What limits search for conjunctions of simple visual features?

Despite of decades of research, we still do not know for sure the roles of internal noise, attention, and crowding in search for conjunctions of simple visual features. In this study, we tried several modifications to the classic design of conjunction-search experiments. In order to match exactly the proportions of simple features, two different targets were presented in target-present trials-vertical red and horizontal blue bars among vertical blue and horizontal red distractors. Both the length of the bars and the number of objects in a display were varied. Positions of objects were selected for minimal crowding effects. Exposure duration was 60 ms, and proportion correct was used as the measure of performance. For conjunction search, the results rejected the unlimited-capacity model and were consistent with limited-capacity attentional processing, and with the Naka-Rushton transform of the target-distractor difference. Qualitatively the same results were obtained when bar length was fixed, and fine orientation difference was used to manipulate target-distractor discriminability. An experiment of feature (orientation) search produced results close to the unlimited-capacity model.