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Visual Agnosia

Visual Agnosia

Visual agnosia is a condition characterized by the inability to recognize visually presented objects despite having normal vision. For instance, a person with visual agnosia can describe the shape and color of an object but cannot identify or name it. This impairment does not affect their visual field, acuity, color vision, brightness discrimination, language, or memory. An example of this condition in a social setting is someone at a dinner party asking for "that silver thing with a round end"...

Vision

Vision

Vision is the result of light being detected and transduced into neural signals by the retina of the eye. This information is then further analyzed and interpreted by the brain. First, light enters the front of the eye and is focused by the cornea and lens onto the retina—a thin sheet of neural tissue lining the back of the eye. Because of refraction through the convex lens of the eye, images are projected onto the retina upside-down and reversed.

Visual System

Visual System

Light enters the eye through the cornea, a transparent, dome-shaped surface covering the surface of the eyeball that helps to direct and focus incoming light. This light is then channeled toward the pupil, an adjustable opening whose size is controlled by the iris. The iris, a pigmented muscle, regulates the amount of light entering the eye by contracting or dilating the pupil, thereby ensuring optimal light levels for clear vision.
Once through the pupil, the light passes through the lens, a...

Association Areas of the Cortex

Association Areas of the Cortex

Association areas are regions of the cerebral cortex that do not have a specific sensory or motor function. Instead, they integrate and interpret information from various sources to enable higher cognitive processes such as memory, learning, and decision-making. Some key association areas include the following:
Prefrontal Association Area: This area is located in the frontal lobe and is involved in planning, decision-making, and moderating social behavior. It connects with primary motor areas,...

Motor and Sensory Areas of the Cortex

Motor and Sensory Areas of the Cortex

The cerebral cortex, the brain's outermost layer, is pivotal in processing complex cognitive tasks, emotions, and various sensory inputs and executing voluntary motor activities. This intricate structure is divided into three primary functional areas: the motor areas, sensory areas, and association areas.
Motor Areas
The motor areas located in the frontal lobe are central to controlling voluntary movements. This region is further subdivided into the primary motor cortex and the premotor cortex.

Photoreceptors and Visual Pathways

Photoreceptors and Visual Pathways

At the molecular level, visual signals trigger transformations in photopigment molecules, resulting in changes in the photoreceptor cell's membrane potential. The photon's energy level is denoted by its wavelength, with each specific wavelength of visible light associated with a distinct color. The spectral range of visible light, classified as electromagnetic radiation, spans from 380 to 720 nm. Electromagnetic radiation wavelengths exceeding 720 nm fall under the infrared category, whereas...

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Related Experiment Video

Updated: Jun 27, 2026

A Gaze-Contingent Display Framework for Perceptual Learning Research with Simulated Central Vision Loss

A Gaze-Contingent Display Framework for Perceptual Learning Research with Simulated Central Vision Loss

Published on: April 11, 2025

Seeing with profoundly deactivated mid-level visual areas: non-hierarchical functioning in the human visual cortex.

Sharon Gilaie-Dotan¹, Anat Perry, Yoram Bonneh

¹Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel.

Cerebral Cortex (New York, N.Y. : 1991)

|November 19, 2008

Summary

Visual processing typically relies on sequential activation from early to high-level areas. However, case LG shows robust high-level visual function despite deactivated intermediate visual areas (V2-V4).

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Area of Science:

Neuroscience
Cognitive Science
Visual Perception

Background:

Object recognition models posit hierarchical processing from early visual cortex (V1) to higher-order areas.
Activity in higher-order visual regions is generally assumed to depend on bottom-up input from earlier retinotopic areas.

Purpose of the Study:

To investigate visual processing in a rare case (LG) of developmental object agnosia and prosopagnosia.
To examine the relationship between activity in early, intermediate, and high-order visual areas.
To explore the neural basis of visual perception and its dissociation from everyday visual functioning.

Main Methods:

Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) were used to assess brain activity.
Visual stimuli were presented to case LG, a patient with developmental object agnosia and prosopagnosia.
Brain activation patterns were analyzed in response to different visual categories (faces, objects, houses, places).

Main Results:

Primary visual cortex (V1) showed robust activation, but intermediate visual areas (V2-V4) were significantly deactivated.
Despite intermediate deactivation, high-order visual areas exhibited robust activity and category selectivity for houses and places.
Selectivity for faces and objects was impaired, with a more pronounced effect in the left hemisphere.
Everyday visual performance and reading abilities were relatively preserved.

Conclusions:

The findings challenge the strict hierarchical model of visual processing.
Proper functioning of intermediate visual areas may be essential for cortical specialization but not for all visual behaviors.
Nonlinear amplification mechanisms may support visual perception and behavior even with deactivated intermediate visual pathways.