Normally, the pineal gland governs a person' s sleep-wake cycle by secreting melatonin in response to the daily cycle of light and darkness as detected by the eye. Nonetheless, many people who are totally blind due to lesions in the visual cortex of the brain easily maintain a 24-hour sleep-wake cycle. So the neural pathway by which the pineal gland receives information from the eye probably does not pass through the visual cortex. For purposes of evaluating the argument it would be useful to establish which of the following? ['Whether there are any people with normal vision whose melatonin levels respond abnormally to periods of light and darkness', 'Whether the melatonin levels of most totally blind people who successfully maintain a 24-hour sleep-wake cycle change in response to changes in exposure to light and darkness', 'Whether melatonin is the only substance secreted by the pineal gland', 'Whether melatonin supplements help people who have difficulty maintaining a 24-hour sleep cycle to establish such a pattern']