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Biomedical subjects

H A Mallot

Publications and source records attributed to H A Mallot.

15 recordsLinked to original sources
PubMed2005

Rats are able to navigate in virtual environments.

Virtual reality (VR) systems are useful tools that enable users to alter environmental settings and the location of landmarks in an accurate and fast way. Primates have been shown to be able to navigate in virtual environments. For rodents, however, all previous attempts to develop VR systems in which rats behave in the same way as in corresponding 3-D environments have failed. The question arises as to whether, in principle, rodents can be trained to navigate in a properly designed virtual environment (VE), or whether this peculiarity is limited to primates and humans. We built a virtual reality set-up that takes the wide-angle visual system of rats into account. We show for the first time that rats learn spatial tasks in this VE quite readily. This set-up opens up new opportunities for investigations of information processing in navigation (e.g. the importance of optic flow or vestibular input).

Animals↗
PubMed2000

Route navigating without place recognition: what is recognised in recognition-triggered responses?

The use of landmark information in a route-navigation task has been investigated in a virtual environment. After learning a route, subjects were released at intermediate points along the route and asked to indicate the next movement direction required to continue the route. At each decision point, three landmarks were present, one of which was viewed centrally and two which appeared in the periphery of the visual field when approaching the decision point. In the test phase, landmarks could be replaced either within or across places. If all landmarks combined into a new place had been associated with the same movement direction during training, subjects performed as in the control condition. This indicates that they did not need to recognise places as configurations of landmarks. If, however, landmarks that had been associated with conflicting movement directions during training were combined, subjects' performance was reduced. We conclude that local views and objects are recognised individually and that the associated directions are combined in a voting scheme. No evidence was found for a recognition of places as panoramic views or configurations of objects.

Humans↗
PubMed1998

Navigation and acquisition of spatial knowledge in a virtual maze.

Spatial behavior in humans and animals includes a wide variety of behavioral competences and makes use of a large number of sensory cues. Here we studied the ability of human subjects to search locations, to find shortcuts and novel paths, to estimate distances between remembered places, and to draw sketch maps of the explored environment; these competences are related to goal-independent memory of space, or cognitive maps. Information on spatial relations was restricted to two types: a visual motion sequence generated by simulated movements in a virtual maze and the subject's own movement decisions defining the path through the maze. Visual information was local (i.e., no global landmarks or compass information was provided). Other position and movement information (vestibular or proprioceptive) was excluded. The amount of visual information provided was varied over four experimental conditions. The results indicate that human subjects are able to learn a virtual maze from sequences of local views and movements. The information acquired is local, consisting of recognized positions and movement decisions associated to them. Although simple associations of this type can be shown to be present in some subjects, more complete configurational knowledge is acquired as well. The results are discussed in a view-based framework of navigation and the representation of spatial knowledge by means of a view graph.

Adolescent↗
PubMed1997

Spatial scale in stereo and shape from shading: image input, mechanisms, and tasks.

The problem of spatial scale in depth perception can be considered at three stages: image input, mechanisms, and depth descriptors specific for certain tasks. A review is presented of a number of earlier experiments supporting the distinction between a coarse, correlation-based mechanism and a feature-matching mechanism of stereopsis in terms of their respective inputs and outputs. In order to measure the influence of the correlation mechanism on the perception of three-dimensional shape, a shape-probe experiment was designed. For smooth intensity profiles with constant disparity, the results show that perceived shape is largely independent of overall disparity but does make use of shape from shading as well as the assumption that brighter parts are in front (proximity-luminance covariance).

Depth Perception↗
PubMed1996

Is correspondence search in human stereo vision a coarse-to-fine process?

One possible strategy for the solution of the correspondence problem of stereo matching is the coarse-to-fine mechanism: The matching process starts with a lowpass-filtered version of the stereogram where only a few, high-contrast image features can be extracted and the probability of false matches is therefore low. In subsequent stages, information from higher spatial frequencies is used gradually to improve the correspondence data obtained on the coarser scales. Coarse-to-fine strategies predict that information from coarse scale is used to disambiguate matching information on finer scales. We have tested this prediction by means of the wallpaper illusion using periodic intensity-profiles with different matching ambiguities on different spatial scale. Our psychophysical experiments show (i) that unambiguous information at coarse scale is not always used to disambiguate finer scale information, (ii) that unambiguous fine-scale information can be used to disambiguate coarse-scale information and (iii) that low spatial frequency is more efficient for disambiguation than higher frequency. We conclude that the human stereo vision system does not always proceed from coarse to fine. As an alternative scheme for scale-space integration, we discuss more symmetric schemes such as maximum likelihood combinations of data from different channels.

Adult↗
PubMed1996

A psychophysical and computational analysis of intensity-based stereo.

We describe two psychophysical experiments testing predictions of the square difference mechanism we have previously proposed for intensity-based stereo. Experiment 1 assesses the relative contributions of disparity and contrast to intensity-based stereo by measuring detection thresholds. The product of disparity and contrast at threshold is shown to be constant. In experiment 2, we measure quantitatively the global depth position perceived in stereograms of curved, smoothly shaded surfaces. The results show that disparity averaging over the surface involves a contrast-dependent weighting function. The results from both experiments are consistent with predictions derived from the square difference mechanism. The relation of this mechanism to feature correspondence stereopsis and shape-from-shading is discussed and a general framework for assessing the modularity of stereopsis is presented.

Adult↗
PubMed1996

Population networks: a large-scale framework for modelling cortical neural networks.

Artificial neural networks are usually built on rather few elements such as activation functions, learning rules, and the network topology. When modelling the more complex properties of realistic networks, however, a number of higher-level structural principles become important. In this paper we present a theoretical framework for modelling cortical networks at a high level of abstraction. Based on the notion of a population of neurons, this framework can accommodate the common features of cortical architecture, such as lamination, multiple areas and topographic maps, input segregation, and local variations of the frequency of different cell types (e.g., cytochrome oxidase blobs). The framework is meant primarily for the simulation of activation dynamics; it can also be used to model the neural environment of single cells in a multiscale approach.

Animals↗
PubMed1996

Disparity-evoked vergence is driven by interocular correlation.

Disparity-evoked vergence is studied in stereograms showing one or two depth planes which are defined by isolated dots of varying density and contrast. Vergence position immediately after stimulus presentation was measured using dichoptic nonius lines. Since the stimulus was not visible after the onset of the vergence movement, the experiment accesses the initiation of vergence rather than its eventual result. In the unequivocal stimuli (one depth plane), elicited vergence tends to reduce disparity. Disparities of 0.5-1 deg are most effective which is in accordance with earlier findings. If two depth planes are presented, elicited vergence lies between the two planes, approaching the plane with higher dot density and/or dot contrast. In quantitative measurements, we show that the depth-averaging mechanism uses signal power per depth plane as a weight. Therefore, the relative pulling strength of dot density compared with dot contrast follows a power law with exponent 2. We propose a population code for vergence control based on disparity-tuned pools of units.

Adult↗
PubMed1995

Human stereovision without localized image features.

Many theories of human stereovision are based on feature matching and the related correspondence problem. In this paper, we present psychophysical experiments indicating that localized image features such as Laplacian zerocrossings, intensity extrema, or centroids are not necessary for binocular depth perception. Smooth one-dimensional intensity profiles were combined into stereograms with mirror-symmetric half-images such that these localized image features were either absent or did not carry stereo information. In a discrimination task, subjects were asked to distinguish between stereograms differing only by an exchange of these half-images (ortho- vs. pseudoscopic stereograms). In a depth ordering task, subjects had to judge which of the two versions appeared in front. Subjects are able to solve both tasks even in the absence of the mentioned image features. The performance is compared to various possible stereo mechanisms. We conclude that localized image features and the correspondences between them are not necessary to perceive stereoscopic depth. One mechanism accounting for our data is correlation or mean square difference.

Contrast Sensitivity↗
PubMed1991

Inverse perspective mapping simplifies optical flow computation and obstacle detection.

We present a scheme for obstacle detection from optical flow which is based on strategies of biological information processing. Optical flow is established by a local "voting" (non-maximum suppression) over the outputs of correlation-type motion detectors similar to those found in the fly visual system. The computational theory of obstacle detection is discussed in terms of space-variances of the motion field. An efficient mechanism for the detection of disturbances in the expected motion field is based on "inverse perspective mapping", i.e., a coordinate transform or retinotopic mapping applied to the image. It turns out that besides obstacle detection, inverse perspective mapping has additional advantages for regularizing optical flow algorithms. Psychophysical evidence for body-scaled obstacle detection and related neurophysiological results are discussed.

Algorithms↗
PubMed1990

Binocular vergence influences the assignment of stereo correspondences.

A modified version of the double-nail illusion was used to measure the influence of binocular vergence on stereoscopic matching. The method is more sensitive than previous procedures and allows quantitative results for various stimulus markers. The results show that matches in the fixation plane are strongly preferred over matches with high disparities.

Convergence, Ocular↗
PubMed1988

Integration of depth modules: stereo and shading.

We studied the integration of image disparities, edge information, and shading in the three-dimensional perception of complex yet well-controlled images generated with a computer-graphics system. The images showed end-on views of flat- and smooth-shaded ellipsoids, i.e., images with and without intensity discontinuities (edges). A map of perceived depth was measured by adjusting a small stereo depth probe interactively to the perceived surface. Our data show that disparate shading (even in the absence of disparate edges) yields a vivid stereoscopic depth perception. The perceived depth is significantly reduced if the disparities are completely removed (shape-from-shading). If edge information is available, it overrides both shape-from-shading and disparate shading. Degradations of depth perception corresponded to a reduced depth rather than to an increased scatter in the depth measurement. The results are compared with computer-vision algorithms for both single cues and their integration for three-dimensional vision.

Computer Graphics↗
PubMed1987

Characteristics of neuronal systems in the visual cortex.

The coupling complexity of cortical areas makes it very difficult to analyse them experimentally. Studies of model systems provide the possibility of adapting the analysis to the available data base and elaborating the fundamental properties that depend on the structure of the system. We propose a model system of variable complexity that is spatially two-dimensional and time-dependent, uses feedback for iteration and smoothing, includes the mapping of the cortical networks and can be nonlinear as the case requires. Combining such elementary systems on the basis of neuroanatomical findings enables us to simulate cortical mappings and to interpret neurophysiological data. The decisive factor is that the dynamics of the system and the neuroanatomically based spatial coupling are closely connected with each other.

Animals↗
PubMed1985

An overall description of retinotopic mapping in the cat's visual cortex areas 17, 18, and 19.

Mathematical functions are derived which model the retinotopic mapping in the cat's visual cortical areas 17, 18, and 19. All three mappings are simple modifications of a complex power function with an exponent of 0.43. This function is decomposed so as to give an intermediate stage which is common to all three mappings and can be regarded as a model of the lateral geniculate nucleus mapping. The influence of retinotopic mapping on visual receptive fields was studied. The results show that a dependence of the receptive field properties on the position in the visual field is to be expected.

Animals↗