Haluk Ogmen - US grants

This research aims to develop neural model of preattentive visual motion perception. A deeper understanding of design principles involved in biological vision and unification of the description of perceptual neural phenomena into a common theoretical language with a small number of dynamic laws and organizational principles are expected long-range outcomes of this research project. As a result of a variety of factors-such as the movements of the eyes and that of external objects, integration time, vergence, accommodation, and non-uniform sampling the retinal image is highly transient, blurred, and distorted. Yet, this problem received very little attention, for most of the models proposed in the literature axe built around the analysis of static (or steady-state) and uniformly in focus images. Thus, a fundamental problem in vision consists of the analysis of image transients to achieve dynamic yet sharp percepts despite these blurring effects. In this proposal, we will address this question by developing continuous-time form and motion channel models. We will carry out theoretical studies by using nonlinear analysis techniques and computer simulations to test and extend the predictions of specialized neural network architectures that we proposed to explain a wide range of data on motion and form perception in invertebrate and vertebrate species. The work will consider on the one hand motion perception without complex form perception in invertebrates and on the other hand motion perception with complex form perception in vertebrates. The study of the former will consist of the refinement of our earlier work by analyzing recent data and in particular, data on object tracking by motion cues. The study of the latter will consist of a motion channel and a form channel. For the motion channel we will extend, with suitable modifications, our invertebrate model to vertebrates. The fundamental properties of our model, namely nonlinear preprocessing, transient behavior, and habituation-sensitization will be studied by considering experimental paradigms probing these properties, i.e. Fourier & non-Fourier motion, apparent motion, and segregation by motion contrast respectively. For the form channel, we will test the predictions of the model by studying related experimental data: masking data to test the role of transient-sustained interactions in dynamic form perception and deblurring data to test the proposed temporal phases and their neural correlates. We will also carry out a bifurcation analysis to establish the properties of a more general class of extraretinal positive feedback interactions of the model. Modeling and testing will be based on extensive data ranging from single cell recordings to psychophysical experiments from a variety of species.

DESCRIPTION: (Adapted from the Investigator's Abstract) The ability to perceive the sharpness of abject boundaries is central to the quality of vision yet very little is know about the underlying neural mechanisms. The broad long-term objective of this research proposal is to reach a more profound understanding of neural mechanisms that determine the perceived form of moving objects in human vision. Under normal viewing conditions, moving objects appear much less blurred than what one would predict from the long duration of visual persistence. This phenomenon is known as motion deblurring. A specific goal of this research is to study the mechanisms underlying motion deblurring and their implications for the perceived form of moving objects. The approach will combine computational and psychophysical methods to test the mechanisms proposed in a neural network model of retino-cortical dynamics. The model leads to the following specific hypotheses: Hypothesis 1 (spatio-temporal profiles): (i) The metacontrast masking function for spatially localized stimuli is oscillatory; (ii) these oscillations are a by-product of the retino-cortical system that occur when it is driven externally by high luminance inputs or internally by focused-arousal/attention; and (iii) the smooth character of the "classical" metacontrast function-extensively reported in the literature-results from spatio-temporal averaging in the post-retinal network. Hypothesis 2 (spatial extent of motion blur): The primary mechanism that determines the length of perceived smear for moving targets is an inhibition from transient cells to sustained cells. Hypothesis 3 (perceived form of motion blur): The brightness profile produced by the retino-cortical dynamics model in response to an isolated moving target will match the psychophysically measured brightness profile of the phenomenon known as "Charpentier's bands."

DESCRIPTION: (Adapted from the Investigator's Abstract) The ability to perceive the sharpness of abject boundaries is central to the quality of vision yet very little is know about the underlying neural mechanisms. The broad long-term objective of this research proposal is to reach a more profound understanding of neural mechanisms that determine the perceived form of moving objects in human vision. Under normal viewing conditions, moving objects appear much less blurred than what one would predict from the long duration of visual persistence. This phenomenon is known as motion deblurring. A specific goal of this research is to study the mechanisms underlying motion deblurring and their implications for the perceived form of moving objects. The approach will combine computational and psychophysical methods to test the mechanisms proposed in a neural network model of retino-cortical dynamics. The model leads to the following specific hypotheses: Hypothesis 1 (spatio-temporal profiles): (i) The metacontrast masking function for spatially localized stimuli is oscillatory; (ii) these oscillations are a by-product of the retino-cortical system that occur when it is driven externally by high luminance inputs or internally by focused-arousal/attention; and (iii) the smooth character of the "classical" metacontrast function-extensively reported in the literature-results from spatio-temporal averaging in the post-retinal network. Hypothesis 2 (spatial extent of motion blur): The primary mechanism that determines the length of perceived smear for moving targets is an inhibition from transient cells to sustained cells. Hypothesis 3 (perceived form of motion blur): The brightness profile produced by the retino-cortical dynamics model in response to an isolated moving target will match the psychophysically measured brightness profile of the phenomenon known as "Charpentier's bands."

Visual masking occurs when the visibility of one stimulus, called the target, is reduced by the presence of another stimulus, designated as the mask. Like the techniques of binocular rivalry and of multi-stable percepts of the same stimulus, visual masking provides a way to dissociate neural processes that are merely stimulus-dependent but not correlated with conscious perception, from neural processes that are percept-dependent and thus correlated with conscious perception of the stimulus. The broad long-term objective of this research is to use the visual masking paradigm to study the temporal dynamics in the micro-genesis of pattern processing from the time of stimulus presentation to the time of its full registration in consciousness. A modified dual-channel model of visual masking that incorporates mutual inhibitory interactions between sustained parvocellular (P) and transient magnocellular (M) pathways will be used to study how the cortical response evoked by the mask interacts with the early and late components of the cortical response evoked by the target. The project will rely on the masking paradigms of para- and metacontrast, theoretical tools based on neural-network modeling, and the additional psychophysical techniques of target disinhibition, binocular rivalry, and unconscious priming by a masked (perceptually suppressed) target. It will investigate where and when in the stream of processing the mechanisms implicated in suppression and disinhibition of target visibility are located. It will also probe whether and when the mechanisms implicated in suppression and disinhibition of target visibility relate to stimulus-dependent (unconscious) or percept-dependent (conscious) levels of neural processing. The research is expected to provide a better understanding of the perceptual processes leading to conscious registration of stimuli. The potential applications include the development of novel biomimetic engineering design principles for autonomous perceptual devices and the development of clinical diagnostic or vulnerability markers for disorders such as dyslexia and schizophrenia.

DESCRIPTION (provided by applicant): The perception of form is one of the most basic functions of the visual system. Because motion and dynamic occlusions are ubiquitous in our environment, understanding how the human visual system computes the form of moving objects in the presence of occlusions is a fundamental problem in vision science. An analysis of dynamic aspects of vision shows that non-retinotopic computational principles and mechanisms are needed to compute the form of moving objects. We designate as "non-retinotopic" those mechanisms that can generate perception of form in the absence of a retinotopic image. Indeed, perceptual data demonstrate that a retinotopic image is neither necessary nor sufficient for the perception of form. The broad long-term objective of this research is to elucidate the mechanisms underlying visual form perception under its natural dynamic conditions. In particular, we want to characterize non-retinotopic computational principles and mechanisms that allow the visual system to compute the form of moving objects. We hypothesize that a synergy between masking, perceptual grouping, and motion estimation mechanisms can provide a unified visual processing framework that can be applied to natural dynamic viewing conditions. We will address the following two specific aims. Specific Aim 1: Elucidate non-retinotopic mechanisms of dynamic form perception by using anorthoscopic perception. Anorthoscopic perception provides a clear demonstration of the existence of non- retinotopic mechanisms. Do such mechanisms also contribute to dynamic perception in the absence of occlusions? By using a stimulus paradigm, known as the Ternus-Pikler display, we have recently discovered a new illusion that shows non-retinotopic feature perception for moving objects in the absence of occlusions. We will use this paradigm to generalize non-retinotopic mechanisms to dynamic vision: Specific Aim 2: Elucidate the role of non-retinotopic mechanisms in dynamic form perception in the absence of occlusions by using variants of Ternus-Pikler displays. PUBLIC HEALTH RELEVANCE: The proposed studies are expected to improve our understanding of how the human visual system works. This understanding can be instrumental in diagnosing, treating disorders of the visual system in particular, and the nervous system in general.