Visual imagery selectively reduces vernier acuity.

Mental imagery interferes with perception. This, an example of the 'Perky effect', was studied for vernier acuity. Mean accuracy for reporting the offset of vertical line targets declined from 80% to 65% when subjects were requested to imagine vertical lines near fixation. Images of horizontal lines or of a grey mist in the fixation region lowered accuracy to a similar extent. However, accuracy was barely affected when the image was requested 1.5 deg or more from the target. The Perky effect remained strong for at least 4 s after an instruction to 'clear' the image away. The results were not due to imagery-induced changes in fixation, pupil diameter, or accommodation, or (at least primarily) to central attentional or decisional factors. Rather, imagery produces a local, pattern-insensitive, and relatively long-lasting reduction in visual sensitivity. The sensitivity loss may be mimicked by a 0.24 log unit reduction in target energy.