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Brain Freeze "Brain Freeze" Headaches-Known to Scientists as Sphenopalatine Ganglioneuralgia -Are Most

question 127

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Brain Freeze
"Brain Freeze" headaches-known to scientists as sphenopalatine ganglioneuralgia -are most often triggered when cold food or drink comes into contact with the roof of your mouth. In order to discover more about this phenomenon, researchers at Harvard Medical School hooked volunteers up to trans-cranial Dopplers, machines that allow scientists to study blood flow in the brain. The volunteers were then given cups of ice water and instructed to drink it using a straw held right up against the roof of the mouth. The volunteer gave a signal when he or she felt a "brain freeze" headache begin and another signal when it ended.
This experiment allowed the scientists to observe changes in brain blood flow before, during, and after a "brain freeze" headache. The scientists found that when the subjects were experiencing a "brain freeze", the anterior cerebral artery (a major blood vessel in the brain) dilated rapidly. Because the artery becomes wider, more blood flows into the brain. This creates an increase in pressure inside the skull, which is what causes the pain. As soon as the artery shrinks to its normal size, the "brain freeze" headache disappears.
This may explain why "brain freeze" headaches hurt, but it does not tell us why they happen in the first place. Scientists do not yet have any definite answers to this question, but they do have some ideas. One theory suggested by the researchers at Harvard Medical School is that a brain freeze is a way of protecting the brain from cold. When the anterior cerebral artery dilates, it allows a flood of warm blood inside the brain, thus keeping it warm.
Which phrase would best replace 'shrinks' in paragraph 2?

Definitions:

Variable Interval

A schedule in operant conditioning where a response is rewarded after an unpredictable amount of time has passed.

Premack Principle

A theory that more probable behaviors can reinforce less probable behaviors.

Primary Reinforcer

A stimulus that satisfies basic survival needs and is naturally reinforcing, such as food, water, or sleep.

Secondary Reinforcer

A stimulus that has become rewarding through its association with a primary reinforcer, which does not inherently satisfy a biological need.