Perky effect

The Perky Effect describes the relationship between real visual information (perception) and mental imagery. Discovered by C. W. Perky in 1910, her experiments were able to show that visualization of images can depress the sensitivity of perception of real visual targets. In her study, participants were asked to imagine certain objects on a white screen, such as a fruit, a tree, or a book. After a few trials, a faint, real image was projected on the screen of the subject matter. Participants were quite often unable to distinguish between their imagined projections and the true images. For example, one participant was surprised that the banana appeared to be in a different orientation than the one they were attempting to imagine it in. Another maintained that the title of the book they were imagining was legible, while there was no title in the actual, projected image. The Perky effect has since been updated to describe the reduction in visual performance due to the maintenance of mental imagery.

A research group was interested in both replicating Perky’s findings and in investigating the influence of contrast on this effect. Before the experiment, researchers asked some participants to imagine lines on a white board and choose a color chip (from a prepared set between black and light grey) that matched most closely with the color of their imagined lines. These lines were called “fixation lines.” Other participants were given a specific color of either black or grey fixation lines (displayed on the white target), while the control group had no fixation lines. The imagined/real color of the fixation line was referred to as the participant’s “mask.”

During the task, each participant received a picture showing an image of two black circles. The picture was removed from view and a target with two black lines was shown, one originating from each previous fixation point with a half-inch gap between the lines.

Figure 1. An image displaying both the black circles and black lines “On,” “Close,” and “Far” from the imaginary/real lines. These fixation lines are shown as they appear in the grey condition, as dashed lines in the figure.

For the imagined condition, the participant was asked to imagine the previous image ‘ON’ the two fixation lines (nearest line $0.1\text{ cm}$‍), ‘CLOSE’ to the fixation lines (nearest line $0.8\text{ cm}$‍), or ‘FAR’ from the fixation lines (nearest line 2 cm). For the black and grey line conditions, the four fixation lines were shown continuously and the black circles were removed and replaced with black lines. The task was to decide whether fixation lines were offset either to the left or to the right. The results are shown in Figure 2.

Figure 2: The mean percent of correct responses for each condition over all participants was calculated. The duration of each trial was between $3\text{ ms}$‍ and $25\text{ ms}$‍. The baseline condition (‘None’) was practiced until the participant was at least $90\mathrm{\%}$‍ accurate. The effect of distance, line type, and the interaction between distance and line type were all significant at $p<0.001$‍.

Reeves, A., & Craver-Lemley, C. (2012). Unmasking the Perky Effect: Spatial Extent of Image Interference on Visual Acuity. Frontiers in Psychology, 3, 296. doi:10.3389/fpsyg.2012.00296

Thomas, N. (2014). The Perky Experiment. Stanford Encyclopedia of Philosophy.