Contrast detection facilitation by spatially separated targets and inducers

We measured contrast detection thresholds for a small (3.6 x 3.6 arc min) square target in the presence and absence of spatially identical pedestal stimuli, and of a spatially non-overlapping inducing line (3.6 x 23 arc min). Results for the pedestal stimuli replicated the classical "dipper function", thresholds being reduced by near-threshold pedestals and increased at higher pedestal contrasts. An inducer without a pedestal also decreased detection thresholds. When the inducer and pedestal were combined, their effects were additive. Thus the inducer facilitated target detection when the pedestal was absent but raised detection thresholds when the pedestal contrast was sufficient by itself to lower threshold. Inducers of opposite polarity to the target did not consistently decrease target thresholds, even when they were clearly visible, arguing against spatial uncertainty as the explanation of the inducer effect. The inducer effect was independent of the length of the inducer except with small (< 3.6 arc min) stimuli, and was abolished by increasing target-inducer separation beyond about 10 arc min.