Refractory e¡ects on auditory-evoked responses in children with reading disorders Mridula Sharma a , Suzanne C. Purdy a,b,c , Philip Newall c , Kevin Wheldall c,d and Robyn Beaman c,d a Department of Psychology, Tamaki Campus, University of Auckland, Auckland, New Zealand, b National Acoustic Laboratories, c Audiology Section, Linguistics Department, Macquarie University, Sydney, Australia and d Macquarie University Special Education Centre Correspondence and requests for reprints to Dr Mridula Sharma, BSc, MSc, PhD, Speech Science, Department of Psychology, Tamaki Campus, The University of Auckland, Private Bag 92019, Auckland, New Zealand Tel: + 64 9 373 7599 ext. 89260; fax: + 64 9 303 5978; e-mail: m.sharma@auckland.ac.nz The data collection was undertaken at National Acoustic Laboratories, Sydney, Australia. Received 20 July 2006; accepted17 August 2006 The current study investigated neural refractory e¡ects in children (8^12 years) with reading disorders and a control group. Cortical responses (P1and N250) to the sound /da / were measured at inter- stimulus intervals of 538, 1072 and 2152 ms. As expected, owing to slow neural recovery periods, both groups showed longer cortical response latencies at the shortest interstimulus interval of 538 ms. N250 showed a slower neural refractory period at the short interstimulus interval (538 ms) for children with reading disorders than the control group, however. Only control group children showed interhemispheric di¡erences for the N250 peak. No group di¡erences were evident for P1. The results suggest that children with reading disorders have di¡erent and slower underlying neural responses than typically developing children. NeuroReport 18:133^136  c 2007 Lippincott Williams & Wilkins. Keywords: auditory-evoked cortical potential, interstimulus interval, N250, neural refractory e¡ects, obligatory cortical responses, P1, reading disorder Introduction Event-related potentials are measures of electrical brain activity ‘time locked’ to a sensory stimulus or a specific event [1,2]. Auditory-evoked potentials are a subset of event-related potentials in which an auditory stimulus is the ‘event’ to which brain activity is locked [2]. These include the ‘obligatory’ (P1–N1–P2) cortical responses, referred to as obligatory because they depend on sensory aspects of the stimulus and are minimally affected by attention. The obligatory cortical responses include a positivity at around 60 ms, referred to as P1, followed by negativity at around 120 ms referred to as N1, followed by another positive peak at around 170 ms referred to as P2. The auditory cortical responses P1, N1 and P2 are believed to be principally generated by the primary and secondary auditory cortices in the superior and lateral surface of the superior temporal gyrus (see review in [1]). Obligatory cortical responses represent elementary levels of auditory sensory coding [3] that are minimally affected by attention [2], do not depend on the participant respond- ing to the stimulus, are sensitive to auditory processing deficits in children [4,5], and hence are ideal for objectively assessing populations such as young infants who cannot easily be tested reliably using behavioural techniques. The obligatory responses can be evoked by a range of speech and nonspeech stimuli. It is suggested that P1 originates from the auditory thalamic and primary and/or secondary auditory cortices [6,7]. For faster stimulus repetition rates (i.e. shorter interstimulus intervals), P2 emerges in the cortical response waveform at about 8–10 years. P2 shows significant variability in children below 15 years of age [4,7]. The auditory N1 wave is a prominent negative peak in the cortical waveform from an early age. School-aged children can show both an early and a late ‘N1’ [8]. In young children, morphology of earlier (‘N160’) and later (‘N250’) N1 peaks varies with the interstimulus interval (ISI) [8]. At short ISIs (less than 1 s), children show P1 and N250 response most consistently to auditory stimulation [8–10]. Evidence exists that, in children, N250 is generated in the region of Heschl’s gyrus in the supratemporal plane [10]. Although obligatory cortical responses are robust and can be elicited by a wide range of stimuli, the N1 response, in particular, reduces with faster stimulus repetition [11,12]. This phenomenon has been related to neural recovery cycles or refractory periods. The concept of a recovery cycle or refractory period derives from studies of single nerve cells where there is a recovery period following an action potential, during which time further stimulation does not result in an action potential [12]. The refractory period is an index of processing rates within cortical sensory areas [13]. Although the nature of the neural mechanisms underlying the refractory period are based on conjecture, N1 amplitude decrements are observed possibly because of the ‘temporal limitations inherent in the physiochemical mechanisms’ [12]. If auditory stimuli are presented during the period when there is no adequate neural recovery, a decline in N1 amplitude should occur [11,12]. LEARNING AND MEMORY NEUROREPORT Vol 18 No 2 22 January 2007 133 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.