Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil--revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: (a) the target was moved manually by the experimenter; (b) the target was moved manually by the subject; (c) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from 'slow' to 'very fast', corresponding to changes in target vergence from about 10 degrees s-1 to about 100 degrees s-1. The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When 'very fast' changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100 degrees s-1. Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i.e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i.e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the 'slow' and 'medium' conditions (stimulus speeds about 10-40 degrees s-1) tracking was accurate to within 1-2 degrees, irrespective of the way in which the target was moved. In the 'fast' and 'very fast' conditions (stimulus speeds about 40-100 degrees s-1), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects.(ABSTRACT TRUNCATED AT 400 WORDS)     

基于视差敏感复杂细胞的聚散式眼动的生理模型

根据心理物理实验和电生理实验的结果,选用对非零视差有选择性反应的视差敏感复杂细胞来检测视差信息,并把所提取的视差信息直接投射到聚散式眼动细胞（ｖｅｒｇｅｎｃｅｃｅｌ）以控制聚散式眼动。同时,综合考虑了复杂细胞编码范围的限制、近细胞和远细胞的相位关系以及跳跃式眼动后增强效应（ｐｏｓｔ－ｓａｃｃａｄｉｃｅｎｈａｎｃｅｍｅｎｔ）等因素。得到的模拟结果与心理物理实验结果定性地符合     

Model of the cone-horizontal cell circuit in the catfish retina

A model of the cone-L-type horizontal cell circuit of the catfish contains 3 stages. The outer segment consists of a compression factor producing the Naka-Rushton relationship between amplitude of response and intensity and 7 low-pass filters in tandem that produces an absolute delay of about 15 ms. The cone pedicle consists of an internal negative feedback circuit in series with a low-pass filter. The L-type horizontal cell acts as a linear low-pass filter and forms the external negative feedback circuit with the cone pedicle. The system shows peicewise linearity with the feedback gain of the external negative feedback circuit directly proportional to the dc level of the horizontal cell. Thus, at any given mean illuminance the impulse response of the cone and L-HC adequately defines the dynamics of the responses. The conversion of a slow monophasic to a faster biphasic impulse response due to either an increase in mean illuminace or use of a steady annulus results from the change in the characteristic equation as the effective value of the feedback gain changes. By proper adjustement of gains and time constants, the cone-L-HC circuit of the catfish retina simulates the experimental data.     

A dual-mode dynamic model of the human accommodation system

The function of the accommodation system is to provide a clear retinal image of objects in the visual scene. The system was previously thought to be under simple continuous (i.e., single mode of operation) feedback control, but recent research has shown that it is under discontinuous (i.e., two stimulus-dependent modes of operation) feedback control by means of fast and slow processes. A model using MATLAB/SIMULINK was developed to simulate this dual-mode behavior. It consists of fast and slow components in a feedback loop. The fast component responds to step target disparity with an open-loop movement to nearly reach the desired level, and then the slow component uses closed-loop feedback to reduce the residual error to an acceptable small level. For slow ramps, the slow component provides smooth tracking of the stimulus, whereas for fast ramps, the fast component provides accurate staircase-like step responses. Simulation of this model using a variety of stimuli, including pulse, step, ramp, and sinusoid, showed good agreement with experimental results. Thus, this represents the first dynamic model of accommodation that can accurately simulate the complex dual-mode behavior seen experimentally. The biological significance of this model is that it can be used to quantitatively analyze clinical deficits such as amblyopia and accommodative insufficiency.     

An ASIC-chip for stereoscopic depth analysis in video-real-time based on visual cortical cell behavior

In a stereoscopic system both eyes or cameras have a slightly different view. As a consequence small variations between the projected images exist ("disparities") which are spatially evaluated in order to retrieve depth information. We will show that two related algorithmic versions can be designed which recover disparity. Both approaches are based on the comparison of filter outputs from filtering the left and the right image. The difference of the phase components between left and right filter responses encodes the disparity. One approach uses regular Gabor filters and computes the spatial phase differences in a conventional way as described already in 1988 by Sanger. Novel to this approach, however, is that we formulate it in a way which is fully compatible with neural operations in the visual cortex. The second approach uses the apparently paradoxical similarity between the analysis of visual disparities and the determination of the azimuth of a sound source. Animals determine the direction of the sound from the temporal delay between the left and right ear signals. Similarly, in our second approach we transpose the spatially defined problem of disparity analysis into the temporal domain and utilize two resonators implemented in the form of causal (electronic) filters to determine the disparity as local temporal phase differences between the left and right filter responses. This approach permits video real-time analysis of stereo image sequences (see movies at http://www.neurop.ruhr-uni-bochum.de/Real- Time-Stereo) and a FPGA-based PC-board has been developed which performs stereo-analysis at full PAL resolution in video real-time. An ASIC chip will be available in March 2000.     

Ocular vergence under natural conditions. II. Gaze shifts between real targets differing in distance and direction

Horizontal binocular eye movements of three subjects were recorded with the scleral sensor coil--revolving magnetic field technique during voluntary shifts of gaze between pairs of stationary, real, continuously visible targets. The target pairs were located either along the median plane (requiring symmetrical vergence), or on either side of the median plane (requiring asymmetrical vergence). Symmetrical vergence was primarily smooth, but it was often assisted by small, disjunctive saccades. Peak vergence speeds were very high; they increased from about 50 degrees s-1 for vergence changes of 5 degrees to between 150 and 200 degrees s-1 for vergence changes of 34 degrees. Differences between convergence and divergence were idiosyncratic. Asymmetrical vergence, requiring a vergence of 11 degrees combined with a version of 45 degrees, was largely saccadic. Unequal saccades mediated virtually all (95%) of the vergence required in the divergent direction, whereas 75% of the vergence required in the convergent direction was mediated by unequal saccades, with the remaining convergence mediated by smooth vergence, following completion of the saccades. Peak divergence speeds during these saccades were very high (180 degrees s-1 for a change of vergence of 11 degrees); much faster than the smooth, symmetrical vergence change of comparable size (14 degrees). Peak convergent saccadic speeds were about 20% lower. This difference in peak speed was caused by an initial, transient divergence, observed at the beginning of all horizontal saccades. The waveform of disjunctive saccades did not have the same shape as the waveform of conjugate saccades of similar size. The smaller saccade of the disjunctive pair was stretched out in time so as to have the same duration as its larger, companion saccade. These results permitted the conclusion that the subsystems controlling saccades and vergence are not independent. Vergence responses were relatively slow and incomplete with monocular viewing, which excluded disparity as a cue. Monocularly stimulated vergence decreased as a function of the increasing presbyopia of our three subjects. Subjects were able to generate some vergence in darkness towards previously seen and remembered targets. Such responses, however, were slow, irregular and evanescent. In conclusion, vergence shifts between targets, which provided all natural cues to distance, were fast and accurate; they appeared adequate to provide effective binocular vision under natural conditions. This result could not have been expected on the basis of previous observations, all of which had been made with severely reduced cues to depth.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reafferent control of optomotor yaw torque inDrosophila melanogaster

Summary In the flight simulator the optomotor response ofDrosophila melanogaster does not operate as a simple feedback loop. Reafferent and exafferent motion stimuli are processed differently. Under open-loop conditions responses to motion are weaker than under closed-loop conditions. It takes the fly less than 100 ms to distinguish reafferent from exafferent motion. In closed-loop conditions, flies constantly generate torque fluctuations leading to small-angle oscillations of the panorama. This reafferent motion stimulus facilitates the response to exafferent motion but does not itself elicit optomotor responses. Reafference control appears to be directionally selective: while a displacement of the patternm by as little as 0.1° against the expected direction leads to a fast syndirectional torque response, displacements in the expected direction have no comparable effect. Based on the behavior of the mutantrol sol, which under open-loop conditions is directionally motion-blind but in closed-loop conditions still performs optomotor balance, a model is proposed in which the fly's endogenous torque fluctuations are an essential part of the course control process. It is argued that the model may also account for wild type optomotor balance in the flight simulator.     

Modelling the effects of a negative feedback circuit from horizontal cells to cones on the impulse responses of cones and horizontal cells in the catfish retina

A model of the cone-L-HC circuit for the catfish retina is presented with the following features: the outer segment consists of a compression factor and 7 low-pass filters in tandem; the cone pedicle consists of an internal negative feedback circuit in series with a low-pass filter; and the L-HC consists of a low-pass filter and forms a negative feedback circuit with the cone pedicle. By proper adjustment of the various time constants of the low-pass filters and the gain factors, the impulse responses for cones and L-HCs of the catfish retina (and turtle) can be duplicated. The negative feedback gain increases with increasing levels of mean illuminance which causes the monophasic impulse responses to become faster, biphasic and decrease in amplitude, i.e. in gain. This is an expression of the Weber-Fechner law.     

Closed-loop control of muscle length through motor unit recruitment in load-moving conditions

Neuroprostheses aimed at restoring lost movement in the limbs of spinal cord injured individuals are being developed in this laboratory. As part of this program, we have designed a digital proportional-integral-derivative controller integrated with a stimulation system which effects recruitment of motor units according to the size principle. This system is intended to control muscle length while shortening against fixed loads. Feline sciatic nerves were exposed and stimulated with ramp, triangular, sinusoidal, staircase and random signals as test inputs. Changes in muscle length and effective time delay under different conditions were measured and analyzed. Differences of tracking quality between open- and closed-loop conditions were examined through analysis of variance as well as the differences between small (250g) and large (1kg) loads. The results showed that parameters used to compare muscle length output to the input signals were dramatically improved in the closed-loop trials as compared to the open-loop condition. Mean squared correlation coefficients between input and output signals for ramp signals increased by 0.019, and for triangular signals by 0.12. Mean peak cross correlation between input and output signals for sinusoidal waveforms increased by 0.06, with decreases in time to peak cross correlation (effective time delay) from 195 to 38ms. In slow random signals (power up to 0.5Hz), peak cross correlation went from 0.74 to 0.89, and time-to-peak cross correlation decreased from 205 to 55ms. In fast random signals (power up to 1Hz), peak cross correlation went from 0.82 to 0.89, and time-to-peak cross correlation from 200 to 65ms. For staircase signals, both rise times and mean steady-state errors decreased. It was found that, once the length range was set, the load weight had no effect on tracking performance. Analysis of mean square error demonstrated that for all signals tested, the feedback decreased the tracking error significantly, whereas, again, load had no effect. The results suggest that tracking is vastly improved by using a closed-loop system to control muscle length, and that load does not affect the quality of signal tracking as measured by standard control system analysis methods.     

Dynamic baroreflex control of blood pressure: influence of the heart vs. peripheral resistance

The aim in the present experiments was to assess the dynamic baroreflex control of blood pressure, to develop an accurate mathematical model that represented this relationship, and to assess the role of dynamic changes in heart rate and stroke volume in giving rise to components of this response. Patterned electrical stimulation [pseudo-random binary sequence (PRBS)] was applied to the aortic depressor nerve (ADN) to produce changes in blood pressure under open-loop conditions in anesthetized rabbits. The stimulus provided constant power over the frequency range 0-0.5 Hz and revealed that the composite systems represented by the central nervous system, sympathetic activity, and vascular resistance responded as a second-order low-pass filter (corner frequency approximately 0.047 Hz) with a time delay (1.01 s). The gain between ADN and mean arterial pressure was reasonably constant before the corner frequency and then decreased with increasing frequency of stimulus. Although the heart rate was altered in response to the PRBS stimuli, we found that removal of the heart's ability to contribute to blood pressure variability by vagotomy and beta(1)-receptor blockade did not significantly alter the frequency response. We conclude that the contribution of the heart to the dynamic regulation of blood pressure is negligible in the rabbit. The consequences of this finding are examined with respect to low-frequency oscillations in blood pressure.     

Tests of Models for Saccade–Vergence Interaction using Novel Stimulus Conditions

During natural activities, two types of eye movements - saccades and vergence - are used in concert to point the fovea of each eye at features of interest. Some electrophysiological studies support the concept of independent neurobiological substrates for saccades and vergence, namely saccadic and vergence burst neurons. Discerning the interaction of these two components is complicated by the near-synchronous occurrence of saccadic and vergence components. However, by positioning the far target below the near target, it is possible to induce responses in which the peak velocity of the vertical saccadic component precedes the peak velocity of the horizontal vergence component by approximately 75 ms. When saccade-vergence responses are temporally dissociated in this way, the vergence velocity waveform changes, becoming less skewed. We excluded the possibility that such change in skewing was due to visual feedback by showing that similar behavior occurred in darkness. We then tested a saccade-related vergence burst neuron (SVBN) model proposed by Zee et al. in J Neurophysiol 68:1624-1641 (1992), in which omnipause neurons remove inhibition from both saccadic and vergence burst neurons. The technique of parameter estimation was used to calculate optimal values for responses from human subjects in which saccadic and convergence components of response were either nearly synchronized or temporally dissociated. Although the SVBN model could account for convergence waveforms when saccadic and vergence components were nearly synchronized, it could not when the components were temporally dissociated. We modified the model so that the saccadic pulse changed the parameter values of the convergence burst units if both components were synchronized. The modified model accounted for velocity waveforms of both synchronous and dissociated convergence movements. We conclude that both the saccadic pulse and omnipause neuron inhibition influence the generation of vergence movements when they are made synchronously with saccades.     

Does vertical disparity scale the perception of stereoscopic depth?

It has been suggested that a measure of the gradients of vertical disparity over a surface may scale the mapping between horizontal disparity and perceived depth. We have investigated this possibility by obtaining estimates of the depth within stereograms that simulated two apposed fronto-parallel planes placed at different distances from an observer. The gradients of vertical disparity in a stereogram were set to simulate those appropriate to a viewing distance of 12.5 cm, 25 cm, 50 cm or 100 cm, whereas the distance specified by vergence and accommodative cues was always fixed at 50 cm. Judgements of the perceived depth between the two planes were uninfluenced by changes in the gradients of vertical disparity. It thus seems that the human visual system does not employ vertical disparity as a scaling parameter in stereoscopic depth judgements.     

Stereoscopic depth constancy

Depth constancy is the ability to perceive a fixed depth interval in the world as constant despite changes in viewing distance and the spatial scale of depth variation. It is well known that the spatial frequency of depth variation has a large effect on threshold. In the first experiment, we determined that the visual system compensates for this differential sensitivity when the change in disparity is suprathreshold, thereby attaining constancy similar to contrast constancy in the luminance domain. In a second experiment, we examined the ability to perceive constant depth when the spatial frequency and viewing distance both changed. To attain constancy in this situation, the visual system has to estimate distance. We investigated this ability when vergence, accommodation and vertical disparity are all presented accurately and therefore provided veridical information about viewing distance. We found that constancy is nearly complete across changes in viewing distance. Depth constancy is most complete when the scale of the depth relief is constant in the world rather than when it is constant in angular units at the retina. These results bear on the efficacy of algorithms for creating stereo content.This article is part of the themed issue ‘Vision in our three-dimensional world’.     

Temporal Co-Variation between Eye Lens Accommodation and Trapezius Muscle Activity during a Dynamic Near-Far Visual Task

Near work is associated with increased activity in the neck and shoulder muscles, but the underlying mechanism is still unknown. This study was designed to determine whether a dynamic change in focus, alternating between a nearby and a more distant visual target, produces a direct parallel change in trapezius muscle activity. Fourteen healthy controls and 12 patients with a history of visual and neck/shoulder symptoms performed a Near-Far visual task under three different viewing conditions; one neutral condition with no trial lenses, one condition with negative trial lenses to create increased accommodation, and one condition with positive trial lenses to create decreased accommodation. Eye lens accommodation and trapezius muscle activity were continuously recorded. The trapezius muscle activity was significantly higher during Near than during Far focusing periods for both groups within the neutral viewing condition, and there was a significant co-variation in time between accommodation and trapezius muscle activity within the neutral and positive viewing conditions for the control group. In conclusion, these results reveal a connection between Near focusing and increased muscle activity during dynamic changes in focus between a nearby and a far target. A direct link, from the accommodation/vergence system to the trapezius muscles cannot be ruled out, but the connection may also be explained by an increased need for eye-neck (head) stabilization when focusing on a nearby target as compared to a more distant target.     

Direct extraction of curvature-based metric shape from stereo by view-modulated receptive fields

Any computation of metric surface structure from horizontal disparities depends on the viewing geometry, and analysing this dependence allows us to narrow down the choice of viable schemes. For example, all depth-based or slant-based schemes (i.e. nearly all existing models) are found to be unrealistically sensitive to natural errors in vergence. Curvature-based schemes avoid these problems and require only moderate, more robust view-dependent corrections to yield local object shape, without any depth coding. This fits the fact that humans are strikingly insensitive to global depth but accurate in discriminating surface curvature. The latter also excludes coding only affine structure. In view of new adaptation results, our goal becomes to directly extract retinotopic fields of metric surface curvatures (i.e. avoiding intermediate disparity curvature).To find a robust neural realisation, we combine new exact analysis with basic neural and psychophysical constraints. Systematic, step-by-step ‘design’ leads to neural operators which employ a novel family of ‘dynamic’ receptive fields (RFs), tuned to specific (bi-)local disparity structure. The required RF family is dictated by the non-Euclidean geometry that we identify as inherent in cyclopean vision. The dynamic RF-subfield patterns are controlled via gain modulation by binocular vergence and version, and parameterised by a cell-specific tuning to slant. Our full characterisation of the neural operators invites a range of new neurophysiological tests. Regarding shape perception, the model inverts widely accepted interpretations: It predicts the various types of errors that have often been mistaken for evidence against metric shape extraction.     

Effect of phase shifts in pressure-flow relationship on response to inspiratory resistance

Inspiratory prolongation is an integral component of the response to added inspiratory resistance. To ascertain whether this response depends on the relation between inspiratory flow (V) and the pressure perturbation, we compared the responses when this relationship was made progressively less distinct by creating phase shifts between V and the resulting negative mouth pressure (Pm). This was done with an apparatus that altered Pm in proportion to V (J. Appl. Physiol. 62:2491-2499, 1987). V was passed through low-pass electronic filters of different frequency responses before serving as the command signal to the apparatus. In six normal subjects the average neural inspiratory duration (TI) response (delta TI) was sharply (P less than 0.01) reduced (0.32 +/- 0.07 to 0.12 +/- 0.07 s) when the filter's frequency response decreased from 7.5 to 3.0 Hz. The TI response was essentially flat between tube resistance (i.e., no lag, delta TI = 0.36 +/- 0.11 s) and the 7.5-Hz filter, and there was no further change in TI response with filters having a frequency response less than 3.0 Hz, with all TI responses in this range being not significant. Subjects could not consciously perceive a difference between various filter settings. We conclude that the TI response is critically influenced by the phase of the negative pressure wave relative to TI. Furthermore the TI responses are not deliberate, although consciousness is required for their elicitation.     

Where Do the Eyes Really Go in the Hollow-Face Illusion?

The hollow-face illusion refers to the finding that people typically perceive a concave (hollow) mask as being convex, despite the presence of binocular disparity cues that indicate the contrary. Unlike other illusions of depth, recent research has suggested that the eyes tend to converge at perceived, rather than actual, depths. However, technical and methodological limitations prevented one from knowing whether disparity cues may still have influenced vergence. In the current study, we presented participants with virtual normal or hollow masks and asked them to fixate the tip of the face's nose until they had indicated whether they perceived it as pointing towards or away from them. The results showed that the direction of vergence was indeed determined by perceived depth, although vergence responses were both somewhat delayed and of smaller amplitude (by a factor of about 0.5) for concave than convex masks. These findings demonstrate how perceived depth can override disparity cues when it comes to vergence, albeit not entirely.     

The neural basis of catalepsy in the stick insect cuniculina impigra

The femur-tibia control system which is responsible for catalepsy is studied in the open-loop configuration (input: stimulation of the femoral chordotonal organ; output: spike-frequency of FETi and SETi as well as the force produced by the extensor tibiae muscle). Comparison of motor neuron activities and muscle force reveals the input-output relationships of the extensor tibiae muscle. This muscle behaves like a low-pass filter with a small time constant for rising inputs and a large time constant for falling inputs. It forms the decisive low-pass filter for force production of the complete system. For freely moving tibia, the elastic properties of the muscles combined with the inert mass of the tibia contribute to the low-pass filter properties. The muscle does not contribute to the high-pass filter properties of the complete system. During repetitive stimulation FETi habituates quickly.Supported by DFG Ba 578     

频差在立体视觉信息加工中的作用 Ⅱ、频差“克服”视差

双眼立体视觉机制至今不很清楚,存在不少争论,研究它具有深远意义。我们的兴趣是从心理物理、电生理和理论模型三方面开展工作,最终目标是试图搞清楚视觉立体信息处理类机制。本文主要利用心理物理学方法研究频差克差视差的问题。我们利用自己研制的一种多功能立体图形发生器产生左边为非均匀条纹、右边为均匀条纹的一系列具有不同视差的立体图对。在感知到“阶梯”后,用三种方法使得“阶梯”感变平:①改变均匀条纹的频率,②改变均匀条纹与被试的距离,③改变非均匀条纹与被试者的距离。从而实现了频差“克服”视差。我们的结果支持用频差来解释双眼倾斜现象,它使我们相信频差是视差在初级视系统中的表象形式。