In the mid-sixties, John Robson and Christina Enroth-Cugell, without realizing what they were doing, set off a virtual revolution in the study of the visual system. They were trying to apply the methods of linear systems analysis (which were already being used to describe the optics of the eye and the psychophysical performance of the human visual system) to the properties of retinal ganglion cells in the cat. Their idea was to stimulate the retina with patterns of stripes and to look at the way that the signals from the center and the antagonistic surround of the respective field of each ganglion cell (first described by Stephen Kuffier) interact to generate the cell's responses. Many of the ganglion cells behaved themselves very nicely and John and Christina got into the habit (they now say) of calling them I (interesting) cells. However. to their annoyance, the majority of neurons they recorded had nasty, nonlinear properties that couldn't be predicted on the basis of simple summ4tion of light within the center and the surround. These uncoop erative ganglion cells, which Enroth-Cugell and Robson at first called D (dull) cells, produced transient bursts of impulses every time the distribution of light falling on the receptive field was changed, even if the total light flux was unaltered.
Considerable evidence exists that visual sensory information is analyzed simultaneously along two or more independent pathways. In the past two decades, researchers have extensively used the concept of parallel visual channels as a framework to direct their explorations of human vision. More recently, basic and clinical scientists have found such a dichotomy applicable to the way we organize our knowledge of visual development, higher order perception, and visual disorders, to name just a few. This volume attempts to provide a forum for gathering these different perspectives.
The last 20 years of research have been marked by exceptional progress in understanding the organization and functions of the primate visual system. This understanding has been based on the wide application of traditional and newly emerging methods for identifying the functionally significant subdivisions of the system, their interconnections, the
Measurement of visual acuity has been the cornerstone of visual testing since Snellen began quantitating visual acuity using letter optotypes in the 1860s. Bjerrum in the 1880s brought sophistication and quantitation to the assessment of the visual field with tangent screen examination using differently sized and colored targets. Further advances in visual testing did not occur until the Goldmann perimeter and the Farnsworth Munsell 100 Hue test were introduced in the 1940s, permitting further refinement in the detection and quantitation of acquired visual loss. An explosion of interest in sensory visual function testing followed the demonstration by Quigley and his colleagues in 1982 that despite the loss of more than 40% of the axons in the optic nerve, Snellen acuity and kinetic perimetry remained normal. Much of this interest has focused on a search for more sensitive and disease-specific sensory visual tests. Previously, novel tests used to probe visual function remained in the province of the visual physiologist and psychophysicist. These tests are now being introduced by the ophthalmologist into clinical practice. Concomitantly, the mass production of microcomputers and other technical advances have made tests such as automated perimetry and visual evoked response testing affordable for most offices. The clinician is presently being inundated with a plethora of visual function tests that may require a knowledge of visual psychophysics and statistics to understand and interpret. The purpose of this book is to acquaint the clinician with these new tests so that they may be used to maximum benefit.
'Vision and the Visual System' offers students, teachers and researchers a rigorous, yet accessible account of how the brain analyses the visual scene. Schiller and Tehovnik describe key aspects of visual perception such as colour, motion, pattern and depth while explaining the relationship between eye movements and neural structures in the brain.
All together, these studies provide strong evidence for the existence of multiple circuits between LGN and MT. Each pathway receives a different combination of M and P inputs and is uniquely suited to convey visual information with varying degrees of spatial and temporal resolution, contrast sensitivity, and color selectivity. Distinct cell types underlie many of these circuits, overcoming the lack of spatial compartmentalization of V1 outputs. Functional studies that can target these specialized cell types and circuits will be necessary to elucidate the contributions of each pathway to response properties in MT and, ultimately, to visual perception and behavior.
Foremost neurophysiologists and psychophysicists provide pertinent information on the nature of representation at the earliest stages as this will constrain the disposition of all subsequent processing. This processing is discussed in several different types of visual perception.
Perception consists of the brain's single best interpretation of the sensory world at a given moment in time. Multiple channels of visual input - be they from the two eyes or from the many parallel visual pathways that originate as early as the retina - must be reconciled to arrive at a unified percept. The fact that this must occur in roughly real time as the visual scene changes poses special challenges and constraints. I investigated two classes of visual processes relevant for the perception of time-varying visual stimuli: prediction, with a probable neural substrate in early visual cortical areas, and parallel processing in the magnocellular (M) and parvocellular (P) pathways. In Chapters 2 and 3, I asked how prediction and parallel pathways, respectively, contribute to perceptual selection using dynamic binocular rivalry stimuli. In binocular rivalry, incompatible images presented to the two eyes result in just one of the images being selected for awareness at any given time. This bistability makes rivalry a useful tool for the study of perceptual selection. In Chapter 2, we found that predictive context in the form of an unambiguous rotating grating biased perceptual selection during subsequent rivalry towards the expected next grating in the rotation sequence, compared to an orthogonal grating. This provided evidence that a prediction-like process influences perceptual selection during rivalry between gratings, which other work has shown is likely resolved at early stages of visual processing. In Chapter 3, we studied spatial, temporal, luminance, and chromatic factors influencing perceptual selection during interocular switch rivalry. In this type of rivalry, flickering orthogonal gratings are periodically exchanged between the two eyes, resulting in either the perception of a fast, regular alternation between orthogonally oriented gratings (similar to the display presented to a single eye) or a slow, irregular alternation, a percept that requires integration across the two eyes over time. We found that stimuli biased toward the M pathway increased the prevalence of fast, regular alternations, while stimuli biased toward the P pathway increased the prevalence of slow, irregular alternations. This finding suggested that the M and P pathways can make distinct contributions to perception during binocular rivalry and led us to propose a new framework for understanding perceptual selection during interocular switch rivalry. Physiological measurement of activity in the M and P pathways can lead to greater understanding of how these pathways contribute to perceptual experience, but methods for measuring functional signals from the M and P pathways of humans have been lacking. Therefore, in Chapter 4, we developed a procedure for functionally mapping the M and P subdivisions of human LGN, the site where these pathways are most clearly segregated, using functional magnetic resonance imaging (fMRI). We observed a gradient of more M-like to more P-like responses across the LGN. Importantly, this gradient had a spatial layout consistent with known LGN anatomical organization. This new method for localizing the M and P subdivisions of the LGN provides a way forward for investigating the function of these pathways in human visual perception, in both healthy and clinical populations. In summary, prediction and parallel processing are two classes of mechanisms that contribute to perception of dynamic visual stimuli. Here we have shown how such mechanisms operating at low levels of the visual system can help resolve competition between percepts, thereby affecting the contents of visual awareness. In addition, we developed a method for the physiological study of the M and P LGN subdivisions in the human brain, which is a promising technique for the future investigation of the roles of the M and P pathways in human visual perception, among other applications.
This is the story of a hugely successful and enjoyable 25-year collaboration between two scientists who set out to learn how the brain deals with the signals it receives from the two eyes. Their work opened up a new area of brain research that led to their receiving the Nobel Prize in 1981. The book contains their major papers from 1959 to 1981, each preceded and followed by comments telling how and why the authors went about the study, how the work was received, and what has happened since. It begins with short autobiographies of both men, and describes the state of the field when they started. It is intended not only for neurobiologists, but for anyone interested in how the brain works-biologists, psychologists, philosophers, physicists, historians of science, and students at all levels from high school to graduate level.
