ABSTRACT
Agnosias are disorders of recognition, specific to one sensory channel, that affect
either the perceptual analysis of the stimulus or the recognition of its meaning.
In the visual modality, objects, faces, and colors can be separately disrupted. Apperceptive
object agnosia refers to failure to achieve a structured description of the shape
of the object. Associative agnosia refers to inability to attribute a meaning to a
correctly perceived stimulus. It must be differentiated from optic aphasia, in which
the object is recognized but cannot be named in the visual modality. Associative agnosia
and optic aphasia are associated with left occipitotemporal damage, and they differ
more quantitatively than qualitatively. The inability to recognize familiar faces
(prosopagnosia) can appear in isolation and be, in some cases, associated with a lesion
confined to the occipitotemporal region of the right hemisphere. These findings are
supportive of the idea that faces have a separate representation in the brain.
Disorders of color cognition can affect color categorization, color-name association,
and color-object association. They are linked to left hemisphere damage. The ability
to recognize objects presented in the visual modality is a hierarchical process in
which several cortical areas, corresponding to about 30% of the cortical mantle, participate.
Their selective lesion results in a gamut of disorders whose identification provides
the experienced neurologist with clues to the locus of damage and contributes to the
understanding of the cognitive architecture underpinning recognition. They can result
either in the inability to detect any change occurring in the visual field or in the
impairment of further stages of the recognition process, from the analysis of the
perceptual properties of the stimulus (form, color, motion, depth, etc.) to the achievement
of its structural description and, eventually, the attribution of a meaning. In this
paper, I focus on the diagnostic and clinical features characterizing the disruption
of the last stage of visual information processing; that is, the failure to identify
what a stimulus represents despite evidence that its three-dimensional structure has
been properly reconstructed. In the literature, this impairment is traditionally referred
to as associative agnosia,[1] a psychological construct that attributes the deficit to the inability to associate
a well-discriminated percept with its semantic attributes, which are stored in separate
cortical areas. In the visual modality, three discrete forms of associative agnosia
have been described, affecting objects, faces, and colors. These are treated separately.
KEYWORD
Color agnosia - prosopagnosia - visual agnosia
