Which of the following checks of the eyes is performed prior to administering HGN to assess possible medical impairment?

The assessment of equal pupil size is crucial prior to administering the Horizontal Gaze Nystagmus (HGN) test because it ensures that the baseline condition of the eyes is established. Equal pupil size indicates that there are no underlying medical issues affecting both eyes symmetrically. This is important for accurately interpreting the results of the HGN test, as unequal pupils can suggest central nervous system issues or other medical impairments that may influence the test outcomes, making it difficult to determine if any observed nystagmus is due to alcohol consumption or other medical factors.

While color perception, pupil reactivity, and depth perception are all relevant aspects of an eye examination, they are not specifically focused on as part of the HGN test protocol. Color perception is not typically changed by impairment from alcohol or drugs, and while pupil reactivity can provide insights into neurological condition, the primary concern before HGN is to confirm that the pupils are equal in size to establish a reliable testing environment. Depth perception is also not directly assessed in the context of the HGN test. Thus, equal pupil size stands out as a key component in ensuring that the assessment can proceed accurately and yield valid results.