How Do We See?

The study of visual perception is difficult because it happens so fast. Somehow light comes into our eyes, makes contact with our memory and, lo and behold, we see a meaningful scene. A recent article1 in the New Scientist provides an overview of how this occurs, or, at least,with out current state of knowledge how we think this occurs.

Since perception happens so quickly, agnosias, specific disorders, can be quite informative. The previous blog post explored Propagnosia. Other types of agnosias include:

Simultanagnosia – seeing one object at a time when viewing a scene comprised of many items.

Integrative agnosia – Inability to recognize whole objects, tending instead to focus on individual features of an object.

Visual form agnosia – Inability to describe the shape, size, or orientation of objects, but still being able to manipulate them.

Optic ataxia – Ability to report the shape and size of an object, though manipulating the object clumsily.

Pure alexia (aka agnosia for words) – inability to identify individual characters or even text, although sometimes being able to write.

Topographical agnosia – Inability to recognize known landmarks or scenes.

Color agnosia – Ability to perceive colors without being able to identify, name, or group them according to similarity.

Research using brain scans can be quite useful in identifying the specific areas in the brain that accomplish these functions. Brain scans have revealed that people with visual form agnosia tend to have damage to the ventral (lower) part of the brain’s visual area. However, people with optic ataxia tend to have damage to the dorsal (upper part) of the brain’s visual area. So it appears that we have two streams of visual processing. The ventral pathway recognizes the object, while the dorsal pathway determines where that object is located in the visual field.

Some neuroscientists think that the brain binds all the different features of the ventral stream to a “master map of location”, which is held in the dorsal stream. They believe that this binding process is so fundamental that this link needs to be formed before an image can pop into consciousness.

So our perceptual system seems to be highly modular with many different modules contributing to conscious experience. All this activity occurs below the level of consciousness to yield the conscious world we do experience.

1Robeson, D. (2010). Seeing Isn’t Believing. New Scientist, 28 August, 30-33.

© Douglas Griffith and, 2010. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Douglas Griffith and with appropriate and specific direction to the original content.


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