A patient had an ischemic infarction of the right medial thalamus, with a resultant contralateral neglect syndrome. We propose that the medial thalamic nuclei, particularly centromedian and parafascicularis (CMPF), are normally involved in the arousal-activation process by which an organism can be aroused by and respond to novel or important stimuli. Specifically, we propose that (1) the mesencephalic reticular formation subserves tonic arousal to novel or painful stimuli by inhibiting the nucleus reticularis thalami (NR) and (2) that selective attention is mediated by cortical input to NR. The CMPF is closely associated with motor systems (basal ganglia, ventrolateral nucleus of the thalamus [VL], and frontal lobes). A pathway involving CMPF, the frontal cortex, and the portion of NR associated with VL may be important in preparing the tonically aroused organism for action. Unilateral lesions of CMPF therefore induce an asymmetric hypokinesia, and bilateral lesions may induce akinetic mutism.