The Pareidolia Test

We previously devised two versions of the tests, the scene and noise pareidolia tests, to evoke and measure pareidolic illusions. The scene pareidolia test originally consisted of 25 blurred natural scene images. We created an abbreviated version in the current study that consisted of the 10 images that produced illusory responses most frequently in our previous study [17]. A detailed explanation and two training trials were given immediately before administering the test. Subjects were instructed to point to and describe in as much detail as possible the objects shown on each image. Each image was presented for a maximum of 60 seconds. No feedback was given to subjects, regardless of whether the responses were correct. Subjects’ responses were classified into three types: (1) correct responses; (2) illusory responses, in which subjects falsely identified objects that were not on the images; and (3) other responses, in which subjects provided no response or said “I don’t know”. The correct answers for each image were defined a priori. When subjects responded with such comments as “It looks like X”, we asked the subjects whether the object (X) was actually in the picture or whether the subject saw something that just looked similar to X. Only the former type of responses was regarded as illusory responses. The number of all illusory responses was used as a measure of pareidolic illusions in our previous study. However, we counted the number of images on which subjects made one or more illusory responses in the current study to ease test administration and scoring (maximum score of 10).

The noise version contained 40 black and white images that consist of visual noise with a spatial frequency of 1/f³. Black and white images of human faces were embedded in 8 of the 40 images. Subjects were requested to state whether a face was present and point to the place where they observed a face after a detailed explanation and three training trials were given. Each picture was presented for a maximum of 30 seconds. No feedback was given to subjects, regardless of whether the responses were correct. The responses were classified into three types: (1) illusory responses, in which subjects falsely found faces in images without a face; (2) detection misses, in which subjects did not detect the embedded faces, and (3) correct responses, in which subjects correctly responded “nothing exists” to the noise stimuli or correctly detected the embedded image in the images that contained faces. The number of images in which subjects made illusory responses was used as a measure of pareidolic illusions [18]. In scoring pareidolic responses, we originally used images with a face and images without a face [17]. In the current study, we used only the 32 images without a face to ease scoring (maximum score of 32).

We defined the pareidolia score as the sum number of images with illusory responses in the scene and noise versions.