Contrast integration across space

Contrast integration across space was studied in respect to stimulus extent and the spatial layout, using high-contrast stimuli. Contrast discriminati on thresholds were measured (2AFC) by either increasing the size of a perip heral (2.4 degrees) Gabor signal (GS: lambda = 0.08 degrees) or by increasi ng the number of GS elements in a circular arrangement. The supra-threshold mask (pedestal) was either increased with the target or fixed at maximal s ize and had 30% contrast. For stimuli with an increasing size of both the p edestal and the increment target, we find approximately constant discrimina tion thresholds. Contrast discrimination improved linearly on a log-log sca le with slopes average of -1/4 (fourth-root summation) when the size of the Gabor target was increased but the mask was kept at maximal size, indicati ng contrast integration across space. Taken together, these results indicat e balanced spatial integration of both contrast increment and pedestal, res ulting object-size invariant contrast discrimination. Contrast discriminati on was found to improve as well when the number of aligned Gabor elements w as increased (both pedestal and increment), pointing to independent contras t normalization for disconnected (sparsely positioned) stimuli. The results indicate a complex pattern of spatial integration involved in contrast dis crimination, possibly depending on image segmentation. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.