Biomedical subjects

L M Vaina

Publications and source records attributed to L M Vaina.

27 records · Page 2Linked to original sources

PubMed1995

Two stages of visual processing for radial and circular motion.

As we move through our environment, the flow of the deforming images on our retinae provides rich information about ego motion and about the three-dimensional structure of the external world. Flow-fields comprise five independent components, including radial and circular motion. Here we provide psychophysical evidence for the existence of neural mechanisms in human vision that integrate motion signals along these complex trajectories. Signal-to-noise sensitivity for discriminating the direction of radial, circular and translational motion increased predictably with the number of exposed sectors, implying the existence of specialized detectors that integrate motion signals of different directions from different locations. However, contrast sensitivity for complex motion did not increase greatly with sector number, implying that the specialized detectors are preceded by a first stage of local-motion mechanisms that impose a contrast threshold. These findings fit well with recent electrophysiological evidence in monkey showing that whereas motion-sensitive neurons in primary visual cortex respond best to local translation, many neurons in the medial superior temporal cortex have large receptive fields tuned to radial, circular or spiral motion.

Contrast Sensitivity↗

PubMed1995

Learning to ignore: psychophysics and computational modeling of fast learning of direction in noisy motion stimuli.

The effects of practice on the discrimination of direction of motion in briefly presented noisy dynamic random dot patterns are investigated in several forced-choice psychophysical tasks. We found that the percentage of correct responses on any specific task increases linearly with repetition of trials within roughly 200 trials from about chance to a performance of 90% or better. The level of performance remained constant or improved over several days, and in most instances it did not transfer when stimulus parameters changed. We used a modified Radial Basis Function (RBF) representation to model the psychophysical tasks. The performance of the model is functionally similar to the psychophysical results. We propose a Hebbian learning algorithm which deactivates the inputs from neurons responding to motion noise in the stimulus. Our computational model suggests that to solve this task in biological systems, neurons (perhaps in MT) improve their performance by 'learning to ignore' noise in the image.

Acoustic Stimulation↗

PubMed1994

Segregation of computations underlying perception of motion discontinuity and coherence.

Motion is one of the most important cues for detecting discontinuities in images. The major dichotomy among theories of motion-defined discontinuity concerns whether the computations related to the extraction of discontinuity and large scale integration of motion signals are organized hierarchically or occur simultaneously in the brain. In this study we investigated the hierarchical nature of these computations using data from two patients with unilateral brain lesions on two psychophysical tasks: one requiring motion for spatial integration of direction in a stochastic motion field, and the other requiring motion to extract discontinuities in the same type of stimuli. The results showed a surprising double dissociation of deficits on these motion tasks which suggests that models for discontinuity detection requiring a single neural substrate for computing coherence and discontinuity are unlikely to be applicable to the human visual system. We discuss the computational implications of these results. Using morphometric three-dimensional reconstructions of the lesions from the magnetic resonance imaging data we suggest possible anatomical sites mediating these computations.

Adult↗

PubMed1990

The largest convex patches: a boundary-based method for obtaining object parts.

The importance of boundaries for shape decomposition into component parts has been discussed from different points of view by Koenderink and van Doorn (1982), and by Hoffman and Richards (1984). The former define part boundaries as parabolic contours, whereas the latter propose that part boundaries should be defined by contours of negative minima (or maxima) of principal curvature. In this article, building on aspects of both approaches, we develop a new method for shape decomposition. This method relies exclusively on global properties of the surface which are fully characterized by local surface properties. We propose that a useful parcellation of shapes into parts can be obtained by decomposing the shape boundary into the largest convex surface patches and the smallest nonconvex surface patches. The essential computational steps of this method are the following: (i) build initial parts from the largest locally convex patches, (ii) consider an initial part as a constituent part if it is essentially convex, and (iii) obtain the remaining constituent parts by merging adjacent initial parts generated by the largest locally convex and the smallest nonconvex patches of nearly the same sizes. The method is illustrated on both smooth and continuous shapes. We show that the decomposition of shapes into the largest convex patches aims to maximize the "thingness" in an object, and to minimize its "non-thingness". The method is conducive to a natural parcellation of shapes into constituent parts useful for recognition and for inferring function.

Form Perception↗

PubMed1990

Intact "biological motion" and "structure from motion" perception in a patient with impaired motion mechanisms: a case study.

A series of psychophysical tests examining early and later aspects of image-motion processing were conducted in a patient with bilateral lesions involving the posterior visual pathways, affecting the lateral parietal-temporal-occipital cortex and the underlying white matter (as shown by magnetic resonance imaging studies and confirmed by neuro-ophthalmological and neuropsychological examinations). Visual acuity, form discrimination, color, and contrast-sensitivity discrimination were normal whereas spatial localization, line bisection, depth, and binocular stereopsis were severely impaired. Performance on early motion tasks was very poor. These include seeing coherent motion in random noise (Newsome & Paré, 1988), speed discrimination, and seeing two-dimensional form from relative speed of motion. However, on higher-order motion tasks the patient was able to identify actions from the evolving pattern of dots placed at the joints of a human actor (Johansson, 1973) as well as discriminating three-dimensional structure of a cylinder from motion in a dynamic random-dot field. The pattern of these results is at odds with the hypothesis that precise metrical comparison of early motion measurements is necessary for higher-order "structure from motion" tasks.

Cerebral Cortex↗

PubMed1989

Selective impairment of visual motion interpretation following lesions of the right occipito-parietal area in humans.

A group of eighteen patients selected on the basis of the anatomical locus of the lesion, normal visual acuity and the ability to discriminate visual motion, were assessed on the perception of Julesz Random-Dot stereograms and on three tasks of visual motion interpretation: Speed Discrimination, 3D-Structure-from-Motion and 2D-Form-from-motion. The results on these experimental tasks demonstrate a double dissociation of deficits on the visual analysis of motion and stereopsis in the patients with lesions to the posterior right hemisphere. The right occipital-parietal (ROP) group failed on the Stereopsis task and showed a dramatic impairment on the Speed Comparison and on the Structure-from-Motion experiments. They performed in the normal range, however, on the 2D-Form-from-Motion task. The right occipital-temporal (ROT) group, on the other hand, were severely impaired on the identification of two dimensional forms from motion or stereopsis. In both cases, however, they were able to obtain a coarse segregation of the figure from the background. The ROT group did not present significant deficits on the Speed Discrimination and the Structure-from-Motion tasks. The results are discussed in the light of recent physiological and psychophysical findings, and it is hypothesized that, in the human brain, visual deficits of motion interpretation and of stereopsis are associated with right occipital-parietal lesions.

Adult↗

PubMedDate not supplied

The selective impairment of the perception of first-order motion by unilateral cortical brain damage.

First-order (Fourier) motion consists of stable spatiotemporal luminance variations. Second-order (non-Fourier) motion consists instead of spatiotemporal modulation of contrast, flicker, or spatial frequency. In spite of extensive psychophysical and computational analysis of the nature and relationship of these two types of motion, it remains unclear whether they are detected by the same mechanism or whether separate mechanisms are involved. Here we report the selective impairment of first-order motion, on a range of local and global motion tasks, in the contralateral visual hemifield of a patient with unilateral brain damage centered on putative visual areas V2 and V3 in the medial part of the occipital lobe. His perception of second-order motion was unimpaired. As his disorder is the obverse of that reported after damage in the vicinity of human visual area MT (V5), the results support models of motion processing in which first- and second-order motion are, at least in part, computed separately at the extrastriate cortical level.

Aged↗

PubMedDate not supplied

A laterally interconnected neural architecture in MST accounts for psychophysical discrimination of complex motion patterns.

The complex patterns of visual motion formed across the retina during self-motion, often referred to as optic flow, provide a rich source of information describing our dynamic relationship within the environment. Psychophysical studies indicate the existence of specialized detectors for component motion patterns (radial, circular, planar) that are consistent with the visual motion properties of cells in the medial superior temporal area (MST) of nonhuman primates. Here we use computational modeling and psychophysics to investigate the structural and functional role of these specialized detectors in performing a graded motion pattern (GMP) discrimination task. In the psychophysical task perceptual discrimination varied significantly with the type of motion pattern presented, suggesting perceptual correlates to the preferred motion bias reported in MST. Simulated perceptual discrimination in a population of independent MST-like neural responses showed inconsistent psychophysical performance that varied as a function of the visual motion properties within the population code. Robust psychophysical performance was achieved by fully interconnecting neural populations such that they inhibited nonpreferred units. Taken together, these results suggest that robust processing of the complex motion patterns associated with self-motion and optic flow may be mediated by an inhibitory structure of neural interactions in MST.

Animals↗

PubMedDate not supplied

Functional segregation of color and motion processing in the human visual cortex: clinical evidence.

Anatomical and physiological investigations indicate two major distinct functional streams within the extrastriate visual cortex of the macaque monkey, and behavioral observations suggest that the ventral (occipitotemporal) pathway is the cornerstone for object recognition whereas the dorsal (occipitoparietal) pathway is primarily involved in visuospatial perception and visuomotor performance. In the context of this dichotomy we conducted a psychophysical and neuropsychological study of visual perceptual abilities in two stroke patients, each with lesions involving several extrastriate areas. Magnetic resonance imaging demonstrated bilateral lesions; in one patient (E.W.) the lesion involves the ventral medial portions of the occipital and temporal lobes, and in the other (A.F.) the lesion involves dorsally the occipital-parietal area, including the region of the temporal-parietal-occipital junction. E.W. suffers from achromatopsia of central origin, prosopagnosia, visual agnosia, and alexia without agraphia. His depth and motion perception, including recognition of moving objects, are normal. He has superior visual field loss bilaterally, and slightly impaired acuity, and complains that the world appears in a deep twilight even on a sunny day. In contrast, A.F. shows specific deficits of stereopsis, spatial localization, and several aspects of motion perception. He is also impaired at recognizing objects presented from unconventional views, but recognition of prototypical views of objects, and color and form discrimination are normal, as is his ability to recognize faces. The anatomical characteristics of the lesions of these two patients permit a direct experimental comparison of the effects of lesions confined to the parietal or temporal pathways. E.W.'s and A.F.'s performance on the psychophysical and neuropsychological tasks discussed here supports the functional distinction between a dorsal and a ventral extrastriate system but additionally suggests the existence of a pathway involved in identification-from-motion that is separate from both the dorsal early motion/spatial analysis pathway and the ventral color/static-form pathway.

Cerebrovascular Disorders↗