Two Types of Crowding Effects Revealed by Word Inversion Experiments

Crowding refers to the phenomenon that the visual performance to a visual target deteriorates with the presence of nearby flankers. To investigate the mechanisms underlying crowding in object perception, we used Chinese characters as stimuli such that we can estimated the role of grouping and familiarity. The target was presented at 6 deg from the fixation. The flankers were placed to both sides of the target with a 1.5 deg center-to-center distance. The target and the flankers were either upright or inverted characters. When the target and the flankers were iso-orientation, in half of the trials, they might be combined into a compound character. The task of the observer was to decide whether the target is upright or inverted. Compared with the no flanker condition, the presence of the flankers reduced the percentage of correct response (accuracy) regardless of the character type or orientation. The accuracy for the iso-orientation conditions was always higher than that for the aniso-orientation conditions. The result is the same regardless whether the target and the flankers can be grouped into a compound word, hence cannot be explained by the word superior effect. Our result suggests two types of flanker effects: The first is a general crowding effect that reduces the visibility of the target whenever a flanker is presented and may result from an early visual mechanism. The second is specific to character orientation and thus should be mediated by a mechanism that tuned to visual word forms.