The pretectum is located between the thalamus and the midbrain. It is primarily responsible for receiving sensory information from the retinal ganglion cells in both eyes. After receiving information it projects to the Edinger-Westphal nucleus and the Cilio-spinal nucleus, which regulate the size of the pupil by controlling the pupillary sphincter muscle and the pupillary dilator muscle, respectively.
The word pretectum is quite unique in its syncopation. To remember the function, I rhyme "pretectum" with "detect 'em", which evokes the idea of picking up sensory information. Other rhymes with this word aid in the mnemonic, such as rectum which is another sphincter muscle and "flex 'em" to indicate a changing the relative size of the pupil. Hopefully these accessible rhymes will help you remember that the pretectum is involved in the pupilliary reflex, which serves as a reminder that the pretectum is "detecting" changes in light.
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Another mnemonic is:
Nurse Thalamus and Dr. Midbrain say that pupils are fragile; they suggest that you protect them from ridiculous light intensity