Agnosia
Agnosia is the inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia only affects a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual information.
Associative visual agnosia
Teuber described the associative agnostic as having a "percept stripped of its meaning," because the afflicted individual cannot generate unique semantic information to identify the percept, since though it is fully formed, it fails to activate the semantic memory associated with the stimulus. Warrington (1975) offered that the problem lies in impaired access to generic engrams (memory traces) that describe categories of objects made up of a multitude of similar elements. Essentially, damage to a modality-specific meaning process (semantic system), is proposed, either in terms of defective access to or a degradation of semantic memory store for visual semantic representations themselves. The fact that agnosias are often restricted to impairments of particular types of stimuli, within distinct sensory modalities, suggests that there are separate modality specific pathways for the meaningful representation of objects and pictures, written material, familiar faces, and colors.