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Use of the Hermann Grid Illusion in the Measurement of Contrast Perception in Dyslexia

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Use of the Hermann Grid Illusion in the Measurement of Contrast Perception in Dyslexia CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Vision Research 45 (2005) 1–8 www.elsevier.com/locate/visres Rapid communications Use of the Hermann grid illusion in the measurement of contrast perception in dyslexia James M. Gilchrist a,*, Barbara K. Pierscionek b, William M. Mann a a Department of Optometry, University of Bradford, Richmond Road, Bradford, BD7 1DP West Yorkshire, UK b School of Biomedical Sciences, University of Ulster, Cromore Road, Coleraine, BT52 1SA Northern Ireland, UK Received 12 March 2004; received in revised form 28 July 2004 Abstract We measured contrast thresholds for perception of the Hermann grid illusion, using different contrast polarities and mean lumi- nances, in dyslexics and non-dyslexics. Both groups of subjects gave significantly lower thresholds with grids having dark squares and light paths, but there was no significant threshold difference between groups. Perceived strength of illusion was also measured in grids at suprathreshold contrast levels. Dyslexics perceived the illusion to be significantly stronger than non-dyslexics when the grid had light paths and low luminance. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Dyslexia; Contrast perception; Hermann grid; Magnocellular 1. Introduction (Livingstone, Rosen, Drislane, & Galaburda, 1991) and psychophysical studies which indicate that some Dyslexia has been defined as a specific reading diffi- dyslexics suffer reduced sensitivity to contrast, flicker culty, inexplicable by any deficit in intelligence, learning and/or motion (Cornelissen, Richardson, Mason, & opportunity, motivation or sensory acuity (Critchley, Stein, 1995; Demb, Boynton, Best, & Heeger, 1998; 1970). The signs of dyslexia are varied and include Everatt, Bradshaw, & Hibbard, 1999; Lovegrove, abnormal phonological awareness, difficulties with writ- 1991; Lovegrove, Bowling, Badcock, & Blackwood, ing, spelling, auditory discrimination and visual process- 1980). However, the magnocellular deficit theory of dys- ing (Habib, 2000). Aspects of vision that appear lexia is by no means universally accepted, particularly correlated with dyslexia are oculomotor instability, im- with regard to contrast perception. In a review of the paired magnocellular processing and increased visual evidence from contrast sensitivity, Skottun (2000) re- discomfort/disturbance due to pattern glare effects ported that, while some studies have found contrast sen- (Evans, 2001). The last two of these may be associated sitivity loss in dyslexics consistent with a magnocellular with changes in contrast perception at threshold and/ deficit, these are outnumbered by studies that show or suprathreshold levels. either no loss of sensitivity, or loss of sensitivity incon- There is evidence that a deficit of the magnocellular sistent with a magnocellular deficit (see also Stuart, system is a significant correlate of dyslexia (Stein & McAnally, & Castles, 2001). More recent studies, too, Walsh, 1997). Support for this comes from anatomical cast doubt on the basic claim for contrast sensitivity loss in dyslexia, with Williams, Stuart, Castles, and McA- * Corresponding author. Tel.: +44 1274 234630. nally (2003) finding no significant difference in con- E-mail address: [email protected] (J.M. Gilchrist). trast sensitivity between dyslexics and controls, while 0042-6989/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2004.08.020 2 J.M. Gilchrist et al. / Vision Research 45 (2005) 1–8 Bednarek and Grabowska (2002) report increased con- achieved faster visual search performance at low con- trast sensitivity in subjects with poor reading and writ- trast, consistent with observations by Meares (1980), ing skills (see also Lovegrove et al., 1982). but OÕBrien, Mansfield, and Legge (2000) found that Differences in contrast perception may also be impli- the dependence of reading speed on text contrast was cated in the occurrence of visual discomfort and distur- the same for dyslexics and controls. bance that is often reported by dyslexics. Some Overall, therefore, we have no definitive picture of individuals are susceptible to such effects when viewing how the contrast responses of dyslexics differ from those repetitive patterns, including text (Wilkins, 1991, 1993, of non-dyslexics, with or without the effects of pattern 1995; Wilkins & Nimmo-Smith, 1987; Wilkins et al., glare. In this study we explore an alternative approach 1984). Wilkins et al. (1984) found headaches, visual to direct psychophysical measurement of contrast re- discomfort and illusory effects in observers viewing sponses in dyslexics and non-dyslexics using the Her- grating patterns, and demonstrated that their occur- mann grid illusion. This is the appearance of illusory rence was influenced by a range of parameters includ- grey spots at the intersections of pathways in a two ing spatial frequency, contrast, location in the visual dimensional array of squares. Baumgartner (1960, cited field, and whether viewed with one or both eyes. Visual by Spillmann & Levine, 1971) explained the illusion in discomfort and disturbance associated with repetitive, terms of receptive fields of on- or off-centre visual neu- high contrast stimuli has been called Ôpattern glareÕ rons, depending on the polarity of the grid. For a grid (Wilkins, 1993), and its effects may be significant, with dark squares and light paths (Fig. 1), it was sug- reducing the clarity and comfort of printed text (Wil- gested that an on-centre receptive field centred on a path kins & Nimmo-Smith, 1987), and the speed and accu- intersection would receive about twice as much inhibi- racy of visual search (Conlon et al., 1998). Meares tion from the field surround as an on-centre receptive (1980) was the first to report that pattern glare effects field illuminated by a light bar. Hence the firing rate are common in children with reading disabilities, and of the neuron viewing the Hermann grid would be re- she found that the reading performance of such chil- duced and the path intersection should appear darker. dren may be improved with smaller print, coloured The opposite effect would be produced in grids with paper or reduced contrast. Alleviation of pattern glare dark paths due to the responses of off-centre neurons lo- by coloured filters was proposed by Irlen (1983) who cated at path intersections. The disappearance of the also claimed that the condition is particularly common illusion on fixation at grid intersections has been attrib- in dyslexia, affecting 12% of the general population but uted to the small size of foveal receptive field centres 65% of dyslexics (Irlen, 1991). Evans, Cook, Richards, (Spillmann, 1994). Grid contrast influences detection and Drasdo (1994) also suggest that susceptibility to of the illusion (Spillmann & Levine, 1971) and sensitiv- pattern glare may be more prevalent in dyslexics, par- ity to it can be reduced in certain disease states that dis- ticularly those who also have an oculomotor deficit turb contrast perception, such as diabetes (Davies & (Evans, Busby, Jeanes, & Wilkins, 1995; Evans, Morland, 2002). Drasdo, & Richards, 1994). Like gratings, Hermann grid patterns enable a ÔpureÕ The pattern glare effect, or Meares–Irlen Syndrome measure of contrast perception, avoiding the effects of (MIS) (Evans et al., 1995), has been described as a form typography, as well as the other sensory and cognitive of physiological hyperexcitability (Wilkins, 1993) which factors involved in text reading. In addition, the grid might be demonstrated by differences in pattern contrast stimulus has general characteristics that are most likely sensitivity. However, an investigation into the aetiology to elicit a pattern glare response; a high contrast, repet- of MIS found no significant difference in static contrast itive pattern structure containing predominantly low to sensitivity between MIS subjects and controls over a medium spatial frequencies. Also, since the illusory grid range of spatial frequencies from 1 to 12cdegÀ1 (Evans response derives from the characteristics of visual et al., 1996). On the other hand, Conlon, Lovegrove, receptive fields, it is possible to selectively stimulate Barker, and Chekaluk (2001) measured perceived distor- receptive fields of different sizes by manipulating the tion and contrast sensitivity in three groups of subjects dimensions of grid paths and squares. If dyslexics are classified according to their visual discomfort level. They indeed more sensitive than non-dyslexics to the struc- found that those who reported high visual discomfort ture and contrast of printed text and other repetitive saw greater distortion with low frequency gratings, patterns, then there may be some manifestation of this and gave reduced static contrast sensitivity in the range in their responses to the Hermann grid illusion. This 1–12cdegÀ1 but no loss of contrast sensitivity for 6Hz work therefore sought to investigate whether dyslexic temporally modulated gratings. and non-dyslexic subjects differ in their contrast sensi- Studies using suprathreshold contrast levels have also tivity for perception of the Hermann grid illusion, reported results that are difficult to interpret consist- and to determine whether there is any difference in ently. Williams, May, Solman, and Zhou (1995), for the perceived strength of illusion between the two example, found that subjects with reading difficulty groups. J.M. Gilchrist et al. / Vision Research 45 (2005) 1–8 3 Fig. 1. (a) Hermann grid patterns used for Experiment 1 (threshold
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