How can the presence of multiple substances affect the HGN response?

The presence of multiple substances can significantly influence the Horizontal Gaze Nystagmus (HGN) response by intensifying the effects of each substance involved. When an individual ingests or is exposed to multiple intoxicating substances, the combined effects can lead to a more pronounced manifestation of nystagmus, which is an involuntary eye movement. This intensification may make the HGN signs more vivid, but it also complicates the interpretation of those signs.

For instance, alcohol may cause the eyes to exhibit nystagmus to a certain degree, while other drugs—such as certain prescription medications or stimulants—may further enhance or alter this nystagmus. As a result, the test administrator may observe increased eye movement or instability, which signals impairment but could be misleading due to the cumulative effects of multiple substances. Thus, the intricacy of interpreting these responses is heightened when more than one substance is present in the individual's system.