This study will recruit a total of 40 evaluable subjects (20 cognitively depressive, and 20 AD depressive); each evaluable subject involved in this study must fulfill all the inclusion and exclusion criteria according the subject grouping.

Safety measurement will be evaluated by medical history, vital signs, physical examinations, laboratory examinations and collecting of adverse events.

This study is expected to be completed in a period of 3 years.

Depression and dementia are the two common psychiatric disorders in the elder subjects. Alzheimer's disease (AD) is the most frequent cause of dementia and about 20% of them have depression. Depression subjects are associated with more rapid cognitive decline, a poorer response to treatment. Post mortem study showed close relationship between AD and disruptions of the serotonergic system, including loss of serotnergic neurons at brain stem. However, the alternations in presynaptic serotonin function relative to demented or non-demented subjects remain to be investigated in living subjects. In this study, the investigators will collect 40 elder subjects (i.e., age above or equal to 50 years old). The serotonin transporter activity will be compared between subjects with or without dementia using I-123 ADAM images. The single photon emission tomography (SPECT) will be compared to recent (within 6 months) F-18 FDG PET images for further investigation.

Inclusion Criteria:

1. Patients may be enrolled in the AD depressive group if they:

   • Are males or females at least 50 years of age;
   • Fulfilled the DSM-IV criteria for Major depressive disorder (APA, 1994) by MINI interview.
   • Meet the NINCDS/ADRDA(National institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association) criteria for probable AD
   • A Mini Mental State Examination (MMSE) score at screening between 10 and 24 inclusive;
   • Give informed consent. If the patient is incapable of giving informed consent, the caregiver may consent on behalf of the patient (the patient must still confirm assent).

2. Patients may be enrolled in the cognitively depressive group if they:

   • Are males or females at least 50 years of age;
   • Fulfilled the DSM-IV criteria for Major depressive disorder (APA, 1994) by MINI interview.
   • Memory function above the lower normal limits (i.e. 1.5 SD above the mean) on tests for episodic memory.
   • Clinical Dementia Rating = 0. Memory Box score must be 0.
   • Cognitively normal, based on an absence of significant impairment in cognitive functions or ADL.
   • A MMSE score at screening > 24 for those with education level of 6 years or above and > 17 for those are illiterate;
   • Give informed consent.

Exclusion Criteria:

• Pregnant or becoming pregnant during the study (as documented by pregnancy testing at screening or at any date during the study according to the PI discretion) or current breast feeding.
• Conditions affecting brain structure or function (e.g., stroke, diabetes, head trauma, depression) or use of cognitively
• Substance abuse.
• Alcohol dependence