How does the frequency of a visual stimulus affect the steady-state visually evoked potential?

I want to make a project for EEG signal processing, and in my research I found the concept of SSVEP, which means that if you have a stimulus with low frequency applied to the eye, the electrical activity (measured via EEG) of the brain will change.

My question is whether is it known how the electric activity of brain will change? And a second thing, does it mean that each frequency applied the eye has a fixed reaction for the brain?

-

1 Answer

The steady state response is a sustained oscillatory response to a stimulus that is varying in strength with some regularity. But it is not a true oscillation, it's closer to an ERP in nature: it is a stimulus-driven response to variations in the stimulation.

In auditory processing, which I am familiar with, people take a tone and then add an amplitude modulation to it. This means that the sound level increases and decreases regularly at a certain frequency. Increases and decreases in neural stimulation are clearly visible in the brain, so if you average the frequency decomposition of your signal over all trials, you get a response at the frequency of the amplitude modulation. If you then manage to change something about this frequency response (e.g. the power) over different experimental manipulations, then most likely you've found some true top-down effect.

In vision I assume it would be similar if you flicker a picture: the stimulation will increase and decrease regularly and you should get a response at the frequency of the flicker. I haven't got a clue what would happen if you would split the frequency rate for the two eyes though.

One thing that mystifies me is that in audition, the steady-state response is particularly well picked up if the amplitude modulation is around 20 or 40 Hz. I have no idea why this is. I also don't really understand what the advantages are over just recording brain activity and looking at the ERP. With the steady-state response, you're in danger of covering up genuinely interesting oscillatory activity with the frequency of the flicker. Also, I'd expect that you'd also pick up a flicker that matches the frequency of the refresh rate of your display. I don't know how that would influence the steady state response (we don't have that problem in audition).

-
i'm get confused , is amplitude modulation that you talked is the same amplitude modulation that is used in electrical communication systems ?(i don't think so). also , what is refresh rate of a display , is it the delay needed to display new "thing" ? –  Learner Jul 26 '14 at 21:12
By amplitude modulation I mean this. In this picture the carrier signal is the frequency of the tone, whereas the modulating sine wave signal is what I call amplitude modulation. The result is an amplitude modulated signal: it's a tone of a given pitch, that gets louder and softer at the frequency of the modulating sine wave signal. This frequency should also be visible in the neural response. –  Ana Jul 26 '14 at 22:15
The refresh rate is the rate at which the image on your screen gets renewed. I think that for most monitors it's 60 Hz. But it's often more complicated than that, because screens often don't refresh the entire image at once (e.g. they might do it line by line, or odd vs. even lines, or center to surround). But some of this flicker would probably be visible in the neural response. I'm not too sure about that though, maybe it's too subtle to be registered as neural activity. –  Ana Jul 26 '14 at 22:18