People with face blindness might not be able to recognize familiar faces, but tests of their skin conductance, which reveal the body's physiological reaction to a stimulus, show that they still react to these faces emotionally. It is likely that people with face blindness, the "overt," or direct pathway, that sustains face recognition is impaired, Diard-Detoeuf said.
In contrast, patients with regular Capgras syndrome can identify familiar faces, but don't show any emotional reaction to them in skin conductance tests, Diard-Detoeuf said. It's likely people with regular Capgras have brain damage to a different pathway, called the covert pathway, which sustains familiarity. If they see a familiar face but don't feel a sense of familiarity, a subsequent feeling of strangeness will ensue, Diard-Detoeuf said. [10 Things You Didn't Know About the Brain]