A Study of RO4602522 in Participants With Moderate Severity Alzheimer Disease on Background Alzheimer Disease Therapy

Inclusion Criteria: * Probable Alzheimer disease, based on the National Institute of Neurological and Communicative Disorders and Stroke (NINCDS)/Alzheimer's Disease and Related Disorders Association (ADRDA) and Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition (DSM-IV-TR) criteria * Mini-Mental State Exam (MMSE) score at screening between 13 and 20, inclusive * Body mass index (BMI) between 18 and 36 kilograms per square meter (kg/m\^2) (inclusive) at screening * Modified Hachinski Ischemia Score of less than or equal to (\</=) 4 * Participants with Cornell Scale for Depression in Dementia (CSDD) scores \</= 13 at screening * Receiving treatment with donepezil, rivastigmine, galantamine or any AChEIs in combination with memantine for at least 4 months before screening, with their dose and formulation stabilized at least 3 months before screening. All formulation and dosages are allowed except donezepil 23 mg (alone or in combination) * Females of childbearing potential must have a negative pregnancy test and must agree to use effective contraception * Generally healthy and ambulatory or ambulatory-aided (i.e., walker or cane) * Have a reliable caregiver or some other identified responsible person who has frequent contact with the participant Exclusion Criteria: * Any neurological or psychiatric condition that may occur currently or during the course of the study that can impair cognition or functioning that is not associated with Alzheimer's disease * Background of mental retardation * Uncontrolled behavioral symptoms incompatible with compliance or evaluability * Alcohol and/or substance abuse or dependence (DSM-IV-TR) in the past 2 years, except nicotine use which is allowed. However, smokers treated with nicotine replacement therapy or bupropion are excluded * Unstable or poorly controlled hypertension as assessed by the investigator regardless of whether or not the participant is taking antihypertensive medications * Unstable or clinically significant cardiovascular disease that could be expected to progress, recur, or change during study period to such an extent that it could bias the assessment of the clinical or mental status of the participant * Inadequate hepatic, renal or thyroid function * Positive for hepatitis B, hepatitis C or human immunodeficiency virus (HIV) infection * Poorly controlled diabetes (glycosylated hemoglobin \[HbA1c\] greater than or equal to \[\>/=\] 9 percent at screening) * Requiring nursing home care. Participants living in assisted living facilities are allowed if a reliable caregiver is available (see inclusion criteria) * Current treatment for Alzheimer's disease other than those listed in inclusion criteria * Participation at any time in an active Alzheimer's disease vaccine study * Participation in a passive Alzheimer's disease immunization study less than 1 year before screening except for a) participants where documented medical history indicate that they were randomized to the placebo group in these studies, b) participants treated with bapineuzumab where a 6-month exclusion period applies * Recent (\</= 12 weeks) or concomitant use of other Monoamine oxidase inhibitors (selective or not) including selegiline or rasagiline * Antidepressant treatments are not allowed except for citalopram up to 20 mg daily, escitalopram up to 10 mg daily, paroxetine up to 30 mg daily, sertraline up to 100 mg daily and trazodone up to 100 mg daily. If treated with one of these antidepressants, the treatment should be present for at least 6 weeks at screening. All other antidepressants including other SSRIs, tricyclic antidepressants (TCAs), serotonin-norepinephrine reuptake inhibitors (SNRIs), St. John's wort and bupropion are excluded * Anti-psychotic use within 4 weeks before screening is not permitted except risperidone up to 1.5 mg/day, quetiapine up to 100 milligrams per day (mg/day), olanzapine up to 5 mg/day, and aripiprazole up to 10 mg daily * Anxiolytics/ hypnotics use is not permitted except for benzodiazepines of short or intermediate half-life for anxiety/sleeping disorders. Zolpidem (up to 5 mg/day), zopiclone (up to 7.5 mg/day), eszopiclone (up to 2 mg/day), trazodone (up to 50 mg/day, at bedtime) or zaleplon (up to 5 mg/day) is permitted for insomnia * Anti-Parkinson's agents within 2 weeks before screening are not permitted * Recent (less than 4 weeks prior to screening) or concomitant use of anticonvulsants * Anticholinergics/ antihistaminics within 2 weeks before screening are not permitted, except i) if used episodically more than 3 days before the screening cognitive measurement, ii) non-sedating antihistaminic medications (without anticholinergic effects such as cetirizine) or peripheral anticholinergics without central anticholinergic effects (such as, trospium for the treatment of hyperactive bladder), which are permitted * Recent (less than 1 week prior to screening) or concomitant use of opioid drugs (tramadol, methadone, propoxyphene, or meperidine), cyclobenzaprine and dextromethorphan * Concomitant use of sympathomimetic drugs, including sympathomimetics in local anesthetics and ephedra supplements