Handout on Farah's "The Apperceptive Agnosias"

Apperceptive Agnosia (general sense)
"…any failure of object recognition in which perceptual impairment seems clearly at fault, despite relative preserved elementary visual functions such as acuity, brightness discrimination, and color vision" (Farah, p.7)

FOUR TYPES OF APPERCEPTIVE AGNOSIA

1) Apperceptive agnosia (narrow sense)
Basic description: Subjects see color, size, brightness, local contour, movement and depth, but they do not see shape*. (*Some subjects see shape if the objects are moving.)

Subjects with this deficiency cannot distinguish or recognize objects in terms of their shapes by vision alone. (Even simple objects like "X" and "O".) They can't copy letters or figures. They act like they are blind—they can't navigate their environment. If asked to identify a static object, they will make a guess on the basis of its color, size, etc., or they will trace its outline using their finger or their head. This tracing has a "slavish" quality to it.

2) Dorsal Simultanagnosia
Basic Description: Subjects are limited to seeing (and recognizing) one object at a time*—all other objects in the environment are completely unseen. (* Some subjects can simultaneously see and recognize two really small objects that are placed really close together.)

These subjects also act like they are blind—they can't navigate their environment. They have major difficulty recognizing or describing complex objects/scenes. They can't count objects by vision alone. Accompanied by "visual disorientation"—they can't localize the perceived object relative to other objects.

3) Ventral Simultanagnosia
Basic Description: Subjects are limited to recognizing one object at a time. Unlike dorsal simultanagnosia, they can see other objects but they can only recognize objects one at a time.

Unlike previous two cases, these subjects don't act like they are blind—they can navigate their environment. They can also count using vision alone. They have difficulty recognizing/describing complex objects/scenes (it takes them more time than it takes a normal subject). They also have difficulty reading—they are "letter-by-letter readers".

4) Perceptual Categorization Deficit
Subjects have abnormal difficulty in matching three-dimensional objects across changes in perspective or lighting.

All references are from Farah, M. Visual Agnosia. Cambridge: The MIT Press, 1991 (2nd print)

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