Abstract
The visual system’s ability to extract statistical information such as mean or variance from groups of objects is generally referred to as ensemble perception. In a standard setup, observers are presented with a group of, for example, lines and are asked to indicate their average orientation. Hitherto, ensemble perception is only studied using isolated and randomly displayed features. However, three horizontal lines and a vertical line in the right spatial arrangement can also form a more complex object such as the letter E. One might consequently ask what are the limitations of averaging individual features when they are grouped into objects? Here, we investigate how extracting summary statistics is affected by the arrangement of the individual elements of ensembles. If elements are grouped into complex objects, can these individual elements still be averaged or does grouping break the averaging ability? We presented observers with letters and various types of letter manipulations such as scrambled or disassembled letters and asked them to judge the average orientation by rotating a single line afterwards. Our results revealed strong effects of hierarchical complexity: Compared to scrambled or disassembled letters, averaging performance was impaired when lines were grouped into letters. Moreover, adjustment time and the number of line rotations revealed similar patterns: Observers required more time and rotations to complete their adjustment when lines formed letters compared to the identical, but dissembled lines. Our results show for the first time that once features are part of objects, extracting their summary statistics is impaired showing both, larger adjustment errors and longer reaction times. These results allow for a new perspective on the extraction of statistical information and the question at which level of object processing the averaging of features happens. Our results indicate that summary statistics are extracted before features are combined to more complex objects.