This study will evaluate the influence of non-invasive brain stimulation on different elements of cognitive function in healthy persons between the ages of 18-90 years. It will specifically use transcranial direct current stimulation (tDCS) to evaluate the relationship between certain brain structures and cognitive abilities, like attention, working memory, visual search, etc.

The research team conducting the research procedures will perform several tests. Investigators will ask participants if participants are right or left handed, how far participants went in school, their job, and a medical history including brain and nervous system disorders, psychiatric disorders, heart related disorders, and other significant disorders. Investigators will ask participants for their current medications. Investigators will ask participants about their history of cognitive function. If participants are a woman of child bearing potential, Investigators will perform a pregnancy test and the result must be negative to receive stimulation. Prior to stimulation, Investigators will have participants complete pre-stimulation computerized cognitive tests involving tasks where participants are asked to respond to images or sounds presented by a computer. Investigators will also collect measures of physiological function during the testing session, specifically heart rate, blood pressure, skin conductance, respiration rate, and pupillary response. These will be measured using recording electrodes attached to the fingertip, arm, and chest and a respiration band around the chest. A small camera will record changes in their pupil size. Investigators will also collect measures of brain structure and function using magnetic resonance imaging. Investigators will also ask participants to fill out a questionnaire describing their experience during each study session.

If participants decide to take part in this study, participants will be asked to participate in up to four different testing sessions separated by at least 24 hours. In each of these testing sessions, participants will be asked to receive either electrical stimulation or placebo stimulation during different testing sessions. Placebo stimulation looks like and is performed in the same way as electrical stimulation, but stimulation is stopped before it can have an effect. Placebo stimulation is used in research studies to show what effect a treatment has compared with receiving a lower level of stimulation or nothing at all. Studies have shown, however, that about 1 in 3 persons who take a placebo have an effect, if only for a short time. Participants will not know whether participants are receiving placebo stimulation or electrical stimulation, but that information is available if it is needed. In the remainder of the description of what will be done, both the electrical stimulation and the placebos stimulation will be called "stimulation."

Stimulation will be completed with a Transcranial Direct Current Stimulator. This is an experimental device that is not currently approved by the Food and Drug Administration (FDA) for any treatment. Participants will receive stimulation for up to 20 minutes. To perform the stimulation, Investigators will either apply two salt-water soaked sponges to their head or five smaller gel covered disk electrodes. Gel-covered disk electrodes are placed in specially designed holders held in a cap that fits on their head. Caps are fitted to their head size for comfort. Following the stimulation, Investigators will remove the sponges or electrodes and have participants perform a series of computerized tests to assess their cognitive ability. These involve tasks where participants will have to respond to images or sounds presented by a computer screen or headphones.

Participants may also be asked to undergo magnetic resonance imaging (MRI) of their brain before, during, and after undergoing transcranial direct current stimulation. Investigators will use an MRI safe Transcranial Direct Current Stimulator to perform stimulation and evaluate how their brain responds to stimulation. Magnetic resonance imaging (MRI) is a procedure that allows doctors to look inside the body by using a scanner that sends out a strong magnetic field and radio waves. This procedure is used routinely for medical care and is very safe for most people, but participants will be monitored during the entire MRI scan in case any problems occur. The risks of MRI are:

The MRI scanner contains a very strong magnet. Therefore, participants may not be able to have the MRI if participants have any type of metal implanted in their body, for example, any pacing device (such as a heart pacer), any metal in their eyes, or certain types of heart valves or brain aneurysm clips. Someone will ask participants questions about this before participants have the MRI.

There is not much room inside the MRI scanner. Participants may be uncomfortable if participants do not like to be in close spaces ("claustrophobia"). During the procedure, participants will be able to talk with the MRI staff through a speaker system, and, in the event of an emergency, participants can tell them to stop the scan.

The MRI scanner produces a loud hammering noise, which has produced hearing loss in a very small number of patients. Participants will be given earplugs to reduce this risk.

Inclusion Criteria:

• Healthy adult
• Eligible to undergo an MRI

Exclusion Criteria:

• History of neurologic disease or dementia
• History of psychiatric disease which required hospitalization, or
• Current depression
• History of head trauma with unconsciousness
• Developmental learning disability (e.g., dyslexia, dyscalculia)
• History of vital organ failure (e.g., heart, lungs, kidneys, liver)
• Currently taking any psycho-active medications
• Pregnancy