Inclusion Criteria:  LBD patients (DLB or PDD) who have cognitive fluctuations and who are on stable doses of cholinesterase inhibitors (i.e., at least 4 weeks) will be recruited to participate in this study. DLB patients will meet the Fourth consensus report of the DLB Consortium inclusion criteria for probable DLB. Subjects will be identified according to the following recognized DLB features: spontaneous parkinsonian motor signs, fluctuating attention and concentration, recurrent well-formed visual hallucinations, presence of REM behavioral sleep disturbance, anosmia/hyposmia,or autonomic dysfunction. PDD patients will meet the criteria by Emre et al. (Cognitive deficits in at least two of four of the following cognitive domains: Impaired attention, impaired executive functions, impairment in visuo-spatial functions, impaired free recall memory typically improved with cuing. Must also meet criteria for at least one behavioral symptom: apathy, depressed or anxious mood, hallucinations, delusions, excessive daytime sleepiness). Lack of behavioral symptoms does not exclude the diagnosis. Must also have none of Group III features present: (1) Co-existance of any other abnormality which might cause impairment, but judged not to be the cause of dementia. (2) Time interval between development of motor and cognitive symptoms not known. Must also have none of Group IV symptoms present: (1) Cognitive and behavioral symptoms appear solely in the context of other conditions such as acute confusion caused by systemic diseases or abnormalities, drug intoxication, or major depression according to DSM IV. (2) Features compatible with Probable Vascular Dementia criteria accordingly to NINDS-AIREN.  Exclusion Criteria:  Subjects with contra-indications to MR imaging and/or tDCS, including pacemakers or claustrophobia Evidence of large vessel stroke or mass lesion on MRI Use of anti-cholinergic or neuroleptic drugs Evidence of atypical parkinsonism on neurological exam Major psychiatric illness, such as bipolar disorder Neurological conditions such as epilepsy, stroke, multiple sclerosis, or moderate to severe brain injury Sensory impairments that significantly limit one's ability to see or hear A significant history of recent alcohol or drug dependence Previous major radiation exposure Pregnancy