Using Contextual Effects To Test Theories Of Coding In Visual Cortex
Funder
National Health and Medical Research Council
Funding Amount
$200,500.00
Summary
The visual cortex is the main structure in the brain that processes the visual scene. Cells in the cortex respond selectively to features of the scene such as the orientation of objects, the direction they move and their brightness relative to the background. Cortical cells are arranged in a topographic map of visual space, so that nearby cells respond to light from nearby parts of the image. Recent advances have shown that cells talk to each other so a stimulus in one part of the visual field c ....The visual cortex is the main structure in the brain that processes the visual scene. Cells in the cortex respond selectively to features of the scene such as the orientation of objects, the direction they move and their brightness relative to the background. Cortical cells are arranged in a topographic map of visual space, so that nearby cells respond to light from nearby parts of the image. Recent advances have shown that cells talk to each other so a stimulus in one part of the visual field can influence the responses of cells looking at other regions. This communication between cells is important in guiding the brain to focus on areas of the visual scene that are most important, a process known as attention. An example would be that a mouse moving through the periphery of someone's vision would attract their attention away from objects elsewhere in the scene. This project is designed to study the way that cells in the visual cortex cooperate to guide attention. Attention is important because it reduces the need to process all the detail in the visual scene with the same level of accuracy, leaving more resources free to process what is important. Attention deficits are a problem for people with dyslexia, so understanding the physiological basis of attention is an important goal. As well as attention, the visual system has a range of other mechanisms to select important information from the visual scene. For example, visual adaptation tends to improve the ability to code changes in the visual scene at the expense of reducing the sensitivity of the system overall. This project will investigate the relationship between attentional and adaptive mechanisms in the visual cortex. We expect to establish the precise physiological mechanisms that drive adaptive and attentional mechanisms in the mammalian brain.Read moreRead less
Multidimensional Coding Of Visual Information In The Retina
Funder
National Health and Medical Research Council
Funding Amount
$359,431.00
Summary
Although both the retina and visual cortex are part of the central nervous system, the coding of visual information in the two laminar structures differs markedly in that all three dimensions of the cortical sheet are used to code multiple response axes but only one dimension of the retinal sheet. This project examines how visual response properties are mapped through the depth of the retina and this will provide a comparatively simple paradigm of complex information processing in the brain.
Experimental Validation Of A Clinical Indicator Of Utricular Function.
Funder
National Health and Medical Research Council
Funding Amount
$198,689.00
Summary
The vestibular system is responsible for our sense of balance, it is located in the inner ear and is responsible for maintaining posture and helping an organism to make appropriate eye movements when the head moves. Damage to the vestibular system by disease or accident is extremely debilitating for a patient. A chief goal of our research program is to develop simple tests of the vestibular system that can be used in the clinic to diagnose vestibular disorders. Most present clinical tests only t ....The vestibular system is responsible for our sense of balance, it is located in the inner ear and is responsible for maintaining posture and helping an organism to make appropriate eye movements when the head moves. Damage to the vestibular system by disease or accident is extremely debilitating for a patient. A chief goal of our research program is to develop simple tests of the vestibular system that can be used in the clinic to diagnose vestibular disorders. Most present clinical tests only test the part of the vestibular system that responds to angular rotations (the semicircular canals). There are few good tests that can reliably diagnose problems concerned with sensed position (the otoliths). Recent evidence from human studies has shown that by delivering small electrical currents (galvanic stimulation) via electrodes located on the surface of the skin behind the ears, a characteristic pattern of eye movement occurs. Our hypothesis is that the distinctive pattern of eye movement produced is a result of otolith stimulation. The aim of this project is to determine exactly how these small currents produce the eye movements and if these eye movements are in fact mediated by the otoliths. To determine the physiology that underlies these types of responses we are unable to conduct these experiments in humans. Our present program will therefore use both behavioural and physiological experiments on guinea pigs to test our hypotheses about galvanic stimulation and otolith function. The significance of this work lies in that it will lead to a new way of evaluating the function of the gravity sensing part of the human vestibular system which appear to be uniquely important for patients with balance problems.Read moreRead less
Electroretinogram Recordings Of Human Scotopic Dark Adaptation Following Intense Bleaching Exposures
Funder
National Health and Medical Research Council
Funding Amount
$272,250.00
Summary
After a human subject has been exposed to intense illumination, it can take many minutes for the eye to regain full sensitivity, as one experiences (for example) when entering a dark cave after being out on a bright sunny beach. This project will investigate the processes that occur in the cells of retina lining the back of the eye, that prevent the instantaneous recovery of vision following intense illumination. Electrical recordings will be made from the eyes of normal individuals, using new t ....After a human subject has been exposed to intense illumination, it can take many minutes for the eye to regain full sensitivity, as one experiences (for example) when entering a dark cave after being out on a bright sunny beach. This project will investigate the processes that occur in the cells of retina lining the back of the eye, that prevent the instantaneous recovery of vision following intense illumination. Electrical recordings will be made from the eyes of normal individuals, using new techniques that allow the activity of different types of nerve cell in the retina to be monitored. The study will determine how it is that events in the light-detector cells of the eye (the rod and cone photoreceptors) influence the activity of subsequent nerve cells in the visual system, and how these events contribute to the poor vision that one experiences following bright lights.Read moreRead less
Behavioural And Physiological Tests Of Clinical Indicators Of Vestibular Function.
Funder
National Health and Medical Research Council
Funding Amount
$205,500.00
Summary
Part of the inner ear is responsible for our sense of balance and for maintaining clear vision and stable posture. Recurring vertigo attacks and persistent imbalance can disrupt the most productive years of patients' lives and balance-related falls account for more the half of accidental deaths in the elderly. Despite this most doctors find dizziness difficult to diagnose and impossible to treat, so specialist Balance Disorders Clinics see many patients a year but still have very long waiting ti ....Part of the inner ear is responsible for our sense of balance and for maintaining clear vision and stable posture. Recurring vertigo attacks and persistent imbalance can disrupt the most productive years of patients' lives and balance-related falls account for more the half of accidental deaths in the elderly. Despite this most doctors find dizziness difficult to diagnose and impossible to treat, so specialist Balance Disorders Clinics see many patients a year but still have very long waiting times. There is then clearly a major need to improve the understanding and the treatment of dizzy patients and improvement will come about through scientific understanding of vestibular dysfunction, and technological realization of improvements in the assessment and treatment of patients. This project seeks to identify whether 3 simple new indicators of balance function are valid, using tests on guinea pigs to understand the way in which they work.Read moreRead less
Oculomotor Tests Of Otolith Function Using Human Centrifugation
Funder
National Health and Medical Research Council
Funding Amount
$284,450.00
Summary
The effects of dysfunction of the balance system of the inner ear (the vestibular system) can range from mild brief episodes of dizziness to almost complete incapacitation: where the patient feels unstable, their posture is unstable, their vision is unstable. In order to guide appropriate treatment and rehabilitation, diagnostic tests are needed to identify whether the dysfunction is due to a brain problem or a problem in the inner ear, and if an ear problem - which part of the inner ear system ....The effects of dysfunction of the balance system of the inner ear (the vestibular system) can range from mild brief episodes of dizziness to almost complete incapacitation: where the patient feels unstable, their posture is unstable, their vision is unstable. In order to guide appropriate treatment and rehabilitation, diagnostic tests are needed to identify whether the dysfunction is due to a brain problem or a problem in the inner ear, and if an ear problem - which part of the inner ear system is affected. By combining basic research with clinically feasible methods my colleagues and I have, with NH and MRC support, developed some of the most widely used clinical tests of balance function available today (see Halmagyi, Cremer and Curthoys 2003 for a review). In this Project I seek to continue the development of a test of part of the balance system (the utricular macula) for which there are few acceptable tests. I will do that by using the unique facilities available at Sydney University and RPA Hospital, presenting controlled stimuli (linear accelerations) to healthy human subjects and patients with known vestibular losses, and measuring precisely their eye movement responses to these stimuli using computerized image processing techniques. The experiments will test an hypothesis about how part of the vestibular system, the utricular macula, works and controls eye movements. One eye movement response, rolling of the eye around the line of sight (called ocular torsion) during maintained roll head-tilts (to the patients left or right) may be due to the joint activation of two regions of the balance receptors: the utricular and saccular maculae. As a result of a recent direct test we have managed to conduct on two normal subjects, we think that hypothesis should be thoroughly re-tested on subjects and patients with known vestibular losses and in doing so provide new understanding of otolith function which may lead to new clinical tests.Read moreRead less
Role Of Cortico-cortical Connections In Mediating Cerebral Cortex Plasticity: Visual Cortex Model
Funder
National Health and Medical Research Council
Funding Amount
$362,500.00
Summary
In mammals injury to the retina not only affects the neurones within the eye but also induces changes in the other parts of the brain, particularly in the visual cortex. It has been found that after retinal injury cells in the visual cortex, normally receiving an input from the injured part of the retina, now receive an input from adjacent normal retina ( ectopic receptive field ). All mammals with well developed vision have a large number of separate visual cortical areas (more than 30 in prima ....In mammals injury to the retina not only affects the neurones within the eye but also induces changes in the other parts of the brain, particularly in the visual cortex. It has been found that after retinal injury cells in the visual cortex, normally receiving an input from the injured part of the retina, now receive an input from adjacent normal retina ( ectopic receptive field ). All mammals with well developed vision have a large number of separate visual cortical areas (more than 30 in primates). These areas are arranged in a hierarchy in which it is thought that as features of the visual stimuli become more complex they are discriminated in the areas higher in the hierarchy. These higher-order areas also project back to lower-order areas. This feedback activity from the higher areas can be reversibly abolished by cooling a given area to about 10oC and then rewarming it back to its normal temperature. We will try to determine if in cats (animals with well developed vision) following damage to a small region of the retina the feedback activity from higher visual cortical areas affects the ectopic receptive fields in the lower visual cortical areas. Another possibility is that the ectopic receptive field apparent following retinal damage might depend on horizontal connections within the particular cortical area, running from normal cortex to the area of cortex affected by the lesion. We propose to test this idea by blocking reversibly (with chemical agents) transmission of these horizontal fibres and determining the characteristics of neurones in the area affected by the lesion. Understanding the role of feedback and horizontal cortico-cortical connections in establishing new ectopic receptive fields following spatially delineated damage to the retina will help us to understand the mechanisms underlying perceptual distortions and visual hallucinations which occur following retinal traumas or some age-related retinal degenerations.Read moreRead less
Neuronal Basis Of Stimulus Dependent Receptive Field Properties And The Role Of Feedback Projections
Funder
National Health and Medical Research Council
Funding Amount
$258,000.00
Summary
In mammals with a number of distinct visual cortical areas the processing of information in the visual cortex largely follows a hierarchical order. It has been widely assumed that the neurones at the highest processing level in the visual system are capable of extracting behaviorally significant features from the external visual world by virtue of their large receptive fields. However, there are massive and dense inter-connections between the cortical areas and intra-connections between the neur ....In mammals with a number of distinct visual cortical areas the processing of information in the visual cortex largely follows a hierarchical order. It has been widely assumed that the neurones at the highest processing level in the visual system are capable of extracting behaviorally significant features from the external visual world by virtue of their large receptive fields. However, there are massive and dense inter-connections between the cortical areas and intra-connections between the neurones within the same cortical area. For example the information at the higher processing levels may flow back to the lower ones via the feedback connections. Thus, it is conceivable that the neurones in the primary visual cortex (at the first stage of cortical processing) may posses the properties allowing them to integrate a considerable amount of information from a large area in visual space due to the existence of a dense web of connections. We wish to study the neuronal basis of perceptually related properties in primary visual cortex by examining the detailed receptive field properties of individual neurons and their response characteristics when more complicated visual stimuli are presented in visual space. We will also examine the influence of the feedback connections on the properties of these neurones by silencing the higher-order visual cortical areas which inversely connect to primary visual cortex. It is hoped that by relating our understanding of the basic neuronal properties to their functional roles in visual processing we will obtain further insights concerning the contributions of individual visual cortical areas (primary visual cortex in this project) to the function of visual perception.Read moreRead less
Orientation-specific Contextual Modulation In Human Visual Cortex
Funder
National Health and Medical Research Council
Funding Amount
$290,413.00
Summary
Context has a strong infuence on our visual perception. We will study patterns of activity in the normal human brain to identify the cortical signature of contextual modulation in vision. The correspondences between patterns of brain activity and visual perception in the normal human brain will provide data against which brain activity in disorders such as schizophrenia and bipolar disorder can be assessed.