How caffeine affects the visual brain

Twenty healthy young (18-35 years) adults will be recruited and screened to ensure healthy eyes and vision. The screening includes a visual acuity test, determination of spectacle prescription, and brief ocular health examination. The main task is a measure of ocular dominance, where observers will look at patterns on a computer monitor and make judgments (by pressing a button) about the colour of a pattern that alternates between red and green. Both eyes need to be open (blinking allowed) during the main task for ocular dominance to be measured. There will be two test sessions, a treatment session and control session. The order of the sessions will be randomised and the participants blinded to the treatment. Participants will be instructed not to consume caffeine for 12 hours prior to attending each test session. At the beginning of each test session, participants will undergo baseline testing of the main ocular dominance task (15 minutes). This will be followed by: (1) a controlled dose of caffeine in the form of a single ‘No Doz Plus’ pill, which consists of 100 mg caffeine, 10 mg thiamine hydrochloride (Vitamin B1) and 10 mg nicotinic acid (Vitamin B3), or (2) a placebo pill (single ‘Betamin’ pill, which consists of 100 mg thiamine hydrochloride (Vitamin B1). Participants will wait 30 minutes for the pill to reach its peak effect, and then the participant's dominant eye will be patched for 90 minutes. Post-patching ocular dominance tests (repeat of baseline tests, 15 minutes) will be conducted at the end of the session. During the 120 minutes of waiting, participants will be asked to sit quietly in a room and can undertake activities such as reading, watching a movie, or working on a laptop. At the second visit at least one week later, participants will undergo the same protocol (caffeine washout, baseline ocular dominance testing, tablet ingestion, patching of dominant eye, post-tablet ocular dominance testing) such that all participants will have consumed both the caffeine pill and placebo pill. The pills are both white, uncoated, round tablets with no engravings of approximately the same size, which will assist in masking the identity of the pill at each test session.