Agnosia: Visual

Visual agnosia refers to a class of disorders in the domain of visual recognition, in which the affected person cannot arrive at the meaning of some or all categories of visual stimuli. In such cases, knowledge of the shape, form, or other stored knowledge of the physical attributes of an object cannot be accessed, with the result that the stimulus is not successfully identified. The failure to recognize the object occurs under a whole range of conditions and independent of whether the stimulus is presented as a real three-dimensional object, as a black-and-white line drawing, or as a photograph. Dramatically, these perceptual failures occur despite the individual exhibiting normal or near-normal elementary visual functions such as acuity, brightness discrimination, and color vision, along with normal or near-normal semantic and memory functioning. These individuals also have intact alertness, intelligence, and language, thus setting aside questions about whether agnosia is simply a manifestation of reduced elementary visual function and intelligence.

Visual agnosia may result from a lesion (e.g., as a consequence of a stroke or tumor or other form of brain damage) sustained by an adult who possessed normal premorbid perceptual abilities, or it may be apparent in a child who sustained a brain lesion early in life, a disorder usually referred to as developmental agnosia. Visual agnosia may also occur in the context of a progressive deterioration of perceptual skills, as in the visually selective progressive posterior cortical atrophy, and finally, it may be evident in individuals who are apparently impaired at recognition (predominantly of faces) from birth in the absence of any obvious neurological concomitant (see Table 1).

Not surprisingly, patients may fail to recognize objects if they have a deficit in semantics (knowledge or meaning of objects and their properties such as what an object might be used for). Interestingly, however, and the focus of this short review, agnosic patients typically retain the long-term representation of the object but still fail to recognize the stimulus presented. Thus, several investigators have reported data from visual imagery tasks that demonstrate that stored visual knowledge may be at least relatively preserved in patients with severe visual agnosia and that, despite this, the patients do not recognize the presented object; for example, patients may be able to visualize in their mind's eye a particular object but, presented with the same object, may fail to identify it. Consistent with this, these patients usually demonstrate normal recognition of objects through modalities other than vision (touch, audition, verbal definition, or description of its function), further indicating that the deficit is not simply a difficulty in retrieving names or in accessing the necessary semantic/knowledge information. In short, the problem appears to be one of accessing meaning from visual input.

Visual agnosia can be general, affecting the recognition of all visual stimuli, or it can be more specific: for example, some agnosias are (relatively selective) for objects, for faces or prosopagnosia, for words or pure alexia, for body parts, for colors, and for environmental scenes, including landmarks. Finally, the disorder of simultanagnosia—an [Page 33]inability to “see” more than one object at a time—is also regarded as a type of visual agnosia. Some cases evince fairly “pure” forms of one of these agnosias, but there are many reported cases in whom some subset of the specific forms can co-occur. Some of the more common subtypes of visual agnosia are described in this entry.

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