ON and OFF Pathways and the Human Periphery Effect

Abstract

Abstract: : Purpose: The human periphery effect (PE) is characterized by changes in target thresholds when the luminance of retinal areas that are remote from the target locus is modulated. Physiological studies of the PE in monkey retina and LGN show that remote stimulation affects the responses of both On– and Off–center neurons and by approximately the same amount. We sought to determine if the On and Off–pathways in humans responded similarly and also if observed effects were retinal or cortical in origin. To test this we determined the effect of peripheral stimulation on thresholds for detecting flicker in rapid on and rapid off sawtooth temporal waveforms. Methods: Five subjects set thresholds for detection of flicker in rapid on and rapid off luminance modulated foveal stimuli. Stimuli were 1.2^o and presented on a 4^o degree steady white background of 41.0 cd/m2. Surrounding the background was a luminance matched 16 x 22^o field that was either unmodulated (uniform control) or a 0.25 cpd vertical square wave grating that was counterphase modulated at 7.5 Hz. In experiment 1, the flickering test stimuli were presented on top of the background while in experiment 2, they were modulated around the mean luminance of the background. In experiment 3, the surrounding grating was presented either to the same or opposite eye as the target in a dichoptic viewing situation. Results: Sensitvity was generally greater for the rapid on waveform than for the rapid off. Remote stimulation elevated test flicker thresholds for both rapid on and rapid off sawtooth waveforms. The magnitude of the effect varied with flicker frequency with an average of 0.19 and 0.15 log units for rapid on and rapid off, respectively. Although the thresholds were elevated, there was no significant change in the relationship between the on and off waveforms when tested with the square wave grating. Counterphase modulation of the sawtooth around the mean luminance of the background increased flicker sensitivity but otherwise did not alter the effects of remote stimulation. Presenting the grating and test to the same or different eyes resulted in elevated flicker thresholds at 20 Hz for the former condition (0.094 log units) but not the latter (–0.005 log units). Conclusions: A PE of nearly equal magnitude is found in both the On and Off–pathways in humans. While the effect is equal, the initial sensitivity was slightly and thus preferential for the on waveform. Furthermore, the locus of the effects of remote stimulation on flicker thresholds at higher frequencies appears to be retinal in origin. CI: None Support: VA Medical Research Service, Washington, DC.

Keywords: detection • temporal vision • brightness and lightness