Abstract:

Drifting ramped gratings or shaded diamonds brighter when the direction of movement produces fast-OFF relative to fast-ON luminance profiles. The neural mechanism and locus remain unclear. We recorded DC-electroretinograms from toad eyecups presented with sequential epochs of sawtooth, sine-wave, and square-wave gratings drifting horizontally across the retina with drift temporal frequencies of 2.5 - 20Hz. A sustained DC increase in trans-tissue potential during drift (plateau region), plus a peak at drift offset were observed. Consistent with human perceptual experience, the DC plateau was greater for fast-OFF cf fast-ON sawtooth stimuli.  All gratings produced an increase in DC potential as a function of temporal frequency. Modelling suggested that changes in root mean square (RMS) temporal luminance contrast contributed to these effects, but could not explain the divergence in response amplitudes for the two sawtooth profiles. Pharmacological dissection of retinal responses indicated that the response differences for the two sawtooth profiles remained following suppression of post-photoreceptoral activity with Tetrodotoxin (TTX), 2-Amino-4-Phosphonobutric acid (APB), and cis-Piperidine-2,3-dicarboxylic Acid (PDA). This points to a photoreceptoral locus as the origin of DC potential differences resulting from asymmetries in the response to fast-ON and fast-OFF sawtooth profiles, and as the likely locus for the drifting sawtooth brightness illusion.