Definition of flicker rates

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1

Hi there,

In coding a flicker-based psychopy study, I’m getting confused by what should be a straightforward concept. I’m sorry if this question belongs on a different message board, I don’t really know where to put it!

The best example of my confusion is in this link to a stack overflow page: python - Where can i find flickering functions for stimuli on psychopy? and how do i use it? - Stack Overflow

The idea is this: Say we want to present a stimulus flickering at 10 hz. I’ve found a few sources, like the page above, suggest that in order to achieve this on a 60 hz monitor, you can set the opacity on each frame by: StimObject.opacity = ((frameN % 12) >= 6)

This presents the image for 6 frames in a row, and then makes the image invisible for 6 frames in a row, and repeats over and over again.

My intuition tells me that this is actually a 5 hz flicker, because the actual cycle takes 12 frames, and you can do 5 full cycles in one second on a 60 hz refresh rate. But online, I find multiple pages calling it a 10 hz flicker.

Does the vision science community generally define the frequency hz of flicker by the rate of change (something changes every 6 frames, so 10 “changes” occur per second), instead of the full number of cycles completed within a second (one complete on-off cycle finishes 5 times a second)?

Thank you!

2

You are correct. A cycle is one complete waveform - it has to be measured from one point on a wave to the same point on the next cycle - e.g. peak-to-peak or trough-to-trough (in your case light-onset to next light-onset, or light-offset to next light-offset).

Particularly when a waveform is composed of discrete events (e.g. a square wave like this rather than a continuous sine wave function), it’s an all too common misconception in experimental psychology to count the constituent events of the waveform (e.g. light on/light off) to calculate the frequency rather than to regard them as part of an overall cycle. You’ll encounter this many times, even in the published literature - it doesn’t make it right though. We should be guided by the proper definition that comes from maths/physics rather than the folk error that has crept into to some psychology studies.

Because some readers will also misunderstand, it is worth being very explicit in your methods description. e.g. “The stimulus alternated between being white for 50 ms and the background colour for 50 ms (i.e. a flicker frequency of 10 Hz)”.

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Thank you for this. I appreciate this context which will be useful when interpreting results (mine and others) moving forward!