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Biomedical subjects

E Brenner

Publications and source records attributed to E Brenner.

At least 19 recordsLinked to original sources
PubMed1997

Internal mammary veins: classification and surgical use in free-tissue transfer.

The internal mammary artery has been well-investigated due to its frequent use in cardiac surgery. However, in reconstructive microsurgery in the thoracic region, the internal mammary vessels have been used rarely as recipient vessels, due to difficulties and lack of knowledge about the internal mammary veins. This study was designed to investigate the anatomy and topography of the veins. Its goal was to gain sufficient information about their availability in free-tissue transfer. Dissection of the vessels was performed in 86 cadavers bilaterally to the sternum. At the level of the fourth rib, which is the most desired access for microvascular anastomosis in reconstructive breast surgery with free flaps, the veins were found to be adequately large (range 0.64 to 4.45 mm). Results were in close agreement with 100 bilateral measurements obtained by color Doppler sonography in 16 patients preoperatively, and additionally in 34 healthy volunteers. Preoperative detection of vessels with the aid of sonography facilitated planning of surgical procedures. According to these findings, the internal mammary veins may be used as suitable recipient veins for free microvascular tissue transfer, especially for established autologous breast reconstruction with the free transverse rectus abdominis myocutaneous flap, or for reconstruction of complex thoracic-wall defects.

Adolescent↗
PubMed1996

Size illusion influences how we lift but not how we grasp an object.

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information on the object's position and orientation guides the hand to the object, while information on the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object-on the basis of its shape, surface texture, and so on- and then moving one's thumb and fingers to these positions. This could lead to the same performance without requiring distinct visual information on the object's orientation or size. If so, an illusory change in size need not influence the distance between thumb and fingers when reaching out for an object. However, as the object's size is used to estimate its weight, the illusory change in size should influence the force that is exerted to lift the object. To find out whether this is so, eight subjects were asked to pick up brass disks from a fixed position straight in front of them. The illusory change in size was brought about by presenting five converging lines in two different configurations under the disks. As predicted, the illusion influenced the force used to lift the disks, but not the distance between the subjects' thumbs and fingers when reaching for the disks.

Hand Strength↗
PubMed1996

Perceived motion in depth.

We examine how various sources of information contribute to the percept of motion in depth. Subjects were presented with targets moving in depth, and were asked to judge their velocities and final positions. On each presentation, the target's position relative to the two eyes (target vergence), the size of the target's retinal image and the difference in this image's position relative to that of the background in the two eyes (relative disparity), each either changed as they normally would for a target moving at a fixed speed towards the observer, or did not change at all. Subjects' judgements for various such combinations show that all three sources of information influence both the perceived velocity of motion in depth and the final perceived position, but in different ways. This is not too surprising, because the assumptions that the use of each source of information are based on, are different for the two tasks. We propose that the way the different sources are combined is governed by the likelihood of the assumptions, that are required to use that information, being true under the given circumstances.

Depth Perception↗
PubMed1996

The special role of distant structures in perceived object velocity.

How do we judge an object's velocity when we ourselves are moving? Subjects compared the velocity of a moving object before and during simulated ego-motion. The simulation consisted of moving the visible environment relative to the subject's eye in precisely the way that a static environment would move relative to the eye if the subject had moved. The ensuing motion of the background on the screen influenced the perceived target velocity. We found that the motion of the "most distant structure" largely determined the influence of the moving background. Relying on retinal motion relative to that of distant structures is usually a reliable method for accounting for rotations of the eye. It provides an estimate of the object's movement, relative to the observer. This strategy for judging object motion has the advantage that it does not require metric information on depth or detailed knowledge of one's own motion.

Depth Perception↗
PubMed1996

Is judging time-to-contact based on 'tau'?

An investigation was undertaken into whether judgments of time-to-contact between a laterally moving object and a bar are based on the direct perception of an optical variable (tau), or on the ratio between the perceived distance and perceived velocity of the object. A moving background was used to induce changes in the perceived velocities without changing the optical variables that specify time-to-contact. Background motion induced large systematic errors in the estimated time-to-contact. It is concluded that the judgment of time-to-contact is primarily based on the ratio between the perceived distance and the perceived velocity, and not on tau.

Acceleration↗
PubMed1995

Prediction of a moving target's position in fast goal-directed action.

Subjects made fast goal-directed arm movements towards moving targets. In some cases, the perceived direction of target motion was manipulated by moving the background. By comparing the trajectories towards moving targets with those towards static targets, we determined the position towards which subjects were aiming at movement onset. We showed that this position was an extrapolation in the target's perceived direction from its position at that moment using its perceived direction of motion. If subjects were to continue to extrapolate in the perceived direction of target motion from the position at which they perceive the target at each instant, the error would decrease during the movements. By analysing the differences between subjects' arm movements towards targets moving in different (apparent) directions with a linear second-order model, we show that the reduction in the error that this predicts is not enough to explain how subjects compensate for their initial misjudgments.

Arm↗
PubMed1995

Moving one's finger to a visually specified position: target orientation influences the finger's path.

It has previously been shown that, when subjects are instructed to move their finger slowly from one point to another the finger follows a path that deviates systematically from a straight line connecting the two points. The deviation depends on the angle between this fictive line and a line connecting the subject's finger with his body. In the present study, we examined whether the deviation also depends on the target's orientation. In two experiments, subjects were instructed to move a finger slowly towards five targets. We recorded the finger's movements. In one experiment, the targets were aligned. In the other, they were oriented radially around the starting point. Otherwise, conditions were the same. The difference in target orientation influenced the finger's path. Most importantly, when the targets were oriented radially around the starting point, the finger's path was straight. We conclude that pointing is more than moving the finger to a specified position.

Fingers↗
PubMed1995

Perception and action are based on the same visual information: distinction between position and velocity.

Ss were presented with spiders running from left to right at various velocities over a structured background. Motion of the background influenced the perceived velocity of the spider: Motion of the background in the opposite direction than the spider increased the perceived velocity. The perceived position of the spider was not influenced by background motion. Ss were asked to hit the spiders as quickly as possible. Fast spiders were hit with a higher velocity than slow spiders. The same effect was found if the spiders only differed in apparent velocity, induced by motion of the background. The trajectory of the hit was not influenced by motion of the background. The authors concluded that although velocity is nothing but the change of position in time, velocity and position are processed independently. Furthermore, these two separately processed sources of information are used in both perception and action.

Acceleration↗
PubMed1995

Simultaneous colour constancy revisited: an analysis of viewing strategies.

We examined whether matching instructions influenced the eye movements that subjects made during a colour constancy experiment. The instructions changed the average duration of exposure to the spectrally biased surround. We also measured the influence that small changes in exposure duration have on the perceived colour. Eye movement and adaptation data were combined to predict differences in colour matches. Two of the five subjects showed an instructional effect that was much larger than that predicted. Analysis of the eye movements, and an experiment with dynamic surrounding colours, reveal that several viewing strategies do not account for the influence of the instruction.

Adaptation, Ocular↗
PubMed1994

Why two eyes are better than one for judgements of heading.

Are two eyes needed for judging direction of self-motion? Traditional analyses stress that the pattern of optic flow in one eye is sufficient. The main difficulty is how to deal with the eye or head rotation. Extraretinal signals help, but humans can also discount the effect of rotation purely on the basis of monocular flow provided the scene contains depth. Depth differences give rise to changing binocular disparities when the observer moves. These disparities are ignored in monocular theories of judgements of heading. Using computer generated displays, we investigated whether stereoscopic presentation improves heading judgements for conditions that pose problems to the monocular observer. We found that adding disparities to simulated ego-motion through a cloud of dots made heading judgements up to four times more tolerant to motion noise. The same improvement was found when the disparities specify the initial distances throughout the motion sequence. We conclude that binocular disparities improve judgements of heading by imposing a depth order on the elements of the scene, not because they provide additional information on the elements' motion in depth.

Depth Perception↗
PubMed1994

Judging object velocity during smooth pursuit eye movements.

Our tendency to constantly shift our gaze and to pursue moving objects with our eyes introduces obvious problems for judging objects' velocities. The present study examines how we deal with these problems. Specifically, we examined when information on rotations (such as eye movements) is obtained from retinal, and when from extra-retinal sources. Subjects were presented with a target moving across a textured background. Moving the background allowed us to manipulate the retinal information on rotation independently of the extra-retinal information. The subjects were instructed to pursue the target with their eyes. At some time during the presentation the target's velocity could change. We determined how various factors influence a subject's perception of such changes in velocity. Under more or less natural conditions, there was no change in perceived target velocity as long as the relative motion between target and background was maintained. However, experiments using conditions that are less likely to occur outside the laboratory reveal how extra-retinal signals are involved in velocity judgements.

Humans↗
PubMed1994

Humans combine the optic flow with static depth cues for robust perception of heading.

The retinal flow during normal locomotion contains components due to rotation and translation of the observer. The translatory part of the flow-pattern is informative of heading, because it radiates outward from the direction of heading. However, it is not directly accessible from the retinal flow. Nevertheless, humans can perceive their direction of heading from the compound retinal flow without need for extra-retinal signals that indicate the rotation. Two classes of models have been proposed to explain the visual decomposition of the retinal flow into its constituent parts. One type relies on local operations to remove the rotational part of the flow field. The other type explicitly determines the direction and magnitude of the rotation from the global retinal flow, for subsequent removal. According to the former model, nearby points are most reliable for estimating one's heading. In the latter type of model the quality of the heading estimate depends on the accuracy with which the ego-rotation is determined and is therefore most reliable when based on the most distant points. We report that subjects underestimate the eccentricity of heading, relative to the fixated point in the ground plane, when the visible range of the ground plane is reduced. Moreover we find that in perception of heading, humans can tolerate more noise than the optimal observer (in the least squares sense) would do if only using optic flow. The latter finding argues against both schemes because ultimately both classes of model are limited in their noise tolerance to that of the optimal observer, which uses all information available in the optic flow. Apparently humans use more information than is present in the optic flow. Both aspects of human performance are consistent with the use of static depth information in addition to the optic flow to select the most distant points. Processing of the flow of these selected points provides the most reliable estimate of the ego-rotation. Subsequent estimates of the heading direction, obtained from the translatory component of the flow, are robust with respect to noise. In such a scheme heading estimates are subject to systematic errors, similar to those reported, if the most distant points are not much further away than the fixation point, because the ego-rotation is underestimated.

Algorithms↗
PubMed1994

The difference between the perception of absolute and relative motion: a reaction time study.

We used a reaction-time paradigm to examine the extent to which motion detection depends on relative motion. In the absence of relative motion, the responses could be described by a simple model based on the detection of a fixed change in position. If relative motion was present, the responses could be modelled using characteristics of motion detectors. Comparing reaction times when relative and absolute velocity are equal with ones when relative velocity is twice the absolute velocity reveals that these detectors measure relative motion.

Humans↗
PubMed1994

Detecting changes in one's own velocity from the optic flow.

Experiments were designed to establish whether we can use the optic flow to detect changes in our own velocity. Subjects were presented with simulations of forward motion across a flat surface. They were asked to respond as quickly as possible to a step increase in simulated ego-velocity. The smallest change for which subjects could respond within 500 ms was determined. At realistic simulated speeds of locomotion, the simulated ego-velocity had to increase by about 50%. The threshold for detecting changes in simulated ego-velocity was hardly better than the threshold for detecting other changes in the acceleration of the dots on the screen. It made little difference whether the surface across which the subject appeared to move was built up of dots, lines, or triangles; neither did it matter whether subjects saw the same image with both eyes, or whether the simulation was presented in stereoscopic depth. The results show that we are very poor at detecting changes in our own velocity on the basis of visual input alone.

Acceleration↗
PubMed1993

Judging an object's velocity when its distance changes due to ego-motion.

This paper examines how one accounts for ones own movements when judging the velocity of a moving object, with emphasis on ego-motion perpendicular to the direction in which the object is moving. The "object" was a square that was tracked with smooth pursuit eye movements as it moved horizontally across a computer screen. Half-way through the presentation, the image on the screen changed in a manner simulating ego-motion in depth. At the same time, the speed with which the square moved across the screen also changed. Subjects were asked to report whether the target moved faster, at the same speed, or more slowly after the simulated ego-motion. The change in target velocity that was required for it to appear to continue to move at the same speed was determined for simulations containing different aspects of the information that is normally at our disposal. The results show that the change in the size of the image of the target, the expansion or contraction of the image of the surrounding, and differences in target motion between the two eyes (giving rise to vergence eye movements), all contribute to rendering the perceived object velocity independent of ego-motion.

Depth Perception↗
PubMed1992

Development of visually guided behaviour requires oriented contours.

Kittens do not learn to use visual information to guide their behaviour if they are deprived of the optic flow that accompanies their own movements. We show that the optic flow that is required for developing visually guided behaviour is derived from changes in contour orientations, rather than from velocity patterns. We used several tests to assess visually guided behaviour. The performance of kittens that had only been allowed to see isolated dots of light was indistinguishable from that of kittens that had received no visual exposure at all. Kittens that had seen streaks of light performed better on several tasks. We discuss this finding with relation to the visual pathways that are presumably involved.

Animals↗