Although much information exists on the connection between memory and biology, it is far from complete. At the neuron level, a deficiency of the neurotransmitter acetylcholine is a factor in the dementia (memory disorder) known as Alzheimer's disease (administration of the neurotransmitter has slowed the disease's progress but not prevented it). Reduced levels of the neurotransmitter glutamate have also been associated with the disorder. Serotonin, another neurotransmitter, is also thought to be important in memory functioning. Eric Kandel and James Schwartz, in a study of sea snails, found that serotonin is released as they learn. At the structural level, damage to the hippocampus, part of the limbic system, has been associated with memory difficulties.