Intact crowding and temporal masking in dyslexia

Abstract

Phonological deficits in dyslexia are well documented. However, there is an ongoing discussion about whether visual deficits limit the reading skills of people with dyslexia. Here, we investigated visual crowding and backward masking. We presented a Vernier (i.e., two vertical bars slightly offset to the left or right) and asked observers to indicate the offset direction. Vernier stimuli are visually similar to letters and are strongly affected by crowding, even in the fovea. To increase task difficulty, Verniers are often followed by a mask (i.e., backward masking). We measured Vernier offset discrimination thresholds for the basic Vernier task, under crowding, and under backward masking, in students with dyslexia (n = 19) and age and intelligence matched students (n = 27). We found no group differences in any of these conditions. Controls with fast visual processing (good backward masking performance), were faster readers. By contrast, no such correlation was found among the students with dyslexia, suggesting that backward masking does not limit their reading efficiency. These findings indicate that neither elevated crowding nor elevated backward masking pose a bottleneck to reading skills of people with dyslexia.

Letter and symbol identification: No evidence for letter-specific crowding mechanisms

“…It seems difficult to reconcile this basic finding with the notion that outer letters would be processed by specialized crowding processes that would boost their processing and outer symbols would be dramatically disadvantaged because they would be processed by ''normal'' crowding processes. The present results are consistent with the more parsimonious and traditional view that symbols are just less optimal stimuli for the visual system than letters presumably because of differences in structural properties or in expertise (Changizi & Shimojo, 2005;Doron et al, 2015;Krueger, 1975;Mewhort et al, 1981;Norris, 2013;Norris & Kinoshita, 2012;Wiley et al, 2016;Wong et al, 2009). The present work also shows that using the partial report task may not be suitable for isolating crowding processes per se.…”

“…We only observed an overall reduced performance for symbols compared to letters. This latter finding might reflect a difference in familiarity between letters and symbols, a result whose standard explanations have nothing to do with crowding (Changizi & Shimojo, 2005;Doron et al, 2015;Mewhort et al, 1981;Norris, 2013;Norris & Kinoshita, 2012;Wiley, Wilson, & Rapp, 2016;Wong, Jobard, James, James, & Gauthier, 2009). Overall, our work is not consistent with the hypothesis that letters would be processed by specialized crowding processes having the function of boosting processing of outer letters.…”

“…Some recent reviews show the difficulty of providing a clear-cut understanding of crowding (Levi, 2008;Strasburger, Rentschler, & Jüttner, 2011). Crowding has also been invoked to account for reading deficits either in amblyopia (Levi, Song, & Pelli, 2007), in low vision (Wallace, Chiu, Nandy, & Tjan, 2013;Wallace, Chung, & Tjan, 2017), or in developmental dyslexia (Martelli, Filippo, Spinelli, & Zoccolotti, 2009) although this remains a controversial issue both in low vision (Bernard, Scherlen, & Castet, 2007;Calabrèse et al, 2010;Chung, 2004;Scherlen, Bernard, Calabrèse, & Castet, 2008) and dyslexia (Doron, Manassi, Herzog, & Ahissar, 2015;Ramus & Ahissar, 2012).…”

Mixture model investigation of the inner-outer asymmetry in visual crowding reveals a heavier weight towards the visual periphery

“…Crowding refers to our inability to identify an object (primarily in the peripheral visual field) because it is presented with nearby objects [1][2][3][4][5][6] . Crowding is considered to be an impediment to reading 7 , face recognition 8 , eye saccade and hand movement, visual search 2 as well as deficits like macular degeneration 9 , amblyopia 10 and dyslexia 11 . Researches have characterized crowding and distinguished it from other spatial interferences, such as masking 5,12 , lateral interaction 13 and surround suppression 14 .…”

Visual perception in dyslexia is limited by sub-optimal scale selection

“…The spatial extent or “receptive field size” of these detectors is thought to decrease during reading acquisition. If this is the case, one might expect visual crowding to be more pronounced in readers with dyslexia than relatively good readers but results are mixed 23, 24 . The modified receptive field hypothesis assumes that letter and shape stimuli are processed differently in V1, despite no evidence to support this.…”