Does a sensory processing deficit explain counting accuracy on rapid visual sequencing tasks in adults with and without dyslexia? Elizabeth G. Conlon a,⇑ , Craig M. Wright a , Karla Norris a , Eugene Chekaluk b a School of Psychology, Behavioural Basis of Health Research Centre, Griffith University, Gold Coast Campus, Queensland 4222, Australia b Department of Psychology, Macquarie University, Sydney, Australia article info Article history: Accepted 29 October 2010 Available online 23 March 2011 Keywords: Rapid sequential processing Dyslexia Magnocellular ISI Processing speed abstract The experiments conducted aimed to investigate whether reduced accuracy when counting stimuli pre- sented in rapid temporal sequence in adults with dyslexia could be explained by a sensory processing deficit, a general slowing in processing speed or difficulties shifting attention between stimuli. To achieve these aims, the influence of the inter-stimulus interval (ISI), stimulus duration, and sequence length were evaluated in two experiments. In the first that used skilled readers only, significantly more errors were found with presentation of long sequences when the ISI or stimulus durations were short. Experiment 2 used a wider range of ISIs and stimulus durations. Compared to skilled readers, a group with dyslexia had reduced accuracy on two-stimulus sequences when the ISI was short, but not when the ISI was long. Although reduced accuracy was found on all short and long sequences by the group with dyslexia, when performance on two-stimulus sequences was used as an index of sensory processing efficiency and con- trolled, group differences were found with presentation of stimuli of short duration only. We concluded that continuous, repetitive stimulation to the same visual area can produce a capacity limitation on rapid counting tasks in all readers when the ISIs or stimulus durations are short. While reduced accuracy on rapid sequential counting tasks can be explained by a sensory processing deficit when the stimulus dura- tion is long, slower processing speed in the group with dyslexia explains the greater inaccuracy found as sequence length is increased when the stimulus duration is short. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Dyslexia is a neurobiological disorder (Brunswick, McCory, Price, Frith, & Frith, 1999) that affects between 5% and 17% of indi- viduals in English speaking countries (Shaywitz, Escobar, Shaywitz, Fletcher, & Makugh, 1992). Adults with dyslexia have poor reading fluency and spelling, persistent phonological difficulties and aver- age or below average word reading skills (Bruck, 1990, 1992). Although it is well accepted that dyslexia is caused by one or more underlying neural deficits (Eckert, 2004), the view that dyslexia is the result of a visual sensory processing deficit in the magnocellu- lar functional stream which can in turn explain visual attentional deficits at a parietal level has been hotly debated (Hari & Renvall, 2001; Lallier et al., 2009; Lassus-Sangosse, N’guyen-Morel, & Valdois, 2008). The evidence for a sensory processing deficit in the magnocellu- lar/dorsal functional stream in dyslexia is based on convergent evi- dence of reduced sensitivity to stimuli that are rapidly presented or contain motion, but not in response to stationary or near stationary stimuli about form (Lovegrove, Martin, & Slaghuis, 1986; Stein, 2001). Stimuli presented in rapid sequences are preferentially pro- cessed in the magnocellular/dorsal (M) functional stream, with stimuli about form predominantly processed in the parvocellular/ ventral functional stream (Merigan & Maunsell, 1993). Supporting evidence for an M stream deficit has been obtained in studies which have shown poorer sensitivity on tests of M stream function in groups with dyslexia relative to controls using electrophysio- logical (Demb, Boynton, & Heeger, 1998; Eden et al., 1996; Schulte-Korne, Bartling, Diemel, & Remschmikdt, 2004a) and psychophysical techniques (Johnson et al., 2008; Slaghuis & Ryan, 2006; Wright & Conlon, 2009). Many studies have found that groups with dyslexia have re- duced accuracy compared to typical readers on tasks that measure speed of visual processing, such as temporal order judgements, vis- ible persistence and double flash detection (Boden & Brodeur, 1999; Chase & Jenner, 1993; Hari, Renvall, & Tanskanen, 2001; Harrison, Burdette, Flowers, Wood, & Wallace, 2005; Kinsbourne, Rufo, Gamzu, Palmer, & Berlinger, 1991; Meyler & Breznitz, 2005; Slaghuis & Pinkus, 1993; Van Ingelghem et al., 2001). These studies which have used two-stimulus sequences have shown that groups with dyslexia have poor accuracy when the inter-stimulus interval (ISI) used between stimulus presentations is short, but not when the ISI is long. These findings have been used to support the 0278-2626/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2010.10.014 ⇑ Corresponding author. Fax: +61 755 528291. E-mail address: e.conlon@griffith.edu.au (E.G. Conlon). Brain and Cognition 76 (2011) 197–205 Contents lists available at ScienceDirect Brain and Cognition journal homepage: www.elsevier.com/locate/b&c