cerebral achromatopsia The inability to perceive and discriminate colors due to pathology within the cerebral cortex. It is to be distinguished from the more common form of color blindness caused by anomalies in the photoreceptors of the eye.

cerebral akinetopsia A rare deficit in the ability to perceive and discriminate movement thought to arise from lesions of cortical visual areas involved in the processing of visual motion information.

functional magnetic resonance imaging (fMRI) Several neuroi-maging techniques capable of showing areas of the brain that are activated during sensory, motor, or cognitive tasks. The most common technique highlights areas of the brain in which blood flow and oxygenation are altered by the experimental task. fMRI images are typically displayed as an overlay on conventional structural MRI images depicting the anatomical structure of the brain.

positron emission tomography (PET) Like functional MRI, PET imaging is capable of showing areas of the brain that are active during sensory, motor, or cognitive tasks. PET is based on the injection and measurement of radioactive tracers whose concentrations are altered at brain sites where activity is experimentally altered. Depending on the tracer, this technique can be sensitive to changes in oxygen or glucose utilization or other chemical factors.

prosopagnosia Deficit in the ability to recognize previously familiar faces. It is thought to be associated with lesions of ventral occipitotemporal cortex.

receptive field That region of the retina (one in each eye), or equivalently the visual field, within which a stimulus is able to alter the response of a visual system neuron.

retinotopy The representation of retinal topography within the connections, nuclei, and cerebral cortex of the visual system. A

retinotopic map is a collection of neurons whose receptive fields, in the aggregate, preserve the two-dimensional topography of the photoreceptors that provide their input, albeit to a variable degree of precision and completeness depending on the brain site. A retinotopic map may be distorted, typically due to expansion of the area devoted to the visual field near the center of gaze.

stereopsis The phenomenon of simultaneous vision with two eyes in which there is a vivid perception of the distances of objects from the viewer; it is present because the two eyes view objects in space from two points, so that the retinal image patterns of the same object are slightly different in the two eyes.

V1, V2, V3, VP, V4, hMT+, V8, FFA, LO, and KO Abbreviated names of human cortical visual areas. V1 is the primary visual cortex, also known as striate cortex. Other visual areas are collectively referred to as extrastriate visual cortex. Some extrastriate visual areas are simply numbered (e.g., V2, V3, V4, and V8); others have more descriptive names. VP, ventral posterior visual area; hMT+, middle temporal1 visual area, also known as V5; FFA, fusiform face area; LO, lateral occipital visual complex; KO, kinetic occipital region.

visual field That portion of the world visible to the observer at any given instant. The foveal visual field refers to the portion of the visual field near the center of gaze (strictly that portion imaged on the fovea of the retina).

The occipital lobe encompasses the most posterior portion of the human cerebral cortex and is primarily responsible for vision. Direct electrical stimulation of the occipital lobe produces overtly visual sensations. Damage to the occipital lobe typically results in either complete or partial blindness or visual agnosias, depending on the location and severity of the lesion. This article will summarize the structure and function

1Although hMT+ derives its name from the homologous monkey visual area, its location is near the junction of the occipital, parietal, and temporal lobes on the lateral aspect of the brain, not in the middle of the temporal lobe as its name would seem to imply.

Encyclopedia of the Human Brain Volume 3

Copyright 2002, Elsevier Science (USA).

All rights reserved.

of the human occipital lobe, stressing its role in vision. Although the discussion is primarily based on studies of normal human subjects, clinical reports of braindamaged patients and animal studies are discussed when they directly illuminate particular issues.

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