Attention: the selective aspect of perception, requires wakefulness (organism), activation (behavior) and inhibition (neuronal systems). It may be observed momentarily (concentration), over time (vigilance) and in the selection between channels (e.g. the rejection of irrelevance in focussed and divided conditions). Anatomical Organisation: Information ascending through the thalamus not only alerts the sensory cortices, but thalamic projections to association areas receive direct feedback allowing gating and preparation of sensory areas for further analysis. A frontal organizational role is subserved by descending pathways allowing for valuation (orbitofrontal cortex, amygdala), association (hippocampal complex) and response possibilities (basal forebrain). Interactions between the latter allow for automatic processes, but with the former limbic connections bring the possibility of conscious control. Neurotransmitters: Additional involvement of brainstem mechanisms allows for volume-control (serotonin, 5HT), tuning (noradrenaline, NA) and switching (dopamine, DA) mechanisms in determining priority in selective mechanisms. Failures of brainstem mechanisms can impair the modulation of several systems ranging from affect (e.g. 5HT, hostility) to the assessment of information relevance (DA, perseveration, switching). Such non-specific features of psychotic processes can be incurred by unusual amino-acid transmission (e.g. Glu, Asp, GABA) from the neo- and archicortices. Psychopathology: The locus of supra- or sub-liminal damage may provide specificity to the symptom and speculatively the schizophrenic syndromes of poverty, disorganization and reality distortion. Useful "attentional" paradigms for the study of the nature and distribution of these processes include sustained attention, the influence of irrelevant stimuli (learned inattention -"blocking"), the covert orienting of attention (cost/benefits of cueing), dichotic and multidiscrimination (allocation), prepulse inhibition and masking (sensory gating) and concept formation. Psychophysiology: Our own work shows, for example, that mismatch negativity is severely reduced in young patients with schizophrenia with or without reality distortion (by 50% and again by 50%, respectively). But only the non-paranoid patients show an abnormal loosening of selection processes, reflected in reduced conditioned blocking (that in turn reflects switching between relevant and irrelevant stimuli and learning about both). This latter result proved to be related to changes of daily DA utilization (measured in 24h-urine samples) consistent with a DA role in switching.