This research project is intended to understand the mechanisms of action for the pain-relieving properties of visual green light exposure.

The investigators have shown previously that greenlight exposure decreased acute and chronic pain in both animals and humans. However, the investigators do not yet understand how green light exposure is capable of such function.

Pain is a biological function. There are several pain centers in the brain that modulate the intensity of the pain signal perceived by the brain. Part of the modulations takes place through two separate pathways. The first pathway mediates pain signals and is known as the ascending facilitator pain pathway. The main function of this pathway is to relay the pain signal to the brain to be processed.

The second pathway is known as the descending inhibitory pain pathway. The main function of this pathway is to inhibit the pain signal mediated by the first pathway.

Normal humans typically have a balance between the activities of the ascending and descending pain pathways.

The investigator animal studies suggest that green light therapy exhibits its pain-relieving effects through altering the balance between the ascending and descending pain pathway in a manner to result in an overall reduction or normalization in pain perception.

To test this hypothesis, the investigators propose recruiting 30 healthy human volunteers. The participants will be equally randomized into either green light (treatment) or white light (control) exposure groups. The investigators will initially test the activities of the ascending and descending pathways using thermal and mechanical stimulations. The investigators will also test brain wave activities using a non-invasive EEG device. After establishing the baseline activities of the pain pathways and brain waves, the volunteers will be exposed to the green light or white light at low intensity (4-100 lux) for two hours in our laboratory. After two hours of light exposure, the investigators will again test the activities of the pain pathways using thermal and mechanical stimulation and measure the brain wave EEG activities.

Inclusion Criteria:

1. 18-50 years healthy men or women
2. Able to speak and understand English
3. No history of Chronic pain
4. No known neuropathies in areas that testing will take place, (forearms, muscle at the base of thumb on the palm of the hand and upper back/shoulder area.

Exclusion Criteria:

1. Serious mental illness defined as distortions of perception, delusions, hallucinations, and unusual behaviors resulting in loss of contact with reality. This will be assessed during the screening interview.
2. Colorblindness or uncorrected cataracts.
3. Tissue damage in areas that testing will take place, (forearms, muscle at the base of the thumb on the palm of the hand, and upper back/should area.