“…Whereas DA or DA agonists improve memory, selective depletion, dysfunction, or lesion of forebrain DA systems strongly impair acquisition and retention, as well as the ability to focus and adapt to environmental changes. In support of its role in cognitive processes, DA has been shown to exert a major modulatory input in synaptic plasticity in different brain areas, including the striatum (Calabresi et al, 1992;Arbuthnott et al, 2000), hippocampus (Lisman and Otmakhova, 2001), and prefrontal cortex (Gurden et al, 1999;Jay, 2003). However, clinical and experimental models of both excessive and deficient DA transmission have revealed an inverted 'U-shaped' relationship between DA levels and cognitive performance, and suggested that the beneficial or detrimental effects of DA depend on basal DA levels, task demands, and regional differences (Cools, 2006).…”