Haluk Ogmen

Recent models and findings in visual backward masking: A comparison, review, and update

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

Fuzzy PID controller: Design, performance evaluation, and stability analysis

Abstract This paper presents a design for a new fuzzy logic proportional-integral-derivative (PID) controller. The main motivation for this design was to control some known nonlinear systems, such as robotic manipulators, which violate the conventional assumption of the linear PID controller. This controller is developed by first describing the discrete-time linear PID control law and then progressively deriving the steps necessary to incorporate a fuzzy logic control mechanism into the modifications of the PID structure. The final version of this new fuzzy PID controller is a computationally efficient analytic scheme suitable for implementation in a real-time closed-loop digital control. Numerous computer simulations are included to demonstrate the effectiveness of the controller for both linear and nonlinear systems. Finally, a brief analysis is presented to prove that the controller has bounded-input/bounded-output (BIBO) stability.

Moving ahead through differential visual latency

The time it takes to transmit information along the human visual pathways introduces a substantial delay in the processing of images that fall on the retina. This visual latency might be expected to cause a moving object to be perceived at a position behind its actual one, disrupting the accuracy of visually guided motor actions such as catching or hitting, but this does not happen. It has been proposed that the perceived position of a moving object is extrapolated forwards in time to compensate for the delay in visual processing.

The what and where in visual masking.

A metacontrast mask suppresses the visibility of, without influencing the reaction time (RT) to, the target. We investigated whether this dissociation results from a sensori-motor pathway immune to masking effects or from the characteristics of stimulus timing in mutually inhibitory sustained and transient channels. For target visibility, para- and metacontrast yielded the usual U-shaped functions. Peak paracontrast occurred at stimulus onset asynchronies (SOAs) of -150 to -100 ms. RTs were relatively low for metacontrast and did not show a systematic change as a function of SOA. The RT contribution from contour-masking was greatest at an SOA of -150 ms (paracontrast) and declined to near zero in the metacontrast regime. The dissociation between visibility and RT seen in metacontrast did not occur in paracontrast, rejecting the theory that RTs are elicited by a single sensori-motor pathway immune to masking. The dependence of the dissociation on stimulus timing can be explained by RECOD, a dual-pathway model wherein fast and slow activities interact.

A neural theory of retino-cortical dynamics

Abstract As a result of a variety of factors—the movements of the eyes, those of external objects, integration time, vergence, accommodation, and nonuniform retinal sampling—the retinal encoding is highly transient, blurred, and distorted. Yet this problem received very little attention, for most of the models proposed in the literature are built around the analysis of static (or steady-state) and uniformly focused images. Thus, a fundamental problem in visual perception consists of the understanding of the processes underlying the synthesis of phenomenally static, sharp percepts from transient, blurred activities. We present a continuous-time neural theory that proposes two major roles for the retinal transient activity: First, we propose that the nonmonotonic behavior of retinal neurons serves as a simple form of memory that adaptively filters visual signals to guide attentional mechanisms. Second, we propose that the transient activity is essential in achieving sharp dynamic percepts while preserving a good sensitivity to light. Theoretical analysis shows that an extraretinal on-center off-surround feedback anatomy is required to sharpen the “blurred output” from the retinal level. Mathematical properties of such feedback loops indicate that a transient reset mechanism is necessary to avoid smearing. It is proposed that transient retinal cells realize the reset by sending inhibitory signals to sustained activity distributions at higher levels (extraretinal areas: e.g., lateral geniculate nucleus (LGN) and/or visual cortical areas). In this theoretical framework, the continuous time behavior of the visual system can be analyzed in three major phases. In the first phase, sustained retinal signals are sharpened by feedback dominant extra-retinal loops. When the input moves, a second phase is engaged. In this phase, transient retinal cells reset rapidly and briefly extra-retinal activities. In the third phase, extra-retinal loops enter a feedforward mode thereby transferring a faithful copy of retinal activity into their own cells. The feedforward mode is maintained by the transient components of the retinal sustained units. When retinal units enter their steady-state mode, the overall system returns to the first phase where sharpening occurs through feedback dominant extra-retinal loops. The predictions of the theory are compared with various experimental data with emphasis on masking and motion deblurring phenomena.

Identifying chaotic systems via a Wiener-type cascade model

In this article we first show a theory that the Wiener-type cascade dynamical model, in which a simple linear plant is used as the dynamic subsystem and a three-layer feedforward artificial neural network is employed as the nonlinear static subsystem, can uniformly approximate a continuous trajectory of a general nonlinear dynamical system with arbitrarily high precision on a compact time domain. We then report some successful simulation results, by training the neural network using a model-reference adaptive control method, for identification of continuous-time and discrete-time chaotic systems, including the typical Duffing, Henon, and Lozi systems. This Wiener-type cascade structure is believed to have great potential for chaotic dynamics identification, control and synchronization.

Neural network architectures for motion perception and elementary motion detection in the fly visual system

Abstract Two distinct but complementary neural architectures for motion perception are presented. Part I of the paper describes a directionally selective, local motion detector based on the fly visual system. Model predictions are compared with experimental data and the relationship of the neural model with the Reichardt functional model is discussed. Part II of the paper introduces a motion-sensitive network, insensitive to the direction of motion. First, local temporal discrimination problem is addressed in shunting networks and the limitations of the local scheme are used to motivate spatial interactions. Then, asynchronous interactions are studied in center-surround antagonistic shunting networks. Some properties of resulting asynchronous shunting networks are established. Finally, extensions of the work and integration of these architectures into a global model are discussed.

Perceptual grouping induces non-retinotopic feature attribution in human vision

The human visual system computes features of moving objects with high precision despite the fact that these features can change or blend into each other in the retinotopic image. Very little is known about how the human brain accomplishes this complex feat. Using a Ternus-Pikler display, introduced by Gestalt psychologists about a century ago, we show that human observers can perceive features of moving objects at locations these features are not present. More importantly, our results indicate that these non-retinotopic feature attributions are not errors caused by the limitations of the perceptual system but follow rules of perceptual grouping. From a computational perspective, our data imply sophisticated real-time transformations of retinotopic relations in the visual cortex. Our results suggest that the human motion and form systems interact with each other to remap the retinotopic projection of the physical space in order to maintain the identity of moving objects in the perceptual space.

Meta- and paracontrast reveal differences between contour- and brightness-processing mechanisms.

We investigated meta- and paracontrast masking using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. The contour task revealed strongest metacontrast at SOAs shorter than those obtained for the brightness task. Paracontrast revealed related temporal differences between the tasks. Additionally, the paracontrast results support the existence not only of prolonged inhibitory effects but also of facilitatory effects. The combined results comport with the existence of cortical mechanisms for: (i) fast contour processing, (ii) slow surface-brightness processing, (iii) prolonged inhibition, and (iv) facilitation.

The flight path of the phoenix--the visible trace of invisible elements in human vision

How features are attributed to objects is one of the most puzzling issues in the neurosciences. A deeply entrenched view is that features are perceived at the locations where they are presented. Here, we show that features in motion displays can be systematically attributed from one location to another although the elements possessing the features are invisible. Furthermore, features can be integrated across locations. Feature mislocalizations are usually treated as errors and limits of the visual system. On the contrary, we show that the nonretinotopic feature attributions, reported herein, follow rules of grouping precisely suggesting that they reflect a fundamental computational strategy and not errors of visual processing.