Effects of Transcranial Direct Current Stimulation on Tobacco Withdrawal Symptoms

Inclusion Criteria: * Good general health * Age between 18 and 55 yrs * Smokers must smoke at least 15 cigarettes per day for the past two years (as indicated by self-report) * Must give a breath CO sample of at least 15 ppm to verify current tobacco consumption during screen * Must give a breath CO sample of 7 or less ppm to verify tobacco abstinence during the abstinence session * Cocaine and alcohol patients should meet DSM-IV criteria, and is confirmed after clinical interview with a structured interview with the SCI-PG and SCID-P. * Right-handed, as indicated by the Edinburgh Handedness Questionnaire * Smokers should have no illicit drug use, as indicated by negative results on urine drug screens of cocaine, methamphetamine, opiates, or benzodiazepines at all sessions * Smokers should consume less than or equal to 10 standard drinks of alcohol per week (one standard drink consists of one 12 oz. beer, 6 oz. of wine, or one shot (1.5 oz.) of hard liquor (80 proof) (as indicated by self-report) * Absence of any past or current DSM-IV Axis I diagnoses (as verified by the SCID) * English as a first language (subjects may be bilingual), and able to read and speak English fluently (as indicated by self-report) Exclusion Criteria: * Any medically significant acute or chronic medical condition as indicated by self-report (significance is determined by the P.I.) * English as a second language (as indicated by self-report) * Left-handed or ambidextrous as indicated on the Edinburgh Handedness Questionnaire * Marijuana use of greater than one joint once per week. The subject's urine must screen negative at each session, so the subject is instructed to refrain from using marijuana for the 72 hours prior to each session. * Pregnant, as indicated by a positive result on the urine pregnancy test given at each session * Below normal intelligence (Shipley estimated IQ of 85 or less) as indicated by a combined score of \<45 on the Shipley Institute of Living Scale * Evidence of head injury involving loss of consciousness for 5 minutes or more and/or requiring hospitalization (as indicated by self-report) * Depression, as assessed by the SCID or a score of \>18 on the BDI (see appendix) * Probable childhood ADHD as indicated by a score of \>46 on the Wender Utah Rating Scale and/or by self-report * Self-report of learning disability or dyslexia * Current or past use of psychotropic drugs (i.e., any antidepressants, antipsychotics, psychostimulants (i.e. Ritalin), benzodiazepines (i.e., Valium, Xanax), or use of any other drugs known to affect cognitive functioning (as indicated by self-report). * HIV positive (as indicated by self-report) * Positive TB test (as indicated by self-report) * For subjects participating in the smoking portion, consumption of more than 10 standard drinks of alcohol per week (see inclusion criteria for description of standard drink) * Presence of any past or current DSM-IV Axis I Diagnoses, as verified by the SCID * Abnormal uncorrected vision or hearing which would affect performance on cognitive tests (as indicated by self-report) * Smokers group: * Has not smoked 15 or more cigarettes per day for a minimum of the past 2 years * A CO breath sample of \<15 ppm during any session (except the abstinence session) * Presence of a pacemaker, catheter or device implant that might be interfered with by the current applied during tDCS. b) Eligibility to participate in the study will be determined by trained research personnel who will screen potential participants over the phone. c) Minors are excluded because the study involves cigarette smoking. Participants over 55 are excluded due to their increased probability of smoking-related and age-related health problems. Pregnant women or those of childbearing potential who are not using birth control will be excluded because the studies involve cigarette smoking and therefore pose a risk to a fetus.