Although alcohol is commonly thought of purely as a depressant, at low concentrations it can actually stimulate certain areas of the brain. Alcohol sensitises the N-methyl-D-aspartate (NMDA) system of the brain, making it more receptive to the neurotransmitter glutamate. Stimulated areas include the cortex, hippocampus and nucleus accumbens, which are responsible for thinking and pleasure seeking. Another one of alcohol's agreeable effects is body relaxation, possibly caused by heightened alpha brain waves surging across the brain. Alpha waves are observed (with the aid of EEGs) when the body is relaxed. Heightened pulses are thought to correspond to higher levels of enjoyment.