Apraxia, agnosias, and higher visual function abnormalities
- Correspondence to: Dr John D W Greene Southern General Hospital, Institute of Neurological Sciences, 1345 Govan Road, Glasgow, G51 4TF, UK;
Cognitive 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 cover disorders of perception and of higher order motor output, both in terms of pathological loss and pathological gain of function.
PERCEPTION AND ITS DISORDERS
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 attend selectively 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 is processed 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 …