A double dissociation of the acuity and crowding limits to letter identification, and the promise of improved visual screening

Abstract

Here, we systematically explore the size and spacing requirements for identifying a letter among other letters. We measure acuity for flanked and unflanked letters, centrally and peripherally, in normals and amblyopes. We find that acuity, overlap masking, and crowding each demand a minimum size or spacing for readable text. Just measuring flanked and unflanked acuity is enough for our proposed model to predict the observer's threshold size and spacing for letters at any eccentricity. We also find that amblyopia in adults retains the character of the childhood condition that caused it. Amblyopia is a developmental neural deficit that can occur as a result of either strabismus or anisometropia in childhood. Peripheral viewing during childhood due to strabismus results in amblyopia that is crowding limited, like peripheral vision. Optical blur of one eye during childhood due to anisometropia without strabismus results in amblyopia that is acuity limited, like blurred vision. Furthermore, we find that the spacing:acuity ratio of flanked and unflanked acuity can distinguish strabismic amblyopia from purely anisometropic amblyopia in nearly perfect agreement with lack of stereopsis. A scatter diagram of threshold spacing versus acuity, one point per patient, for several diagnostic groups, reveals the diagnostic power of flanked acuity testing. These results and two demonstrations indicate that the sensitivity of visual screening tests can be improved by using flankers that are more tightly spaced and letter like. Finally, in concert with Strappini, Pelli, Di Pace, and Martelli (submitted), we jointly report a double dissociation between acuity and crowding. Two clinical conditions-anisometropic amblyopia and apperceptive agnosia-each selectively impair either acuity A or the spacing:acuity ratio S/A, not both. Furthermore, when we specifically estimate crowding, we find a double dissociation between acuity and crowding. Models of human object recognition will need to accommodate this newly discovered independence of acuity and crowding.

Variations in crowding, saccadic precision, and spatial localization reveal the shared topology of spatial vision

“…A range of behavioral studies also support a later-stage locus for visual crowding (3,13,(51)(52)(53). Furthermore, although acuity and crowding are correlated in the "normal" periphery (as in our gap-resolution data) and within-group for cases of amblyopia (54,55), between-group comparisons reveal a clear acuitycrowding dissociation (56,57). It may be that low-level spatial precision sets the precedence for crowding in development, but that disruptions to the visual system through additional factors such as a loss of binocularity (54) can dissociate these two factors.…”

Dancing Letters and Ticks That Buzz around Aimlessly: On the Origin of Crowding

“…In the remainder I would thus like to draw attention to a few points in this respect. For one, the concept of feature integration is still a major contender for letter recognition and (thus) letter crowding (cf Freeman et al, 2012;Pelli et al, 2004;Song, Levi, & Pelli, 2014;Strasburger et al, 2011;Tyler & Likova, 2007). It is perhaps instructive to go back to the roots of that and remind ourselves that Selfridge's (1959) Pandemonium Model (figure 4) was already computational and close to a neural implementation (since the demons Selfridge's (1959) computational Pandemonium Model, as depicted by Lindsay and Norman (1972) (illustration: Leanne Hinton).…”

Size or spacing: Which limits letter recognition in people with age-related macular degeneration?

“…By measuring the threshold letter size (the smallest letter size required to reach a given level of recognition accuracy) required for observers to recognize a letter flanked by other letters for a range of letter spacings, Coates, Chin and Chung (2013) and Song, Levi and Pelli (2014) independently showed that the threshold letter size depends on letter spacing, but only when the letter spacing is within the critical spacing. Within this regime, letter recognition is limited by letter spacing but not letter size.…”