Abstract
For calibrating eye movement recordings, a regression between spatially defined calibration points and corresponding measured raw data is performed. Based on this regression, a confidence interval (CI) of the actually measured eye position can be calculated in order to quantify the measurement error introduced by inaccurate calibration coefficients. For calculating this CI, a standard deviation (SD) - depending on the calibration quality and the design of the calibration procedure - is needed. Examples of binocular recordings with separate monocular calibrations illustrate that the SD is almost independent of the number and spatial separation between the calibration points – even though the later was expected from theoretical simulation. Our simulations and recordings demonstrate that the SD depends critically on residuals at certain calibration points, thus robust regressions are suggested.
Highlights
IntroductionEye movements can be measured with different technical systems (for a review see Collewijn, 1999) that all need a calibration procedure to provide the angular position of the eyes
Eye movements can be measured with different technical systems that all need a calibration procedure to provide the angular position of the eyes
We investigated the effect of the number and the angular separation of the calibration points on the standard deviation (SD) in two ways: 1. We performed simulations according to equation (3) and 2. we compared the simulations with empirical data measured under experimental variations of the calibration procedure
Summary
Eye movements can be measured with different technical systems (for a review see Collewijn, 1999) that all need a calibration procedure to provide the angular position of the eyes. A linear regression is calculated between spatially defined calibration points xi (deg) and corresponding raw data yi (arbitrary units), measured during fixation of calibration points. For any measured raw data Ym (arbitrary units) within the calibration range, the corresponding eye position Xm (deg) can be calculated by: Xm. Figure 1 shows examples of typical calibration curves for the two eyes (relative to the x-axis, the position of the two eyes is indicated at the bottom). Both curves have been recorded separately for each eye with 7 calibration targets that have been presented monocularly. (1) The actual angular position of the eye (relative to the calibration centre) is important: the more the eye position ( xm ) deviates from central fixation ( x ), the larger the SD
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