This project is a collaboration between Nick Hill, Yvonne Griffiths, Peter Bailey and Maggie Snowling. The work is funded by the Wellcome Trust (Project Grant No.55671) and is being carried out in the Hearing & Speech Laboratory and the Centre for Reading and Language of the Department of Psychology at the University of York.
The project will seek to confirm the presence of an auditory deficit and clarify its nature using a theoretically motivated battery of psychoacoustic tests. An early focus of the project will be a detailed examination of the hypothesis that dyslexics may have less precise synchronisation between auditory nerve discharges and the temporal fine structure of the stimulating waveform (i.e. phase locking).
Later stages of the project will assess the consequences of observed auditory impairments for speech perception. Finally, correlations between measures of performance on the psychoacoustic and phonological tests will be computed in an attempt to identify reliable psychoacoustic indicators of phonological performance.
More direct evidence of a link between dyslexia and the presence of a low-level auditory impairment has been reported by McAnally and Stein (1996, 1997), Hari and Kiesla (1996), and Protopapas et al. (1997). McAnally and Stein (1996) compared the performance of dyslexic and normal adults on a set of four psychoacoustic tests and found that while dyslexic adults were unimpaired on two measures of auditory acuity (gap detection and detection of a tone in noise), they were poorer at discriminating tone frequency, and exhibited less binaural release from masking than controls. McAnally and Stein interpreted this pattern of results as evidence that dyslexics may have less precise phase locking in the firing pattern of auditory nerve fibres, and went on to show that an electrophysiological indicator of phase locking, the amplitude of the frequency following response (FFR), was smaller for the dyslexic group.
If it can be confirmed that at least some aspects of dyslexia may result from impaired auditory processing, and the precise nature of the auditory impairment(s) identified, it may be possible to develop clinical tests allowing early detection of those children at risk from developing reading difficulties. Precise knowledge of the auditory impairments underlying dyslexia may also allow the impact of these impairments to be reduced through appropriate remedial training. As yet, however, the available data are only suggestive of a link between low-level auditory deficits and dyslexia. Moreover, one of the principal hypotheses put forward to account for some of the observed impairments, namely, that dyslexics have less precise phase locking in the firing pattern of auditory nerve fibres, has yet to be subjected to detailed examination, and cannot readily account for the data of Tallal (1980) concerning temporal order judgements.
The proposed research will focus specifically on adults with reading impairments. It is well established that the phonological processing deficits considered to be at the core of dyslexia persist into adulthood even in cases where reading difficulties may be compensated (Bruck, 1990; Felton et al., 1990; Pennington et al., 1990). Moreover, neuroimaging studies have shown reduced activation in brain areas associated with phonological processing in dyslexic adults (Paulesu et al., 1996). We assume that any auditory deficits in childhood to which impaired acquisition of reading skills may be attributable will likewise be unlikely to resolve with age. The focus on adult listeners will allow us to use the complex listening tasks and repetitive testing regimes that detailed psychoacoustical investigations require.
Peter Bailey Department of Psychology, University of York, York YO10 5DD Tel: 01904-433141, Fax: 01904-433181, Email: pjb1@york.ac.uk
updated sep99