Associative visual agnosia is a form of visual agnosia. It is an impairment in recognition or assigning meaning to a stimulus that is accurately perceived and not associated with a generalized deficit in intelligence, memory, language or attention. The disorder appears to be very uncommon in a "pure" or uncomplicated form and is usually accompanied by other complex neuropsychological problems due to the nature of the etiology. Associative visual agnosia refers to a subtype of visual agnosia, which was labeled by Lissauer (1890), as an inability to connect the visual percept (mental representation of something being perceived through the senses) with its related semantic information stored in memory, such as, its name, use, and description. This is distinguished from the visual apperceptive form of visual agnosia, apperceptive visual agnosia, which is an inability to produce a complete percept, and is associated with a failure in higher order perceptual processing where feature integration is impaired, though individual features can be distinguished. In reality, patients often fall between both distinctions, with some degree of perceptual disturbances exhibited in most cases, and in some cases, patients may be labeled as integrative agnostics when they fit the criteria for both forms. caused by ischemic stroke, head injury, cardiac arrest, brain tumour, brain hemorrhage, or demyelination.
thumb|The separate streams of the visual processing system. The ventral "what" stream is in purple and the dorsal "where" stream is in green.
Most cases have injury to the occipital and temporal lobes and the critical site of injury appears to be in the left occipito-temporal region, often with involvement of the splenium of the corpus callosum. The etiology of the cognitive impairment, as well the areas of the brain affected by lesions and stage of recovery are the primary determinants of the pattern of deficit. Goldberg suggested that the associative visual form of agnosia results from damage to the ventral stream of the brain, the occipito-temporal stream, which plays a key role in object recognition as the so-called "what" region of the brain, as opposed to the "where," dorsal stream. described the associative agnostic as having a "percept stripped of its meaning," because the affected 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. 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.
Object recognition model
thumb|Depiction of the object recognition model, adapted from Bauer's Clinical Application of a Cognitive Neuropsychological Model of Object Recognition The cognitive system for visual object identification is a hierarchal process, broken up into multiple steps of processing. the process begins with sensory perception (vision) of the object, which results in an initial representation via feature extraction of basic forms and shapes. This is followed by an integration stage, where elements of the visual field combine to form a visual percept image, the 'primary sketch'. This is a dimensional (D) stage with a 'viewer-centered' object representation, where the features and qualities of the object are presented from the viewer's perspective. [[episodic memory|episodic and semantic memory arise from the same memory traces, and no semantic representations are stored permanently in memory. By this view, the meaning of any stimulus emerges momentarily from reactivation of one's previous experiences with that entity. Each episode is made of several components of many different sensory modalities that are typically engaged during interactions with an object. In this scenario, a retrieval cue triggers reactivation of all episodic memory traces, in proportion to the similarity between the cue its 'echo,' the components shared by most activated traces. In a process called 're-injection,' the first echo acts as a further retrieval cue, evoking the 'second echo,' the less frequently associated components of the cue. Thus, the 're-injection' process provides a more complete meaning for the object. According to this model, different types of stimuli will evoke differential 'echos' based on typical interactions with them.
|| Bilateral occipito-temporal cortex