Effect of locus coeruleus stimulation on extracellular NA and DA levels in the medial prefrontal and occipital cortex. Stimuli were delivered in bursts, as described in Methods, for a duration of 20 min (horizontal solid lines). Data are means ± SE, expressed as percent of mean basal value. Filled symbols indicate p < 0.05 with respect to basal values (Dunnett test)