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M. D. Crossland, A. F. Macedo, G. S. Rubin; Calibration of the MP1 Microperimeter for Fixation Measurement of Complex Stimuli. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4735.
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Retinal-specific fixation can be measured on the MP1 microperimeter for static bitmap images. However, current MP1 software will not allow measurement of fixation during rapidly changing visual stimuli such as those presented in a psychophysical experiment. We report a technique which allows the MP1 to measure the retinal fixation location used for observing dynamic or rapidly changing stimuli.
Fixation was recorded using the MP1 microperimeter (Nidek Technologies, Italy) whilst five subjects with good vision observed targets which were presented on the LCD screen of the MP1 but generated on a second computer. The relationship between the displayed target and the fixation data recorded by the MP1; between the fundus image and the fixation data; and between the fundus image and the displayed target was calculated. Nonlinearity and raster distortion were assessed by examining the transfer functions between these values. A Matlab (Mathworks, Natick, MA) program was developed which used these transfer functions to superimpose the fixation position or the target on the retinal image.
Each pixel of target decentration resulted in between 4.8 and 5.3 minutes of motion on the fixation data in the horizontal meridian and 5.0 - 5.4 minutes in the vertical meridian. This decentration moved the retinal image by 2.32 - 2.48 pixels horizontally and 2.42 - 3.12 pixels vertically. No significant nonlinearity or raster distortion existed for either of these factors. There was no significant interobserver variability for either of these factors (fixation data: F(4,11)=0.32, P=0.85. Image data: F(4,11)=0.88, P=0.52). These conversion factors were not significantly different for a subject with high myopia (-13.00DS).
By calibrating the MP1 microperimeter and calculating the transfer functions, the MP1 can be used to record fixation position whilst subjects view any scene which can be created on a LCD screen. These can include movies, rapidly changing words (such as those presented in the rapid serial visual presentation paradigm) or stimuli created by an adaptive staircase in a psychophysical experiment. We demonstrate how this technique can be used to determine the retinal area used for target fixation and reading in control subjects and people with retinal disease.
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