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Introduction

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  • Visual Perception Is a Constructive Process

  • Visual Perception Is Mediated by the Geniculostriate Pathway

  • Form, Color, Motion, and Depth Are Processed in Discrete Areas of the Cerebral Cortex

  • The Receptive Fields of Neurons at Successive Relays in an Afferent Pathway Provide Clues to How the Brain Analyzes Visual Form

  • The Visual Cortex Is Organized into Columns of Specialized Neurons

  • Intrinsic Cortical Circuits Transform Neural Information

  • Visual Information Is Represented by a Variety of Neural Codes

  • An Overall View

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We are so familiar with seeing, that it takes a leap of imagination to realize that there are problems to be solved. But consider it. We are given tiny distorted upside-down images in the eyes and we see separate solid objects in surrounding space. From the patterns of stimulation on the retina we perceive the world of objects and this is nothing short of a miracle.

—Richard L. Gregory, Eye and Brain, 1966

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Most of our impressions of the world and our memories of it are based on sight. Yet the mechanisms that underlie vision are not at all obvious. How do we perceive form and movement? How do we distinguish colors? Identifying objects in complex visual environments is an extraordinary computational achievement that artificial vision systems have yet to duplicate. Vision is used not only for object recognition but also for guiding our movements, and these separate functions are mediated by at least two parallel and interacting pathways.

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The existence of parallel pathways in the visual system raises one of the central questions of cognition, the binding problem. How are different types of information carried by discrete pathways brought together into a coherent visual image?

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Visual Perception Is a Constructive Process

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Vision is often incorrectly compared to the operation of a camera. Unlike a camera, however, the visual system is able to create a three-dimensional representation of the world from the two-dimensional images on the retina. In addition, an object is perceived as the same under strikingly different visual conditions.

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A camera reproduces point-by-point the light intensities in one plane of the visual field. The brain, in contrast, parses scenes into distinct components, separating foreground from background, to determine which light stimuli belong to one object and which to others. In doing so it uses previously learned rules about the structure of the world. In analyzing the incoming stream of visual signals the brain guesses at the scene presented to the eyes based on past experience.

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This constructive nature of visual perception has only recently been fully appreciated. Earlier thinking about sensory perception was greatly influenced by the British empiricist philosophers, notably John Locke, David Hume, and George Berkeley, who thought of perception as an atomistic process in which simple sensory elements, such as color, shape, and brightness, were assembled in an additive way, component by component. The modern view that ...

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