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Apraxia, agnosias, and higher visual function abnormalities
J D W Greene J. Neurol. Neurosurg. Psychiatry 2005;76;25-34 doi:10.1136/jnnp.2005.081885

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J D WGreene v25
J Neurol Neurosurg Psychiatry 2005; 76(Suppl V):v25–v34. doi: 10.1136/jnnp.2005.081885

ognitive neurology deals mainly with disorders of memory (for example, is the patient’s poor memory due to early dementia or to anxiety/depression?) or language (as in stroke). It should be remembered, however, that other areas of cognition may be selectively impaired. This review will coverdisorders of perception and of higher order motor output, both in terms of pathological loss and pathological gain of function.



A patient must be conscious in order to perceive the world around them. An exploration of consciousness is outside the remit of this article though recently reviewed by others.1 2 The patient must also have the capacity to attendselectively in order to focus on one part of the sensorium. Perceptual processing is then necessary to identify what is being perceived through the various sensory modalities (namely vision, hearing, touch, smell, taste), thus allowing access to semantic knowledge and through this understanding of the environment. Initially, perceptual information is basic and modality specific, but as it isprocessed by higher order centres, meaning is ascribed to percepts, and information becomes multi-modal (fig 1). Ultimately, semantic knowledge is accessed using the various sensory streams. For example, if standing in the path of an oncoming train, basic perception will involve visual information, hearing the train coming, and feeling vibration from the ground. These separate streams then come together,accessing relevant semantic knowledge and thus allowing the individual to understand what is taking place. In discussing perception, I shall focus mainly on vision and hearing, as the other three forms of perception are of lesser clinical importance. Perception is not a passive process, but is modulated by attention. There is feedback from higher order centres down to primary sensory cortex.Similarly, attention influences what is perceived. Attention itself has many levels or subcomponents, including selective, divided, sustained attention, etc. Of relevance to perception is selective attention, a process by which the individual focuses specifically on particular areas of sensory experience, rather than simply passively absorbing all such experience. Patients with subcortical dementia canlose this ability to attend selectively, resulting in increased distractibility due to inability to ignore background extraneous stimuli. To some extent, we see what we expect to see. For example, if waiting for someone at a crowded venue, we may have several false positive recognitions of strangers because we are primed to expect to see the friend.

Normal visual processing Visual...
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