Biomedical subjects

A L Yuille

Publications and source records attributed to A L Yuille.

8 recordsLinked to original sources

PubMed2000

Probabilistic motion estimation based on temporal coherence.

We develop a theory for the temporal integration of visual motion motivated by psychophysical experiments. The theory proposes that input data are temporally grouped and used to predict and estimate the motion flows in the image sequence. This temporal grouping can be considered a generalization of the data association techniques that engineers use to study motion sequences. Our temporal grouping theory is expressed in terms of the Bayesian generalization of standard Kalman filtering. To implement the theory, we derive a parallel network that shares some properties of cortical networks. Computer simulations of this network demonstrate that our theory qualitatively accounts for psychophysical experiments on motion occlusion and motion outliers. In deriving our theory, we assumed spatial factorizability of the probability distributions and made the approximation of updating the marginal distributions of velocity at each point. This allowed us to perform local computations and simplified our implementation. We argue that these approximations are suitable for the stimuli we are considering (for which spatial coherence effects are negligible).

Bayes Theorem↗

PubMed1990

A model for the estimate of local image velocity by cells in the visual cortex.

Some computational theories of motion perception assume that the first stage en route to this perception is the local estimate of image velocity. However, this assumption is not supported by data from the primary visual cortex. Its motion sensitive cells are not selective to velocity, but rather are directionally selective and tuned to spatio-temporal frequencies. Accordingly, physiologically based theories start with filters selective to oriented spatio-temporal frequencies. This paper shows that computational and physiological theories do not necessarily conflict, because such filters may, as a population, compute velocity locally. To prove this point, we show how to combine the outputs of a class of frequency tuned filters to detect local image velocity. Furthermore, we show that the combination of filters may simulate 'Pattern' cells in the middle temporal area (MT), whereas each filter simulates primary visual cortex cells. These simulations include three properties of the primary cortex. First, the spatio-temporal frequency tuning curves of the individual filters display approximate space-time separability. Secondly, their direction-of-motion tuning curves depend on the distribution of orientations of the components of the Fourier decomposition and speed of the stimulus. Thirdly, the filters show facilitation and suppression for responses to apparent motions in the preferred and null directions, respectively. It is suggested that the MT's role is not to solve the aperture problem, but to estimate velocities from primary cortex information. The spatial integration that accounts for motion coherence may be postponed to a later cortical stage.

Animals↗

PubMed1989

Quadrature and the development of orientation selective cortical cells by Hebb rules.

We describe a new theoretical scenario for the development of orientation selective cells in a self-organizing feedforward network with modifiable synapses. A suitable choice of Hebb rule leads to a system that develops symmetric and antisymmetric response fields (quadrature pairs) at the same time as directional selectivity occurs using inhibition between neighboring cells. Quadrature phase relationships between the response properties of adjacent cortical cells is suggestive of several highly efficient information processing strategies.

Animals↗

PubMed1989

Energy functions for early vision and analog networks.

This paper describes attempts to model the modules of early vision in terms of minimizing energy functions, in particular energy functions allowing discontinuities in the solution. It examines the success of using Hopfield-style analog networks for solving such problems. Finally it discusses the limitations of the energy function approach.

Models, Neurological↗

PubMed1988

A computational theory for the perception of coherent visual motion.

When we see motion, our perception of how one image feature moves depends on the behaviour of other features nearby. In particular, the Gestaltists proposed the law of shared common fate, in which features tend to be perceived as moving together, that is, coherently. Recent psychophysical findings, such as the cooperativity of the motion system and motion capture, support this law. Computationally, coherence is a sensible assumption, because if two features are close then they probably belong to the same object and thus tend to move together. Moreover, the measurement of local motion may be inaccurate and so the integration of motion information over large areas may help to improve the performance. Present theories of visual motion, however, do not account fully for these coherent motion percepts. We propose here a theory that does account for these phenomena and also provides a solution to the aperture problem, where the local information in the image flow is insufficient to specify the motion uniquely.

Humans↗

PubMed1988

Spontaneous symmetry-breaking energy functions and the emergence of orientation selective cortical cells.

The mammalian visual cortex is comprised of "hypercolumns" of orientation selective cells. The developmental process by which cells are generated with receptive fields tuned to a variety of orientations has so far remained a mystery. We present a model for the production of orientation selective cells that requires no external stimuli and a minimum of input parameters. The process involves spontaneous symmetry-breaking in an energy function that governs the maturation of the cortical cells in a multi-layer network of Hebb-type feedforward neurons. An important feature is that the symmetry breaking occurs for each cell separately and is not due to global organizing effects. We present examples of receptive field profiles calculated with the symmetry-breaking procedure and note that the results seem robust and may be useful in the study of development in several types of cortical cells. The inclusion of long range lateral (intra-layer) correlations in the energy function could result in the development of cell groups with correlated preferred orientations that resemble the hypercolumns seen in the visual cortex.

Animals↗

PubMed1988

Massively parallel implementations of theories for apparent motion.

Two solutions for the correspondence problem for long-range motion are investigated. The first is a modification of the Minimal Mapping Theory (S. Ullman: The Interpretation of Visual Motion, MIT Press, Cambridge, 1979) that is implemented by a massively parallel network. In this network, every two units are interconnected, and thus, its convergence is fast and relatively independent of the number of image features. Computer simulations show that our method accounts as well as the Minimal Mapping Theory for apparent-motion phenomena, although some differences exist. Mathematical proofs provide conditions for the convergence of the network. The second 'solution' for the correspondence problem is called the Structural Theory. This theory assumes that the three-dimensional structure of viewed objects does not change fast in time. Then, the theory looks for the correspondence and three-dimensional structure that best fulfill this assumption. A massively parallel network implementation of this theory is also possible. However, its performance is poor due to the high complexity of its solution space. This supports Ullman's (1979) suggestion that the visual system separates the structure-from-motion process into two stages. First, a stage for motion measurement, and then a stage for structure recovery.

Humans↗

PubMedDate not supplied

Dependence of speed and direction perception on cinematogram dot density.

In the present experiments, we find that with abrupt decreases in dot density of random-dot cinematograms, perceived speed decreases, while with abrupt increases in dot density, perceived speed increases. Further, in steady-state conditions, perceived speed is also affected in the same way, but to a lesser degree, by the dot density of cinematograms. Direction discrimination of random-dot cinematograms is enhanced when dot density increases abruptly from one stimulus to the next, but is degraded when dot density decreases abruptly. Finally, speed discrimination remains constant even when density changes abruptly. The perceived-speed and direction-discrimination data are consistent with the Motion Coherence theory which motivated this study, and with models that include a smoothing stage similar to this theory. Of the other models that we consider, most predict that increasing dot density reduces perceived speed. The speed-discrimination data could not distinguish between the different theories.

Discrimination, Psychological↗