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What are the determining facial features when recognizing a person?

December 18, 2019

We humans still seem to be unbeatable when it comes to recognizing familiar faces. What do we remember about a familiar face to achieve such a performance? Isabelle Bülthoff, Head of the research group for Recognition and Categorization at the Max Planck Institute for Biological Cybernetics, and her team have looked at this question more closely.

We are best at recognizing the faces of well-acquainted people. Within a family we do not confuse twins, although they may be very similar. However, we are just able to differentiate them if we know them well enough. In order to get accurate results on how facial features are stored in our memory, it is therefore essential to conduct studies with faces of people who are personally familiar to the participants.

An experiment among old acquaintances
In her study, Isabelle Bülthoff first created 3D face scans of her colleagues and then gradually changed them: each original face, regardless of its gender, was gradually made more feminine: first, by 10 percent, then by 20 percent, and then up to 50 percent. The same face was also gradually made more masculine, again up to 50 percent.

In a second series, she gradually gave the original Caucasian and Asian faces a more Asian and Caucasian look in a similar way. "We then asked some team members to look at the ten image variations of a colleague together with the original picture and invited them to find the true, unaltered face. We found that the images could be quickly assigned to individual persons, but the participants found it very difficult to correctly identify the original – regardless of the parameters we had changed.

The known unknown
In a third series of experiments, the faces of familiar people were mixed with the faces of strangers. Here, too, the team proceeded step by step: first, they mixed 90 percent of the original face with 10 percent of the unknown face, then they mixed at a ratio of 80:20 up to a ratio of 50:50. Once again, colleagues were asked to identify the "real" face from the series of faces shown. In comparison to the previous series, it was much easier for the participants to identify the original face.
A computer analysis of the images ruled out that the images from the first two tests were more similar to each other than those from the third test. Objectively speaking, the tasks of all three experiments were equally difficult.

Quick Identification
What conclusions can now be drawn from this study?
People remember the face of a certain person on the basis of external characteristics, such as the skin, the shape of eyes or the nose and how these individual parts of the face relate to each other. "These personal characteristics that differ for each face are called idiosyncrasy", explains Isabelle Bülthoff. "We remember them very accurately when we know a person well".

Not only we can recognize the identity of a face, we can also easily categorize each face as male or female, as Asian or Caucasian. This categorical information is apparently not stored as precisely as the idiosyncratic information: We remember the characteristic features of a face more closely than its exact gender-specific or ethnical appearance.

Why this distinction? The scientist explains this with the fact that in social life this exact categorical information is not so important for recognizing a person. For example, it is usually sufficient to know that a person is male or female – how masculine or feminine that person really is, is irrelevant. Another hypothesis is that these types of facial changes are treated similarly to facial expressions: A face may look different depending on the facial expression, but these variations are not perceived as a change in identity. We know very well that it is the same person and ultimately it is precisely this tolerance that makes constant recognition possible.

Bülthoff, I., & Zhao, M. (2019). Personally familiar faces: Higher precision of memory for idiosyncratic than for categorical information. Journal of Experimental Psychology: Learning, Memory, and Cognition. Advance online publication. http://dx.doi.org/10.1037/xlm0000784

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