The aim of this project is to test the effects of an environmental factor (mild stress) on prefrontal cortex (PFC) and the cognitive functions that depend on PFC (collectively called executive functions [EFs]), and to test our predictions concerning how those effects differ by biological factors (hormones and genotype). To test our hypotheses concerning mechanism, the investigators will model the effects of mild stress on EFs pharmacologically. The purpose is to pharmacologically model the effects of mild stress on the cognitive functions (collectively called "executive functions" [EFs]) dependent on the frontal lobe. The investigators would also like to investigate how gender differences and genotype mediate the effect of methylphenidate (MPH) on EFs.

The investigators propose to test each young adult twice; once on the lowest clinically relevant dose of MPH and once on a placebo (Vitamin C), with the order counterbalanced. The two testing sessions will be scheduled 4 weeks apart, once 90 minutes after taking 10mg MPH and once 90 minutes after taking a placebo (90 mg of Vitamin C). Peak plasma concentration of MPH is reached approximately 2 h after ingestion, thus to gain the maximum effect of MPH, the investigators will test subjects 90 minutes after ingestion. They will tell subjects that they are studying the effects of low-dose MPH on EFs, and that they expect it to help the EFs of some but hinder those of others, without telling them the study's predictions. Before each testing session, subjects will come to the lab 90 minutes in advance of the testing period. First, they will be given a pill they will take in front of a research assistant (MPH or placebo). The conditions will be counterbalanced across subjects within each subject group. This is a double-blind design, where neither subject nor tester will know which condition they will be in. The pills (10mg MPH, and 90mg Vitamin C) are made to be identical. They will be over-encapsulated with an opaque coloured capsule. The pills will be in kept in sealed containers that are labeled in such a way that neither the subjects, nor the testers, nor the PI, will not know which pill is which, until it is time for the data analysis. Only a pharmacist, at the compounding pharmacy, will know which container contains which pill.

At low doses the mode of action of MPH increases DA efflux specifically in PFC and preferentially enhances signal processing in PFC. Low dosages of MPH are often effective in improving EFs and specifically because of their effect on PFC. This slight increase of DA in the PFC is similar to the effects of mild stress on the brain, which is why the study uses a low dose of MPH as the pharmacological model of mild stress. Even mild stress markedly increases DA levels in PFC, impairing PFC function and EFs. The use of MPH is meant to mimic the effect of mild stress on the PFC and executive functions.

Purpose/objective: This double-blind study aims to compare performance on tasks of executive function between males, females when their estrogen levels are high, and females when their estrogen levels are low when they have undergone a pharmacological model of mild stress (low dosage of MPH) and when they have not undergone this stress. It also aims to compare the same across the three variants of the catechol-O-methyltransferase (COMT) genotype (methionine-methionine, methionine-valine, and valine-valine).

Hypotheses: MPH, like mild-stress, should raise PFC DA levels in COMT-Vals to optimal, but raise PFC DA levels in COMT-Mets past optimal (since low dose of MPH increases DA levels in PFC). While COMT-Mets generally show better EFs than COMT-Vals without MPH, on MPH that should be reversed with COMT-Vals performing better. An increase in PFC DA levels pharmacologically should mimic the sex difference in the effect of mild stress on EFs, harming females with high estrogen level's performance while enhancing males' performance on tasks of executive functions.

Inclusion Criteria:

Between the ages of 20 and 35 years old

Exclusion Criteria:

• a neurological impairment or disorder, learning disability, or psychological syndrome that might affect EF performance (such as ADHD)
• inability to understand the task instructions (which are in English), or difficulty hearing the instructions, seeing the stimuli, or executing a manual response.
• a serious adverse event during pregnancy or birth.
• an injury (such as a head injury with loss of consciousness) that might affect EF performance.
• a major trauma that might affect current EFs and stress responsivity
• undue current life stress level
• taking any medication that affects thinking, memory, mental clarity, or any other EF ability.
• taking any medication that influences circulating gonadal hormone levels (such as oral contraceptives [birth control pill]).
• having taken such medications within the preceding four months.
• smokers
• use of recreational drugs or consumption of alcohol 24 hours prior to the testing sessions
• women without a period that occurs roughly every month (predicting the onset of the next menses in women who don't have their period monthly is difficult)
• women who are pregnant or who are nursing.
• having the eye problem glaucoma
• having a heart condition
• being anxious, tense or agitated
• taking or have taken within the past 14 days an anti-depression medicine called a monoamine oxidase inhibitor or MAOI.
• taking pressor agents (for hypotension treatment)
• taking coumarin anticoagulants
• taking anticonvulsants (phenobarbital, diphenylhydantoin, primidone)
• taking phenylbutazone (nonsteroidal anti-inflammatory drug)
• taking tricyclic antidepressants (imipramine, desipramine)
• taking cold or allergy medicine that contain decongestants
• being allergic to anything in either the MPH or Vitamin C capsules (methylphenidate HCL, ascorbic acid, lactose, gelatin, Titanium dioxide, D&C Red #28, FD&C Blue #1, FD&C Red #40)

Data will be analyzed at the group level

Double-blind.