Symptoms of Alzheimer's disease usually begin to appear around the age of 60. Alzheimer's disease is primarily marked by the onset and progression of dementia, a decline in mental function that may interfere with the ability to perform daily functions. Dementia involves both cognitive (intellectual) and psychiatric symptoms. Cognitive symptoms of Alzheimer's disease may include:

• Memory loss. Memory can be categorized in two ways: short-term memory and long-term memory. Short-term memory is the ability to remember events that occurred a short time ago or to recall things that were recently learned (such as a person’s telephone number or the name of a restaurant). Short-term memory is stored in the temporal lobe in the brain. This is often the first part of the brain to be affected by Alzheimer's disease. Long-term memory is the ability to remember events that happened in the distant past or recall things that were learned earlier in life. Long-term memory is stored in both the temporal and parietal lobes of the brain. It is usually lost during later stages of the disease.
  
  
• Aphasia. Loss of the ability to use and understand language. This is usually the result of neuron damage to the left side of the brain, which is associated with language. People with Alzheimer's disease may forget words and have difficulty communicating with others (extensive aphasia). They may also have problems understanding spoken or written words (receptive aphasia).
  
  
• Agnosia. Loss of ability to recognize familiar people, places or things. It is usually the result of neuron damage in the occipital or parietal lobes. Agnosia also involves an inability to interpret signals from the body, such as when the bladder is full or pain signals that imply serious conditions (e.g., chest pain).
  
  
• Apraxia. An impaired ability to carry out motor activities, even when motor functioning remains intact.

The psychiatric symptoms of dementia are most likely caused by imbalances in brain chemistry and are often the most distressing for the patient and the patient’s family and friends. Some people with Alzheimer's disease do not experience all or any of these symptoms. Psychiatric symptoms may be treated using antipsychotic, anti-anxiety or anti-depressant medication. They may include:

• Personality changes. This is often a marker of the early stages of Alzheimer's disease. Changes in personality may include irritability, apathy and a tendency to withdraw from the company of friends and relatives.
  
  
• Depression. Most people with Alzheimer's disease show some signs of depression throughout the progression of the disease. This may be mistaken as a reflection of recent events, especially if the person has recently lost a loved one. The development of depression is a risk factor for further psychiatric symptoms, including hallucinations and delusions.
  
  
• Hallucinations and delusions. These usually occur during the middle stage of Alzheimer's disease. Hallucinations may be visual or aural and may be exacerbated by loss of hearing or sight. Although people with Alzheimer's disease may sometimes be aware that the hallucination is false, in later stages of the disease they may have a strongly held belief in things that are not real. This is called delusion. A significant percentage of Alzheimer's patients suffer from paranoid delusions, in which they imagine their home has been invaded, that personal items have been stolen, or that loved ones have been replaced by impostors. These can be extremely upsetting to both care-givers and Alzheimer's patients.

People with Alzheimer's disease may also experience apraxia, which is an inability to perform tasks on demand. This symptom occurs when the neuron connections in the parietal lobe of the brain are affected. The first element that is affected by Alzheimer's disease is usually complex motor skills such as those required to perform most daily tasks (e.g., writing, brushing hair). Next, a person may be unable to complete tasks that were learned during childhood, such as tying a shoelace or brushing teeth. The last element of motor function to be lost is instinctive motor function such as chewing, swallowing or walking.

Certain symptoms of Alzheimer's disease may be mistaken for the changes in memory function and mild confusion that people may experience as they get older. Most older people occasionally misplace items or forget the exact details of something that happened in the past. These minor changes are different from Alzheimer's disease because Alzheimer's disease progressively gets worse until the person is unable to function without a permanent caregiver.