Noise and the perceptual filling-in effect.

Background: It was shown that collinear flankers increased the reports of a target present (false alarms, FA), an effect that is attributed to filling-in of the target location due to increased activity induced by the flankers. A filling-in effect is characterized by high FA and a high percent correct, leading to low decision criterion, which indicates high reports of a target present. However, according to the standard notion, FA is attributed to noise in the system. We hypothesized that if the reports for FA are due to noise, then different levels of external noise implemented in the filling-in task should modulate the filling-in effect. Method: Here we investigated the effect of manipulation on the filling-in effect by external noise that was added to the Gabor target, presented between two collinear Gabor flankers in differing target-flanker separations. The target was masked by white noise with differing levels of contrast. Results: The results show that raising the levels of the noise modulate the filling-in effect; increasing FA at low noise levels, but decreasing the filling-in effect for higher noise levels. The visual sensitivity (d') decreases with increasing noise, indicating a masking effect of the noise on the target alone. Conclusions: The results indicate that the filling-in effect is not due to noise induced by flankers on the target location, rather, it can be inferred to stem from real lateral excitation on the target by the lateral flankers.