We systematically tested many conjunction tasks to investigate this additivity assumption. Target and distractors differed in a conjunction of two features. To assess the additivity assumption, the differences between mean conjunction slopes were calculated for each feature pair. If the additivity assumption holds, these differences should be equal. Differences were found not to be equal, hence we could not replicate the additive effect of features on the mean conjunction slope.
Single features seem to contribute differently to different conjunctions.
If both features are not processed separately in conjunction tasks, then observers might have restricted their search to elements defined by only one feature type. It is not clear, however, which feature the subjects might have chosen, since even for each individual observer no single feature led to constantly small conjunction slopes. This indicates that the choice of the searched feature type might change from condition to condition. Alternatively, the combination of two features might lead to conjunction tasks with new properties not inherent in the single features.