Correcting "confusability regions" in face morphs.

The visual system represents summary statistical information from a set of similar items, a phenomenon known as ensemble perception. In exploring various ensemble domains (e.g., orientation, color, facial expression), researchers have often employed the method of continuous report, in which observers select their responses from a gradually changing morph sequence. However, given their current implementation, some face morphs unintentionally introduce noise into the ensemble measurement. Specifically, some facial expressions on the morph wheel appear perceptually similar even though they are far apart in stimulus space. For instance, in a morph wheel of happy-sad-angry-happy expressions, an expression between happy and sad may not be discriminable from an expression between sad and angry. Without accounting for this confusability, observer ability will be underestimated. In the present experiments we accounted for this by delineating the perceptual confusability of morphs of multiple expressions. In a two-alternative forced choice task, eight observers were asked to discriminate between anchor images (36 in total) and all 360 facial expressions on the morph wheel. The results were visualized on a "confusability matrix," depicting the morphs most likely to be confused for one another. The matrix revealed multiple confusable images between distant expressions on the morph wheel. By accounting for these "confusability regions," we demonstrated a significant improvement in performance estimation on a set of independent ensemble data, suggesting that high-level ensemble abilities may be better than has been previously thought. We also provide an alternative computational approach that may be used to determine potentially confusable stimuli in a given morph space.