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There is a PowerPoint file available to accompany this presentation. The BDA Conference 2001 CD-ROM contains 61 PowerPoint files. For details of how to obtain the CD-ROM, please contact the BDA.

Symposium; Infancy

Thursday stream 2 Session 09.00 - 11.10 Length 25 minutes

Early development of visual movement detection in infants at risk for dyslexia and controls

Pieter H. Been

University of Groningen Pieter@radix.let.rug.nl

Abstract

The seminal work of Vellutino (1979) seemed to establish once and for all that in the majority of dyslexics reading problems are linguistic and not visual. Nowadays the predominant view is that dyslexia is a developmental language disorder which is related to deficient skills for segmenting the temporal structure of language. This applies particularly to the ability of segmenting words into phonemes, labeled as 'phonological awareness' or 'phonological sensitivity'. However, a number of experiments has shown that the magno-part of the visual system of dyslexics suffer from impairment when compared to normal readers. For that reason development of the magno-cellular part of the visual system is assessed in the Dutch Dyslexia program at regular intervals starting at the age of 2 months. One of the paradigms assesses visual potentials of movement detection evoked by presenting moving dots. The movement detection paradigm is robust. First, differences between developmental dyslexics and normal readers have been replicated repeatedly. Second, the paradigm is well suited for use with infants of two months and older. Experimental drop out is minimal. The assessment of movement detection in infants should take into account that movement introduces also temporal modulation, e.g., a moving dot shown at successive temporal intervals at a computer screen has the spatial property of movement and the temporal property of flicker. Sensitivity to temporal properties develops earlier than sensitivity to spatial properties. A paradigm that disentangles the temporal and directional properties of movement has been developed by Wattam-Bell. In the paradigm a shifting random dot pattern is shown in which a movement pattern is embedded. The pattern shifts in a random fashion at a frequency of 8 Hz while the direction of movement is changed at a frequency of 4 Hz. At the age of two month only the temporal properties of the stimulus show up in the VEP. Starting at the age of 3.5 month also the movement reversals can be detected in the averaged VEP. In the Dutch Dyslexia program assessments are made at the ages of 2 and 5 months. If developmental dyslexia is associated with a developmental delay or an abnormal development of the magno-cellular part of the visual system, the detection of movement reversals in the at risk group should be impaired compared to control infants. Results corroborating or rejecting this prediction shall be presented.