Despite a hectic day, could not resist looking into this a tad. There
appears to be a vast literature on "critical flicker fusion."
There is even a section of psychophysics and physiological optics at
the Univ Eye Hosp in Munich! The phenomenon was noted at least as far
back as Ptolemy in the 2nd century who described the apparant
immobility of wheels at certain speeds. The early attempts at
quantification related to the frequency of the light stimulus were
reported by an Englishman approx 1834 (Talbot) and a Belgian, Plateau
whose thesis is the apparent landmark in this field. From a cursory
review of 20 abstracts while quite distracted it does appear the mean
critical flicker fusion frequency shows a linear loss with age ---thus
your interesting observation qbout your son. Interestingly mean light
difference sensitivity decreases only slightly up to age 46 or so with
a marked acceleration above that (slope in 1 abstract goes from
-0.0153 dB/decade to -2.06 dB/decade.)
I could not find out the neurological basis of any of this. The tests
must be done very carefully with standarized conditions as many
nonsensory confounding variables can render the results useless.
Mike
For whatever it's worth, and from my anecdotal evidence only: the
detectible or even subliminally bothersome refresh rate of monitors
(clearly
related to the general neurological function of "flicker
fusion"--utilized
well in motion pictures at a rate at least as low as 24 fps) is
variable
across individuals. For example, I can live with 60hz, though I
CAN--when
asked--detect flicker on bright backgrounds. On the other hand, my
20-something son is driven NUTS by any rate below 70hz--lower rates
must
look like a disco strobe light show to him. Makes me wonder sometimes
how
all the PAL standard countries have been able to stand 50hz standard TV
for
years--oh, yeah, it's the interlacing. Nevermind.
Gary Holder
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