Neural dynamics of invariant object recognition: relative disparity, binocular fusion, and predictive eye movements

How does the visual cortex learn invariant object categories as an observer scans a depthful scene? Two neural processes that contribute to this ability are modeled in this thesis. The first model clarifies how an object is represented in depth. Cortical area V1 computes absolute disparity, which is the horizontal difference in retinal location of an image in the left and right foveas. Many cells in cortical area V2 compute relative disparity, which is the difference in absolute disparity of two visible features. Relative, but not absolute, disparity is unaffected by the distance of visual stimuli from an observer, and by vergence eye movements. A laminar cortical model of V2 that includes shunting lateral inhibition of disparity-sensitive layer 4 cells causes a peak shift in cell responses that transforms absolute disparity from V1 into relative disparity in V2. The second model simulates how the brain maintains stable percepts of a 3D scene during binocular movements. The visual cortex initiates the formation of a 3D boundary and surface representation by binocularly fusing corresponding features from the left and right retinotopic images. However, after each saccadic eye movement, every scenic feature projects to a different combination of retinal positions than before the saccade. Yet the 3D representation, resulting from the prior fusion, is stable through the post-saccadic re-fusion. One key to stability is predictive remapping: the system anticipates the new retinal positions of features entailed by eye movements by using gain fields that are updated by eye movement commands. The 3D ARTSCAN model developed here simulates how perceptual, attentional, and cognitive interactions across different brain regions within the What and Where visual processing streams interact to coordinate predictive remapping, stable 3D boundary and surface perception, spatial attention, and the learning of object categories that are invariant to changes in an object's retinal projections. Such invariant learning helps the system to avoid treating each new view of the same object as a distinct object to be learned. The thesis hereby shows how a process that enables invariant object category learning can be extended to also enable stable 3D scene perception

Visual Cortex

The neurosciences have experienced tremendous and wonderful progress in many areas, and the spectrum encompassing the neurosciences is expansive. Suffice it to mention a few classical fields: electrophysiology, genetics, physics, computer sciences, and more recently, social and marketing neurosciences. Of course, this large growth resulted in the production of many books. Perhaps the visual system and the visual cortex were in the vanguard because most animals do not produce their own light and offer thus the invaluable advantage of allowing investigators to conduct experiments in full control of the stimulus. In addition, the fascinating evolution of scientific techniques, the immense productivity of recent research, and the ensuing literature make it virtually impossible to publish in a single volume all worthwhile work accomplished throughout the scientific world. The days when a single individual, as Diderot, could undertake the production of an encyclopedia are gone forever. Indeed most approaches to studying the nervous system are valid and neuroscientists produce an almost astronomical number of interesting data accompanied by extremely worthy hypotheses which in turn generate new ventures in search of brain functions. Yet, it is fully justified to make an encore and to publish a book dedicated to visual cortex and beyond. Many reasons validate a book assembling chapters written by active researchers. Each has the opportunity to bind together data and explore original ideas whose fate will not fall into the hands of uncompromising reviewers of traditional journals. This book focuses on the cerebral cortex with a large emphasis on vision. Yet it offers the reader diverse approaches employed to investigate the brain, for instance, computer simulation, cellular responses, or rivalry between various targets and goal directed actions. This volume thus covers a large spectrum of research even though it is impossible to include all topics in the extremely diverse field of neurosciences

Recognition by Flickering Components: The Effect of Temporal Modulation on Image Recognition

A primary goal of vision is to identify objects rapidly and efficiently. Successful object and scene recognition results from the integration of both feed-forward and feedback processes that correspond a two-dimensional retinal image to a representation of its content stored in memory (Bar, 2003). One general organizing principle may be that the visual system analyzes images and scenes according to their spatial components in a coarse- (low spatial frequency) to-fine (high spatial frequency) sequence (Bullier, 2001; Hegde, 2008). An individual’s sensitivity to these spatial components is described by contrast sensitivity function (CSF), which indicates the minimum contrast required for the detection of patterns of various sizes. A consistent finding is that, when temporally modulated at a moderate rate, sensitivity to lower spatial frequency gratings is heightened relative to its static counterpart (Robson, 1966; Abramov et al., 2012). This suggests that temporal modulation may enhance image detectability, especially so for those coarsest spatial components—the lower spatial frequencies—that seem to be most important for detection and categorization of objects and scenes. We presented participants with an array of grayscale images depicting objects and scenes under 3 spatial (3cpd-filtered, 4cpd-filtered, and unfiltered full-spectrum) and 3 temporal (static, 6Hz counterphase flicker, and 250ms presentation) conditions, at 4 levels of near-threshold contrast. Responses were open-ended identifications. Temporal modulation was hypothesized to improve recognition in all spatial conditions, while short-duration presentation was predicted to result in performance comparable to the full-duration static condition. Males were hypothesized to perform better than females based on previously reported (Abramov, et al., 2012) sensitivity differences. Results partially supported our hypotheses: temporal modulation at 6Hz had recognition enhancement effects only for the lowest spatial frequency-cutoff—3 cpd. We describe a distinction among the neurological underpinnings of the CSF into two separate mechanisms responsible for the lower and upper halves of the CSF

Perception of Color Break-Up

Hintergrund. Ein farbverfälschender Bildfehler namens Color Break-Up (CBU) wurde untersucht. Störende CBU-Effekte treten auf, wenn Augenbewegungen (z.B. Folgebewegungen oder Sakkaden) während der Content-Wiedergabe über sogenannte Field-Sequential Color (FSC) Displays oder Projektoren ausgeführt werden. Die Ursache für das Auftreten des CBU-Effektes ist die sequenzielle Anzeige der Primärfarben über das FSC-System. Methoden. Ein kombiniertes Design aus empirischer Forschung und theoretischer Modellierung wurde angewendet. Mittels empirischer Studien wurde der Einfluss von hardware-, content- und betrachterbasierten Faktoren auf die CBU-Wahrnehmung der Stichprobe untersucht. Hierzu wurden zunächst Sehleistung (u. a. Farbsehen), Kurzzeitzustand (u. a. Aufmerksamkeit) und Persönlichkeitsmerkmale (u. a. Technikaffinität) der Stichprobe erfasst. Anschließend wurden die Teilnehmenden gebeten, die wahrgenommene CBU-Intensität verschiedener Videosequenzen zu bewerten. Die Sequenzen wurden mit einem FSC-Projektor wiedergegeben. Das verwendete Setup ermöglichte die Untersuchung folgender Variablen: die Größe (1.0 bis 6.0°) und Leuchtdichte (10.0 bis 157.0 cd/m2) des CBU-provozierenden Contents, das Augenbewegungsmuster des Teilnehmenden (Geschwindigkeit der Folgebewegung: 18.0 bis 54.0 °/s; Amplitude der Sakkade: 3.6 bis 28.2°), die Position der Netzhautstimulation (0.0 bis 50.0°) und die Bildrate des Projektors (30.0 bis 420.0 Hz). Korrelationen zwischen den unabhängigen Variablen und der subjektiven CBU-Wahrnehmung wurden getestet. Das ergänzend entwickelte Modell prognostiziert die CBU-Wahrnehmung eines Betrachters auf theoretische Weise. Das Modell rekonstruiert die Intensitäts- und Farbeigenschaften von CBU-Effekten zunächst grafisch. Anschließend wird die visuelle CBU-Rekonstruktion zu repräsentativen Modellindizes komprimiert, um das modellierte Szenario mit einem handhabbaren Satz von Metriken zu quantifizieren. Die Modellergebnisse wurden abschließend mit den empirischen Daten verglichen. Ergebnisse. Die hohe interindividuelle CBU-Variabilität innerhalb der Stichprobe lässt sich nicht durch die Sehleistung, den Kurzzeitzustand oder die Persönlichkeitsmerkmale eines Teilnehmenden erklären. Eindeutig verstärkende Bedingungen der CBU-Wahrnehmung sind: (1) eine foveale Position des CBU-Stimulus, (2) eine reduzierte Stimulusgröße während Sakkaden, (3) eine hohe Bewegungsgeschwindigkeit des Auges und (4) eine niedrige Bildrate des Projektors (Korrelation durch Exponentialfunktion beschreibbar, r2 > .93). Die Leuchtdichte des Stimulus wirkt sich nur geringfügig auf die CBU-Wahrnehmung aus. Generell hilft das Modell, die grundlegenden Prozesse der CBU-Genese zu verstehen, den Einfluss von CBU-Determinanten zu untersuchen und ein Klassifizierungsschema für verschiedene CBU-Varianten zu erstellen. Das Modell prognostiziert die empirischen Daten innerhalb der angegebenen Toleranzbereiche. Schlussfolgerungen. Die Studienergebnisse ermöglichen die Festlegung von Bildraten und Eigenschaften des CBU-provozierenden Content (Größe und Position), die das Überschreiten vordefinierter, störender CBU-Grenzwerte vermeiden. Die abgeleiteten Hardwareanforderungen und Content-Empfehlungen ermöglichen ein praxisnahes und evidenzbasiertes CBU-Management. Für die Vorhersage von CBU kann die Modellgenauigkeit weiter verbessert werden, indem Merkmale der menschlichen Wahrnehmung berücksichtigt werden, z.B. die exzentrizitätsabhängige Netzhautempfindlichkeit oder Änderungen der visuellen Wahrnehmung bei unterschiedlichen Arten von Augenbewegungen. Zur Modellierung dieser Merkmale können teilnehmerbezogene Daten der empirischen Forschung herangezogen werden.Background. A color-distorting artifact called Color Break-Up (CBU) has been investigated. Disturbing CBU effects occur when eye movements (e.g., pursuits or saccades) are performed during the presentation of content on Field-Sequential Color (FSC) display or projection systems where the primary colors are displayed sequentially rather than simultaneously. Methods. A mixed design of empirical research and theoretical modeling was used to address the main research questions. Conducted studies evaluated the impact of hardware-based, content-based, and viewer-based factors on the sample’s CBU perception. In a first step, visual performance parameters (e.g., color vision), short-term state (e.g., attention level), and long-term personality traits (e.g., affinity to technology) of the sample were recorded. Participants were then asked to rate the perceived CBU intensity for different video sequences presented by a FSC-based projector. The applied setup allowed the size of the CBU-provoking content (1.0 to 6.0°), its luminance level (10.0 to 157.0 cd/m2), the participant’s eye movement pattern (pursuits: 18.0 to 54.0 deg/s; saccadic amplitudes: 3.6 to 28.2°), the position of retinal stimulation (0.0 to 50.0°), and the projector’s frame rate (30.0 to 420.0 Hz) to be varied. Correlations between independent variables and subjective CBU perception were tested. In contrast, the developed model predicts a viewer’s CBU perception on an objective basis. The model graphically reconstructs the intensity and color characteristics of CBU effects. The visual CBU reconstruction is then compressed into representative model indices to quantify the modeled scenario with a manageable set of metrics. Finally, the model output was compared to the empirical data. Results. High interindividual CBU variability within the sample cannot be explained by a participant’s visual performance, short-term state or long-term personality traits. Conditions that distinctly elevate the participant’s CBU perception are (1) a foveal stimulus position on the retina, (2) a small-sized stimulus during saccades, (3) a high eye movement velocity, and (4) a low frame rate of the projector (correlation expressed by exponential function, r2 > .93). The stimulus luminance, however, only slightly affects CBU perception. In general, the model helps to understand the fundamental processes of CBU genesis, to investigate the impact of CBU determinants, and to establish a classification scheme for different CBU variants. The model adequately predicts the empirical data within the specified tolerance ranges. Conclusions. The study results allow the determination of frame rates and content characteristics (size and position) that avoid exceeding predefined annoyance thresholds for CBU perception. The derived hardware requirements and content recommendations enable practical and evidence-based CBU management. For CBU prediction, model accuracy can be further improved by considering features of human perception, e.g., eccentricity-dependent retinal sensitivity or changes in visual perception with different types of eye movements. Participant-based data from the empirical research can be used to model these features

Activity in area V3A predicts positions of moving objects

No description supplie

Aerospace medicine and biology: A continuing bibliography with indexes, supplement 239, December 1982

This bibliography lists 318 reports, articles and other documents introduced into the NASA scientific and technical information system in November 1982

Novel photoreceptor cells, pupillometry and electrodiagnosis in orbital, vitreo-retinal and refractive disorders

Imperial Users onl

Detection of higher visual function deficits and validation of multifocal pupillography in stroke, chiasmal compression and anterior ischemic optic neuropathy.

It is well established that neural damage can result in visual dysfunction, both visual field loss and in higher visual function (HVF) loss such as perceptions of depth, colour, motion and faces. This thesis examines these visual deficits in the common neurological diseases of stroke, chiasmal compression, and anterior ischemic optic neuropathy (AION). While it is established that isolated complete HVF deficits do occur in stroke, they are also known to be rare. However, as HVFs are not routinely tested in clinical practice, it is unknown how common more subtle defects are, and what tools are effective in detecting these. Chapter 3 explores these questions, outlining that colour and depth perceptions are the most commonly affected, that Ishihara (colour) and stereofly and randot (depth), are the most useful tests, and outlines recommendations for improvement in some of these tools. The relatively new invention of multifocal pupillographic objective perimetry (mfPOP) provides a number of benefits from other forms of perimetry. It measures both eyes at once, allowing measures of direct and consensual responses, it is objective, and it allows repeat measures of each region giving a measure of error. This advancement opens up new opportunities to investigate pupillary physiology in neurological disorders and adds new challenges in how to combine these signals into a single meaningful measure. Chapter 4 investigates the physiology of the pupil in stroke, chiasmal compression, and AION, and investigates how these components can be appropriately combined into a single measure. Results show naso-temporal differences are consistent with known physiology in control subjects and provides evidence that denser nasal retinal input may underpin the greater contraction anisocoria seen in temporal fields than in nasal fields. With the intention that mfPOP be used in clinical practice, it must demonstrate it can perform as well as traditional perimetry, such as Humphrey and Matrix devices, in a wide range of disorders. Currently mfPOP testing neurological disorders has been very limited, and this represents a large gap in the literature. Chapter 5 compares mfPOP to Humphrey and Matrix perimeters, showing they mfPOP does not correlate well with these devices, and compares their utility in neurological disease. It shows that Humphrey appears the most useful device overall, with Matrix being exceptionally good in chiasmal compression, while mfPOP does not appear effective in these disorders. With the first mfPOP approach having limitations in its diagnostic ability, a second stimulus protocol was designed using colour opponency with the measure of response latency (rather than amplitude), thought to preferentially stimulate cortical input to the pupil response, and may allow detection of cortical lesions. Chapter 6 investigates this new colour exchange protocol and latency measure, contrasting with the more common luminance approach used in chapter 5. It shows that the colour protocol shows a number of subtle differences compared to the luminance protocol, but does not show any greater utility in neurological disease. It reveals that latency and amplitude appear to have a weak positive relationship, and that mfPOP repeats appear to correlate well, but all measures have substantial variation. These finding open up a number of future directions, from a larger and more focused HVF study into colour and depth perception, to considering retinal density as contributing towards biases in pupillary components, exploring hemifield ratios as a measure of early detection of chiasmal compression, and trialling other mfPOP methods to determine whether neurological disorders can be detected through pupillometry

Cerebral Metamorphopsia: Perceived spatial distortion from lesions of the adult human central visual pathway

Metamorphopsia is the perceived visual illusion of spatial distortion. Cerebral causes of metamorphopsia are much less common than retinal or ocular causes. Cerebral metamorphopsia can be caused by lesions along the central visual pathway or as a manifestation of epileptogenic discharges. Geometric visual distortions may result from structural lesions of the central visual pathway after reorganisation of the retinotopic representation in the cortex. Very few experimental investigations have been performed regarding cerebral metamorphopsia as it is often viewed as a clinical curiousity and analysis of the perceived distortion is difficult due to its subjective nature. Investigations have been undertaken to understand cortical plasticity as an explanation for visual filling-in. There has been much interest in cortical reorganisation after injuries to the peripheral and central visual pathway. Behavioural experiments aimed at quantifying the possible visual spatial distortion surrounding homonymous paracentral scotomas may be able to demonstrate cortical reorganisation after brain-damage and provide clues regarding the neural processes of visual perception. The aims of the thesis are: 1. To identify which cases of metamorphopsia, both published and unpublished, might be a consequence of cortical spatial reorganisation of retinotopic projections. 2. To investigate perceptual spatial distortion surrounding homonymous paracentral scotomas in adults with isolated unilateral injuries of the striate cortex. A review of the literature describing cases of cerebral metamorphopsia was performed. Metamorphopsia caused by retinal or ocular pathology, psychiatric conditions, drugs or medications were excluded. A retrospective case series of eight patients with metamorphopsia from a cerebral cause was performed in two clinical neurology practices specialising in vision disorders. Two cases who suffered from paracentral homonymous scotomas due to isolated unilateral primary visual cortex (V1) lesions were identified from a Neuro-ophthalmology practice. Neuropsychophysical experiments to investigate visual spatial perception surrounding their scotomas were developed and tested using MATLAB and Psychtoolbox. The use of the term 'metamorphopsia' was only in reference to cases in which contours or lines were experienced as distorted. In the published literature, few cases of cerebral metamorphopsia have been identified as being potentially due to cortical reorganisation. The main result is a statistically significant visual spatial distortion in the visual field surrounding a paracentral homonymous scotoma when compared to a normal control. There is also significant distortion of perception in the subjects' "unaffected" visual hemifield. After lesions of V1, visual perceptual spatial distortions may occur in the visual field surrounding homonymous paracentral scotomas. The spatial distortion may also occur in the normal hemifield possibly due to long-range cortical connections crossing to the other hemisphere through the corpus callosum. A collaborative approach across disciplines within vision science is required to further investigate the mechanisms responsible for perceptual visual illusions. Behavioural testing in brain-damaged cases remains important in developing theories of normal visual processing. New neuroimaging and neuroscience techniques could then test these theories, furthering our understanding of visual perception. An understanding of normal visual perception could allow future modification of neuronal processes to harness cortical reorganisation and potentially restore functional vision in humans with lesions of the central visual pathway