type input) had horizontal receptive-field disparities that were significantly more divergent than those of the cells in area 17 and 17-18 'border region'. Referred to the area centralis, the mean horizontal receptive-field disparity in area 19 was -0.5 degrees (+/- 0.8 degrees). The mean horizontal receptive-field disparity of area 17 (receiving X-, Y- and W-type input) was convergent with respect to the visual axis at +2 degrees (+/- 0.5 degrees). Finally, the mean horizontal receptive-field disparity of the cells in the 17-18 border region (which receive mainly Y-type input) was even more convergent (2.6 degrees +/- 1.5 degrees) than that of area 17. Binocular interactions of cortical neurons were tested with the Risley biprism technique. Area 19 cells had maximal responses to binocular stimulation when the receptive-field disparities were either close to zero or slightly divergent. In contrast, area 17 cells tended to respond optimally to disparities that were either slightly or strongly convergent. At the level of the lateral geniculate nucleus there were significant differences between the receptive-field disparities inferred from the comparison of receptive-field positions of adjacent neurons recorded on either side of the border between the A and A1 geniculate laminae and those inferred from a similar comparison at the C1-C2 border. The mean horizontal disparities inferred from the interlaminar comparison at the A-A1 border were +2.1 degrees (+/- 0.3 degrees); those inferred from the interlaminar comparison at the C1-C2 border -0.2 (+/- 0.2 degrees) were more divergent.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between insect larvae and tadpoles in tropical rain pools

Abstract. 1. Ephemeral rain pools on rock surfaces are common in Africa and are inhabited by dense populations of aquatic dipteran larvae. About 30% of the pools also support large numbers of tadpoles of the frog Ptychadena anchietae .
2. Experiments reveal that the presence of tadpoles suppress eclosion of the rock pool dwelling midge Chironomus imicola .
3. However, the presence of tadpoles also shortens the larval life-span of C. imicola by speeding up growth rates. Since the larval stages must be completed before the pool dries tadpoles may help 'fine-tune' the dipteran to the rock pool habitat.
4. A change in the diet of Cimicola larvae is associated with the accelerated development. Grazing by tadpoles results in algae, growing on the water surface, reaching the mud in tadpole faeces. These algae represent a high protein and energy food not otherwise accessible to mud dwelling dipteran larvae.
5. When pools dry tadpoles are killed, but a second species of dipteran Dasyhelea thompsoni have larvae able to survive to dry phase in situ . These larvae are scavengers so that on reflooding dead tadpoles are available as food. Experiments show that the presence of tadpoles again increase larval growth rates.
6. Both alive and dead tadpoles therefore are responsible for increasing the quality and quantity of food available to dipteran larvae and are thus among the mechanisms ensuring an abundance of food for rock pool dwelling insect larvae.     

Connections between wide-field monocular and binocular movement detectors in the brain of a hawk moth

Summary Recordings were made in the brain of Sphinx ligustri of pairs of directionally selective movement detectors, and the spike trains analysed with a computer for possible synaptic connections between two classes of movement detector. (1) Neurones with large binocular fields which arise in the medial protocerebrum and project to the medulla or lobula of one optic lobe, or to the ventral nerve cord. (2) Movement detectors which project from the lobula complex of one optic lobe to the opposite medial protocerebrum. The majority of the second group had back-to-front preferred directions over the ipsilateral eye, and of these many were weakly sensitive to stimuli to the opposite eye. The ipsilateral receptive field covered most of the eye.Optic lobe output cells with the appropriate preferred direction provide a powerful excitatory input to the binocular movement detectors centrifugal to the medulla. Each centrifugal movement detector probably receives excitatory inputs from no more than two optic lobe output cells with back-to-front preferred direction. The same set of optic lobe output neurones probably feeds several cells projecting to the medulla and lobula of both optic lobes, and, possibly, to the ventral nerve cord.Evidence was obtained that the optic lobe output cells themselves receive few excitatory inputs, and that therefore the receptive fields of their input cells are large.Two moving stimuli were presented in different areas of the receptive field. Movement through the null direction in one area inhibited the response to movement in the preferred direction in another area. This suppression was stronger in optic lobe output cells with front-to-back preferred direction than in units with back-to-front preferred direction. Thus the optic lobe output cells, or wide-field units feeding them, receive inhibitory inputs from wide-field units with the opposite preferred direction.Similar tests in which moving stimuli were presented to both eyes gave results indicating that the binocular centrifugal movement detectors may receive inhibitory inputs from movement detectors with back-to-front preferred direction. The possible functional significance of these inhibitory inputs is discussed.I am very greatful to F. A. Miles for helpful discussion and criticism. Financial support came from the U. K. Science Research Council.     

Adaptation to natural binocular disparities in primate V1 explained by a generalized energy model

Sensory processing in the brain is thought to have evolved to encode naturally occurring stimuli efficiently. We report an adaptation in binocular cortical neurons that reflects the tight constraints imposed by the geometry of 3D vision. We show that the widely used binocular energy model predicts that neurons dedicate part of their dynamic range to impossible combinations of left and right images. Approximately 42% of the neurons we record from V1 of awake monkeys behave in this way (a powerful confirmation of the model), while about 58% deviate from the model in a manner that concentrates more of their dynamic range on stimuli that obey the constraints of binocular geometry. We propose a simple extension of the energy model, using multiple subunits, that explains the adaptation we observe, as well as other properties of binocular neurons that have been hard to account for, such as the response to anti-correlated stereograms.     

Interaction between mitogens upon intracellular Ca2+ pools in murine fibroblasts.

A comparison of the effect of platelet-derived growth factor (PDGF) and bombesin on intracellular Ca2+ stores was carried out in Swiss 3T3 cells loaded with Fura-2. It was found that the tumor promoter thapsigargin (Tg) almost completely inhibited both the PDGF- and the bombesin-induced intracellular Ca2+ concentration ([Ca2+]i) rise, indicating that the two mitogens mobilize Ca2+ from intracellular pool(s) sensitive to the tumor promoter. It was also found that pre-treatment with PDGF almost totally and persistently (up to at least 30 min) inhibited the bombesin-, Tg- and ionomycin-induced rise in [Ca2+]i, whereas pre-treatment with bombesin had only a partial inhibitory effect on the PDGF, Tg and ionomycin [Ca2+]i response, both in the absence and in the presence of external Ca2+. On the other hand, vasopressin and bradykinin, which also stimulate hydrolysis of phosphoinositides in these cells, did not affect the [Ca2+]i response induced by the same agents. These results indicate that, despite the poor production of inositol 1,4,5-trisphosphate (InsP3), PDGF was capable of totally discharging and maintaining discharged the InsP3-sensitive stores of intracellular Ca2+, regardless of whether extracellular Ca2+ was present in the medium. Bombesin only partially caused this effect. On the contrary, bradykinin and vasopressin, after releasing intracellular Ca2+ allowed an almost total refilling of the pools. It is interesting to note that, at variance with PDGF and bombesin, neither bradykinin nor vasopressin are able to induce a mitogenic response in Swiss 3T3 cells.     

Isotropic integration of binocular disparity and relative motion in the perception of three-dimensional shape

Richards (1985) showed that veridical three-dimensional shape may be recovered from the integration of binocular disparity and retinal motion information, but proposed that this integration may only occur for horizontal retinal motion. Psychophysical evidence supporting the combination of stereo and motion information is limited to the case of horizontal motion (Johnston et al., 1994), and has been criticised on the grounds of potential object boundary cues to shape present in the stimuli. We investigated whether veridical shape can be recovered under more general conditions. Observers viewed cylinders that were defined by binocular disparity, two-frame motion or a combination of disparity and motion, presented at simulated distances of 30 cm, 90 cm or 150 cm. Horizontally and vertically oriented cylinders were rotated about vertical and horizontal axes. When rotation was about the cylinder's own axis, no boundary cues to shape were introduced. Settings were biased for the disparity and two-frame motion stimuli, while more veridical shape judgements were made under all conditions for combined cue stimuli. These results demonstrate that the improved perception of three-dimensional shape in these stimuli is not a consequence of the presence of object boundary cues, and that the combination of disparity and motion is not restricted to horizontal image motion.     

Distribution of exogenous 25-hydroxycholesterol in Hep G2 cells between two different pools

Binding of [26,27-(3)H]25-hydroxycholesterol (25HC) to human hepatoma Hep G2 cells was saturated within 120 min. Two intracellular pools of 25HC were identified in a pulse-chase experiment: (i) an exchangeable pool which was in dynamic equilibrium with 25HC in the medium (t(1/2) of reversible exchange 15 min) and (ii) an unexchangeable pool which remained in cells during incubation in medium containing LPDS. 25HC from the exchangeable pool inhibits cholesterol biosynthesis, decreases the HMG CoA reductase mRNA level and stimulates cholesterol acylation. 25HC from the unexchangeable pool was partially bound to cytosolic proteins and apparently utilized for metabolic transformation. Incubation of Hep G2 cells with [26,27-(3)H]25HC in the presence of a 30-fold molar excess of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one was found to cause (i) 2-fold decrease in the binding of [26,27-(3)H]25HC to cytosolic proteins (sedimentation constant of radioactive complex was 4-5 S) and (ii) the 35% inhibition of 25HC transformation to polar metabolites.     

Efflux of cholesterol from different cellular pools

Free cholesterol is very efficiently removed from cells by 2-hydroxypropyl-beta-cyclodextrins. The efflux of cholesterol occurs from two distinct kinetic pools: the half-times (t(1/2)) for the two pools in CHO-K1 cells are 15 +/- 5 s and 21 +/- 6 min and they represent 25% +/- 5% and 75% +/- 5% of the readily exchangeable cell cholesterol, respectively. In this study we have determined that the fast pool and the majority of the slow kinetic pool for cholesterol efflux are apparently present in the plasma membrane. Numerous agents that inhibit intracellular cholesterol trafficking are unable to affect either the size or the t(1/2) for efflux of either kinetic pool. In contrast, treatment of the cells with N-ethylmaleimide (NEM), exogenous lipases such as sphingomyelinase and phospholipase C, calcium ionophore A23187, or heat resulted in the dramatic increase in the size of the fast kinetic pool of cholesterol. These changes in the kinetics of cholesterol efflux are not specific to the nature of the extracellular acceptor indicating that they are a consequence of changes in the cell plasma membrane. The above treatments disrupt the normal organization of the lipids in the plasma membrane via either hydrolysis or randomization. The phosphatidylcholine and sphingomyelin present in the plasma membrane are critical for maintaining the two kinetic pools of cholesterol; any alteration in the amount or the location of these phospholipids results in an enhancement of efflux by redistributing cholesterol into the fast kinetic pool.     

Interaction between calmodulin and five different spin-labelled chlorophenothiazines.

The binding to purified calmodulin of five spin-labelled derivatives of chlorophenothiazine was investigated by e.s.r. spectrometry and by the antagonizing potency on the calmodulin-dependent activation of myosin light chain kinase. The results of a comparative study and the influence of pH and ionic strength on the binding support the occurrence of an electrostatic binding involving the terminal amino group of the side-chain of the chlorophenothiazine. These results are discussed in relation to the specificity of the interaction that holds the antipsychotic drug-calmodulin complex together.     

A model for neural representation of binocular disparity in striate cortex: distributed representation and veto mechanism

A model in striate cortex is proposed for a distributed neural representation of binocular disparity with a simple cell. In the model, disparity is represented by far, near and tuned inhibitory simple cells. However, the representation will be vetoed by model cells where disparity is excessively large. The veto mechanism consists of a neural network of the model cell which received output from simple cells and which interacts with neighbors. The mechanism is necessary, the model cell responds like a simple cell, and the network is physiologically plausible in the brain. Computer simulation on the neural network model with random dot stereography indicates reasonable performance.     

Interaction between RNAs from different sources and egg lecithin liposomes

The interaction of RNAs of molecular weight ranging between 15,000 and 30,000 with egg lecithin large and small unilamellar liposomes has been investigated. Torula and tRNA proved to be incorporated to a similar extent but no linear relationship between molecular weight and percent of incorporation was demonstrated. A relationship between percent of secondary structure of RNA and its ability to be trapped in SUV liposomes is suggested.     

Interaction between fulvic acids of different origins and active oxygen radicals

Using the spin trapping technique, the interaction between fulvic acids (FAs) of different origins and the active oxygen radicals was studied. The active oxygen radicals under study included superoxide anion (O2 · -) produced by xanthine oxidase (XOD) and stimulated polymorphonuclear leukocytes (PMN) of human being and hydroxyl radical ( ·OH) produced from Fenton's reaction. It has been found that the FAs from both Kaschin-Beck disease (KBD) region and non-KBD region can accelerate the production of ·OH and scavenge O2 ·- . FA from peat can scavenge both O2·- and ·OH. The results show that the behavior of KBD and non-KBD FAs differs clearly from peat FA. It has been concluded that the superoxidation damage of KBD induced by FA is mainly due to hydroxyl radical reaction initiated in biological system.