Recent attention models view exogenous and endogenous attention as separate components of attention. Exogenous attention is defined as automatic, involuntary, directed by external stimulation and unaffected by memory load, while endogenous attention is defined as executive, voluntary, directed by voluntary acts and affected by memory load. Methods. Two studies were designed to examine if decline in these two components of attention is similar in normal aging and Huntington's disease (HD). Standardized tests derived from Posner's model of visuospatial attention were administered to normal elderly subjects (n=13), patients with HD (n = 17) and matched control subjects (n = 42).

In healthy elderly subjects, both exogenous and endogenous attention were found to decline within normal limits, and the decrease was greater for endogenous attention, particularly in situations of perceptual conflict. Patients with HD showed marked impairment of endogenous or voluntary attention components, while exogenous or automatic components were preserved.

Our results suggest that anterior executive and posterior automatic neuronal networks for attention are differentially vulnerable to the effects of normal aging and neurodegenerative diseases, despite the fact that both normal aging and HD are characterized by decreased endogenous attention in situations of perceptual conflict.