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PJ Clark, T Kuyk; Sawtooth Flicker and the Human Periphery Effect . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4728.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In the human periphery effect (PE), psychophysical thresholds for visual targets are elevated by modulating the luminance of retinal areas that are far removed from the target locus. Studies of the physiological PE in monkey show that remote stimulation yields similar excitatory responses in on and off-center neurons in retina and lateral geniculate nucleus. We hypothesized that in human observers, responses to both light increments and decrements would be affected by peripheral stimulation and by the same amount. To test this, we determined the effect of peripheral stimulation on thresholds for detecting flicker in rapid on and rapid off sawtooth temporal waveforms. These waveforms preferentially stimulate on and off ganglion cells. Methods: The stimulus configuration consisted of 1.2 deg achromatic test stimuli presented on 4 deg achromatic backgrounds of 41.0, 4.1, and 1.3 cd/m2. Test stimuli were luminance modulated, rapid on or rapid off, at 3, 10, 20, or 24 Hz and presented foveally. Surrounding the background was a 16 x 22 deg field of the same luminance as the background. It was either unmodulated (uniform, no mask condition) or a 0.25 cpd vertical square wave grating that was counterphase modulated at 7.5 Hz (remote mask). In 2 observers, thresholds for detection of flicker in the sawtooth waveforms was determined in the presence of each surround. Results: With the uniform surround, flicker thresholds for both rapid on and rapid off waveforms declined as a function of modulation rate and adaptation level. In one subject, flicker thresholds measured on the uniform surround were lower for rapid on than rapid off modulation, but in the other subject they were the same. However, in both observers, the remote mask elevated rapid on and rapid off flicker thresholds and by approximately the same amount (0.15 log units). Also, the amount of threshold elevation was similar across test frequencies and adaptation levels. Conclusion: The masking results support our hypothesis that the PE would be similar for light increments and decrements. This suggests that, as in monkey, the neural mechanism (presumably amacrine cells) generating the PE signal in humans and transmitting it laterally across retina, exerts similar influences on the on and off channels.
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