On the evidence for a 'pure' binocular process in human vision

Wolfe (1986, Psychol. Rev. 93, 269-282) proposed a model of human binocular vision based on the assumption of two functionally distinct classes of binocular neuron. These neurons may be regarded as logical AND and OR gates. In the present paper we assess the evidence relevant to this assumption. We find that while both types of binocular neuron have been described in the cortex of cat and monkey, there is no indication that they form functionally separate populations. Critical analysis of the psychophysical evidence for AND and OR channels in human vision suggests that much of the data presented in favor of an AND channel is subject to alternative interpretations. We conclude that the available data are not consistent with the existence of separate channels as proposed by Wolfe.     

On the role of transducin GTPase in the quenching of a phosphodiesterase cascade of vision

The rate of GTP hydrolysis in the active site of transducin and that of the release of the phosphate thus formed have been measured. The former step has been found to be a rate-limiting one. The rate constant for GTP hydrolysis is equal to 0.027 s-1 at 23 degrees C, and 0.07 s-1 at 37 degrees C. Besides, it has been shown that the rate of GTPase reaction on the transducin alpha-subunit does not depend on the concentration of a complex of transducin beta- and gamma-subunits or on the presence of cGMP phosphodiesterase and a 48 kDa protein from rod outer segments. According to the results, GTP hydrolysis on transducin proceeds too slowly to account for the rapid quenching of a phosphodiesterase cascade in rod outer segments.     

Material characterization of human medial collateral ligament.

The objectives of this study were to determine the longitudinal and transverse material properties of the human medial collateral ligament (MCL) and to evaluate the ability of three existing constitutive models to describe the material behavior of MCL. Uniaxial test specimens were punched from ten human cadaveric MCLs and tensile tested along and transverse to the collagen fiber direction. Using load and optical strain analysis information, the tangent modulus, tensile strength and ultimate strain were determined. The material coefficients for each constitutive model were determined using nonlinear regression. All specimens failed within the substance of the tissue. Specimens tested along the collagen fiber direction exhibited the typical nonlinear behavior reported for ligaments. This behavior was absent from the stress-strain curves of the transverse specimens. The average tensile strength, ultimate strain, and tangent modulus for the longitudinal specimens was 38.6 +/- 4.8 MPa, 17.1 +/- 1.5 percent, and 332.2 +/- 58.3 MPa, respectively. The average tensile strength, ultimate strain, and tangent modulus for the transverse specimens was 1.7 +/- 0.5 MPa, 11.7 +/- 0.9 percent, and 11.0 +/- 3.6 MPa, respectively. All three constitutive models described the longitudinal behavior of the ligament equally well. However, the ability of the models to describe the transverse behavior of the ligament varied.     

Repeated contractions alter the geometry of human skeletal muscle.

The aim of this study was to investigate the effect of repeated contractions on the geometry of human skeletal muscle. Six men performed two sets (sets A and B) of 10 repeated isometric plantarflexion contractions at 80% of the moment generated during plantarflexion maximal voluntary contraction (MVC), with a rest interval of 15 min between sets. By use of ultrasound, the geometry of the medial gastrocnemius (MG) muscle was measured in the contractions of set A and the displacement of the MG tendon origin in the myotendinous junction was measured in the contractions of set B. In the transition from the 1st to the 10th contractions, the fascicular length at 80% of MVC decreased from 34 +/- 4 (means +/- SD) to 30 +/- 3 mm (P < 0.001), the pennation angle increased from 35 +/- 3 to 42 +/- 3 degrees (P < 0.001), the myotendinous junction displacement increased from 5 +/- 3 to 10 +/- 3 mm (P < 0.001), and the average fascicular curvature remained constant (P > 0.05) at approximately 4.3 m(-1). No changes (P > 0.05) were found in fascicular length, pennation angle, and myotendinous junction displacement after the fifth contraction. Electrogoniometry showed that the ankle rotated by approximately 6.5 degrees during contraction, but no differences (P > 0.05) were obtained between contractions. The present results show that repeated contractions induce tendon creep, which substantially affects the geometry of the in-series contracting muscles, thus altering their potential for force and joint moment generation.     

Viewing geometry determines how vision and haptics combine in size perception

Vision and haptics have different limitations and advantages because they obtain information by different methods. If the brain combined information from the two senses optimally, it would rely more on the one providing more precise information for the current task. In this study, human observers judged the distance between two parallel surfaces in two within-modality experiments (vision-alone and haptics-alone) and in an intermodality experiment (vision and haptics together). In the within-modality experiments, the precision of visual estimates varied with surface orientation, as expected from geometric considerations; the precision of haptic estimates did not. An ideal observer that combines visual and haptic information weights them differently as a function of orientation. In the intermodality experiment, humans adjusted visual and haptic weights in a fashion quite similar to that of the ideal observer. As a result, combined size estimates are finer than is possible with either vision or haptics alone; indeed, they approach statistical optimality.     

On the biophysics of vision

A photochemical system is considered which is similar to that of S. Hecht but which includes an intermediate substance between visual purple and visual white. This intermediate is assumed to change the embrane voltage of the photoreceptors. A simple model is presented from which one may calculate the frequency of the impulses resulting from the application of light. It is found that this physicochemicai model enables one to account for a considerable number of physiological phenomena of vision. Among these are the light and dark adaptation phenomena. By introducing a simple assumption regarding spatial interaction one can account for some psychological phenomena such as the effect of intensity, light-dark ratio, shape, and area on the critical flicker frequency. A number of other phenomena are discussed in terms of the model. This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command under Contract No. AF 18(600)-1454. Reproduction in whole or in part is permitted for any purpose of the United States Government.     

A computational theory of human stereo vision.

An algorithm is proposed for solving the stereoscopic matching problem. The algorithm consists of five steps: (1) Each image is filtered at different orientations with bar masks of four sizes that increase with eccentricity; the equivalent filters are one or two octaves wide. (2) Zero-crossings in the filtered images, which roughly correspond to edges, are localized. Positions of the ends of lines and edges are also found. (3) For each mask orientation and size, matching takes place between pairs of zero-crossings or terminationss of the same sign in the two images, for a range of disparities up to about the width of the mask's central region. (4) Wide masks can control vergence movements, thus causing small masks to come into correspondence. (5) When a correspondence is achieved, it is stored in a dynamic buffer, called the 2 1/2-D sketch. It is shown that this proposal provides a theoretical framework for most existing psychophysical and neurophysiological data about stereopsis. Several critical experimental predictions are also made, for instance about the size of Panum's area under various conditions. The results of such experiments would tell us whether, for example, cooperativity is necessary for the matching process.     

Contrast adaptation and contrast masking in human vision.

After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.     

The role of the medial prefrontal cortex in achieving goals

Achieving goals in changing environments requires the course of action to be selected on the basis of goal expectation and memory of action-outcome contingency. It is often also essential to evaluate action on the basis of immediate outcomes and the discrimination of early action steps from the final step towards the goal. Recently, in single-cell recordings in monkeys, the neuronal activity that appears to underlie these processes has been noted in the medial part of the prefrontal cortex. Medial prefrontal cells were also active when the subjects extracted the rules of a task in a novel environment. The processes described above might play important roles in rule learning.     

Subject-specific knee joint geometry improves predictions of medial tibiofemoral contact forces

Estimating tibiofemoral joint contact forces is important for understanding the initiation and progression of knee osteoarthritis. However, tibiofemoral contact force predictions are influenced by many factors including muscle forces and anatomical representations of the knee joint. This study aimed to investigate the influence of subject-specific geometry and knee joint kinematics on the prediction of tibiofemoral contact forces using a calibrated EMG-driven neuromusculoskeletal model of the knee. One participant fitted with an instrumented total knee replacement walked at a self-selected speed while medial and lateral tibiofemoral contact forces, ground reaction forces, whole-body kinematics, and lower-limb muscle activity were simultaneously measured. The combination of generic and subject-specific knee joint geometry and kinematics resulted in four different OpenSim models used to estimate muscle–tendon lengths and moment arms. The subject-specific geometric model was created from CT scans and the subject-specific knee joint kinematics representing the translation of the tibia relative to the femur was obtained from fluoroscopy. The EMG-driven model was calibrated using one walking trial, but with three different cost functions that tracked the knee flexion/extension moments with and without constraint over the estimated joint contact forces. The calibrated models then predicted the medial and lateral tibiofemoral contact forces for five other different walking trials. The use of subject-specific models with minimization of the peak tibiofemoral contact forces improved the accuracy of medial contact forces by 47% and lateral contact forces by 7%, respectively compared with the use of generic musculoskeletal model.     

Matched filters in human vision

The technique of subthreshold addition of sinusoidal gratings was used to analyse the visual system of man during the perception of edges, lines, and bars. The experimentally obtained sensitivity function varies in close relationship to the test pattern, and can be factorized into the conjugate complex spectrum of the test pattern at threshold and a pattern-invariant function of spatial frequency. Interpreting the sensitivity function as transfer function, which is possible under certain conditions, we can describe the visual system as a matched filter which extracts an input signal contaminated with noise of specified spectral energy density. Questions discussed refer to the spatial operations occurring in matched filters, the relationship between the modulation transfer function for sine-wave gratings and the pattern-invariant transfer component, the exact determination of elements within the theoretical concept, and the realization of matched filters by the nervous system.These investigations were supported as a project of the Sonderforschungsbereich 50 Kybernetik, München.     

Kinetic evidence for a role of heme geometry on the modulation of carbon monoxide reactivity in human hemoglobin

Kinetics of CO binding to human hemoglobin (Hb) has been followed below neutrality. With respect to the behavior observed at pH 7.0, CO binding to deoxy-Hb at pH 2.3 displays a much faster second-order combination rate constant (1.2 x 10(-7) M-1 s-1) and loss of the autocatalytic character of the kinetic progress curve. The spectroscopic features of the transient deoxy-Hb at pH 2.75 indicate the phenomenon to be related to the cleavage of the proximal histidine N epsilon-Fe bond, as reported for monomeric hemoproteins (Coletta, M., Ascenzi, P., Traylor, T. G., and Brunori, M. (1985) J. Biol. Chem. 260, 4151-4155). The faster CO binding rate constant, higher than that characteristic of the R state, cannot be attributed to either (i) an enhanced dimerization of deoxy-Hb at low pH, or (ii) a quaternary switch of the unliganded form to the R0 state. The data indicate that interaction(s) of the heme on the proximal side is crucial in accounting for the difference in the CO binding rate constant between the two quaternary conformations of hemoglobin.     

The instructive role of binocular vision in the Xenopus tectum

This review presents the fascinating neurobiology underlying the development of the frog optic tectum, the brain structure where the two separate inputs from the two eye are combined into a single, integrated map. In the species Xenopus laevis, binocular visual information has a dramatic impact on axon growth and connectivity, and the formation of binocular connections in this system provides a rich basis for both theoretical and experimental investigations.     

The role of geometry and elastic strains in dynamic states of proteins.

A theory is developed, where a linear macromolecule with geometrically constrained ends, elastically strained, exchanging energy with the solvent molecules through random collisions may provide a mechanism for the following specific functions in proteins: a) Induction of transient, oriented strains in substrates during transition between conformations. b) External variation of the rigidity and geometry of the active site. More generally, a macromolecule in solution possessing appropriate geometrical and elastic properties constitutes a machine, whose possible operations have common features with biological function such as passive transport, enzymatic catalysis and active transport. The theory suggests a quantitative law by which new information about the dynamical state of the protein molecule can be elucidated from the Arrhenius plot. It predicts a relationship between the rate of catalysis and the local viscosity of the solution.     

Three-dimensional geometry of the human carotid artery

Accurate characterization of carotid artery geometry is vital to our understanding of the pathogenesis of atherosclerosis. Three-dimensional computer reconstructions based on medical imaging are now ubiquitous; however, mean carotid artery geometry has not yet been comprehensively characterized. The goal of this work was to build and study such geometry based on data from 16 male patients with severe carotid artery disease. Results of computerized tomography angiography were used to analyze the cross-sectional images implementing a semiautomated segmentation algorithm. Extracted data were used to reconstruct the mean three-dimensional geometry and to determine average values and variability of bifurcation and planarity angles, diameters and cross-sectional areas. Contrary to simplified carotid geometry typically depicted and used, our mean artery was tortuous exhibiting nonplanarity and complex curvature and torsion variations. The bifurcation angle was 36 deg?±?11 deg if measured using arterial centerlines and 15 deg?±?14 deg if measured between the walls of the carotid bifurcation branches. The average planarity angle was 11 deg?±?10 deg. Both bifurcation and planarity angles were substantially smaller than values reported in most studies. Cross sections were elliptical, with an average ratio of semimajor to semiminor axes of 1.2. The cross-sectional area increased twofold in the bulb compared to the proximal common, but then decreased 1.5-fold for the combined area of distal internal and external carotid artery. Inter-patient variability was substantial, especially in the bulb region; however, some common geometrical features were observed in most patients. Obtained quantitative data on the mean carotid artery geometry and its variability among patients with severe carotid artery disease can be used by biomedical engineers and biomechanics vascular modelers in their studies of carotid pathophysiology, and by endovascular device and materials manufacturers interested in the mean geometrical features of the artery to target the broad patient population.     

The role of background movement in goldfish vision

In experiments described in the literature objects presented to restrained goldfish failed to induce eye movements like fixation and/or tracking. We show here that eye movements can be induced only if the background (visual surround) is not stationary relative to the fish but moving. We investigated the influence of background motion on eye movements in the range of angular velocities of 5–20° s⁻¹. The response to presentation of an object is a transient shift in mean horizontal eye position which lasts for some 10 s. If an object is presented in front of the fish the eyes move in a direction such that it is seen more or less symmetrically by both eyes. If it is presented at ±70° from the fish's long axis the eye on the side of the object moves in the direction that the object falls more centrally on its retina. During these object induced eye responses the typical optokinetic nystagmus of amplitude of some 5° with alternating fast and slow phases is maintained, and the eye velocity during the slow phase is not modified by presentation of the object. Presenting an object in front of stationary or moving backgrounds leads to transient suppression of respiration which shows habituation to repeated object presentations. Accepted: 14 April 2000     

The geometry of the human trochlea tali

The geometrical shape of the trochlea tali is responsible for two completely different courses of motion in the ankle joint setting out from the neutral position: dorsiflexion and plantar flexion. Dorsiflexion: The tibia leads the talus, whereas the fibula is pushed laterally by the screw-shaped lateral articular facet of the talus. The malleoli tightly embrace the trochlea tali, whilst an obvious cleft appears dorsally and medially between the superior articular surface of the talus and the tibial roof. Plantar flexion: The fibula leads the talus which withdraws from the medial malleolus by stretching the anterior talofibular ligament. At the same time the superior articular face of the talus closely contacts the tibial roof.