A ruler placed between one student's fingers and thumb is released without warning. A second student catches the ruler. The distance the ruler falls is recorded. This experiment is most likely designed to determine the --

The experiment is most likely designed to determine the second student's reaction time to stimulus.

Explanation:

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The ruler test is designed to measure a person's reaction time to a stimulus, that is, the time it takes someone to perform visual-motor coordination to respond to a stimulus appropriately.

On the second student, the nerve pathways involved are the optic nerve (perception of the stimulus, afferent pathway), the visual and motor cortexes (association) in the Central Nervous System and the efferent pathway to the effector, the muscles of the hand.

When the first student drops the ruler, the second student catches it visually and reacts by closing his hand to prevent it from falling to the floor. What the test really measures is how long it takes for the second student to react to the stimulus of the ruler falling, which is called reaction time.

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Reaction time - yarstick test link