The lingering effects of artificial scotomata

Abstract

Prolonged fixation produces artificial scotomata in dynamic random noise: peripheral, zero-contrast regions disappear. When the noise's contrast is suddently zeroed, dynamic afterimages are seen where the scotomata had been (Ramachandran & Gregory, Nature, 1991). We measured contrast increment thresholds for dynamic noise coextensive with these afterimages, using various pedestal and adapting contrasts. When the adapting contrast was zero, increment thresholds formed a dipper-shaped function of pedestal contrast. When the adapting contrast was high, the dip disappeared, but there was no change to the left or right side of the function. Inverting popular theories for dipper-shaped increment-threshold functions, we infer linear transduction and/or intrinsic certainty from the lack of dipper. However, changes in transduction or certainty should also be reflected in absolute threshold. To explain adaptation's lack of effect on detection without a pedestal, we are forced to believe in a lingering effect of the sensitivity loss responsible for the artificial scotomata (Morgan & Solomon, ECVP, 2003), one that perfectly balances the effect of linear transduction and/or uncertainty reduction on absolute threshold.