Suggested protocols or Comments on protocols

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Regarding the photopic frequency series, my comments concern the data analysis. Compulsory aspects of frequency analysis are as in Bach & Meigan 1999 paper, i.e. a whole number (and >10) stimulus periods contained in data epoch; sampling frequency should be at least twice the stimulus frequency; stimulus period should contain integer number of sample periods.

There are two further possible constraints, the first of which is related to equipment. Some equipment can deliver flickering stimuli at only integer values of stimulus period (in milliseconds), i.e. not at 30Hz (33.33333..ms stimulus period) but at 30.3030...Hz (33ms stimulus period). This small difference is important when looking for small signals in the frequency domain.

Secondly, many people may choose to use a Fast Fourier Transform to look at their signals in the frequency domain, since it is available on almost every desktop as part of Microsoft Excel!. This adds the further contraint that 2^n data points per data epoch are required - achieving this (without resampling data) further reduces those frequencies it is possible to record.

As an example, to record flicker ERGs with an Espion (I believe) and to meet all the above constraints without resampling, and using an FFT, only five frequencies within perceptual range are possible: 3.906250Hz, 7.81250Hz, 15.6250Hz, 31.250Hz and 62.50Hz, with data epochs of 4096ms, 2048ms, 1024ms, 512ms and 256ms respectively.

-- RuthHamilton - 13 Jul 2009

(1) Please don't use the term "FFT" -- it is misleading, not even the fastest etc. Use "DFT" (discrete Fourier transform) instead -- thus immediately loosing the tie to orders of 2, which the original butterfly algorithm entails. (2) The restriction to powers of 2 is totally ridiculous, especially given current computing resources. Just use a DFT implementation which is not constrained by this. (3) Yet another reason NOT to use Excel for science...

-- MichaelBach - 15 Jul 2009