Attention is a distributed cognitive function, important for the operation of many other cognitive domains; the terms concentration, vigilance, and persistence may be used synonymously with attention.
A distinction may be made between different types of attention, viz.:

Divided/executive function.

It is generally accepted that attention is effortful, selective, and closely linked to intention.
Impairment of attentional mechanisms may lead to distractibility (with a resulting complaint of poor memory, better termed aprosexia, q.v.), disorientation in time and place, perceptual problems, and behavioral problems (e.g., disinhibition), as in the cardinal disorder of attention, delirium (q.v.).
The neuroanatomical substrates of attention encompass the ascending reticular activating system of the brainstem, the thalamus, and the prefrontal (multimodal association) cerebral cortex (especially on the right). Damage to any of these areas may cause impaired attention.
Attentional mechanisms may be tested in a variety of ways. Those adapted to "bedside" use all essentially look for a defect in selective attention, also known as working memory or short term memory (although this does not necessarily equate with lay use of the term "short term memory"):

Orientation in time/place
Digit span forwards/backward
Reciting months of the year backward, counting back from 30 to 1
Serial sevens (serial subtraction of 7 from 100, = 93, 86, 79, 72, 65).

In the presence of severe attentional disorder (as in delirium) it is difficult to make any meaningful assessment of other cognitive domains (e.g., memory).
Besides delirium, attentional impairments may be seen following head injury, and in ostensibly "alert" patients, for example, with Alzheimer’s disease (the dysexecutive syndrome of impaired divided attention).



Parasuraman R. Attentional functioning in Alzheimer’s disease. In: Morris RG, Becker JT (eds.). Cognitive neuropsychology of Alzheimer’sdisease (2nd edition). Oxford: OUP, 2004: 81-102
Perry RJ, Hodges JR. Attention and executive deficits in Alzheimer’s disease: a critical review. Brain 1999; 122: 383-404
Scholey A. Attention. In: Perry E, Ashton H, Young A (eds.). Neurochemistry of consciousness: neurotransmitters in mind. Amsterdam: John Benjamins, 2002: 43-63


Cross References

Aprosexia; Delirium; Dementia; Disinhibition; Dysexecutive syndrome; Frontal lobe syndromes; Pseudodementia