Dependence of Reading Speed on Letter Spacing in Central Vision Loss

Reading is difficult and slow for many low vision patients, especially those who have lost their central vision, and thus have to rely on their peripheral vision for visual tasks. The leading cause of central vision loss is age-related macular degeneration (AMD), which is also the leading cause of visual impairment in developed countries.1–4 Given that reading is the most frequent clinical complaint as well as the primary goal of patients with AMD seeking visual rehabilitation,1,5,6 it is of utmost importance to understand the limiting factors on reading in peripheral vision, and to devise rehabilitation methods or strategies that could improve reading performance for people with central vision loss. Following the onset of central vision loss, most people with the condition eventually adopt a retinal location close to the border of the central scotoma, the preferred retinal locus (PRL), as the reference location for visual tasks.7–10 The location of the PRL could change for different tasks and under different lighting levels,11–14 but in virtually all cases, the PRL is located away from the anatomical fovea. Because reading involves the recognition of letters, and that it has been known for a long time that letters are easier to read when they are in isolation than when they are presented with nearby letters (as in the case of words),15 especially in the periphery,16–18 the phenomenon of crowding has long been suggested as a viable explanation of slow reading in peripheral vision. However, the more pronounced crowding effect found in peripheral vision are mostly findings obtained using letter recognition tasks. There exist very few studies that compared the magnitude and/or the spatial extent of crowding with reading performance, by manipulating the stimulus in a comparable way. One study modulated the spatial extent of crowding in normal peripheral vision by manipulating the contrast polarity of adjacent letters.19 Letters were rendered black on a gray background (normal contrast polarity), or white on a gray background (reversed contrast polarity). When observers were asked to identify the orientation of the target (a letter T oriented in different orientations), the spatial extent of crowding was found to be greater when the target and its flankers shared the same contrast polarity (i.e. all letters were either black-on-gray, or white-on-gray), and smaller when the target and its flankers had opposite contrast polarity (i.e., a mix of black-on-gray and white-on-gray letters; see also Kooi et al20). However, when the same observers were asked to read text with alternate letters rendered in opposite contrast polarity, their reading speeds were virtually identical to those obtained when all the letters had the same contrast polarity. This finding implies that despite a reduction of crowding when adjacent letters had opposite contrast polarities, the manipulation did not benefit reading. Another study, capitalizing on the classical finding that the magnitude of crowding between letters is reduced (or the performance accuracy of letter identification is higher) when the separation between letters increases,16,21 found that reading speed in normal peripheral vision does not benefit from increased letter spacing beyond the standard spacing found in printed text.22 Although these studies convincingly show that the reduction in crowding does not benefit reading speed in normal peripheral vision, there is no published evidence showing whether or not a similar lack of benefit on reading speed would be found in people with central vision loss. There are at least two reasons to believe that people with central vision loss might show a different dependence of reading speed on letter spacing. First, to date, we still do not have conclusive evidence that the normal periphery is a good model for people with central vision loss. Second, given that most of the PRL locations are close to the central scotomas, the retinal regions corresponding to the PRL locations may not be as healthy as the normal retina. It is conceivable that the visual system may need to pool information over a larger area in order to establish a more reliable signal of a visual target, thus increasing the spatial extent of crowding. Therefore, the primary goal of this study was to examine whether or not a simple manipulation of text to reduce crowding, viz., increasing the letter spacing as in a previous study22 would benefit reading for people with central vision loss. From a scientific point of view, our result would indicate whether or not the normal periphery is a good model for visual processing for people with central vision loss. From a practical point of view, the result would provide us with information as to what the optimal letter spacing in text should be for people with central vision loss. The secondary goal of the study was to relate our finding to the current understanding of crowding. A current account of crowding is that because the visual system integrates features from objects within regions referred to as integration fields,a if features from nearby objects fall within the same integration field, then these features would be integrated erroneously, forming an incorrect percept of the objects.23,24,26,27,29–31 To avoid this undesirable inappropriate feature integration, nearby objects need to be separated from one another by a distance greater than the dimension of the integration field.26,27 In the normal periphery, some of the known properties of the integration field include (1) the shape of the integration field is anisotropic and oriented radially with respect to the fovea, with the radial dimension being greater than the tangential dimension;18,26,27,32 (2) the size of the integration field scales with eccentricity, with the half radial dimension approximating half the eccentricity at which the field is centered;16,18,26,27,32 and (3) inward-outward asymmetry whereby along the radial dimension, the flanker further away from fixation needs to be separated from the target letter by a distance greater than that between the target and the flanker closer to fixation.16,18,32,33 Applying the concept to reading, letters of a word may need to be separated from one another by a distance such that adjacent letters do not fall within the same integration field. Figure 1 is a schematic representation of how the size and shape of an integration field changes according to its location with respect to the fovea, and how the number of letters falling within an integration field could affect whether or not the letters are recognizable. In this figure, a phrase “their house” is presented above the fovea (represented by the gray cross). Three integration fields are depicted as ellipses a, b and c, according to properties 1 and 2 as stated above (although property 3 is relevant, in real life text is printed with regular spacing among letters, instead of having scaled spacing depending on where observers look, therefore in this paper, integration fields will be represented by ellipses that are symmetrical with respect to the center, instead of egg-shaped to take into consideration the inward-outward asymmetry). Ellipse a, centered directly above the fovea, is small enough that only letter t falls within this integration field, therefore, this letter should be easily recognizable. For ellipse b that centers on letter e, the horizontal dimension of the integration field matches the spacing between e and its adjacent letters h and i such that these adjacent letters fall just on the border of the integration field, allowing recognition of these three letters. However, for ellipse c, the size of the integration field is so large that many letters fall within the integration field. In this case, individual letters would not be recognizable and thus reading would suffer. One way to make these letters recognizable would be to increase the separation between adjacent letters. In other words, the size of the integration field places a direct limitation on the letter spacing for reading, at least in the normal periphery. Figure 1 A cartoon representation of how the size and shape of the integration field changes according to its location with respect to the fovea. A phrase “their house” is presented above the fovea (represented by the gray cross). Three ellipses ... In relation to our study, if the properties of the integration field at the PRL of an observer with central vision loss are similar to those at the same retinal location in the normal periphery, and if the size of the integration field limits the optimal letter spacing for reading, then the optimal letter spacing for reading should match the size of the integration field based on the retinal location of the PRL. Here, we attempted to determine whether the size of the integration field, which changes in a systematic way based on the retinal location in the normal periphery, could explain the optimal letter spacing for reading for our group of observers with central vision loss.