On the second level, Sociology, the vision is significant because it represents the passing from childhood into adulthood. Thus, the hope to see a vision, a 'good' vision, is an anxious one.
One step down to Psychology...
The young man tells his vision to the medicine man who interprets it for him. Since this quest is meant in part to represent the character of the person, the vision could be seen as a projection of the person's own spirit. It is told that Black Elk said "What is received ... is determined in part by the character of the person ... for it is only those people who are very qualified who receive the great visions." Thus visions are often of strong, brave, or otherwise powerful animals (such as an eagle, a bear, or a wolf).
Biologically, what is happening when the vision is seen?
The person is sent into nature alone, without food, and often with a pipe. They ceremoniously smoke their pipe and point the staft at the four directions of the wind. They are instructed not to sleep. They will cry out for a vision, and fast until they see one (this could take days).
And Chemically, it has been shown that lack of sleep causes neurons in the brain to be triggered, and "the individual may hallucinate". Lack of food causes the body to go into hypoglycemia - the decline in blood sugar. Hypoglycemia can cause what appears to be 'psychiatric symptoms', and/or an altered mental status. In addition, "if blood sugar levels fall sharply ... it may result in the release of adrenaline."
Thus, visions could possibly be reduced to low blood sugar and high levels of adrenaline.
Additional reading:
The Neuropsychology of Religion
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