arXiv subjects

Matthias S. Keil

Publications and source records attributed to Matthias S. Keil.

9 recordsLinked to original sources

arXiv2025

Auto-Associative Memories for Direct Signalling of Visual Angle During Object Approaches

Being hit by a ball is usually not a pleasant experience. While a ball may not be fatal, other objects can be. To protect themselves, many organisms, from humans to insects, have developed neuronal mechanisms to signal approaching objects such as predators and obstacles. The study of these neuronal circuits is still ongoing, both experimentally and theoretically. Many computational proposals rely on temporal contrast integration, as it encodes how the visual angle of an approaching object changes with time. However, mechanisms based on contrast integration are severely limited when the observer is also moving, as it is difficult to distinguish the background-induced temporal contrast from that of an approaching object. Here, I present results of a new mechanism for signaling object approaches, based on modern content-addressable (auto-associative) memories. Auto-associative memories were first proposed by Hopfield in 1982, and are a class of simple neuronal networks which transform incomplete or noisy input patterns to complete and noise-free output patterns. The memory holds different sizes of a generic pattern template that is efficient for segregating an approaching object from irrelevant background motion. Therefore, the model's output correlates directly with angular size. Generally, the new mechanism performs on a par with previously published models. The overall performance was systematically evaluated based on the network's responses to artificial and real-world video footage.

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arXiv2018

From Neuronal Models to Neuronal Dynamics and Image Processing

This paper is an introduction to the membrane potential equation for neurons. Its properties are described, as well as sample applications. Networks of these equations can be used for modeling neuronal systems, which also process images and video sequences, respectively. Specifically, (i) a dynamic retina is proposed (based on a reaction-diffusion system), which predicts afterimages and simple visual illusions, (ii) a system for texture segregation (texture elements are understood as even-symmetric contrast features), and (iii) a network for detecting object approaches (inspired by the locust visual system).

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arXiv2018

A neural model of the locust visual system for detection of object approaches with real-world scenes

In the central nervous systems of animals like pigeons and locusts, neurons were identified which signal objects approaching the animal on a direct collision course. Unraveling the neural circuitry for collision avoidance, and identifying the underlying computational principles, is promising for building vision-based neuromorphic architectures, which in the near future could find applications in cars or planes. At the present there is no published model available for robust detection of approaching objects under real-world conditions. Here we present a computational architecture for signalling impending collisions, based on known anatomical data of the locust \emph{lobula giant movement detector} (LGMD) neuron. Our model shows robust performance even in adverse situations, such as with approaching low-contrast objects, or with highly textured and moving backgrounds. We furthermore discuss which components need to be added to our model to convert it into a full-fledged real-world-environment collision detector. KEYWORDS: Locust, LGMD, collision detection, lateral inhibition, diffusion, ON-OFF-pathways, neuronal dynamics, computer vision, image processing

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arXiv2011

Computation of Object Approach by a Biophysical Model of a Wide-Field Visual Neuron: Dynamics, Peaks, and Fits

Many species show avoidance reactions in response to looming object approaches. In locusts, the corresponding escape behavior correlates with the activity of the lobula giant movement detector (LGMD) neuron. During an object approach, its firing rate was reported to gradually increase until a peak is reached, and then it declines quickly. The ETA-function predicts that the LGMD activity is a product between an exponential function of angular size and angular velocity, and that peak activity is reached before time-to-contact (ttc). The ETA-function has become the prevailing LGMD model because it reproduces many experimental observations, and even experimental evidence for the multiplicative operation was reported. Several inconsistencies remain unresolved, though. Here we address these issues with a new model (PSI-model), which explicitly connects angular size and angular velocity to biophysical quantities. The PSI-model avoids biophysical problems associated with implementing exp(), implements the multiplicative operation of ETA via divisive inhibition, and explains why activity peaks could occur after ttc. It consistently predicts response features of the LGMD, and provides excellent fits to published experimental data, with goodness of fit measures comparable to corresponding fits with the ETA-function.

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arXiv2008

Does Face Image Statistics Predict a Preferred Spatial Frequency for Human Face Processing?

Psychophysical experiments suggested a relative importance of a narrow band of spatial frequencies for recognition of face identity in humans. There exists, however, no conclusive evidence of why it is that such frequencies are preferred. To address this question, I examined the amplitude spectra of a large number of face images, and observed that face spectra generally fall off steeper with spatial frequency compared to ordinary natural images. When external face features (like hair) are suppressed, then whitening of the corresponding mean amplitude spectra revealed higher response amplitudes at those spatial frequencies which are deemed important for processing face identity. The results presented here therefore provide support for that face processing characteristics match corresponding stimulus properties.

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arXiv2007

Gradient Representations and the Perception of Luminosity

The neuronal mechanisms that serve to distinguish between light-emitting and light reflecting objects are largely unknown. It has been suggested that luminosity perception implements a separate pathway in the visual system, such that luminosity constitutes an independent perceptual feature. Recently, a psychophysical study was conducted to address the question whether luminosity has a feature status or not. However, the results of this study lend support to the hypothesis that luminance gradients are instead a perceptual feature. Here, I show how the perception of luminosity can emerge from a previously proposed neuronal architecture for generating representations of luminance gradients.

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arXiv2007

Local to Global Normalization Dynamic by Nonlinear Local Interactions

Here, I present a novel method for normalizing a finite set of numbers, which is studied by the domain of biological vision. Normalizing in this context means searching the maximum and minimum number in a set and then rescaling all numbers such that they fit into a numerical interval. My method computes the minimum and maximum number by two pseudo-diffusion processes in separate diffusion layers. Activity of these layers feed into a third layer for performing the rescaling operation. The dynamic of the network is richer than merely performing a rescaling of its input, and reveals phenomena like contrast detection, contrast enhancement, and a transient compression of the numerical range of the input. Apart from presenting computer simulations, some properties of the diffusion operators and the network are analyzed mathematically. Furthermore, a method is proposed for to freeze the model's state when adaptation is observed.

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arXiv2006

Face Recognition in the Machine Reveals Properties of Human Face Recognition

Psychophysical studies suggest that face recognition takes place in a narrow band of low spatial frequencies (``critical band''). Here, we examined the recognition performance of an artificial face recognition system as a function of the size of the input images. Recognition performance was quantified with three discriminability measures: Fisher Linear Discriminant Analysis, non Parametric Discriminant Analysis, and mutual information. All of the three measures revealed a maximum at the same image sizes. Since spatial frequency content is a function of image size, our data consistently predict the range of psychophysical found frequencies. Our results therefore support the notion that the critical band of spatial frequencies for face recognition in humans and machines follows from inherent properties of face images.

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arXiv2006

Pushing it to the Limit: Adaptation With Dynamically Switching Gain Control

With this paper we propose a model to simulate the functional aspects of light adaptation in retinal photoreceptors. Our model, however, does not link specific stages to the detailed molecular processes which are thought to mediate adaptation in real photoreceptors. We rather model the photoreceptor as a self-adjusting integration device, which adds up properly amplified luminance signals. The integration process and the amplification obey a switching behavior that acts to locally shut down the integration process in dependence on the internal state of the receptor. The mathematical structure of our model is quite simple, and its computational complexity is quite low. We present results of computer simulations which demonstrate that our model adapts properly to at least four orders of input magnitude.

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