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David Crewther, Nina Riddell, Laila Hugrass, Jude Jayasuriya, Sheila Crewther; Toad lights up the prince of brightness illusions.. Journal of Vision 2016;16(12):561. doi: 10.1167/16.12.561.
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© 2017 Association for Research in Vision and Ophthalmology.
Human psychophysics indicates that drifting gratings or diamonds shaded in a sawtooth pattern appear brighter when the direction of movement produces fast-OFF relative to fast-ON luminance profiles. Although a retinal basis has been speculated, the cellular mechanisms remain unclear. We recorded electroretinograms (ERGs) from toad eyecups presented with sequential epochs of sawtooth (fast-ON_slow-OFF and fast-OFF_slow-ON), sine-wave, and square-wave gratings drifting horizontally across the retina with drift temporal frequencies of 2.5 - 20Hz. ERGs revealed a sustained DC increase in trans-tissue potential during drift, plus a peak at drift offset. For sawtooth gratings, the DC potential effect was greater for fast-OFF cf fast-ON sawtooths, consistent with the perceptual strength of the brightness illusion in human. All gratings produced an increase in DC potential as temporal frequency increased. Modelling of the frame-based stimulus presentation showed changes in root mean square (RMS) temporal luminance contrast as a function of temporal frequency, in way that differs between square, sine and sawtooth profiles. However, these effects could not explain the divergence in ERG response amplitudes for the two sawtooth profiles which remained immune to this frame-based phenomenology. Pharmacological probing indicated that the response amplitude difference for the two sawtooth profiles remained following suppression of post-receptoral activity with Tetrodotoxin (TTX) blocking ganglion cells and spiking amacrines, 2-Amino-4-Phosphonobutyric acid (APB) blocking ON bipolars, and cis-Piperidine-2,3-dicarboxylic Acid (PDA) blocking OFF bipolar cells. Thus, pharmacological dissection indicated that this DC difference originates from asymmetries in the photoreceptoral response to fast-ON and fast-OFF sawtooth profiles, suggesting the outer retina as the neural site of origin for the drifting sawtooth brightness illusion.
Meeting abstract presented at VSS 2016
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