Categorical discrimination of human faces from the other body parts in pigeons

Abstract

We trained pigeons to discriminate photographs of human faces from those of other body parts, using a yes/no procedure. The birds were trained on the discrimination until they showed good transfer to novel stimuli. They were then tested on all-reinforced probe trials with a variety of modified “face” stimuli. The test stimuli lacked some or all of the inner features (eyes, nose, and mouth) in Experiment 1, or had some other feature of faces (color, hair, contour, and orientation) modified in Experiment 2. We found that the pigeons chose the key corresponding to “face” significantly less often for the test stimuli lacking color or hair than for intact faces. The results suggested that it was color and hair that controlled the birds' response. The weight each bird placed on any given feature for solving the task varied among subjects. In Experiment 3, the test stimuli lacked both color and hair. All birds chose the key corresponding to “non-face” in trials with the test stimuli. These results suggest that the pigeons used the two features to discriminate “face” and “non-face” stimuli in an additive way.