Experiment

[0iD]

For me it happens with dorito's but not with wotsits.

[stytagm]

iirc from some ancient Physics lesson, it's your eyeballs vibrating/resonating in sink with the vibrations the crisps cause in your head. Makes your vision go a bit blurry.

<nerd>This is a serious effect in helicopters etc, because if the vibrations in the copter match the resonant frequency of the pilots eyes, their vision suddenly goes very blurry indeed, and hard to spot objects (power lines for instance) can disappear in the blur. </nerd>

[ponders]

i think its something neuronal, and as chewing involves a variety of reflexes and coupled with the texture of cruchy foods- which are not really found in nature- may cause an unnormal effect on vision. its like looking a revolving blades, identifiying one actual blade is easier if you hum or sing a note.

[manwithnoname]

Finally, I have a 15" Hansol CRT set at 800x600 on the desktop with a refresh rate of 75 Hz and 32 bit colour ...

Ouch, I can see the flicker on a monitor with a refresh rate of 60Hz, I cannot use a monitor refresh rate of 75Hz because I can see the flicker in my peripheral vision if I am not looking directly into the monitor - gives me a headache very quickly. I think I'd go along with the resonating effect other are suggesting, as the averge human eye must be close to seeing the flicker.