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17. CONTRAST GAIN OF MAGNO-CELLS REVEALED IN NEW METHOD OF MEASURING CONTRAST SENSITIVITY AND REACTION TIME by G.A. Akinlabi Volume 55 (February 2020 Issue)
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CONTRAST GAIN OF MAGNO-CELLS REVEALED IN NEW METHOD OF MEASURING CONTRAST SENSITIVITY AND REACTION TIME

G.A. Akinlabi

Department of Optometry, University of Benin, Benin City

Abstract

Measuring the speed of responses to low contrast stimuli may provide better insights into functional capabilities, anatomical and physiological dichotomy of the retino- cortical pathways.Two new computer tests are developed to measure response times as the contrast of the stimulus is varied. Firstly, large numbers (1-8) 4-cm high appears on the screen in sequentially reducing contrast. The subject quickly touches the numbers in order and time is recorded. Secondly, a large (4-cm) solid square appears in one of four quadrants on the display screen. As soon as the subject touches a square, it disappears and a new square appears. There are 6 sweeps of the contrast range that can give two presentations at up to 24 contrast levels. The test provides measures of threshold CS and responds times as a function of contrast. In 8 normal subjects, we performed the number-search test and the detection test for both blinking and jumping squares. The result shows that the reaction time increases with increasing contrast sensitivity. Our graph is biphasic in nature, revealing the activity of two mechanisms with different reaction times: transient/sustained, magno/parvo. The first phase with asymptote reaction time revealed activity of parvocells (sustained cells) which dominates at low contrast sensitivity and the steep second phase, revealed contrast gain and the activity of magnocells (transient) at supra threshold contrast. This simple test may be similar to the pedestal paradigm.

Keywords: Contrast sensitivity, contrast gain, transient/sustained, Magno/parvocells

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