How much face identity information is required for face recognition?

Many studies have shown that degradation of face identity information impairs face recognition, however, when such information degradation reaches the limit of our face recognition ability remains unclear. Here we systematically decreased face identity information by morphing an increasing number of faces together and investigated how much identity information is required for recognizing a face in a morph. Our results show that participants could identify half of faces mixed in 3-identity morphs using only their memory of these faces (Experiment 1) and, when perceptual information is available, they could recognize two of three faces mixed in a morph (Experiment 2). When we systematically reduced the contribution of each identity to a face morph from 50 % to 6.25 % (i.e., morphing 2 to 16 faces together; Experiments 3 and 4), participants could still consistently recognize faces in a morph containing as little as 12.5 % of their identity information. Moreover, familiarity with faces enhanced participants' performance, whether they were asked to recognize all faces mixed in a morph in one go (Experiments 1 and 2) or to recognize them individually (Experiments 3 and 4). Finally, image-based similarity between the faces and morphs could predict how decreasing identity information impairs face recognition performance. Together, these results not only help quantify the minimum information required for face recognition but also offer new insights into the representational differences between familiar and unfamiliar faces.