From: A double dissociation of the acuity and crowding limits to letter identification, and the promise of improved visual screening Journal of Vision. 2014;14(5):3. doi:10.1167/14.5.3 Figure Legend: Distinguishing crowding from overlap masking. When a target letter is among flanking letters, the flankers can impair target recognition in two ways: crowding and overlap masking. These effects have different explanations and phenomenology but have not always been easy to distinguish. Here we introduce a new diagnostic test. Making the flankers identical to the target abolishes crowding yet hardly affects overlap masking. Regan, Giaschi, Kraft, and Kothe (1992) introduced a repeated-letter chart to tolerate errors in fixation or selection. Here we exploit its immunity to crowding. The threshold spacing of peripheral vision is normally due to crowding. (The conclusions of this demo depend only on comparing the two charts, side by side, at any viewing distance. If you like, viewing from at least 2 m will eliminate any concern that you might be limited by the resolution of this page.) Please fixate the top plus sign and try to identify the middle letter in the vertical triplets to the left and right. Ignore the two flankers above and below each target letter. On the left, the flankers (R) are different from the target. On the right, they are identical to the target. You will find that you can read more lines in the right column than in the left column. Your left-column threshold is limited by the severe crowding of peripheral vision; your threshold spacing is several times larger than your acuity A. Your right-column threshold is limited by overlap masking, a spacing of 1.4A, which is less than twice your acuity, allowing you to read more lines. In central vision, we saw above that crowding and overlap masking are both viable explanations for the measured threshold spacing of 1.4A. Using central vision to directly fixate each target we find that we can read down to the same level on both sides of the chart. There is no effect of making the flankers identical to the target. Since abolishing crowding has no effect, any crowding present must have a threshold spacing less than or equal to that of the overlap masking. Note that most charts probe critical spacing radially from the point of fixation; the target and flankers all lie on the same radial line. This chart probes critical spacing tangentially; the target and flankers lie on a line that is orthogonal to the radial line connecting target to fixation. Critical spacing tangentially is about half the critical spacing radially (Toet & Levi, 1992). Date of download: 11/10/2017 The Association for Research in Vision and Ophthalmology Copyright © 2017. All rights reserved.