Measuring single-item identification efficiencies for letters and 3-D objects

Identification thresholds and the corresponding efficiencies (ideal/human thresholds) are typically computed by collapsing data across an entire stimulus set within a given task in order to obtain a "multiple-item" summary measure of information use. However, some individual stimuli may be processed more efficiently than others, and such differences are not captured by conventional multiple-item threshold measurements. Here, we develop and present a technique for measuring "single-item" identification efficiencies. The resulting measure describes the ability of the human observer to make use of the information provided by a single stimulus item within the context of the larger set of stimuli. We applied this technique to the identification of 3-D rendered objects (Exp. 1) and Roman alphabet letters (Exp. 2). Our results showed that efficiency can vary markedly across stimuli within a given task, demonstrating that single-item efficiency measures can reveal important information that is lost by conventional multiple-item efficiency measures.