Internal domain generality for ensemble perception of average orientation: Comparing and integrating information from multiple stimulus categories.

Ensemble perception (the rapid extraction of summary statistics from groups of objects) operates across a range of visual features (e.g., average orientation, average emotion) and stimuli (e.g., triangles and faces). Ensemble displays usually contain identical stimuli, although natural environments are composed of more heterogeneous items. This raises the question of whether ensemble mechanisms can generalize across stimulus types (“internal domain generality”). Across 4 Experiments we examine whether ensemble perception for orientation operates similarly across ensemble displays composed of different stimulus types. In Experiment 1, participants compared average orientation (same/different) across two homogeneous ensembles that were made up of either the same or different stimulus types (faces, shoes, triangles). We found that participants were just as accurate when the stimulus types were the same for both ensembles compared to when they were different, suggesting that ensemble abilities are governed by a domain-general mechanism. In Experiments 2A and 2B, participants were shown a single ensemble that was made up of either one stimulus type (e.g., triangles) (pure) or two (e.g., triangles and faces) (mixed) and then determined whether the direction of a subsequent probe arrow was representative of the average orientation. There were minimal differences in the pure condition. However, in the mixed condition, stimulus specificities emerged. Specifically, when the ensemble contained triangles, participant responses were biased in the direction of those triangles. Further evidence of this triangle bias was found via a continuous report task (Experiment 3). When every item within an ensemble is the same, ability to extract summary statistics is similar across ensembles composed of different stimuli, but having more than one type of stimulus present within a single ensemble interferes with the extraction and subsequent report of summary statistics. This suggests that ensemble perception abilities do show internal domain generality, but specificities do emerge under certain circumstances (e.g., stimulus dependencies in mixed displays). We posit that these dependencies are due to differences in the shape and orientability of the stimuli.