Visual Stimulation Under 4 Hz, Not at 10 Hz, Generates the Highest-Amplitude Frequency-Tagged Responses of the Human Brain: Understanding the Effect of Stimulation Frequency

This article addresses an important practical aspect of event-related potential research: which stimulation frequency to choose to obtain the “largest” responses in studies that use frequency-tagging. The authors raise many important points, and I applaud them for having reviewed this highly relevant issue. Their conclusion, as conveyed by the title of the article, is that visual stimulation below 4 Hz generates the highest-amplitude responses.

This is a straightforward conclusion, but unfortunately, the 4 Hz threshold is not universally valid. The article seems to target readers who may not be fully familiar with the mathematical underpinnings of the issue and who may thus be particularly prone to being misled. A more guarded message would be: If the cycle duration is long enough to avoid the superposition of responses to successive individual stimuli, then one can capture the full underlying single-stimulus response through frequency-domain analysis. Otherwise, it depends. Importantly, the “it depends” regimen may extend well below 4 Hz when slow response components are involved.

Because of potentially complex superposition effects that arise when the cycle length is shorter than the total duration of a single-stimulus response, a stimulus frequency that yields a high overall amplitude is not necessarily one that shows a large (absolute or relative) effect as the result of an experimental manipulation or pathophysiological process. Further, if the goal is to optimize the signal-to-noise ratio for a given recording time, even more factors need to be considered.

The authors acknowledge some of these details themselves, for instance, in the Caveats section of the article. It would have been helpful if these crucial insights had been imparted by the prima facie message of this very relevant and timely article.