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Visual Neuroscience

Visual Neuroscience Basics

Definition & Goals

Visual neuroscience is a field of Neuroscience dedicated to the understanding of the structure and function of vision, visual processing, and the visual system which contains the visual cortex. Visual neuroscience studies the anatomy, physiology, psychology, and scientific functions of the visual neural system.

History & Foundations

Hermann Helmholtz, born on August 31, 1821 was a significant pioneer in the field of Visual Neuroscience. Helmholtz, interested in physics and visual optics since childhood, coined the modern psychology visual field of Sensation and Perception. In Helmholtz's Handbook of Physiological Optics (1856-1866), he conducted experiments on subjects that were divided into physical, physiological, and psychological categories to study sensation and perception. Helmholtz was able to distinguish between the "raw elements" of conscious experience (sensations) and "meaningful interpretations" given to sensations called perceptions. Sensations are void of previous education and interpretation; such as various points of light and different colored hues in an individual's vision. Perceptions, however, require comprehension and bring meaning to these sensations, such as the ability to focus the points of light and different colored hues into perception of a picture. Hermann Helmholtz also studied physical, physiological, and psychological elements of the eye which added great contributions to the field of Visual Neuroscience.

Rene Descartes, born on March 31, 1596, had earlier believed that the retina consisted of minute fibers in the eye that created the fibers of the optic nerve. He thought that their size defined the limits of what could be seen. He estimated the diameters of nerve fibers on the basis of human visual acuity. Descartes found that the diameters of nerve fibers in the retina are one-7200th part of an inch (0.0035 mm), which is based on the resolution of one minute of arc as the minimum visible. A minute of an arch of is an angular measurement that equals one-sixtieth of a degree. Because a degree is one three-hundred and sixtieth (1/360) of a rotation, one minute of an arc is 1/21,600 of a rotation. So 1/21,600 of a single rotation is the visual minimum that average visual acuity can see.

Processing of Visual Percept within Visual Cortices

Processing visual information in a real world setting is an intensely complex task. Varying conditions of light, motion, shape, and texture complicate this task even further. Considering this it naturally follows that visual processing is not a single step task, rather it is a complex multi-layered task. As such the visual cortex (the location where visual processing occurs) is not a single layered portion, it is divided up into primary visual cortex, secondary visual cortex, and tertiary visual cortex. Each “layer” of cortex corresponds to increasingly difficult types of processing tasks, and each requires further integration from the previous levels of cortex. Before exploring this complex organization it should be noted that the organization of visual pathways is contralateral. That is to say the right portion of the brain receives signals from the left eye and likewise the left portion of the brain receives signals from the right eye. Beyond this visual processing occurs in a cumulative fashion beginning in the Primary Visual Cortex, followed by more complex integration performed by the Secondary Visual Cortex, and finally the most Complex integration and perception performed by Tertiary Visual Cortex.

Processing in Primary Visual Cortex

The Primary visual cortex, also known as the striate cortex, or simply V1 performs the most basic tasks associated with visual processing. When viewing an image V1 begins to discern boundaries of the objects in view, it does so by responding to basic elementary stimuli. Objects such as “bars” (Vertical boundaries or lines) and “gratings” (Horizontal or other crosshatch type boundaries) elicit the most response from cells in V1, essentially visual information is filtered according to linear and non-linear groupings. This lower level processing creates a basic framework of an image, something analogous to wire mesh recreation of the image.

Processing in Secondary Visual Cortex

Secondary visual cortex, also known as the V2 prestriate cortex, or simply V2 begins the process of assembling visual sensations while simultaneously integrating information from V1 into meaningful percepts. When viewing an image V2 receives the information previously processed by V1 and begins to define the image further by separating objects from their backgrounds. This occurs when V2 detects differences in luminance, differences in contrast, as well as changes in object texture. This occurs as a result of neuronal excitation, neurons within V2 respond to orientation, as well as spatial frequency and directionality or movement within a given image. This second order processing is then integrated with the information passed on from V1 and is combined to create a more complex image. This integration takes the basic framework or an image created within V1 and transforms it into something analogous to an unfinished, working animation if we are to continue on with the model discussed previously. The basic image representation is present at this point, but detail is absent, more importantly identification of objects within the image and association of these.

Processing in Tertiary Visual Cortex

Tertiary Visual cortex, also termed tertiary association cortex and extrastriate cortex, or simply V3 is responsible for the most complex visual processing tasks, and while integrating information previously interpreted by earlier visual cortices. When viewing an image V3 receives the information discerned by V1 and V2 and begins to finalize the image. Final definition of the object beings, coloration, motion, and lighting conditions are integrated. However the most important function of V3 is the integration of all image information while employing higher-level cognition in order to recognize and identify the objects being viewed. At this point in the stream of processing, the image has been completed and objects within the visual field have been compared against existing knowledge and memories and recognition has occurred.

Eye Movement and Attention

Eye movements are extremely important to the human beings visual perception of the world. For example, high acuity vision is only possible within the fovea consisting of only about 1/40th or 1% of the retinal surface area. Eye movements redirect incoming light onto specific regions such as the fovea allowing the visual system to operate correctly.

Eye Movement Types

Eye movements have been categorized in a variety of ways. The list provided below is neither a distinct nor comprehensive categorization of eye movements but a listing of the primary types along with their definitions.

• Saccade: When we read or search for an object, the human eye doesn’t actually move continuously, rather many short and rapid eye movements are made; these movements are called saccades. Saccades are rapid movements of the eyes with velocities as high as 500 angular degrees per second. In fact even when we are simply viewing a scene, although it may seem that we are taking in the whole scene simultaneously our eyes are actually completing many saccadic movements. These saccades allow our vision to transition between all of the important parts of the scene and stitch them together in order to attend the image as a whole.

• Smooth Pursuit: Smooth pursuit eye movements allow primates to rotate their eyes smoothly so that the fovea remains pointed at slowly moving objects. This type of eye movement is experienced while tracking moving objects across your vision. Smooth pursuit is impossible for most people to begin without a moving stimulus.

• Vergence: Vergence is the simultaneous inward and outward movement of both eyes. Like smooth pursuit, vergence allows primates to keep images of interest focused on the foveal region of the retina. The process of vergence is most salient to humans as one attempts to focus on a close object and feels the eyes straining to turn inwards.

• Vestibulo-ocular: Vestibular eye movements occur when the eyes rotate to compensate for head and body movements in order to maintain the same direction of vision. Without the vestibular system humans would be unable to remain upright as this system contributes largely to the process of balance.

• Miniature Eye Movements: Miniature eye movements, also called microsaccades, are extremely small eye movements, so small that they are constantly happening yet we as human beings do not experience them in our vision. Even when gaze is fixed on a specific object the eyes are physically moving ever so slightly. This area of eye movements comprises a category of vision that remains somewhat debated. Some claim that miniature eye movements have no practical application and play no important role in vision. However, others have hypothesized that these minute movements of the eyes prevent the process of adaptation that would be experienced if the eyes did not move at all.

Perception

Color perception

The term color perception is how we experience the hue, saturation and brightness of a color. The manner in which we organize our experience of color must have a correlate with neural activity at some level of the visual system, so the manner in which the information is sampled, integrated and transmitted in the visual system should be related to the representation of color. The historical broad usage of the term color to refer to physiological or physical properties and events, rather than to perceived phenomena, is a common misconception and source of confusion.

As we progress through the visual system, the receptive fields expand to encompass larger portions of the visual field. This suggests that at later stages of visual processing, the responses of cells are influenced by events occurring at locations further away from the centers of receptor fields. For example, the colors that we attribute to lights and surfaces are highly dependent on the spatial, temporal and chromatic properties occurring elsewhere in the visual field. Therefore the neural processes mediating color perception are intertwined with those in object perception.

Depth perception

Identifying the spatial layout of the variety of different objects and surfaces that make up our surroundings is a crucial goal of depth perception. The perception of depth is actively organizing the depth estimates into meaningful sections. To determine an inconsistency of an element, the element must be localized in the visual system in two retinal images. Once the matching image features have been identified, the difference in retinal location is the retinal disparity,which can be utilized to estimate depth.

Lesion Studies

A brain lesion is an area of tissue that has been damaged through injury or disease. Early researchers, Tatsuji Inouye and George Holmes studied soldiers with bullet wounds to their primary visual cortex. Studies showed that the primary visual cortex is crucial for visual function because when parts of the primary visual cortex were damaged from the bullets, scotomas would appear. A scotoma is a loss of vision in the primary vision field. Inouye and Holmes discovered that the parts of the brain that were damaged directly corresponded to where the blind spots of the vision field were located. Some participants reported loss of visual awareness but maintained visual function, known as "blindsight". Humans with blindsight are not able to recall awareness of a stimuli, but are able to describe the location, shape, and direction of movement of the stimuli noted as "black moving on black". Monkeys with unilateral primary visual cortex lesions were able to report stimuli that was present, similar to visual awareness in humans, but were able to characterize properties of the stimuli under forced conditions. As stated in an article by Frank Tong, "Primary Visual Cortex and Visual Awareness" these findings on humans and primates means that processing may be different from awareness, yet it cannot be said that awareness is completely absent under blindsight. In a study conducted by the Department of Psychology at Princeton University, one patient reported seeing "afterimages", images of previously seen stimuli, after the onset of blindsight. Although the patient lost partial sight and visual awareness, they were able to see images of objects they saw before blindsight and could adapt these images to the shapes they saw after blindsight.

Lesion damange in other areas of the visual cortex causes more defined impairments in visual perception. Lesions in the posterior extrastriate areas, directly connected to the primary visual cortex can cause loss of perceptual grouping, motion perception, color perception, and/or face or object recognition. Lesions in the posterior parietal lobe and/or superior temporal lobe of the brain can cause inadequate awareness or visual attention. Other lesions such as unilateral lesions (which occur on only one hemisphere of the brain) can cause spatial neglect which is the failure to report awareness of presentation of a stimuli. Bilateral lesions (which occur on both hemispheres of the brain) can cause Balint's syndrome, the failure to be able to focus on more than one object at a time.

Bilateral brain lesions can occur between both sides of the brain; "simultanagnosia", a visual attention disorder that causes the inability to process more than one object at one time and occurs on the parieto-occipital junction of the brain. Individuals with simultanagnosia can only process one individual object even if multiple objects are present. It has been noted that individuals with this disorder have a "restricted window" of attention which prevents the person from seeing the entire view of what is happening.Unilateral brain lesions can occur on either hemisphere of the brain. Unilateral lesions on the parleto-temporo-occipital and fronto-parleto-temporal cortex can cause "achromatopsia" which is a loss of color perception. Individuals see colors as pale, washed-out, gray, or dim.