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Biomedical subjects

M Lappe

Publications and source records attributed to M Lappe.

At least 37 records · Page 2Linked to original sources
PubMed1999

The use of optical velocities for distance discrimination and reproduction during visually simulated self motion.

Successful navigation through an environment requires precise monitoring of direction and distance traveled ("path integration" or "dead reckoning"). Previous studies in blindfolded human subjects showed that velocity information arising from vestibular and somatosensory signals can be used to reproduce passive linear displacements. In these studies, visual information was excluded as sensory cue. Yet, in our everyday life, visual information is very important and usually dominates vestibular and somatosensory cues. In the present study, we investigated whether visual signals can be used to discriminate and reproduce simulated linear displacements. In a first set of experiments, subjects viewed two sequences of linear motion and were asked in a 2AFC task to judge whether the travel distance in the second sequence was larger or shorter than in the first. Displacements in either movement sequence could be forward (f) or backward (b). Subjects were very accurate in discriminating travel distances. Average error was less than 3% and did not depend on displacements being into the same (ff, bb) or opposite direction (fb, bf). In a second set of experiments, subjects had to reproduce a previously seen forward motion (passive condition), either in light or in darkness, i.e., with or without visual feedback. Passive displacements had different velocity profiles (constant, sinusoidal, complex) and speeds and were performed across a textured ground plane, a 2-D plane of dots or through a 3-D cloud of dots. With visual feedback, subjects reproduced distances accurately. Accuracy did not depend on the kind of velocity profile in the passive condition. Subjects tended to reproduce distance by replicating the velocity profile of the passive displacement. Finally, in the condition without visual feedback, subjects reproduced the shape of the velocity profile, but used much higher speeds, resulting in a substantial overshoot of travel distance. Our results show that visual, vestibular, and somatosensory signals are used for path integration, following a common strategy: the use of the velocity profile during self-motion.

Adult↗
PubMed1999

Ocular responses to radial optic flow and single accelerated targets in humans.

Self-movement in a structured environment induces retinal image motion called optic flow. Optic flow on one hand provides information about the direction of self-motion. On the other hand optic flow presents large field visual motion which will elicit eye movements for the purpose of image stabilization. We investigated oculomotor behavior in humans during the presentation of radial optic flow fields which simulated forward or backward self-motion. Different conditions and oculomotor tasks were compared. In one condition, subjects had to actively pursue single dots in a radial flow pattern. In a second condition, subjects had to pursue single dots over a dark background. These dots accelerated or decelerated similar to single dots in radial optic flow. In a third condition, subjects were asked to passively view the entire optic flow stimulus. Smooth pursuit eye movements with high gain were observed when dots were actively pursued. This was true for single dots moving over a homogeneous background and for single dots in the optic flow. Passive viewing of optic flow stimuli evoked eye movements that resembled an optokinetic nystagmus. Slow phase eye movements tracked the motion of elements in the optic flow. Gain was low for simulated forward self-motion (expanding optic flow) and high for simulated backward movement self-motion (contracting optic flow). Thus, voluntary pursuit and passive optokinetic responses yielded different gain for the tracking of elements of an expanding optic flow pattern. During passive viewing of the optic flow stimulus, gaze was usually at or near the focus of radial flow. Our results give insights into the oculomotor performances and needs for image stabilization during self-motion and in the role of gaze strategy for the detection of the direction of heading.

Eye Movements↗
PubMed1999

Temporal recruitment along the trajectory of moving objects and the perception of position.

The trajectory of a moving object provides information about its velocity, direction and position. This information can be used to enhance the visual system's ability to detect changes in these parameters. We show that the visibility of the trajectory of a moving object influences the perception of its position. This form of temporal recruitment builds up on a long timescale of approximately 500 ms. Temporary occlusion of the trajectory during this time period reduces recruitment, but does not abolish it. Moreover, we found no spatial restrictions on recruitment on the scale of 10 degrees of arc. When the position of objects on trajectories with different degrees of visibility are compared, this recruitment effect causes spatial offsets. This leads to a visual illusion in which the position of moving objects is misperceived.

Attention↗
PubMed1999

How stereovision interacts with optic flow perception: neural mechanisms.

Optic flow, the global visual motion experienced during self-movement, supplies important navigational information. Optic flow analysis in the visual system is aided by several other visual and non-visual signals. Recent psychophysical findings demonstrate an interaction of optic flow perception and stereoscopic depth vision. Retinal disparity strongly affects an optic flow illusion, which can be related to the mechanisms of visual self-motion detection. To investigate the neuronal basis of this interaction, we tested several hypotheses by introducing different disparity contributions in a detailed neurobiological model of optic flow processing in monkey cortex. The disparity-dependent modification, which accounted best for the data suggests a specific contribution of a subset of stereoscopically modulated cortical neurons present in areas MT and MST.

Journal Article↗
PubMed1999

Perception of self-motion from visual flow.

Accurate and efficient control of self-motion is an important requirement for our daily behavior. Visual feedback about self-motion is provided by optic flow. Optic flow can be used to estimate the direction of self-motion ('heading') rapidly and efficiently. Analysis of oculomotor behavior reveals that eye movements usually accompany self-motion. Such eye movements introduce additional retinal image motion so that the flow pattern on the retina usually consists of a combination of self-movement and eye movement components. The question of whether this 'retinal flow' alone allows the brain to estimate heading, or whether an additional 'extraretinal' eye movement signal is needed, has been controversial. This article reviews recent studies that suggest that heading can be estimated visually but extraretinal signals are used to disambiguate problematic situations. The dorsal stream of primate cortex contains motion processing areas that are selective for optic flow and self-motion. Models that link the properties of neurons in these areas to the properties of heading perception suggest possible underlying mechanisms of the visual perception of self-motion.

Journal Article↗
PubMed1999

Optic flow illusion and single neuron behaviour reconciled by a population model.

Radial patterns of optic flow contain a centre of expansion that indicates the observer's direction of self-movement. When the radial pattern is viewed with transparently overlapping unidirectional motion, the centre of expansion appears to shift in the direction of the unidirectional motion [Duffy, C.J. & Wurtz, R.H. (1993) Vision Res., 33, 1481-1490]. Neurons in the medial superior temporal (MST) area of monkey cerebral cortex are thought to mediate optic flow analysis, but they do not shift their responses to parallel the illusion created by transparent overlap. The population-based model of optic flow analysis proposed by Lappe and Rauschecker replicates the illusory shift observed in perceptual studies [Lappe, M. & Rauschecker, J.P. (1995) Vision Res., 35, 1619-1631]. We analysed the behaviour of constituent neurons in the model, to gain insight into neuronal mechanisms underlying the illusion. Single model neurons did not show the illusory shift but rather graded variations of their response specificity. The shift required the aggregate response of the population. We compared the model's predictions about the behaviour of single neurons with the responses recorded from area MST. The predicted distribution of overlap effects agreed with that observed in area MST. The success of the population-based model in predicting the illusion and the neuronal behaviour suggests that area MST uses the graded responses of single neurons to create a population response that supports optic flow perception.

Animals↗
PubMed1999

Dynamical use of different sources of information in heading judgments from retinal flow.

The optic flow arising in the eyes of an observer during self-motion is influenced by the occurrence of eye movements. The determination of heading during eye movements may be based on the pattern of retinal image motion (the retinal flow) or on an additional use of an extraretinal eye-movement signal. Previous research has presented support for either of these hypotheses, depending on the movement geometry and the layout of the visual scene. A special situation in which all previous studies unequivocally have agreed that an extra-retinal signal is required occurs when the visual scene consists of a single frontoparallel plane. In this situation eye movements shift the center of expansion on the retina to a location that does not correspond to the direction of self-movement. Without extraretinal input, human observers confuse the center of expansion with their heading and show a systematical heading estimation error. We reexamined and further investigated this situation. We presented retinal flow stimuli on a large projection screen in the absence of extra-retinal input and varied stimulus size, presentation duration, and orientation of the plane. In contrast to previous studies we found that in the case of a perpendicular approach toward the plane, heading judgments can be accurate. Accurate judgments were observed when the field of view was large (90 degrees x 90 degrees) and the stimulus duration was short (< or = 0.5 s). For a small field of view or a prolonged stimulus presentation, a systematic and previously described error appeared that is related to the radial structure of the flow field and the location of the center of expansion. An oblique approach toward the plane results in an ambiguous flow field with two mathematically possible solutions for heading. In this situation, when the stimulus duration was short, subjects reported a perceived heading midway between these two solutions. For longer flow sequences, subjects again chose the center of expansion. Our results suggest a dynamical change in the analysis or interpretation of retinal flow during heading perception.

Adult↗
PubMed1998

Interaction of stereo vision and optic flow processing revealed by an illusory stimulus.

The influence of stereoscopic vision on the perception of optic flow fields was investigated in experiments based on a recently described illusion. In this illusion, subjects perceive a shift of the center of an expanding optic flow field when it is transparently superimposed by a unidirectional motion pattern. This illusory shift can be explained by the visual system taking the presented flow pattern as a certain self-motion flow field. Here we examined the dependence of the illusory transformation on differences in depth between the two superimposed motion patterns. Presenting them with different relative binocular disparities, we found a strong variation in the magnitude of the illusory shift. Especially when translation was in front of expansion, a highly significant decrease of the illusory shift occurred, down to 25% of its magnitude at zero disparity. These findings confirm the assumption that the motion pattern is interpreted as a self-motion flow field. In a further experiment we presented monocular depth cues by changing dot size and dot density. This caused a reduction of the illusory shift which is distinctly smaller than under stereoscopic presentation. We conclude that the illusory optic flow transformation is modified by depth information, especially by binocular disparity. The findings are linked to the phenomenon of induced motion and are related to neurophysiology.

Adult↗
PubMed1998

Factors influencing hospital transport of patients in continuing cardiac arrest.

STUDY OBJECTIVE: Prior research has established the futility of continued resuscitation efforts for patients in cardiac arrest who fail to respond to out-of-hospital advanced cardiac life support. Determination of both medical and nonmedical factors resulting in the transport of patients in continuing cardiac arrest to the hospital may encourage the development of new systems or strategies to increase the appropriateness of these transports. METHODS: The attending paramedic completed a prospective survey after unsuccessful resuscitation efforts in our urban, hospital-based, two-tier emergency medical services (EMS) system. All nontraumatic adult arrests were included unless they were clearly noncardiac in nature. RESULTS: Paramedics responded to 259 cardiac arrests between September 12, 1996, and April 31, 1997. Seventy-nine patients were pronounced dead without resuscitation efforts. Of the remaining 180 patients, 44 had return of spontaneous circulation and were transported to the hospital, 68 were pronounced dead in the field, and 68 were transported to the hospital in continuing cardiac arrest. The 68 patients transported while in cardiac arrest are the focus of this study. Rare problems with field termination were identified. Reasons for transport of the 68 patients in continuing cardiac arrest included arrest in ambulance or going to ambulance (n = 6), arrest in a public place (n = 17), environmental factors (n = 6), road hazard to paramedics (n = 1), possible reversible cause (n = 4), persistent ventricular dysrhythmia (n = 5), no intravenous access (n = 5), airway difficulties (n = 5), family unable to accept field termination (n = 3), cultural or language barrier (n = 1), EMS physician ordered transport (n = 1), and obesity (n = 1). A protocol allowing pronouncement of death in the ambulance and transport of the body to a designated area could have prevented lights-and-siren transport to the emergency department in 24 of the 68 cases. CONCLUSION: Factors other than medical ones often influence the decision to transport patients in continuing cardiac arrest. In our urban system, physician, medical examiner, and paramedic education and protocols were needed to aid decision-making in this situation.

Cardiopulmonary Resuscitation↗
PubMed1998

The position of moving objects.

Moving objects occupy a range of positions during the period of integration of the visual system. Nevertheless, a unique position is usually observed. We investigate how the trajectory of a stimulus influences the position at which the object is seen. It has been shown before that moving objects are perceived ahead of static objects shown at the same place and time. We show here that this perceived position difference builds up over the first 500 ms of a visible trajectory. Discontinuities in the visual input reduce this buildup when the presentation frequency of a stimulus with a duration of 42 ms falls below 16 Hz. We interpret this relative mislocalization in terms of a spatiotemporal-filtering model. This model fits well with the data, given two assumptions. First, the position signal persists even though the objects are no longer visible and, second, the perceived distance is a 500 ms average of the difference of these position signals.

Computer Graphics↗
PubMed1998

Optokinetic eye movements elicited by radial optic flow in the macaque monkey.

We recorded spontaneous eye movements elicited by radial optic flow in three macaque monkeys using the scleral search coil technique. Computer-generated stimuli simulated forward or backward motion of the monkey with respect to a number of small illuminated dots arranged on a virtual ground plane. We wanted to see whether optokinetic eye movements are induced by radial optic flow stimuli that simulate self-movement, quantify their parameters, and consider their effects on the processing of optic flow. A regular pattern of interchanging fast and slow eye movements with a frequency of 2 Hz was observed. When we shifted the horizontal position of the focus of expansion (FOE) during simulated forward motion (expansional optic flow), median horizontal eye position also shifted in the same direction but only by a smaller amount; for simulated backward motion (contractional optic flow), median eye position shifted in the opposite direction. We relate this to a change in Schlagfeld typically observed in optokinetic nystagmus. Direction and speed of slow phase eye movements were compared with the local flow field motion in gaze direction (the foveal flow). Eye movement direction matched well the foveal motion. Small systematic deviations could be attributed to an integration of the global motion pattern. Eye speed on average did not match foveal stimulus speed, as the median gain was only approximately 0.5-0.6. The gain was always lower for expanding than for contracting stimuli. We analyzed the time course of the eye movement immediately after each saccade. We found remarkable differences in the initial development of gain and directional following for expansion and contraction. For expansion, directional following and gain were initially poor and strongly influenced by the ongoing eye movement before the saccade. This was not the case for contraction. These differences also can be linked to properties of the optokinetic system. We conclude that optokinetic eye movements can be elicited by radial optic flow fields simulating self-motion. These eye movements are linked to the parafoveal flow field, i.e., the motion in the direction of gaze. In the retinal projection of the optic flow, such eye movements superimpose retinal slip. This results in complex retinal motion patterns, especially because the gain of the eye movement is small and variable. This observation has special relevance for mechanisms that determine self-motion from retinal flow fields. It is necessary to consider the influence of eye movements in optic flow analysis, but our results suggest that direction and speed of an eye movement should be treated differently.

Animals↗
PubMed1996

Functional consequences of an integration of motion and stereopsis in area MT of monkey extrastriate visual cortex.

Experimental evidence from neurophysiological recordings in the middle temporal (MT) area of the macaque monkey suggests that motion-selective cells can use disparity information to separate motion signals that originate from different depths. This finding of a cross-talk between different visual channels has implications for the understanding of the processing of motion in the primate visual system and especially for behavioral tasks requiring the determination of global motion. In this paper, the consequences for the analysis of optic flow fields are explored. A network model is presented that effectively uses the disparity sensitivity of MT-like neurons for the reduction of noise in optic flow fields. Simulations reproduce the recent psychophysical finding that the robustness of the human optic flow processing system is improved by stereoscopic depth information, but that the use of this information depends on the structure of the visual environment.

Animals↗
PubMed1996

Optic flow processing in monkey STS: a theoretical and experimental approach.

How does the brain process visual information about self-motion? In monkey cortex, the analysis of visual motion is performed by successive areas specialized in different aspects of motion processing. Whereas neurons in the middle temporal (MT) area are direction-selective for local motion, neurons in the medial superior temporal (MST) area respond to motion patterns. A neural network model attempts to link these properties to the psychophysics of human heading detection from optic flow. It proposes that populations of neurons represent specific directions of heading. We quantitatively compared single-unit recordings in area MST with single-neuron simulations in this model. Predictions were derived from simulations and subsequently tested in recorded neurons. Neuronal activities depended on the position of the singular point in the optic flow. Best responses to opposing motions occurred for opposite locations of the singular point in the visual field. Excitation by one type of motion is paired with inhibition by the opposite motion. Activity maxima often occur for peripheral singular points. The averaged recorded shape of the response modulations is sigmoidal, which is in agreement with model predictions. We also tested whether the activity of the neuronal population in MST can represent the directions of heading in our stimuli. A simple least-mean-square minimization could retrieve the direction of heading from the neuronal activities with a precision of 4.3 degrees. Our results show good agreement between the proposed model and the neuronal responses in area MST and further support the hypothesis that area MST is involved in visual navigation.

Animals↗
PubMed1996

Neuronal responses in the motion pathway of the macaque monkey to natural optic flow stimuli.

Neurones in higher visual motion areas in the superior temporal sulcus (STS) of the macaque monkey respond to abstract random dot optic flow stimuli. Higher motion areas may not only represent, but in a next computational stage also analyse the flow field to determine, for instance, the direction of heading for navigation purposes. Real world visual scenes differ in several aspects from these abstract optic flow stimuli. We tested the neuronal response to naturalistic optic flow stimuli which simulated egomotion in different virtual environments and contained different numbers of visual cues. Neuronal activity depended mainly on the position of the focus of expansion rather than on other visual cues. This finding supports the hypothesis that higher motion areas within the STS analyse optic flow in natural scenes and can thus signal the direction of heading.

Animals↗
PubMed1996

Visual inspection of three-dimensional objects by human observers.

Eye movements are an important aid in active visual exploration of the environment and in performing behavioural tasks. Eye movements might also play a role in human perception of three-dimensional (3-D) objects. Eye-movement strategies were investigated when humans inspected and memorised 3-D objects. Subjects were instructed to memorise the 3-D structure of parts of statues of human figures placed on a turntable free to rotate through 360 degrees. Eye movements and turning behaviour were recorded. Different turning and eye-movement strategies could be observed. Subjects showed individual turning behaviours that were reproducible between trials. Turning strategies ranged from focusing on only a limited number of perspective views to almost continuously rotating the object with only short stops. On average twelve-thirteen views were inspected during memorising. Eye movements also revealed individual strategies. Fixation locations within each inspection view ranged from either closely spaced on isolated parts of the object to distributed over the whole view with large saccades in between. Eye movements were often directed to the same details from different perspectives. The differences in turning and viewing strategy also resulted in differences in the ability to recognise parts of the object later on. In general, successful later recognition required that the subject actually fixated the part to be recognised. A strategy of thoroughly inspecting the object with a series of closely spaced fixations from only a limited number of viewpoints led to best recognition rates. This was especially true for two subjects trained in fine arts with prior experiences in modelling. The results support models of viewpoint-dependent object recognition with viewer-centred, two-dimensional representations of 3-D objects.

Adult↗
PubMed1995

Motion anisotropies and heading detection.

In motion-processing areas of the visual cortex in cats and monkeys, an anisotropic distribution of direction selectivities displays a preference for movements away from the fovea. This 'centrifugal bias' has been hypothetically linked to the processing of optic flow fields generated during forward locomotion. In this paper, we show that flow fields induced on the retina in many natural situations of locomotion of higher mammals are indeed qualitatively centrifugal in structure, even when biologically plausible eye movements to stabilize gaze on environmental targets are performed. We propose a network model of heading detection that carries an anisotropy similar to the one found in cat and monkey. In simulations, this model reproduces a number of psychophysical results of human heading detection. It suggests that a recently reported human disability to correctly identify the direction of heading from optic flow when a certain type of eye movement is simulated might be linked to the noncentrifugal structure of the resulting retinal flow field and to the neurophysiological anisotropies.

Algorithms↗
PubMed1995

An illusory transformation in a model of optic flow processing.

We present results from computer simulations of a biologically plausible model of heading detection in the visual motion pathway of higher mammals. These simulations are closely related to a recently discovered visual illusion in optic flow processing in humans. The model reproduces the results described for humans and suggests a possible explanation, namely that humans interpret the illusory stimuli in terms of egomotion. It provides further indication that the visual system makes use of visual information to cope with eye movement effects in dealing with optic flow.

Algorithms↗