This study will use repetitive transcranial magnetic stimulation, or rTMS (magnetic stimulation to the brain), to examine how the brain distinguishes between two signals that are spaced very closely in time. The ability to tell the difference between sensory signals is important to understanding sensory input. Understanding how this works may help to develop new treatments for sensory deficits.

Healthy volunteers 18 years of age and older may be eligible for this study. Participants undergo the following procedures during three visits to the NIH Clinical Center:

rTMS - all visits (sham rTMS on one visit) :

For TMS, a wire coil is held on the scalp. A brief electrical current passed through the coil creates a magnetic pulse that stimulates the brain. During the stimulation, the subject may be asked to tense certain muscles slightly or perform other simple actions.

Theta burst stimulation (TBS) - all visits (sham TBS on one visit):

Same as TMS, but brief pulses of electrical current are passed through the coil. Subjects undergo intermittent TBS on one visit and continuous TBS on another.

Somatosensory evoked potential (SEP) - all visits:

SEP tests how sensory information travels along the nerves to the spinal cord and brain. A small metal disk electrode placed on an arm delivers a small electrical shock. Electrodes placed on the scalp record how the impulse travels over the nerve pathways to the cerebral cortex of the brain. EEG (see below) records what sensory information the brain is detecting and processing. Paired-pulse SEP is done before and after TBS.

Temporal discrimination threshold - all visits:

This test investigates the brain's ability to discriminate sensory information. Electrodes are placed on the subject's wrist. Two electrical pulses are delivered to the nerve at the wrist at different spaced intervals to determine when the subject feels the two pulses are fused into one. This test is done before and after TBS.

Electroencephalography (EEG) - all visits:

This test records brain waves (electrical activity of the brain). Electrodes are placed on the scalp with an electrode cap. The spaces between the electrodes and the scalp are filled with a gel that conducts electrical activity.