On the functional neuroanatomy of intrinsic and phasic alertness

Intrinsic and phasic alertness are the most basic aspects of attention inte nsity probably constituting the basis for the more complex and capacity-dem anding aspects of attention selectivity. Intrinsic alertness represents the cognitive control of wakefulness and arousal and is typically assessed by simple reaction time tasks without a preceding warning stimulus. Phasic ale rtness, in contrast, is called for in reaction time tasks in which a warnin g stimulus precedes the target, and it represents the ability to increase r esponse readiness subsequent to external cueing, We report PET and fMRI dat a from both the literature and our own experiments to delineate the cortica l and subcortical networks subserving alertness, sustained attention (as an other aspect of attention intensity), and spatial orienting of attention. I rrespective of stimulus modality, there seems to exist a mostly right-hemis pheric frontal, parietal, thalamic, and brain-stem network which is coactiv ated by alerting and orienting attentional demands. These findings corrobor ate both the hypothesis of a frontal modulation of brain-stem activation pr obably via the reticular nucleus of the thalamus and of a coactivation of t he posterior attention system, involved in spatial orienting by the anterio r alerting network. Under conditions of phasic alertness there are addition al activations of left-hemisphere frontal and parietal structures which are interpreted as basal aspects of attention selectivity rather than addition al features of alerting. (C) 2001 Academic Press.