Spatial Frequency and Similarity Modulate Crowding in Letter Identification

Abstract

Visual crowding, which impairs our ability to accurately identify a target stimulus when surrounded by flankers, is ubiquitous across a wide variety of stimulus classes. Target eccentricity and target-flanker distance constitute fundamental factors in crowding. Target-flanker similarity appears as another key factor based on findings obtained with non-linguistic stimuli. The present study investigated the impact of these factors in conjunction with spatial frequency (SF) content on single letter identification performance. We presented SF filtered letters to neurologically intact nondyslexic readers while manipulating target-flanker distance, target eccentricity and target-flanker similarity (metric based on published letter confusion matrices). SF filtering conditions were broadband, low-pass, high-pass and hybrid (i.e. medium SFs, known as optimal for letter recognition, removed from the stimulus). These conditions were matched on overall contrast energy. Participants were required to identify the target letter as fast and as accurately as possible. The results show that high target-flanker similarity enhances crowding, i.e. the joint effects of distance and eccentricity. This extends past findings on the impact of similarity on crowding to the visual identification of linguistic materials. Most importantly, the magnitude of the crowding effect is greatest with low-pass filtering, followed by hybrids, high-pass, and broadband, with all pairwise contrasts significant. We conclude that: 1- medium SFs provide optimal protection from crowding in letter recognition; 2- when medium SFs are absent from the stimulus, low SFs magnify crowding and high SFs protect against it, most likely through their opposite impact on the availability of distinctive feature information.