Abstract
People can rapidly extract average information of the features of an object set by visual statistical summary processing. Previous studies demonstrated independent mechanisms for summarizing low (e.g. color, orientation) and high-level (facial identity, emotion) visual information. Furthermore, evidence suggests a shared capacity for simultaneously summarizing multiple sets along the same feature dimension, but independent capacities for summarizing distinct feature dimensions. Here, we investigated whether statistical summary processing of low-level visual features relies on a feature-specific or a feature-general mechanism and whether there are capacity limitations to simultaneous averaging of different visual features. We asked participants to average one of the features in a set of lines that varied in size and orientation. The relevant feature was either the same throughout a block or mixed within a block. In Experiment 1 we found that participant performance on one feature did not predict performance on the other feature, regardless of whether the task-relevant feature was varied within versus across blocks. Furthermore, errors were similar between single-feature and mixed blocks for both features. These results imply that size and orientation features are independently averaged with no cost of simultaneous processing. In Experiment 2 we reduced the encoding time of the displays from 200 milliseconds to 50 and 100 milliseconds to determine if orientation averaging has an advantage over size at early processing. Results were consistent with Experiment 1: size and orientation averaging errors were similar across encoding durations. In Experiment 3, we added a visual mask to terminate iconic processing after 50 milliseconds. Size summaries remained similarly accurate regardless of whether the task-relevant feature was mixed or blocked. However, orientation averaging errors were higher in mixed than single blocks. Overall, results suggest there are further independent, feature-specific statistical summary processing mechanisms for size and orientation features.