On the statistical significance of electrophysiological steady-state responses

Steady-state stimulation is a useful paradigm in many physiologic and clini cal situations, for ERG, Pattern-ERG and VEP. One of the advantages is the easy evaluation of the response via Fourier analysis. However, the question whether a given response is statistically significant or not has received little attention so far, although it is especially relevant in high noise, low amplitude recordings, as often occur in pathologic conditions. A given response is statistically significant if it is unlikely that its value is d ue to noise fluctuations. Thus appropriate estimates of noise and response are required. We have analytically derived formulas for the statistical sig nificance of a given signal-to-noise-ratio s, based on two different estima tes of noise: (1) Noise estimate by a 'no stimulus' recording, or by a '+/- average'. The former needs an additional recording, the latter can simulta neously be calculated as the standard average. (2) Noise is estimated as th e average of the two neighboring spectral lines (one below, and one above t he response frequency). Analytical solutions were obtained for both noise e stimates that can easily be evaluated in all appropriate recordings. Noise estimate (1) performs much poorer than noise estimate (2), as can be seen f rom the following landmark values: Typical significance levels of 5%, 1%, a nd 0.1% require s values of 4.36, 9.95, and 31.6 (1), and 2.82, 4.55, and 8 .40 (2). The noise estimate based on the neighboring frequencies can be eas ily applied after recording, provided that the noise spectrum is reasonably smooth around the response and frequency-overspill was avoided. It allows a quantitative assessment of low responses in physiological threshold analy ses and pathological conditions, e.g., 'submicrovolt flicker-ERG'.