Factors that influence negative summation in a spatial-search task with pigeons

► A conditioned inhibition procedure was used to evaluate landmark-based spatial search. ► We found that subjects showed no change in overall responding during AX− training trials but did decrease pecks to the goal signaled by A. ► Summation tests revealed that X had a greater suppressive effect than Y on overall responding. ► The effectiveness of a stimulus trained as an inhibitor is dependent on the strength of the association between its training excitor (A) and the US, and the spatial proximity of stimuli during testing.