An intrusion is an inappropriate recurrence of a response (verbal, motor) to a preceding test or procedure after intervening stimuli. Intrusions are thought to reflect inattention, and may be seen in dementing disorders or delirium. These phenomena overlap to some extent with the recurrent type of perseveration.
The term intrusion is also used to describe inappropriate saccadic eye movements which interfere with macular fixation during pursuit eye movements.
Fuld PA, Katzman R, Davies P, Terry RD. Intrusions as a sign of Alzheimer dementia: chemical and pathological verification. Annals ofNeurology 1982; 11: 155-159