Mooney face stimuli for visual perception research

Abstract

In 1957, Craig Mooney published a set of human face stimuli to study perceptual closure: the formation of a coherent percept on the basis of minimal visual information. Images of this type, now known as "Mooney faces", are widely used in cognitive psychology and neuroscience because they offer a means of inducing variable perception with constant visuo-spatial characteristics (they are often not perceived as faces if viewed upside down). Mooney's original set of 40 stimuli has been employed in several studies. However, it is often necessary to use a much larger stimulus set. We created a new set of over 500 Mooney faces and tested them on a cohort of human observers. We present the results of our tests here, and make the stimuli freely available via the internet. Our test results can be used to select subsets of the stimuli that are most suited for a given experimental purpose.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Face and Mooney face stimuli.

(A) This greyscale image is easily perceived as a face although most visual information is covered by shadows. (B) A typical “Mooney” face. (C) An extremely easy “Mooney” face, devoid of cast shadows.

Fig 2. Reaction times.

Subjects were faster when they perceived a face then when they did not, both in the upright and in the inverted condition (mean difference 29.8 ms, T(17) = 3.86, p = 0.001). They also showed a typical face inversion effect, taking longer to perceive inverted than upright Mooney faces (mean difference 38.9 ms, T(17) = 7.54, p<0.001). No reaction time differences between perceived and non-perceived scrambled images were observed (mean difference 2.1 ms, T(17) = 0.11, p = 0.906). Error bars reflect the standard error of the mean, corrected for between-subject variability [34, 35].

Fig 3. Frequency of stimuli perceived as face, upright or inverted.

Most faces were correctly identified as faces by the majority of subjects when presented upright (green) both in our and in the original Mooney face set, but both stimulus sets also contain difficult stimuli that are only perceived as faces by a few subjects. Face inversion (blue) markedly reduced the number of ‘face’ responses. On the right are examples of easy and difficult Mooney faces from the upright and the inverted conditions, respectively. Original Mooney faces reprinted from [1] under a CC BY license, with permission from the Canadian Psychological Association Inc., original copyright 1957.

Fig 4. Inversion effects.

Inversion effects were evident both when considering whether an inverted face was recognized as a face (red) and when considering reaction times (yellow), i.e., longer reaction times to inverted than to upright faces. The latter type of inversion effect was much more frequent than the former. Overall, inversion effects of reaction times showed a median effect of 66 ms in our new stimulus set, and 79 ms in the original Mooney stimulus set (black solid lines). The right shows the stimuli with the most reliable/largest inversion effects for face recognition and reaction times, respectively. Original Mooney faces reprinted from [1] under a CC BY license, with permission from the Canadian Psychological Association Inc., original copyright 1957.