Frequency Tagging
Periodic stimulation
Imagine a light flashing in your eyes at six times per second. If we recorded your brain activity using EEG while this light was flashing, an oscillatory neural response would be detectable at the same frequency as the flashing light (6 Hz) in the occipital region of your brain, which is typically the region associated with visual processing.
If we increased the speed of the flashing light to 10 times per second, the neural response would also increase to 10 Hz. This relationship between periodic stimulation and the neural response at that same frequency is the basis for FPVS experiments. This phenomenon is known as a Steady State Visually Evoked Potential, or SSVEP.
From flickering light to face perception
Frequency tagging has a long history. In an early foundational study, David Regan (1966) measured EEG responses to light whose brightness rose and fell in a smooth, repeating rhythm. He showed that an ongoing response over the back of the head could follow the frequency of that stimulation. This helped establish a key idea behind the SSVEP: a known visual rhythm gives researchers a known frequency at which to look for a neural response.
Decades later, Rossion and Boremanse (2011) applied this approach to a more complex question: how does the brain distinguish one face from another? They presented faces at 3.5 images per second and compared sequences repeating the same face with sequences showing different identities. The response at 3.5 Hz was larger for different faces, particularly over the right occipito-temporal region. This showed that frequency tagging could reveal sensitivity to facial identity, extending the approach beyond simple flickering lights.
A later study by Liu-Shuang, Norcia, and Rossion (2014; first published online in 2013) introduced a periodic oddball approach to individual face discrimination. One face was repeated rapidly, with a different identity appearing every fifth image. A response at the slower identity-change frequency provided a measure of the brain distinguishing those faces. This leads directly to the distinction between the base and oddball frequencies used in many FPVS experiments today.
Base and Oddball frequencies
FPVS oddball experiments typically produce two separate frequencies referred to as the Base Frequency and the Oddball Frequency.
The Base Frequency is the frequency at which stimuli are displayed on screen. For example, if a stimulus is displayed six times per second, like the flashing light used in the previous example, the base frequency would be 6 Hz. The Oddball frequency is the frequency at which the oddball stimulus is displayed. For example, if the oddball stimulus is displayed once every five stimuli at a base presentation rate of 6 Hz, the oddball frequency would be 1.2 Hz.
Harmonics
A harmonic is a frequency that is an exact whole-number multiple of a repeating signal’s fundamental frequency, hereafter referred to as F. For example, if a visual stimulus repeats at 6 Hz, its harmonics occur at 12 Hz (2F), 18 Hz (3F), 24 Hz (4F), and so on.
One way to understand harmonics is to imagine building a repeating waveform by combining several smooth sine waves. A single sine wave at F produces a simple, smooth waveform. Real visual stimuli and brain responses are usually more complex. Additional, faster sine waves are needed to reproduce features such as sharper peaks, asymmetry, or abrupt changes. Because the complete waveform repeats at the same regular interval, these additional waves fit a whole number of cycles within each repetition. This places them at integer multiples of F.
A fast Fourier transform (FFT) separates the recorded EEG waveform into these individual frequency components. Harmonics may reflect various properties of the stimulus in question.
Norcia et al. (2015) provide more detailed explanations and illustrations showing how complex repeating responses produce harmonics at F, 2F, 3F, and so on.