Dementia sounds serious but also covers mild changes in a horses alertness and behavior:

• Depression: a decrease in alertness of the horse.
• Semicoma: Severe depression, a profound lack of reaction to what is happening around the horse.
• Aimless wandering, circling, head pressing: The horse moves around but seems unaware of his surroundings, if he comes to a wall he may continue to try and move forward.
• Seizure: brain dysfunction due to excessive neuronal discharge, and usually associated with some reduction or loss of consciousness. The horse may appear to just faint and there may or may not be spasms of the muscles.
• Delirium: An altered state of consciousness, consisting of confusion, distractibility, disorientation, disordered thinking, defective perception (illusions and hallucinations), prominent hyperactivity, agitation, or aggression.

Behavioral changes indicate a disease of the brain, more specifically the cerebrum or brain stem. Because the brain is closely integrated with the cranial nerves often these are effected also. Not all cases of dementia, particularly mild depression are caused by primary brain disease. Both fever and colic can cause depression and liver disease can induce severe brain dysfunction. Not all extreme behaviors or behaviorial changes are a indication of disease. Differences exist between individuals, sexes, breeds, and ages. Behavioral changes may be seen at the onset of estrus in mares, aggressive behavior may be seen in stallions, and changes in tack may lead to behavior changes (head shaking, chewing) in the performance of race horses. Certainly pain anywhere in the body can cause behavorial changes but usually this is easily interprted as a horse's reaction to peripheral pain.

If dementia is present the brain has a primary lesion whether the horse is ataxic or not. This article describes the diseases of the brain and provides links to articles on these disorders.