Frequency Tagging
Periodic stimulation
Imagine a light flashing in your eyes at six times per second. If you were to record 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). 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 (e.g., the light flashing at a fixed rate of six times per second) and frequency-domain neural responses is the basis for FPVS methodology.
This phenomenon is known as a Steady State Visually Evoked Potential, or SSVEP.
This is also where FPVS gets its name; the “Fast” in FPVS refers to the fact that the stimuli are presented at a fast rate (6 Hz, 10 Hz, etc), and the “Periodic” in FPVS refers to the fact that the stimulin are presented at a constant frequency.
Base and target 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, the base frequency is 6 Hz. When analyzing data in the frequency domain, a strong neural response is expected at the base frequency and its harmonics (in e.g., 6 Hz, 12 Hz, 18 Hz, etc). Responses at the base frequency are typically not of interest in FPVS oddball experiments, as they reflect the brain’s response to the visual stimulation itself rather than the cognitive process of interest.
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, the oddball frequency is 1.2 Hz. When analyzing data in the frequency domain, a strong neural response is expected at the oddball frequency and its harmonics (in e.g., 1.2 Hz, 2.4 Hz, 3.6 Hz, etc). Responses at the oddball frequency are typically of interest in FPVS oddball experiments, as they reflect the brain’s response to the cognitive process of interest.
Harmonics
Harmonics
A harmonic is a frequency that is an exact whole-number multiple of a repeating signal’s fundamental frequency, 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. The FFT does not create the harmonics; it reveals components that are already present in the signal. Harmonics may reflect properties of the stimulus, such as rapid on-off flashing, as well as nonlinear processing in the visual system. Measuring responses across several harmonics can therefore capture neural activity that would be missed by examining only the fundamental frequency.
Norcia et al. (2015) provide more detailed explanations and illustrations showing how complex repeating responses produce harmonics at F, 2F, 3F, and so on.